text:
Conditions for Using Bison
1 The Concepts of Bison
1.1 Languages and Context-Free Grammars
1.2 From Formal Rules to Bison Input
1.3 Semantic Values
1.4 Semantic Actions
1.5 Writing GLR Parsers
1.5.1 Using GLR on Unambiguous Grammars
1.5.2 Using GLR to Resolve Ambiguities
1.5.3 GLR Semantic Actions
126.96.36.199 Deferred semantic actions
220.127.116.11 Restrictions on semantic values and locations
1.5.4 Controlling a Parse with Arbitrary Predicates
1.5.5 Considerations when Compiling GLR Parsers
1.6 Locations
1.7 Bison Output: the Parser Implementation File
1.8 Stages in Using Bison
1.9 The Overall Layout of a Bison Grammar
2 Examples
2.1 Reverse Polish Notation Calculator
2.1.1 Declarations for 'rpcalc'
2.1.2 Grammar Rules for 'rpcalc'
188.8.131.52 Explanation of 'input'
22.214.171.124 Explanation of 'line'
184.108.40.206 Explanation of 'expr'
2.1.3 The 'rpcalc' Lexical Analyzer
2.1.4 The Controlling Function
2.1.5 The Error Reporting Routine
2.1.6 Running Bison to Make the Parser
2.1.7 Compiling the Parser Implementation File
2.2 Infix Notation Calculator: 'calc'
2.3 Simple Error Recovery
2.4 Location Tracking Calculator: 'ltcalc'
2.4.1 Declarations for 'ltcalc'
2.4.2 Grammar Rules for 'ltcalc'
2.4.3 The 'ltcalc' Lexical Analyzer.
2.5 Multi-Function Calculator: 'mfcalc'
2.5.1 Declarations for 'mfcalc'
2.5.2 Grammar Rules for 'mfcalc'
2.5.3 The 'mfcalc' Symbol Table
2.5.4 The 'mfcalc' Lexer
2.5.5 The 'mfcalc' Main
2.6 Exercises
3 Bison Grammar Files
3.1 Outline of a Bison Grammar
3.1.1 The prologue
3.1.2 Prologue Alternatives
3.1.3 The Bison Declarations Section
3.1.4 The Grammar Rules Section
3.1.5 The epilogue
3.2 Symbols, Terminal and Nonterminal
3.3 Grammar Rules
3.3.1 Syntax of Grammar Rules
3.3.2 Empty Rules
3.3.3 Recursive Rules
3.4 Defining Language Semantics
3.4.1 Data Types of Semantic Values
3.4.2 More Than One Value Type
3.4.3 Generating the Semantic Value Type
3.4.4 The Union Declaration
3.4.5 Providing a Structured Semantic Value Type
3.4.6 Actions
3.4.7 Data Types of Values in Actions
3.4.8 Actions in Mid-Rule
18.104.22.168 Using Mid-Rule Actions
188.8.131.52 Mid-Rule Action Translation
188.8.131.52 Conflicts due to Mid-Rule Actions
3.5 Tracking Locations
3.5.1 Data Type of Locations
3.5.2 Actions and Locations
3.5.3 Default Action for Locations
3.6 Named References
3.7 Bison Declarations
3.7.1 Require a Version of Bison
3.7.2 Token Type Names
3.7.3 Operator Precedence
3.7.4 Nonterminal Symbols
3.7.5 Performing Actions before Parsing
3.7.6 Freeing Discarded Symbols
3.7.7 Printing Semantic Values
3.7.8 Suppressing Conflict Warnings
3.7.9 The Start-Symbol
3.7.10 A Pure (Reentrant) Parser
3.7.11 A Push Parser
3.7.12 Bison Declaration Summary
3.7.13 %define Summary
3.7.14 %code Summary
3.8 Multiple Parsers in the Same Program
4 Parser C-Language Interface
4.1 The Parser Function 'yyparse'
4.2 The Push Parser Function 'yypush_parse'
4.3 The Pull Parser Function 'yypull_parse'
4.4 The Parser Create Function 'yystate_new'
4.5 The Parser Delete Function 'yystate_delete'
4.6 The Lexical Analyzer Function 'yylex'
4.6.1 Calling Convention for 'yylex'
4.6.2 Semantic Values of Tokens
4.6.3 Textual Locations of Tokens
4.6.4 Calling Conventions for Pure Parsers
4.7 The Error Reporting Function 'yyerror'
4.8 Special Features for Use in Actions
4.9 Parser Internationalization
5 The Bison Parser Algorithm
5.1 Lookahead Tokens
5.2 Shift/Reduce Conflicts
5.3 Operator Precedence
5.3.1 When Precedence is Needed
5.3.2 Specifying Operator Precedence
5.3.3 Specifying Precedence Only
5.3.4 Precedence Examples
5.3.5 How Precedence Works
5.3.6 Using Precedence For Non Operators
5.4 Context-Dependent Precedence
5.5 Parser States
5.6 Reduce/Reduce Conflicts
5.7 Mysterious Conflicts
5.8 Tuning LR
5.8.1 LR Table Construction
5.8.2 Default Reductions
5.8.4 Unreachable States
5.9 Generalized LR (GLR) Parsing
5.10 Memory Management, and How to Avoid Memory Exhaustion
6 Error Recovery
7 Handling Context Dependencies
7.1 Semantic Info in Token Types
7.2 Lexical Tie-ins
7.3 Lexical Tie-ins and Error Recovery
8 Debugging Your Parser
8.1 Understanding Your Parser
8.2 Visualizing Your Parser
8.3 Visualizing your parser in multiple formats
8.4 Tracing Your Parser
8.4.1 Enabling Traces
8.4.2 Enabling Debug Traces for 'mfcalc'
8.4.3 The 'YYPRINT' Macro
9 Invoking Bison
9.1 Bison Options
9.2 Option Cross Key
9.3 Yacc Library
10 Parsers Written In Other Languages
10.1 C++ Parsers
10.1.1 C++ Bison Interface
10.1.2 C++ Semantic Values
10.1.2.1 C++ Unions
10.1.2.2 C++ Variants
10.1.3 C++ Location Values
10.1.3.1 C++ 'position'
10.1.3.2 C++ 'location'
10.1.3.3 User Defined Location Type
10.1.4 C++ Parser Interface
10.1.5 C++ Scanner Interface
10.1.5.1 Split Symbols
10.1.5.2 Complete Symbols
10.1.6 A Complete C++ Example
10.1.6.1 Calc++ -- C++ Calculator
10.1.6.2 Calc++ Parsing Driver
10.1.6.3 Calc++ Parser
10.1.6.4 Calc++ Scanner
10.1.6.5 Calc++ Top Level
10.2 Java Parsers
10.2.1 Java Bison Interface
10.2.2 Java Semantic Values
10.2.3 Java Location Values
10.2.4 Java Parser Interface
10.2.5 Java Scanner Interface
10.2.6 Special Features for Use in Java Actions
10.2.7 Java Push Parser Interface
10.2.8 Differences between C/C++ and Java Grammars
10.2.9 Java Declarations Summary
11 Frequently Asked Questions
11.1 Memory Exhausted
11.2 How Can I Reset the Parser
11.3 Strings are Destroyed
11.4 Implementing Gotos/Loops
11.5 Multiple start-symbols
11.6 Secure? Conform?
11.7 I can't build Bison
11.8 Where can I find help?
11.9 Bug Reports
11.10 More Languages
11.11 Beta Testing
11.12 Mailing Lists
Appendix A Bison Symbols
Appendix B Glossary
Appendix C Copying This Manual
Index of Terms
This manual (23 October 2013) is for GNU Bison (version 3.0.2), the GNU
parser generator.
Copyright (C) 1988-1993, 1995, 1998-2013 Free Software Foundation,
Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License,
Version 1.3 or any later version published by the Free Software
Foundation; with no Invariant Sections, with the Front-Cover texts
being "A GNU Manual," and with the Back-Cover Texts as in (a)
below. A copy of the license is included in the section entitled
"GNU Free Documentation License."
(a) The FSF's Back-Cover Text is: "You have the freedom to copy and
modify this GNU manual. Buying copies from the FSF supports it in
developing GNU and promoting software freedom."
"Bison" is a general-purpose parser generator that converts an annotated
context-free grammar into a deterministic LR or generalized LR (GLR)
parser employing LALR(1) parser tables. As an experimental feature,
Bison can also generate IELR(1) or canonical LR(1) parser tables. Once
you are proficient with Bison, you can use it to develop a wide range of
language parsers, from those used in simple desk calculators to complex
programming languages.
Bison is upward compatible with Yacc: all properly-written Yacc
grammars ought to work with Bison with no change. Anyone familiar with
Yacc should be able to use Bison with little trouble. You need to be
fluent in C or C++ programming in order to use Bison or to understand
this manual. Java is also supported as an experimental feature.
We begin with tutorial chapters that explain the basic concepts of
using Bison and show three explained examples, each building on the
last. If you don't know Bison or Yacc, start by reading these chapters.
Reference chapters follow, which describe specific aspects of Bison in
Bison was written originally by Robert Corbett. Richard Stallman
made it Yacc-compatible. Wilfred Hansen of Carnegie Mellon University
added multi-character string literals and other features. Since then,
Bison has grown more robust and evolved many other new features thanks
to the hard work of a long list of volunteers. For details, see the
'THANKS' and 'ChangeLog' files included in the Bison distribution.
This edition corresponds to version 3.0.2 of Bison.
Conditions for Using Bison
The distribution terms for Bison-generated parsers permit using the
parsers in nonfree programs. Before Bison version 2.2, these extra
permissions applied only when Bison was generating LALR(1) parsers in C.
And before Bison version 1.24, Bison-generated parsers could be used
only in programs that were free software.
The other GNU programming tools, such as the GNU C compiler, have
never had such a requirement. They could always be used for nonfree
software. The reason Bison was different was not due to a special
policy decision; it resulted from applying the usual General Public
License to all of the Bison source code.
The main output of the Bison utility--the Bison parser implementation
file--contains a verbatim copy of a sizable piece of Bison, which is the
code for the parser's implementation. (The actions from your grammar
are inserted into this implementation at one point, but most of the rest
of the implementation is not changed.) When we applied the GPL terms to
the skeleton code for the parser's implementation, the effect was to
restrict the use of Bison output to free software.
We didn't change the terms because of sympathy for people who want to
make software proprietary. *Software should be free.* But we concluded
that limiting Bison's use to free software was doing little to encourage
people to make other software free. So we decided to make the practical
conditions for using Bison match the practical conditions for using the
other GNU tools.
This exception applies when Bison is generating code for a parser.
You can tell whether the exception applies to a Bison output file by
inspecting the file for text beginning with "As a special exception...".
The text spells out the exact terms of the exception.
Version 3, 29 June 2007
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1 The Concepts of Bison
This chapter introduces many of the basic concepts without which the
details of Bison will not make sense. If you do not already know how to
use Bison or Yacc, we suggest you start by reading this chapter
1.1 Languages and Context-Free Grammars
In order for Bison to parse a language, it must be described by a
"context-free grammar". This means that you specify one or more
"syntactic groupings" and give rules for constructing them from their
parts. For example, in the C language, one kind of grouping is called
an 'expression'. One rule for making an expression might be, "An
expression can be made of a minus sign and another expression". Another
would be, "An expression can be an integer". As you can see, rules are
often recursive, but there must be at least one rule which leads out of
the recursion.
The most common formal system for presenting such rules for humans to
read is "Backus-Naur Form" or "BNF", which was developed in order to
specify the language Algol 60. Any grammar expressed in BNF is a
context-free grammar. The input to Bison is essentially
machine-readable BNF.
There are various important subclasses of context-free grammars.
Although it can handle almost all context-free grammars, Bison is
optimized for what are called LR(1) grammars. In brief, in these
grammars, it must be possible to tell how to parse any portion of an
input string with just a single token of lookahead. For historical
reasons, Bison by default is limited by the additional restrictions of
LALR(1), which is hard to explain simply. *Note Mysterious Conflicts::,
for more information on this. As an experimental feature, you can
escape these additional restrictions by requesting IELR(1) or canonical
LR(1) parser tables. *Note LR Table Construction::, to learn how.
Parsers for LR(1) grammars are "deterministic", meaning roughly that
the next grammar rule to apply at any point in the input is uniquely
determined by the preceding input and a fixed, finite portion (called a
"lookahead") of the remaining input. A context-free grammar can be
"ambiguous", meaning that there are multiple ways to apply the grammar
rules to get the same inputs. Even unambiguous grammars can be
"nondeterministic", meaning that no fixed lookahead always suffices to
determine the next grammar rule to apply. With the proper declarations,
Bison is also able to parse these more general context-free grammars,
using a technique known as GLR parsing (for Generalized LR). Bison's GLR
parsers are able to handle any context-free grammar for which the number
of possible parses of any given string is finite.
In the formal grammatical rules for a language, each kind of
syntactic unit or grouping is named by a "symbol". Those which are
built by grouping smaller constructs according to grammatical rules are
called "nonterminal symbols"; those which can't be subdivided are called
"terminal symbols" or "token types". We call a piece of input
corresponding to a single terminal symbol a "token", and a piece
corresponding to a single nonterminal symbol a "grouping".
We can use the C language as an example of what symbols, terminal and
nonterminal, mean. The tokens of C are identifiers, constants (numeric
and string), and the various keywords, arithmetic operators and
punctuation marks. So the terminal symbols of a grammar for C include
'identifier', 'number', 'string', plus one symbol for each keyword,
operator or punctuation mark: 'if', 'return', 'const', 'static', 'int',
'char', 'plus-sign', 'open-brace', 'close-brace', 'comma' and many more.
(These tokens can be subdivided into characters, but that is a matter of
lexicography, not grammar.)
Here is a simple C function subdivided into tokens:
int /* keyword 'int' */
square (int x) /* identifier, open-paren, keyword 'int',
identifier, close-paren */
{ /* open-brace */
return x * x; /* keyword 'return', identifier, asterisk,
identifier, semicolon */
} /* close-brace */
The syntactic groupings of C include the expression, the statement,
the declaration, and the function definition. These are represented in
the grammar of C by nonterminal symbols 'expression', 'statement',
'declaration' and 'function definition'. The full grammar uses dozens
of additional language constructs, each with its own nonterminal symbol,
in order to express the meanings of these four. The example above is a
function definition; it contains one declaration, and one statement. In
the statement, each 'x' is an expression and so is 'x * x'.
Each nonterminal symbol must have grammatical rules showing how it is
made out of simpler constructs. For example, one kind of C statement is
the 'return' statement; this would be described with a grammar rule
which reads informally as follows:
A 'statement' can be made of a 'return' keyword, an 'expression'
and a 'semicolon'.
There would be many other rules for 'statement', one for each kind of
statement in C.
One nonterminal symbol must be distinguished as the special one which
defines a complete utterance in the language. It is called the "start
symbol". In a compiler, this means a complete input program. In the C
language, the nonterminal symbol 'sequence of definitions and
declarations' plays this role.
For example, '1 + 2' is a valid C expression--a valid part of a C
program--but it is not valid as an _entire_ C program. In the
context-free grammar of C, this follows from the fact that 'expression'
is not the start symbol.
The Bison parser reads a sequence of tokens as its input, and groups
the tokens using the grammar rules. If the input is valid, the end
result is that the entire token sequence reduces to a single grouping
whose symbol is the grammar's start symbol. If we use a grammar for C,
the entire input must be a 'sequence of definitions and declarations'.
If not, the parser reports a syntax error.
1.2 From Formal Rules to Bison Input
A formal grammar is a mathematical construct. To define the language
for Bison, you must write a file expressing the grammar in Bison syntax:
a "Bison grammar" file. *Note Bison Grammar Files: Grammar File.
A nonterminal symbol in the formal grammar is represented in Bison
input as an identifier, like an identifier in C. By convention, it
should be in lower case, such as 'expr', 'stmt' or 'declaration'.
The Bison representation for a terminal symbol is also called a
"token type". Token types as well can be represented as C-like
identifiers. By convention, these identifiers should be upper case to
distinguish them from nonterminals: for example, 'INTEGER',
'IDENTIFIER', 'IF' or 'RETURN'. A terminal symbol that stands for a
particular keyword in the language should be named after that keyword
converted to upper case. The terminal symbol 'error' is reserved for
error recovery. *Note Symbols::.
A terminal symbol can also be represented as a character literal,
just like a C character constant. You should do this whenever a token
is just a single character (parenthesis, plus-sign, etc.): use that same
character in a literal as the terminal symbol for that token.
A third way to represent a terminal symbol is with a C string
constant containing several characters. *Note Symbols::, for more
The grammar rules also have an expression in Bison syntax. For
example, here is the Bison rule for a C 'return' statement. The
semicolon in quotes is a literal character token, representing part of
the C syntax for the statement; the naked semicolon, and the colon, are
Bison punctuation used in every rule.
stmt: RETURN expr ';' ;
*Note Syntax of Grammar Rules: Rules.
1.3 Semantic Values
A formal grammar selects tokens only by their classifications: for
example, if a rule mentions the terminal symbol 'integer constant', it
means that _any_ integer constant is grammatically valid in that
position. The precise value of the constant is irrelevant to how to
parse the input: if 'x+4' is grammatical then 'x+1' or 'x+3989' is
equally grammatical.
But the precise value is very important for what the input means once
it is parsed. A compiler is useless if it fails to distinguish between
4, 1 and 3989 as constants in the program! Therefore, each token in a
Bison grammar has both a token type and a "semantic value". *Note
Defining Language Semantics: Semantics, for details.
The token type is a terminal symbol defined in the grammar, such as
'INTEGER', 'IDENTIFIER' or '',''. It tells everything you need to know
to decide where the token may validly appear and how to group it with
other tokens. The grammar rules know nothing about tokens except their
The semantic value has all the rest of the information about the
meaning of the token, such as the value of an integer, or the name of an
identifier. (A token such as '','' which is just punctuation doesn't
need to have any semantic value.)
For example, an input token might be classified as token type
'INTEGER' and have the semantic value 4. Another input token might have
the same token type 'INTEGER' but value 3989. When a grammar rule says
that 'INTEGER' is allowed, either of these tokens is acceptable because
each is an 'INTEGER'. When the parser accepts the token, it keeps track
of the token's semantic value.
Each grouping can also have a semantic value as well as its
nonterminal symbol. For example, in a calculator, an expression
typically has a semantic value that is a number. In a compiler for a
programming language, an expression typically has a semantic value that
is a tree structure describing the meaning of the expression.
1.4 Semantic Actions
In order to be useful, a program must do more than parse input; it must
also produce some output based on the input. In a Bison grammar, a
grammar rule can have an "action" made up of C statements. Each time
the parser recognizes a match for that rule, the action is executed.
*Note Actions::.
Most of the time, the purpose of an action is to compute the semantic
value of the whole construct from the semantic values of its parts. For
example, suppose we have a rule which says an expression can be the sum
of two expressions. When the parser recognizes such a sum, each of the
subexpressions has a semantic value which describes how it was built up.
The action for this rule should create a similar sort of value for the
newly recognized larger expression.
For example, here is a rule that says an expression can be the sum of
two subexpressions:
expr: expr '+' expr { $$ = $1 + $3; } ;
The action says how to produce the semantic value of the sum expression
from the values of the two subexpressions.
1.5 Writing GLR Parsers
In some grammars, Bison's deterministic LR(1) parsing algorithm cannot
decide whether to apply a certain grammar rule at a given point. That
is, it may not be able to decide (on the basis of the input read so far)
which of two possible reductions (applications of a grammar rule)
applies, or whether to apply a reduction or read more of the input and
apply a reduction later in the input. These are known respectively as
"reduce/reduce" conflicts (*note Reduce/Reduce::), and "shift/reduce"
conflicts (*note Shift/Reduce::).
To use a grammar that is not easily modified to be LR(1), a more
general parsing algorithm is sometimes necessary. If you include
'%glr-parser' among the Bison declarations in your file (*note Grammar
Outline::), the result is a Generalized LR (GLR) parser. These parsers
handle Bison grammars that contain no unresolved conflicts (i.e., after
applying precedence declarations) identically to deterministic parsers.
However, when faced with unresolved shift/reduce and reduce/reduce
conflicts, GLR parsers use the simple expedient of doing both,
effectively cloning the parser to follow both possibilities. Each of
the resulting parsers can again split, so that at any given time, there
can be any number of possible parses being explored. The parsers
proceed in lockstep; that is, all of them consume (shift) a given input
symbol before any of them proceed to the next. Each of the cloned
parsers eventually meets one of two possible fates: either it runs into
a parsing error, in which case it simply vanishes, or it merges with
another parser, because the two of them have reduced the input to an
identical set of symbols.
During the time that there are multiple parsers, semantic actions are
recorded, but not performed. When a parser disappears, its recorded
semantic actions disappear as well, and are never performed. When a
reduction makes two parsers identical, causing them to merge, Bison
records both sets of semantic actions. Whenever the last two parsers
merge, reverting to the single-parser case, Bison resolves all the
outstanding actions either by precedences given to the grammar rules
involved, or by performing both actions, and then calling a designated
user-defined function on the resulting values to produce an arbitrary
merged result.
1.5.1 Using GLR on Unambiguous Grammars
In the simplest cases, you can use the GLR algorithm to parse grammars
that are unambiguous but fail to be LR(1). Such grammars typically
require more than one symbol of lookahead.
Consider a problem that arises in the declaration of enumerated and
subrange types in the programming language Pascal. Here are some
type subrange = lo .. hi;
type enum = (a, b, c);
The original language standard allows only numeric literals and constant
identifiers for the subrange bounds ('lo' and 'hi'), but Extended Pascal
(ISO/IEC 10206) and many other Pascal implementations allow arbitrary
expressions there. This gives rise to the following situation,
containing a superfluous pair of parentheses:
type subrange = (a) .. b;
Compare this to the following declaration of an enumerated type with
only one value:
type enum = (a);
(These declarations are contrived, but they are syntactically valid, and
more-complicated cases can come up in practical programs.)
These two declarations look identical until the '..' token. With
normal LR(1) one-token lookahead it is not possible to decide between
the two forms when the identifier 'a' is parsed. It is, however,
desirable for a parser to decide this, since in the latter case 'a' must
become a new identifier to represent the enumeration value, while in the
former case 'a' must be evaluated with its current meaning, which may be
a constant or even a function call.
You could parse '(a)' as an "unspecified identifier in parentheses",
to be resolved later, but this typically requires substantial
contortions in both semantic actions and large parts of the grammar,
where the parentheses are nested in the recursive rules for expressions.
You might think of using the lexer to distinguish between the two
forms by returning different tokens for currently defined and undefined
identifiers. But if these declarations occur in a local scope, and 'a'
is defined in an outer scope, then both forms are possible--either
locally redefining 'a', or using the value of 'a' from the outer scope.
So this approach cannot work.
A simple solution to this problem is to declare the parser to use the
GLR algorithm. When the GLR parser reaches the critical state, it
merely splits into two branches and pursues both syntax rules
simultaneously. Sooner or later, one of them runs into a parsing error.
If there is a '..' token before the next ';', the rule for enumerated
types fails since it cannot accept '..' anywhere; otherwise, the
subrange type rule fails since it requires a '..' token. So one of the
branches fails silently, and the other one continues normally,
performing all the intermediate actions that were postponed during the
If the input is syntactically incorrect, both branches fail and the
parser reports a syntax error as usual.
The effect of all this is that the parser seems to "guess" the
correct branch to take, or in other words, it seems to use more
lookahead than the underlying LR(1) algorithm actually allows for. In
this example, LR(2) would suffice, but also some cases that are not
LR(k) for any k can be handled this way.
In general, a GLR parser can take quadratic or cubic worst-case time,
and the current Bison parser even takes exponential time and space for
some grammars. In practice, this rarely happens, and for many grammars
it is possible to prove that it cannot happen. The present example
contains only one conflict between two rules, and the type-declaration
context containing the conflict cannot be nested. So the number of
branches that can exist at any time is limited by the constant 2, and
the parsing time is still linear.
Here is a Bison grammar corresponding to the example above. It
parses a vastly simplified form of Pascal type declarations.
%token TYPE DOTDOT ID
%left '+' '-'
%left '*' '/'
type_decl: TYPE ID '=' type ';' ;
'(' id_list ')'
| expr DOTDOT expr
| id_list ',' ID
'(' expr ')'
| expr '+' expr
| expr '-' expr
| expr '*' expr
| expr '/' expr
When used as a normal LR(1) grammar, Bison correctly complains about
one reduce/reduce conflict. In the conflicting situation the parser
chooses one of the alternatives, arbitrarily the one declared first.
Therefore the following correct input is not recognized:
type t = (a) .. b;
The parser can be turned into a GLR parser, while also telling Bison
to be silent about the one known reduce/reduce conflict, by adding these
two declarations to the Bison grammar file (before the first '%%'):
%expect-rr 1
No change in the grammar itself is required. Now the parser recognizes
all valid declarations, according to the limited syntax above,
transparently. In fact, the user does not even notice when the parser
So here we have a case where we can use the benefits of GLR, almost
without disadvantages. Even in simple cases like this, however, there
are at least two potential problems to beware. First, always analyze
the conflicts reported by Bison to make sure that GLR splitting is only
done where it is intended. A GLR parser splitting inadvertently may
cause problems less obvious than an LR parser statically choosing the
wrong alternative in a conflict. Second, consider interactions with the
lexer (*note Semantic Tokens::) with great care. Since a split parser
consumes tokens without performing any actions during the split, the
lexer cannot obtain information via parser actions. Some cases of lexer
interactions can be eliminated by using GLR to shift the complications
from the lexer to the parser. You must check the remaining cases for
In our example, it would be safe for the lexer to return tokens based
on their current meanings in some symbol table, because no new symbols
are defined in the middle of a type declaration. Though it is possible
for a parser to define the enumeration constants as they are parsed,
before the type declaration is completed, it actually makes no
difference since they cannot be used within the same enumerated type
1.5.2 Using GLR to Resolve Ambiguities
Let's consider an example, vastly simplified from a C++ grammar.
#define YYSTYPE char const *
int yylex (void);
void yyerror (char const *);
%token TYPENAME ID
%right '='
%left '+'
| prog stmt { printf ("\n"); }
expr ';' %dprec 1
| decl %dprec 2
ID { printf ("%s ", $$); }
| TYPENAME '(' expr ')'
{ printf ("%s ", $1); }
| expr '+' expr { printf ("+ "); }
| expr '=' expr { printf ("= "); }
TYPENAME declarator ';'
{ printf ("%s ", $1); }
| TYPENAME declarator '=' expr ';'
{ printf ("%s ", $1); }
ID { printf ("\"%s\" ", $1); }
| '(' declarator ')'
This models a problematic part of the C++ grammar--the ambiguity between
certain declarations and statements. For example,
T (x) = y+z;
parses as either an 'expr' or a 'stmt' (assuming that 'T' is recognized
as a 'TYPENAME' and 'x' as an 'ID'). Bison detects this as a
reduce/reduce conflict between the rules 'expr : ID' and 'declarator :
ID', which it cannot resolve at the time it encounters 'x' in the
example above. Since this is a GLR parser, it therefore splits the
problem into two parses, one for each choice of resolving the
reduce/reduce conflict. Unlike the example from the previous section
(*note Simple GLR Parsers::), however, neither of these parses "dies,"
because the grammar as it stands is ambiguous. One of the parsers
eventually reduces 'stmt : expr ';'' and the other reduces 'stmt :
decl', after which both parsers are in an identical state: they've seen
'prog stmt' and have the same unprocessed input remaining. We say that
these parses have "merged."
At this point, the GLR parser requires a specification in the grammar
of how to choose between the competing parses. In the example above,
the two '%dprec' declarations specify that Bison is to give precedence
to the parse that interprets the example as a 'decl', which implies that
'x' is a declarator. The parser therefore prints
"x" y z + T
The '%dprec' declarations only come into play when more than one
parse survives. Consider a different input string for this parser:
T (x) + y;
This is another example of using GLR to parse an unambiguous construct,
as shown in the previous section (*note Simple GLR Parsers::). Here,
there is no ambiguity (this cannot be parsed as a declaration).
However, at the time the Bison parser encounters 'x', it does not have
enough information to resolve the reduce/reduce conflict (again, between
'x' as an 'expr' or a 'declarator'). In this case, no precedence
declaration is used. Again, the parser splits into two, one assuming
that 'x' is an 'expr', and the other assuming 'x' is a 'declarator'.
The second of these parsers then vanishes when it sees '+', and the
parser prints
x T y +
Suppose that instead of resolving the ambiguity, you wanted to see
all the possibilities. For this purpose, you must merge the semantic
actions of the two possible parsers, rather than choosing one over the
other. To do so, you could change the declaration of 'stmt' as follows:
expr ';' %merge
| decl %merge
and define the 'stmtMerge' function as:
static YYSTYPE
stmtMerge (YYSTYPE x0, YYSTYPE x1)
printf (" ");
return "";
with an accompanying forward declaration in the C declarations at the
beginning of the file:
#define YYSTYPE char const *
static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1);
With these declarations, the resulting parser parses the first example
as both an 'expr' and a 'decl', and prints
"x" y z + T x T y z + =
Bison requires that all of the productions that participate in any
particular merge have identical '%merge' clauses. Otherwise, the
ambiguity would be unresolvable, and the parser will report an error
during any parse that results in the offending merge.
1.5.3 GLR Semantic Actions
The nature of GLR parsing and the structure of the generated parsers
give rise to certain restrictions on semantic values and actions.
22.214.171.124 Deferred semantic actions
By definition, a deferred semantic action is not performed at the same
time as the associated reduction. This raises caveats for several Bison
features you might use in a semantic action in a GLR parser.
In any semantic action, you can examine 'yychar' to determine the
type of the lookahead token present at the time of the associated
reduction. After checking that 'yychar' is not set to 'YYEMPTY' or
'YYEOF', you can then examine 'yylval' and 'yylloc' to determine the
lookahead token's semantic value and location, if any. In a nondeferred
semantic action, you can also modify any of these variables to influence
syntax analysis. *Note Lookahead Tokens: Lookahead.
In a deferred semantic action, it's too late to influence syntax
analysis. In this case, 'yychar', 'yylval', and 'yylloc' are set to
shallow copies of the values they had at the time of the associated
reduction. For this reason alone, modifying them is dangerous.
Moreover, the result of modifying them is undefined and subject to
change with future versions of Bison. For example, if a semantic action
might be deferred, you should never write it to invoke 'yyclearin'
(*note Action Features::) or to attempt to free memory referenced by
Another Bison feature requiring special consideration is 'YYERROR'
(*note Action Features::), which you can invoke in a semantic action to
initiate error recovery. During deterministic GLR operation, the effect
of 'YYERROR' is the same as its effect in a deterministic parser. The
effect in a deferred action is similar, but the precise point of the
error is undefined; instead, the parser reverts to deterministic
operation, selecting an unspecified stack on which to continue with a
syntax error. In a semantic predicate (see *note Semantic Predicates::)
during nondeterministic parsing, 'YYERROR' silently prunes the parse
that invoked the test.
22.214.171.124 Restrictions on semantic values and locations
GLR parsers require that you use POD (Plain Old Data) types for semantic
values and location types when using the generated parsers as C++ code.
1.5.4 Controlling a Parse with Arbitrary Predicates
In addition to the '%dprec' and '%merge' directives, GLR parsers allow
you to reject parses on the basis of arbitrary computations executed in
user code, without having Bison treat this rejection as an error if
there are alternative parses. (This feature is experimental and may
evolve. We welcome user feedback.) For example,
%?{ new_syntax } "widget" id new_args { $$ = f($3, $4); }
| %?{ !new_syntax } "widget" id old_args { $$ = f($3, $4); }
is one way to allow the same parser to handle two different syntaxes for
widgets. The clause preceded by '%?' is treated like an ordinary
action, except that its text is treated as an expression and is always
evaluated immediately (even when in nondeterministic mode). If the
expression yields 0 (false), the clause is treated as a syntax error,
which, in a nondeterministic parser, causes the stack in which it is
reduced to die. In a deterministic parser, it acts like YYERROR.
As the example shows, predicates otherwise look like semantic
actions, and therefore you must be take them into account when
determining the numbers to use for denoting the semantic values of
right-hand side symbols. Predicate actions, however, have no defined
value, and may not be given labels.
There is a subtle difference between semantic predicates and ordinary
actions in nondeterministic mode, since the latter are deferred. For
example, we could try to rewrite the previous example as
{ if (!new_syntax) YYERROR; }
"widget" id new_args { $$ = f($3, $4); }
| { if (new_syntax) YYERROR; }
"widget" id old_args { $$ = f($3, $4); }
(reversing the sense of the predicate tests to cause an error when they
are false). However, this does _not_ have the same effect if 'new_args'
and 'old_args' have overlapping syntax. Since the mid-rule actions
testing 'new_syntax' are deferred, a GLR parser first encounters the
unresolved ambiguous reduction for cases where 'new_args' and 'old_args'
recognize the same string _before_ performing the tests of 'new_syntax'.
It therefore reports an error.
Finally, be careful in writing predicates: deferred actions have not
been evaluated, so that using them in a predicate will have undefined
1.5.5 Considerations when Compiling GLR Parsers
The GLR parsers require a compiler for ISO C89 or later. In addition,
they use the 'inline' keyword, which is not C89, but is C99 and is a
common extension in pre-C99 compilers. It is up to the user of these
parsers to handle portability issues. For instance, if using Autoconf
and the Autoconf macro 'AC_C_INLINE', a mere
will suffice. Otherwise, we suggest
#if (__STDC_VERSION__ < 199901 && ! defined __GNUC__ \
&& ! defined inline)
# define inline
1.6 Locations
Many applications, like interpreters or compilers, have to produce
verbose and useful error messages. To achieve this, one must be able to
keep track of the "textual location", or "location", of each syntactic
construct. Bison provides a mechanism for handling these locations.
Each token has a semantic value. In a similar fashion, each token
has an associated location, but the type of locations is the same for
all tokens and groupings. Moreover, the output parser is equipped with
a default data structure for storing locations (*note Tracking
Locations::, for more details).
Like semantic values, locations can be reached in actions using a
dedicated set of constructs. In the example above, the location of the
whole grouping is '@$', while the locations of the subexpressions are
'@1' and '@3'.
When a rule is matched, a default action is used to compute the
semantic value of its left hand side (*note Actions::). In the same
way, another default action is used for locations. However, the action
for locations is general enough for most cases, meaning there is usually
no need to describe for each rule how '@$' should be formed. When
building a new location for a given grouping, the default behavior of
the output parser is to take the beginning of the first symbol, and the
end of the last symbol.
1.7 Bison Output: the Parser Implementation File
When you run Bison, you give it a Bison grammar file as input. The most
important output is a C source file that implements a parser for the
language described by the grammar. This parser is called a "Bison
parser", and this file is called a "Bison parser implementation file".
Keep in mind that the Bison utility and the Bison parser are two
distinct programs: the Bison utility is a program whose output is the
Bison parser implementation file that becomes part of your program.
The job of the Bison parser is to group tokens into groupings
according to the grammar rules--for example, to build identifiers and
operators into expressions. As it does this, it runs the actions for
the grammar rules it uses.
The tokens come from a function called the "lexical analyzer" that
you must supply in some fashion (such as by writing it in C). The Bison
parser calls the lexical analyzer each time it wants a new token. It
doesn't know what is "inside" the tokens (though their semantic values
may reflect this). Typically the lexical analyzer makes the tokens by
parsing characters of text, but Bison does not depend on this. *Note
The Lexical Analyzer Function 'yylex': Lexical.
The Bison parser implementation file is C code which defines a
function named 'yyparse' which implements that grammar. This function
does not make a complete C program: you must supply some additional
functions. One is the lexical analyzer. Another is an error-reporting
function which the parser calls to report an error. In addition, a
complete C program must start with a function called 'main'; you have to
provide this, and arrange for it to call 'yyparse' or the parser will
never run. *Note Parser C-Language Interface: Interface.
Aside from the token type names and the symbols in the actions you
write, all symbols defined in the Bison parser implementation file
itself begin with 'yy' or 'YY'. This includes interface functions such
as the lexical analyzer function 'yylex', the error reporting function
'yyerror' and the parser function 'yyparse' itself. This also includes
numerous identifiers used for internal purposes. Therefore, you should
avoid using C identifiers starting with 'yy' or 'YY' in the Bison
grammar file except for the ones defined in this manual. Also, you
should avoid using the C identifiers 'malloc' and 'free' for anything
other than their usual meanings.
In some cases the Bison parser implementation file includes system
headers, and in those cases your code should respect the identifiers
reserved by those headers. On some non-GNU hosts, '',
'', '', and '' are included as needed to
declare memory allocators and related types. '' is included
if message translation is in use (*note Internationalization::). Other
system headers may be included if you define 'YYDEBUG' to a nonzero
value (*note Tracing Your Parser: Tracing.).
1.8 Stages in Using Bison
The actual language-design process using Bison, from grammar
specification to a working compiler or interpreter, has these parts:
1. Formally specify the grammar in a form recognized by Bison (*note
Bison Grammar Files: Grammar File.). For each grammatical rule in
the language, describe the action that is to be taken when an
instance of that rule is recognized. The action is described by a
sequence of C statements.
2. Write a lexical analyzer to process input and pass tokens to the
parser. The lexical analyzer may be written by hand in C (*note
The Lexical Analyzer Function 'yylex': Lexical.). It could also be
produced using Lex, but the use of Lex is not discussed in this
3. Write a controlling function that calls the Bison-produced parser.
4. Write error-reporting routines.
To turn this source code as written into a runnable program, you must
follow these steps:
1. Run Bison on the grammar to produce the parser.
2. Compile the code output by Bison, as well as any other source
3. Link the object files to produce the finished product.
1.9 The Overall Layout of a Bison Grammar
The input file for the Bison utility is a "Bison grammar file". The
general form of a Bison grammar file is as follows:
The '%%', '%{' and '%}' are punctuation that appears in every Bison
grammar file to separate the sections.
The prologue may define types and variables used in the actions. You
can also use preprocessor commands to define macros used there, and use
'#include' to include header files that do any of these things. You
need to declare the lexical analyzer 'yylex' and the error printer
'yyerror' here, along with any other global identifiers used by the
actions in the grammar rules.
The Bison declarations declare the names of the terminal and
nonterminal symbols, and may also describe operator precedence and the
data types of semantic values of various symbols.
The grammar rules define how to construct each nonterminal symbol
from its parts.
The epilogue can contain any code you want to use. Often the
definitions of functions declared in the prologue go here. In a simple
program, all the rest of the program can go here.
2 Examples
Now we show and explain several sample programs written using Bison: a
reverse polish notation calculator, an algebraic (infix) notation
calculator -- later extended to track "locations" -- and a
multi-function calculator. All produce usable, though limited,
interactive desk-top calculators.
These examples are simple, but Bison grammars for real programming
languages are written the same way. You can copy these examples into a
source file to try them.
2.1 Reverse Polish Notation Calculator
The first example is that of a simple double-precision "reverse polish
notation" calculator (a calculator using postfix operators). This
example provides a good starting point, since operator precedence is not
an issue. The second example will illustrate how operator precedence is
The source code for this calculator is named 'rpcalc.y'. The '.y'
extension is a convention used for Bison grammar files.
2.1.1 Declarations for 'rpcalc'
Here are the C and Bison declarations for the reverse polish notation
calculator. As in C, comments are placed between '/*...*/'.
/* Reverse polish notation calculator. */
int yylex (void);
void yyerror (char const *);
%define api.value.type {double}
%token NUM
%% /* Grammar rules and actions follow. */
The declarations section (*note The prologue: Prologue.) contains two
preprocessor directives and two forward declarations.
The '#include' directive is used to declare the exponentiation
function 'pow'.
The forward declarations for 'yylex' and 'yyerror' are needed because
the C language requires that functions be declared before they are used.
These functions will be defined in the epilogue, but the parser calls
them so they must be declared in the prologue.
The second section, Bison declarations, provides information to Bison
about the tokens and their types (*note The Bison Declarations Section:
Bison Declarations.).
The '%define' directive defines the variable 'api.value.type', thus
specifying the C data type for semantic values of both tokens and
groupings (*note Data Types of Semantic Values: Value Type.). The Bison
parser will use whatever type 'api.value.type' is defined as; if you
don't define it, 'int' is the default. Because we specify '{double}',
each token and each expression has an associated value, which is a
floating point number. C code can use 'YYSTYPE' to refer to the value
Each terminal symbol that is not a single-character literal must be
declared. (Single-character literals normally don't need to be
declared.) In this example, all the arithmetic operators are designated
by single-character literals, so the only terminal symbol that needs to
be declared is 'NUM', the token type for numeric constants.
2.1.2 Grammar Rules for 'rpcalc'
Here are the grammar rules for the reverse polish notation calculator.
| input line
| exp '\n' { printf ("%.10g\n", $1); }
| exp exp '+' { $$ = $1 + $2; }
| exp exp '-' { $$ = $1 - $2; }
| exp exp '*' { $$ = $1 * $2; }
| exp exp '/' { $$ = $1 / $2; }
| exp exp '^' { $$ = pow ($1, $2); } /* Exponentiation */
| exp 'n' { $$ = -$1; } /* Unary minus */
The groupings of the rpcalc "language" defined here are the
expression (given the name 'exp'), the line of input ('line'), and the
complete input transcript ('input'). Each of these nonterminal symbols
has several alternate rules, joined by the vertical bar '|' which is
read as "or". The following sections explain what these rules mean.
The semantics of the language is determined by the actions taken when
a grouping is recognized. The actions are the C code that appears
inside braces. *Note Actions::.
You must specify these actions in C, but Bison provides the means for
passing semantic values between the rules. In each action, the
pseudo-variable '$$' stands for the semantic value for the grouping that
the rule is going to construct. Assigning a value to '$$' is the main
job of most actions. The semantic values of the components of the rule
are referred to as '$1', '$2', and so on.
188.8.131.52 Explanation of 'input'
Consider the definition of 'input':
| input line
This definition reads as follows: "A complete input is either an
empty string, or a complete input followed by an input line". Notice
that "complete input" is defined in terms of itself. This definition is
said to be "left recursive" since 'input' appears always as the leftmost
symbol in the sequence. *Note Recursive Rules: Recursion.
The first alternative is empty because there are no symbols between
the colon and the first '|'; this means that 'input' can match an empty
string of input (no tokens). We write the rules this way because it is
legitimate to type 'Ctrl-d' right after you start the calculator. It's
conventional to put an empty alternative first and to use the (optional)
'%empty' directive, or to write the comment '/* empty */' in it (*note
Empty Rules::).
The second alternate rule ('input line') handles all nontrivial
input. It means, "After reading any number of lines, read one more line
if possible." The left recursion makes this rule into a loop. Since
the first alternative matches empty input, the loop can be executed zero
or more times.
The parser function 'yyparse' continues to process input until a
grammatical error is seen or the lexical analyzer says there are no more
input tokens; we will arrange for the latter to happen at end-of-input.
184.108.40.206 Explanation of 'line'
Now consider the definition of 'line':
| exp '\n' { printf ("%.10g\n", $1); }
The first alternative is a token which is a newline character; this
means that rpcalc accepts a blank line (and ignores it, since there is
no action). The second alternative is an expression followed by a
newline. This is the alternative that makes rpcalc useful. The
semantic value of the 'exp' grouping is the value of '$1' because the
'exp' in question is the first symbol in the alternative. The action
prints this value, which is the result of the computation the user asked
This action is unusual because it does not assign a value to '$$'.
As a consequence, the semantic value associated with the 'line' is
uninitialized (its value will be unpredictable). This would be a bug if
that value were ever used, but we don't use it: once rpcalc has printed
the value of the user's input line, that value is no longer needed.
184.108.40.206 Explanation of 'expr'
The 'exp' grouping has several rules, one for each kind of expression.
The first rule handles the simplest expressions: those that are just
numbers. The second handles an addition-expression, which looks like
two expressions followed by a plus-sign. The third handles subtraction,
and so on.
| exp exp '+' { $$ = $1 + $2; }
| exp exp '-' { $$ = $1 - $2; }
We have used '|' to join all the rules for 'exp', but we could
equally well have written them separately:
exp: NUM ;
exp: exp exp '+' { $$ = $1 + $2; };
exp: exp exp '-' { $$ = $1 - $2; };
Most of the rules have actions that compute the value of the
expression in terms of the value of its parts. For example, in the rule
for addition, '$1' refers to the first component 'exp' and '$2' refers
to the second one. The third component, ''+'', has no meaningful
associated semantic value, but if it had one you could refer to it as
'$3'. When 'yyparse' recognizes a sum expression using this rule, the
sum of the two subexpressions' values is produced as the value of the
entire expression. *Note Actions::.
You don't have to give an action for every rule. When a rule has no
action, Bison by default copies the value of '$1' into '$$'. This is
what happens in the first rule (the one that uses 'NUM').
The formatting shown here is the recommended convention, but Bison
does not require it. You can add or change white space as much as you
wish. For example, this:
exp: NUM | exp exp '+' {$$ = $1 + $2; } | ... ;
means the same thing as this:
| exp exp '+' { $$ = $1 + $2; }
| ...
The latter, however, is much more readable.
2.1.3 The 'rpcalc' Lexical Analyzer
The lexical analyzer's job is low-level parsing: converting characters
or sequences of characters into tokens. The Bison parser gets its
tokens by calling the lexical analyzer. *Note The Lexical Analyzer
Function 'yylex': Lexical.
Only a simple lexical analyzer is needed for the RPN calculator.
This lexical analyzer skips blanks and tabs, then reads in numbers as
'double' and returns them as 'NUM' tokens. Any other character that
isn't part of a number is a separate token. Note that the token-code
for such a single-character token is the character itself.
The return value of the lexical analyzer function is a numeric code
which represents a token type. The same text used in Bison rules to
stand for this token type is also a C expression for the numeric code
for the type. This works in two ways. If the token type is a character
literal, then its numeric code is that of the character; you can use the
same character literal in the lexical analyzer to express the number.
If the token type is an identifier, that identifier is defined by Bison
as a C macro whose definition is the appropriate number. In this
example, therefore, 'NUM' becomes a macro for 'yylex' to use.
The semantic value of the token (if it has one) is stored into the
global variable 'yylval', which is where the Bison parser will look for
it. (The C data type of 'yylval' is 'YYSTYPE', whose value was defined
at the beginning of the grammar via '%define api.value.type {double}';
*note Declarations for 'rpcalc': Rpcalc Declarations.)
A token type code of zero is returned if the end-of-input is
encountered. (Bison recognizes any nonpositive value as indicating
Here is the code for the lexical analyzer:
/* The lexical analyzer returns a double floating point
number on the stack and the token NUM, or the numeric code
of the character read if not a number. It skips all blanks
and tabs, and returns 0 for end-of-input. */
yylex (void)
int c;
/* Skip white space. */
while ((c = getchar ()) == ' ' || c == '\t')
/* Process numbers. */
if (c == '.' || isdigit (c))
ungetc (c, stdin);
scanf ("%lf", &yylval);
return NUM;
/* Return end-of-input. */
if (c == EOF)
return 0;
/* Return a single char. */
return c;
2.1.4 The Controlling Function
In keeping with the spirit of this example, the controlling function is
kept to the bare minimum. The only requirement is that it call
'yyparse' to start the process of parsing.
main (void)
return yyparse ();
2.1.5 The Error Reporting Routine
When 'yyparse' detects a syntax error, it calls the error reporting
function 'yyerror' to print an error message (usually but not always
'"syntax error"'). It is up to the programmer to supply 'yyerror'
(*note Parser C-Language Interface: Interface.), so here is the
definition we will use:
/* Called by yyparse on error. */
yyerror (char const *s)
fprintf (stderr, "%s\n", s);
After 'yyerror' returns, the Bison parser may recover from the error
and continue parsing if the grammar contains a suitable error rule
(*note Error Recovery::). Otherwise, 'yyparse' returns nonzero. We
have not written any error rules in this example, so any invalid input
will cause the calculator program to exit. This is not clean behavior
for a real calculator, but it is adequate for the first example.
2.1.6 Running Bison to Make the Parser
Before running Bison to produce a parser, we need to decide how to
arrange all the source code in one or more source files. For such a
simple example, the easiest thing is to put everything in one file, the
grammar file. The definitions of 'yylex', 'yyerror' and 'main' go at
the end, in the epilogue of the grammar file (*note The Overall Layout
of a Bison Grammar: Grammar Layout.).
For a large project, you would probably have several source files,
and use 'make' to arrange to recompile them.
With all the source in the grammar file, you use the following
command to convert it into a parser implementation file:
bison FILE.y
In this example, the grammar file is called 'rpcalc.y' (for "Reverse
Polish CALCulator"). Bison produces a parser implementation file named
'FILE.tab.c', removing the '.y' from the grammar file name. The parser
implementation file contains the source code for 'yyparse'. The
additional functions in the grammar file ('yylex', 'yyerror' and 'main')
are copied verbatim to the parser implementation file.
2.1.7 Compiling the Parser Implementation File
Here is how to compile and run the parser implementation file:
# List files in current directory.
$ ls
rpcalc.tab.c rpcalc.y
# Compile the Bison parser.
# '-lm' tells compiler to search math library for 'pow'.
$ cc -lm -o rpcalc rpcalc.tab.c
# List files again.
$ ls
rpcalc rpcalc.tab.c rpcalc.y
The file 'rpcalc' now contains the executable code. Here is an
example session using 'rpcalc'.
$ rpcalc
4 9 +
=> 13
3 7 + 3 4 5 *+-
=> -13
3 7 + 3 4 5 * + - n Note the unary minus, 'n'
=> 13
5 6 / 4 n +
=> -3.166666667
3 4 ^ Exponentiation
=> 81
^D End-of-file indicator
2.2 Infix Notation Calculator: 'calc'
We now modify rpcalc to handle infix operators instead of postfix.
Infix notation involves the concept of operator precedence and the need
for parentheses nested to arbitrary depth. Here is the Bison code for
'calc.y', an infix desk-top calculator.
/* Infix notation calculator. */
int yylex (void);
void yyerror (char const *);
/* Bison declarations. */
%define api.value.type {double}
%token NUM
%left '-' '+'
%left '*' '/'
%precedence NEG /* negation--unary minus */
%right '^' /* exponentiation */
%% /* The grammar follows. */
| input line
| exp '\n' { printf ("\t%.10g\n", $1); }
| exp '+' exp { $$ = $1 + $3; }
| exp '-' exp { $$ = $1 - $3; }
| exp '*' exp { $$ = $1 * $3; }
| exp '/' exp { $$ = $1 / $3; }
| '-' exp %prec NEG { $$ = -$2; }
| exp '^' exp { $$ = pow ($1, $3); }
| '(' exp ')' { $$ = $2; }
The functions 'yylex', 'yyerror' and 'main' can be the same as before.
There are two important new features shown in this code.
In the second section (Bison declarations), '%left' declares token
types and says they are left-associative operators. The declarations
'%left' and '%right' (right associativity) take the place of '%token'
which is used to declare a token type name without
associativity/precedence. (These tokens are single-character literals,
which ordinarily don't need to be declared. We declare them here to
specify the associativity/precedence.)
Operator precedence is determined by the line ordering of the
declarations; the higher the line number of the declaration (lower on
the page or screen), the higher the precedence. Hence, exponentiation
has the highest precedence, unary minus ('NEG') is next, followed by '*'
and '/', and so on. Unary minus is not associative, only precedence
matters ('%precedence'. *Note Operator Precedence: Precedence.
The other important new feature is the '%prec' in the grammar section
for the unary minus operator. The '%prec' simply instructs Bison that
the rule '| '-' exp' has the same precedence as 'NEG'--in this case the
next-to-highest. *Note Context-Dependent Precedence: Contextual
Here is a sample run of 'calc.y':
$ calc
4 + 4.5 - (34/(8*3+-3))
-56 + 2
3 ^ 2
2.3 Simple Error Recovery
Up to this point, this manual has not addressed the issue of "error
recovery"--how to continue parsing after the parser detects a syntax
error. All we have handled is error reporting with 'yyerror'. Recall
that by default 'yyparse' returns after calling 'yyerror'. This means
that an erroneous input line causes the calculator program to exit. Now
we show how to rectify this deficiency.
The Bison language itself includes the reserved word 'error', which
may be included in the grammar rules. In the example below it has been
added to one of the alternatives for 'line':
| exp '\n' { printf ("\t%.10g\n", $1); }
| error '\n' { yyerrok; }
This addition to the grammar allows for simple error recovery in the
event of a syntax error. If an expression that cannot be evaluated is
read, the error will be recognized by the third rule for 'line', and
parsing will continue. (The 'yyerror' function is still called upon to
print its message as well.) The action executes the statement
'yyerrok', a macro defined automatically by Bison; its meaning is that
error recovery is complete (*note Error Recovery::). Note the
difference between 'yyerrok' and 'yyerror'; neither one is a misprint.
This form of error recovery deals with syntax errors. There are
other kinds of errors; for example, division by zero, which raises an
exception signal that is normally fatal. A real calculator program must
handle this signal and use 'longjmp' to return to 'main' and resume
parsing input lines; it would also have to discard the rest of the
current line of input. We won't discuss this issue further because it
is not specific to Bison programs.
2.4 Location Tracking Calculator: 'ltcalc'
This example extends the infix notation calculator with location
tracking. This feature will be used to improve the error messages. For
the sake of clarity, this example is a simple integer calculator, since
most of the work needed to use locations will be done in the lexical
2.4.1 Declarations for 'ltcalc'
The C and Bison declarations for the location tracking calculator are
the same as the declarations for the infix notation calculator.
/* Location tracking calculator. */
int yylex (void);
void yyerror (char const *);
/* Bison declarations. */
%define api.value.type {int}
%token NUM
%left '-' '+'
%left '*' '/'
%precedence NEG
%right '^'
%% /* The grammar follows. */
Note there are no declarations specific to locations. Defining a data
type for storing locations is not needed: we will use the type provided
by default (*note Data Types of Locations: Location Type.), which is a
four member structure with the following integer fields: 'first_line',
'first_column', 'last_line' and 'last_column'. By conventions, and in
accordance with the GNU Coding Standards and common practice, the line
and column count both start at 1.
2.4.2 Grammar Rules for 'ltcalc'
Whether handling locations or not has no effect on the syntax of your
language. Therefore, grammar rules for this example will be very close
to those of the previous example: we will only modify them to benefit
from the new information.
Here, we will use locations to report divisions by zero, and locate
the wrong expressions or subexpressions.
| input line
| exp '\n' { printf ("%d\n", $1); }
| exp '+' exp { $$ = $1 + $3; }
| exp '-' exp { $$ = $1 - $3; }
| exp '*' exp { $$ = $1 * $3; }
| exp '/' exp
if ($3)
$$ = $1 / $3;
$$ = 1;
fprintf (stderr, "%d.%d-%d.%d: division by zero",
@3.first_line, @3.first_column,
@3.last_line, @3.last_column);
| '-' exp %prec NEG { $$ = -$2; }
| exp '^' exp { $$ = pow ($1, $3); }
| '(' exp ')' { $$ = $2; }
This code shows how to reach locations inside of semantic actions, by
using the pseudo-variables '@N' for rule components, and the
pseudo-variable '@$' for groupings.
We don't need to assign a value to '@$': the output parser does it
automatically. By default, before executing the C code of each action,
'@$' is set to range from the beginning of '@1' to the end of '@N', for
a rule with N components. This behavior can be redefined (*note Default
Action for Locations: Location Default Action.), and for very specific
rules, '@$' can be computed by hand.
2.4.3 The 'ltcalc' Lexical Analyzer.
Until now, we relied on Bison's defaults to enable location tracking.
The next step is to rewrite the lexical analyzer, and make it able to
feed the parser with the token locations, as it already does for
semantic values.
To this end, we must take into account every single character of the
input text, to avoid the computed locations of being fuzzy or wrong:
yylex (void)
int c;
/* Skip white space. */
while ((c = getchar ()) == ' ' || c == '\t')
/* Step. */
yylloc.first_line = yylloc.last_line;
yylloc.first_column = yylloc.last_column;
/* Process numbers. */
if (isdigit (c))
yylval = c - '0';
while (isdigit (c = getchar ()))
yylval = yylval * 10 + c - '0';
ungetc (c, stdin);
return NUM;
/* Return end-of-input. */
if (c == EOF)
return 0;
/* Return a single char, and update location. */
if (c == '\n')
yylloc.last_column = 0;
return c;
Basically, the lexical analyzer performs the same processing as
before: it skips blanks and tabs, and reads numbers or single-character
tokens. In addition, it updates 'yylloc', the global variable (of type
'YYLTYPE') containing the token's location.
Now, each time this function returns a token, the parser has its
number as well as its semantic value, and its location in the text. The
last needed change is to initialize 'yylloc', for example in the
controlling function:
main (void)
yylloc.first_line = yylloc.last_line = 1;
yylloc.first_column = yylloc.last_column = 0;
return yyparse ();
Remember that computing locations is not a matter of syntax. Every
character must be associated to a location update, whether it is in
valid input, in comments, in literal strings, and so on.
2.5 Multi-Function Calculator: 'mfcalc'
Now that the basics of Bison have been discussed, it is time to move on
to a more advanced problem. The above calculators provided only five
functions, '+', '-', '*', '/' and '^'. It would be nice to have a
calculator that provides other mathematical functions such as 'sin',
'cos', etc.
It is easy to add new operators to the infix calculator as long as
they are only single-character literals. The lexical analyzer 'yylex'
passes back all nonnumeric characters as tokens, so new grammar rules
suffice for adding a new operator. But we want something more flexible:
built-in functions whose syntax has this form:
At the same time, we will add memory to the calculator, by allowing you
to create named variables, store values in them, and use them later.
Here is a sample session with the multi-function calculator:
$ mfcalc
pi = 3.141592653589
=> 3.1415926536
=> 0.0000000000
alpha = beta1 = 2.3
=> 2.3000000000
=> 2.3000000000
=> 0.8329091229
=> 2.3000000000
Note that multiple assignment and nested function calls are
2.5.1 Declarations for 'mfcalc'
Here are the C and Bison declarations for the multi-function calculator.
#include /* For printf, etc. */
#include /* For pow, used in the grammar. */
#include "calc.h" /* Contains definition of 'symrec'. */
int yylex (void);
void yyerror (char const *);
%define api.value.type union /* Generate YYSTYPE from these types: */
%token NUM /* Simple double precision number. */
%token VAR FNCT /* Symbol table pointer: variable and function. */
%type exp
%precedence '='
%left '-' '+'
%left '*' '/'
%precedence NEG /* negation--unary minus */
%right '^' /* exponentiation */
The above grammar introduces only two new features of the Bison
language. These features allow semantic values to have various data
types (*note More Than One Value Type: Multiple Types.).
The special 'union' value assigned to the '%define' variable
'api.value.type' specifies that the symbols are defined with their data
types. Bison will generate an appropriate definition of 'YYSTYPE' to
store these values.
Since values can now have various types, it is necessary to associate
a type with each grammar symbol whose semantic value is used. These
symbols are 'NUM', 'VAR', 'FNCT', and 'exp'. Their declarations are
augmented with their data type (placed between angle brackets). For
instance, values of 'NUM' are stored in 'double'.
The Bison construct '%type' is used for declaring nonterminal
symbols, just as '%token' is used for declaring token types. Previously
we did not use '%type' before because nonterminal symbols are normally
declared implicitly by the rules that define them. But 'exp' must be
declared explicitly so we can specify its value type. *Note Nonterminal
Symbols: Type Decl.
2.5.2 Grammar Rules for 'mfcalc'
Here are the grammar rules for the multi-function calculator. Most of
them are copied directly from 'calc'; three rules, those which mention
'VAR' or 'FNCT', are new.
%% /* The grammar follows. */
| input line
| exp '\n' { printf ("%.10g\n", $1); }
| error '\n' { yyerrok; }
| VAR { $$ = $1->value.var; }
| VAR '=' exp { $$ = $3; $1->value.var = $3; }
| FNCT '(' exp ')' { $$ = (*($1->value.fnctptr))($3); }
| exp '+' exp { $$ = $1 + $3; }
| exp '-' exp { $$ = $1 - $3; }
| exp '*' exp { $$ = $1 * $3; }
| exp '/' exp { $$ = $1 / $3; }
| '-' exp %prec NEG { $$ = -$2; }
| exp '^' exp { $$ = pow ($1, $3); }
| '(' exp ')' { $$ = $2; }
/* End of grammar. */
2.5.3 The 'mfcalc' Symbol Table
The multi-function calculator requires a symbol table to keep track of
the names and meanings of variables and functions. This doesn't affect
the grammar rules (except for the actions) or the Bison declarations,
but it requires some additional C functions for support.
The symbol table itself consists of a linked list of records. Its
definition, which is kept in the header 'calc.h', is as follows. It
provides for either functions or variables to be placed in the table.
/* Function type. */
typedef double (*func_t) (double);
/* Data type for links in the chain of symbols. */
struct symrec
char *name; /* name of symbol */
int type; /* type of symbol: either VAR or FNCT */
double var; /* value of a VAR */
func_t fnctptr; /* value of a FNCT */
} value;
struct symrec *next; /* link field */
typedef struct symrec symrec;
/* The symbol table: a chain of 'struct symrec'. */
extern symrec *sym_table;
symrec *putsym (char const *, int);
symrec *getsym (char const *);
The new version of 'main' will call 'init_table' to initialize the
symbol table:
struct init
char const *fname;
double (*fnct) (double);
struct init const arith_fncts[] =
{ "atan", atan },
{ "cos", cos },
{ "exp", exp },
{ "ln", log },
{ "sin", sin },
{ "sqrt", sqrt },
{ 0, 0 },
/* The symbol table: a chain of 'struct symrec'. */
symrec *sym_table;
/* Put arithmetic functions in table. */
init_table (void)
int i;
for (i = 0; arith_fncts[i].fname != 0; i++)
symrec *ptr = putsym (arith_fncts[i].fname, FNCT);
ptr->value.fnctptr = arith_fncts[i].fnct;
By simply editing the initialization list and adding the necessary
include files, you can add additional functions to the calculator.
Two important functions allow look-up and installation of symbols in
the symbol table. The function 'putsym' is passed a name and the type
('VAR' or 'FNCT') of the object to be installed. The object is linked
to the front of the list, and a pointer to the object is returned. The
function 'getsym' is passed the name of the symbol to look up. If
found, a pointer to that symbol is returned; otherwise zero is returned.
#include /* malloc. */
#include /* strlen. */
symrec *
putsym (char const *sym_name, int sym_type)
symrec *ptr = (symrec *) malloc (sizeof (symrec));
ptr->name = (char *) malloc (strlen (sym_name) + 1);
strcpy (ptr->name,sym_name);
ptr->type = sym_type;
ptr->value.var = 0; /* Set value to 0 even if fctn. */
ptr->next = (struct symrec *)sym_table;
sym_table = ptr;
return ptr;
symrec *
getsym (char const *sym_name)
symrec *ptr;
for (ptr = sym_table; ptr != (symrec *) 0;
ptr = (symrec *)ptr->next)
if (strcmp (ptr->name, sym_name) == 0)
return ptr;
return 0;
2.5.4 The 'mfcalc' Lexer
The function 'yylex' must now recognize variables, numeric values, and
the single-character arithmetic operators. Strings of alphanumeric
characters with a leading letter are recognized as either variables or
functions depending on what the symbol table says about them.
The string is passed to 'getsym' for look up in the symbol table. If
the name appears in the table, a pointer to its location and its type
('VAR' or 'FNCT') is returned to 'yyparse'. If it is not already in the
table, then it is installed as a 'VAR' using 'putsym'. Again, a pointer
and its type (which must be 'VAR') is returned to 'yyparse'.
No change is needed in the handling of numeric values and arithmetic
operators in 'yylex'.
yylex (void)
int c;
/* Ignore white space, get first nonwhite character. */
while ((c = getchar ()) == ' ' || c == '\t')
if (c == EOF)
return 0;
/* Char starts a number => parse the number. */
if (c == '.' || isdigit (c))
ungetc (c, stdin);
scanf ("%lf", &yylval.NUM);
return NUM;
Bison generated a definition of 'YYSTYPE' with a member named 'NUM' to
store value of 'NUM' symbols.
/* Char starts an identifier => read the name. */
if (isalpha (c))
/* Initially make the buffer long enough
for a 40-character symbol name. */
static size_t length = 40;
static char *symbuf = 0;
symrec *s;
int i;
if (!symbuf)
symbuf = (char *) malloc (length + 1);
i = 0;
/* If buffer is full, make it bigger. */
if (i == length)
length *= 2;
symbuf = (char *) realloc (symbuf, length + 1);
/* Add this character to the buffer. */
symbuf[i++] = c;
/* Get another character. */
c = getchar ();
while (isalnum (c));
ungetc (c, stdin);
symbuf[i] = '\0';
s = getsym (symbuf);
if (s == 0)
s = putsym (symbuf, VAR);
*((symrec**) &yylval) = s;
return s->type;
/* Any other character is a token by itself. */
return c;
2.5.5 The 'mfcalc' Main
The error reporting function is unchanged, and the new version of 'main'
includes a call to 'init_table' and sets the 'yydebug' on user demand
(*Note Tracing Your Parser: Tracing, for details):
/* Called by yyparse on error. */
yyerror (char const *s)
fprintf (stderr, "%s\n", s);
main (int argc, char const* argv[])
int i;
/* Enable parse traces on option -p. */
for (i = 1; i < argc; ++i)
if (!strcmp(argv[i], "-p"))
yydebug = 1;
init_table ();
return yyparse ();
This program is both powerful and flexible. You may easily add new
functions, and it is a simple job to modify this code to install
predefined variables such as 'pi' or 'e' as well.
2.6 Exercises
1. Add some new functions from 'math.h' to the initialization list.
2. Add another array that contains constants and their values. Then
modify 'init_table' to add these constants to the symbol table. It
will be easiest to give the constants type 'VAR'.
3. Make the program report an error if the user refers to an
uninitialized variable in any way except to store a value in it.
3 Bison Grammar Files
Bison takes as input a context-free grammar specification and produces a
C-language function that recognizes correct instances of the grammar.
The Bison grammar file conventionally has a name ending in '.y'.
*Note Invoking Bison: Invocation.
3.1 Outline of a Bison Grammar
A Bison grammar file has four main sections, shown here with the
appropriate delimiters:
Comments enclosed in '/* ... */' may appear in any of the sections.
As a GNU extension, '//' introduces a comment that continues until end
of line.
3.1.1 The prologue
The PROLOGUE section contains macro definitions and declarations of
functions and variables that are used in the actions in the grammar
rules. These are copied to the beginning of the parser implementation
file so that they precede the definition of 'yyparse'. You can use
'#include' to get the declarations from a header file. If you don't
need any C declarations, you may omit the '%{' and '%}' delimiters that
bracket this section.
The PROLOGUE section is terminated by the first occurrence of '%}'
that is outside a comment, a string literal, or a character constant.
You may have more than one PROLOGUE section, intermixed with the
BISON DECLARATIONS. This allows you to have C and Bison declarations
that refer to each other. For example, the '%union' declaration may use
types defined in a header file, and you may wish to prototype functions
that take arguments of type 'YYSTYPE'. This can be done with two
PROLOGUE blocks, one before and one after the '%union' declaration.
#define _GNU_SOURCE
#include "ptypes.h"
%union {
long int n;
tree t; /* 'tree' is defined in 'ptypes.h'. */
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(F, N, L) print_token_value (F, N, L)
When in doubt, it is usually safer to put prologue code before all
Bison declarations, rather than after. For example, any definitions of
feature test macros like '_GNU_SOURCE' or '_POSIX_C_SOURCE' should
appear before all Bison declarations, as feature test macros can affect
the behavior of Bison-generated '#include' directives.
3.1.2 Prologue Alternatives
The functionality of PROLOGUE sections can often be subtle and
inflexible. As an alternative, Bison provides a '%code' directive with
an explicit qualifier field, which identifies the purpose of the code
and thus the location(s) where Bison should generate it. For C/C++, the
qualifier can be omitted for the default location, or it can be one of
'requires', 'provides', 'top'. *Note %code Summary::.
Look again at the example of the previous section:
#define _GNU_SOURCE
#include "ptypes.h"
%union {
long int n;
tree t; /* 'tree' is defined in 'ptypes.h'. */
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(F, N, L) print_token_value (F, N, L)
Notice that there are two PROLOGUE sections here, but there's a subtle
distinction between their functionality. For example, if you decide to
override Bison's default definition for 'YYLTYPE', in which PROLOGUE
section should you write your new definition? You should write it in
the first since Bison will insert that code into the parser
implementation file _before_ the default 'YYLTYPE' definition. In which
PROLOGUE section should you prototype an internal function,
'trace_token', that accepts 'YYLTYPE' and 'yytokentype' as arguments?
You should prototype it in the second since Bison will insert that code
_after_ the 'YYLTYPE' and 'yytokentype' definitions.
This distinction in functionality between the two PROLOGUE sections
is established by the appearance of the '%union' between them. This
behavior raises a few questions. First, why should the position of a
'%union' affect definitions related to 'YYLTYPE' and 'yytokentype'?
Second, what if there is no '%union'? In that case, the second kind of
PROLOGUE section is not available. This behavior is not intuitive.
To avoid this subtle '%union' dependency, rewrite the example using a
'%code top' and an unqualified '%code'. Let's go ahead and add the new
'YYLTYPE' definition and the 'trace_token' prototype at the same time:
%code top {
#define _GNU_SOURCE
/* WARNING: The following code really belongs
* in a '%code requires'; see below. */
#include "ptypes.h"
#define YYLTYPE YYLTYPE
typedef struct YYLTYPE
int first_line;
int first_column;
int last_line;
int last_column;
char *filename;
%union {
long int n;
tree t; /* 'tree' is defined in 'ptypes.h'. */
%code {
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(F, N, L) print_token_value (F, N, L)
static void trace_token (enum yytokentype token, YYLTYPE loc);
In this way, '%code top' and the unqualified '%code' achieve the same
functionality as the two kinds of PROLOGUE sections, but it's always
explicit which kind you intend. Moreover, both kinds are always
available even in the absence of '%union'.
The '%code top' block above logically contains two parts. The first
two lines before the warning need to appear near the top of the parser
implementation file. The first line after the warning is required by
'YYSTYPE' and thus also needs to appear in the parser implementation
file. However, if you've instructed Bison to generate a parser header
file (*note %defines: Decl Summary.), you probably want that line to
appear before the 'YYSTYPE' definition in that header file as well. The
'YYLTYPE' definition should also appear in the parser header file to
override the default 'YYLTYPE' definition there.
In other words, in the '%code top' block above, all but the first two
lines are dependency code required by the 'YYSTYPE' and 'YYLTYPE'
definitions. Thus, they belong in one or more '%code requires':
%code top {
#define _GNU_SOURCE
%code requires {
#include "ptypes.h"
%union {
long int n;
tree t; /* 'tree' is defined in 'ptypes.h'. */
%code requires {
#define YYLTYPE YYLTYPE
typedef struct YYLTYPE
int first_line;
int first_column;
int last_line;
int last_column;
char *filename;
%code {
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(F, N, L) print_token_value (F, N, L)
static void trace_token (enum yytokentype token, YYLTYPE loc);
Now Bison will insert '#include "ptypes.h"' and the new 'YYLTYPE'
definition before the Bison-generated 'YYSTYPE' and 'YYLTYPE'
definitions in both the parser implementation file and the parser header
file. (By the same reasoning, '%code requires' would also be the
appropriate place to write your own definition for 'YYSTYPE'.)
When you are writing dependency code for 'YYSTYPE' and 'YYLTYPE', you
should prefer '%code requires' over '%code top' regardless of whether
you instruct Bison to generate a parser header file. When you are
writing code that you need Bison to insert only into the parser
implementation file and that has no special need to appear at the top of
that file, you should prefer the unqualified '%code' over '%code top'.
These practices will make the purpose of each block of your code
explicit to Bison and to other developers reading your grammar file.
Following these practices, we expect the unqualified '%code' and '%code
requires' to be the most important of the four PROLOGUE alternatives.
At some point while developing your parser, you might decide to
provide 'trace_token' to modules that are external to your parser.
Thus, you might wish for Bison to insert the prototype into both the
parser header file and the parser implementation file. Since this
function is not a dependency required by 'YYSTYPE' or 'YYLTYPE', it
doesn't make sense to move its prototype to a '%code requires'. More
importantly, since it depends upon 'YYLTYPE' and 'yytokentype', '%code
requires' is not sufficient. Instead, move its prototype from the
unqualified '%code' to a '%code provides':
%code top {
#define _GNU_SOURCE
%code requires {
#include "ptypes.h"
%union {
long int n;
tree t; /* 'tree' is defined in 'ptypes.h'. */
%code requires {
#define YYLTYPE YYLTYPE
typedef struct YYLTYPE
int first_line;
int first_column;
int last_line;
int last_column;
char *filename;
%code provides {
void trace_token (enum yytokentype token, YYLTYPE loc);
%code {
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(F, N, L) print_token_value (F, N, L)
Bison will insert the 'trace_token' prototype into both the parser
header file and the parser implementation file after the definitions for
'yytokentype', 'YYLTYPE', and 'YYSTYPE'.
The above examples are careful to write directives in an order that
reflects the layout of the generated parser implementation and header
files: '%code top', '%code requires', '%code provides', and then
'%code'. While your grammar files may generally be easier to read if
you also follow this order, Bison does not require it. Instead, Bison
lets you choose an organization that makes sense to you.
You may declare any of these directives multiple times in the grammar
file. In that case, Bison concatenates the contained code in
declaration order. This is the only way in which the position of one of
these directives within the grammar file affects its functionality.
The result of the previous two properties is greater flexibility in
how you may organize your grammar file. For example, you may organize
semantic-type-related directives by semantic type:
%code requires { #include "type1.h" }
%union { type1 field1; }
%destructor { type1_free ($$); }
%printer { type1_print (yyoutput, $$); }
%code requires { #include "type2.h" }
%union { type2 field2; }
%destructor { type2_free ($$); }
%printer { type2_print (yyoutput, $$); }
You could even place each of the above directive groups in the rules
section of the grammar file next to the set of rules that uses the
associated semantic type. (In the rules section, you must terminate
each of those directives with a semicolon.) And you don't have to worry
that some directive (like a '%union') in the definitions section is
going to adversely affect their functionality in some counter-intuitive
manner just because it comes first. Such an organization is not
possible using PROLOGUE sections.
This section has been concerned with explaining the advantages of the
four PROLOGUE alternatives over the original Yacc PROLOGUE. However, in
most cases when using these directives, you shouldn't need to think
about all the low-level ordering issues discussed here. Instead, you
should simply use these directives to label each block of your code
according to its purpose and let Bison handle the ordering. '%code' is
the most generic label. Move code to '%code requires', '%code
provides', or '%code top' as needed.
3.1.3 The Bison Declarations Section
The BISON DECLARATIONS section contains declarations that define
terminal and nonterminal symbols, specify precedence, and so on. In
some simple grammars you may not need any declarations. *Note Bison
Declarations: Declarations.
3.1.4 The Grammar Rules Section
The "grammar rules" section contains one or more Bison grammar rules,
and nothing else. *Note Syntax of Grammar Rules: Rules.
There must always be at least one grammar rule, and the first '%%'
(which precedes the grammar rules) may never be omitted even if it is
the first thing in the file.
3.1.5 The epilogue
The EPILOGUE is copied verbatim to the end of the parser implementation
file, just as the PROLOGUE is copied to the beginning. This is the most
convenient place to put anything that you want to have in the parser
implementation file but which need not come before the definition of
'yyparse'. For example, the definitions of 'yylex' and 'yyerror' often
go here. Because C requires functions to be declared before being used,
you often need to declare functions like 'yylex' and 'yyerror' in the
Prologue, even if you define them in the Epilogue. *Note Parser
C-Language Interface: Interface.
If the last section is empty, you may omit the '%%' that separates it
from the grammar rules.
The Bison parser itself contains many macros and identifiers whose
names start with 'yy' or 'YY', so it is a good idea to avoid using any
such names (except those documented in this manual) in the epilogue of
the grammar file.
3.2 Symbols, Terminal and Nonterminal
"Symbols" in Bison grammars represent the grammatical classifications of
the language.
A "terminal symbol" (also known as a "token type") represents a class
of syntactically equivalent tokens. You use the symbol in grammar rules
to mean that a token in that class is allowed. The symbol is
represented in the Bison parser by a numeric code, and the 'yylex'
function returns a token type code to indicate what kind of token has
been read. You don't need to know what the code value is; you can use
the symbol to stand for it.
A "nonterminal symbol" stands for a class of syntactically equivalent
groupings. The symbol name is used in writing grammar rules. By
convention, it should be all lower case.
Symbol names can contain letters, underscores, periods, and
non-initial digits and dashes. Dashes in symbol names are a GNU
extension, incompatible with POSIX Yacc. Periods and dashes make symbol
names less convenient to use with named references, which require
brackets around such names (*note Named References::). Terminal symbols
that contain periods or dashes make little sense: since they are not
valid symbols (in most programming languages) they are not exported as
token names.
There are three ways of writing terminal symbols in the grammar:
* A "named token type" is written with an identifier, like an
identifier in C. By convention, it should be all upper case. Each
such name must be defined with a Bison declaration such as
'%token'. *Note Token Type Names: Token Decl.
* A "character token type" (or "literal character token") is written
in the grammar using the same syntax used in C for character
constants; for example, ''+'' is a character token type. A
character token type doesn't need to be declared unless you need to
specify its semantic value data type (*note Data Types of Semantic
Values: Value Type.), associativity, or precedence (*note Operator
Precedence: Precedence.).
By convention, a character token type is used only to represent a
token that consists of that particular character. Thus, the token
type ''+'' is used to represent the character '+' as a token.
Nothing enforces this convention, but if you depart from it, your
program will confuse other readers.
All the usual escape sequences used in character literals in C can
be used in Bison as well, but you must not use the null character
as a character literal because its numeric code, zero, signifies
end-of-input (*note Calling Convention for 'yylex': Calling
Convention.). Also, unlike standard C, trigraphs have no special
meaning in Bison character literals, nor is backslash-newline
* A "literal string token" is written like a C string constant; for
example, '"<="' is a literal string token. A literal string token
doesn't need to be declared unless you need to specify its semantic
value data type (*note Value Type::), associativity, or precedence
(*note Precedence::).
You can associate the literal string token with a symbolic name as
an alias, using the '%token' declaration (*note Token Declarations:
Token Decl.). If you don't do that, the lexical analyzer has to
retrieve the token number for the literal string token from the
'yytname' table (*note Calling Convention::).
*Warning*: literal string tokens do not work in Yacc.
By convention, a literal string token is used only to represent a
token that consists of that particular string. Thus, you should
use the token type '"<="' to represent the string '<=' as a token.
Bison does not enforce this convention, but if you depart from it,
people who read your program will be confused.
All the escape sequences used in string literals in C can be used
in Bison as well, except that you must not use a null character
within a string literal. Also, unlike Standard C, trigraphs have
no special meaning in Bison string literals, nor is
backslash-newline allowed. A literal string token must contain two
or more characters; for a token containing just one character, use
a character token (see above).
How you choose to write a terminal symbol has no effect on its
grammatical meaning. That depends only on where it appears in rules and
on when the parser function returns that symbol.
The value returned by 'yylex' is always one of the terminal symbols,
except that a zero or negative value signifies end-of-input. Whichever
way you write the token type in the grammar rules, you write it the same
way in the definition of 'yylex'. The numeric code for a character
token type is simply the positive numeric code of the character, so
'yylex' can use the identical value to generate the requisite code,
though you may need to convert it to 'unsigned char' to avoid
sign-extension on hosts where 'char' is signed. Each named token type
becomes a C macro in the parser implementation file, so 'yylex' can use
the name to stand for the code. (This is why periods don't make sense
in terminal symbols.) *Note Calling Convention for 'yylex': Calling
If 'yylex' is defined in a separate file, you need to arrange for the
token-type macro definitions to be available there. Use the '-d' option
when you run Bison, so that it will write these macro definitions into a
separate header file 'NAME.tab.h' which you can include in the other
source files that need it. *Note Invoking Bison: Invocation.
If you want to write a grammar that is portable to any Standard C
host, you must use only nonnull character tokens taken from the basic
execution character set of Standard C. This set consists of the ten
digits, the 52 lower- and upper-case English letters, and the characters
in the following C-language string:
"\a\b\t\n\v\f\r !\"#%&'()*+,-./:;<=>?[\\]^_{|}~"
The 'yylex' function and Bison must use a consistent character set
and encoding for character tokens. For example, if you run Bison in an
ASCII environment, but then compile and run the resulting program in an
environment that uses an incompatible character set like EBCDIC, the
resulting program may not work because the tables generated by Bison
will assume ASCII numeric values for character tokens. It is standard
practice for software distributions to contain C source files that were
generated by Bison in an ASCII environment, so installers on platforms
that are incompatible with ASCII must rebuild those files before
compiling them.
The symbol 'error' is a terminal symbol reserved for error recovery
(*note Error Recovery::); you shouldn't use it for any other purpose.
In particular, 'yylex' should never return this value. The default
value of the error token is 256, unless you explicitly assigned 256 to
one of your tokens with a '%token' declaration.
3.3 Grammar Rules
A Bison grammar is a list of rules.
3.3.1 Syntax of Grammar Rules
A Bison grammar rule has the following general form:
where RESULT is the nonterminal symbol that this rule describes, and
COMPONENTS are various terminal and nonterminal symbols that are put
together by this rule (*note Symbols::).
For example,
exp: exp '+' exp;
says that two groupings of type 'exp', with a '+' token in between, can
be combined into a larger grouping of type 'exp'.
White space in rules is significant only to separate symbols. You
can add extra white space as you wish.
Scattered among the components can be ACTIONS that determine the
semantics of the rule. An action looks like this:
This is an example of "braced code", that is, C code surrounded by
braces, much like a compound statement in C. Braced code can contain
any sequence of C tokens, so long as its braces are balanced. Bison
does not check the braced code for correctness directly; it merely
copies the code to the parser implementation file, where the C compiler
can check it.
Within braced code, the balanced-brace count is not affected by
braces within comments, string literals, or character constants, but it
is affected by the C digraphs '<%' and '%>' that represent braces. At
the top level braced code must be terminated by '}' and not by a
digraph. Bison does not look for trigraphs, so if braced code uses
trigraphs you should ensure that they do not affect the nesting of
braces or the boundaries of comments, string literals, or character
Usually there is only one action and it follows the components.
*Note Actions::.
Multiple rules for the same RESULT can be written separately or can
be joined with the vertical-bar character '|' as follows:
They are still considered distinct rules even when joined in this way.
3.3.2 Empty Rules
A rule is said to be "empty" if its right-hand side (COMPONENTS) is
empty. It means that RESULT can match the empty string. For example,
here is how to define an optional semicolon:
semicolon.opt: | ";";
It is easy not to see an empty rule, especially when '|' is used. The
'%empty' directive allows to make explicit that a rule is empty on
| ";"
Flagging a non-empty rule with '%empty' is an error. If run with
'-Wempty-rule', 'bison' will report empty rules without '%empty'. Using
'%empty' enables this warning, unless '-Wno-empty-rule' was specified.
The '%empty' directive is a Bison extension, it does not work with
Yacc. To remain compatible with POSIX Yacc, it is customary to write a
comment '/* empty */' in each rule with no components:
/* empty */
| ";"
3.3.3 Recursive Rules
A rule is called "recursive" when its RESULT nonterminal appears also on
its right hand side. Nearly all Bison grammars need to use recursion,
because that is the only way to define a sequence of any number of a
particular thing. Consider this recursive definition of a
comma-separated sequence of one or more expressions:
| expseq1 ',' exp
Since the recursive use of 'expseq1' is the leftmost symbol in the right
hand side, we call this "left recursion". By contrast, here the same
construct is defined using "right recursion":
| exp ',' expseq1
Any kind of sequence can be defined using either left recursion or right
recursion, but you should always use left recursion, because it can
parse a sequence of any number of elements with bounded stack space.
Right recursion uses up space on the Bison stack in proportion to the
number of elements in the sequence, because all the elements must be
shifted onto the stack before the rule can be applied even once. *Note
The Bison Parser Algorithm: Algorithm, for further explanation of this.
"Indirect" or "mutual" recursion occurs when the result of the rule
does not appear directly on its right hand side, but does appear in
rules for other nonterminals which do appear on its right hand side.
For example:
| primary '+' primary
| '(' expr ')'
defines two mutually-recursive nonterminals, since each refers to the
3.4 Defining Language Semantics
The grammar rules for a language determine only the syntax. The
semantics are determined by the semantic values associated with various
tokens and groupings, and by the actions taken when various groupings
are recognized.
For example, the calculator calculates properly because the value
associated with each expression is the proper number; it adds properly
because the action for the grouping 'X + Y' is to add the numbers
associated with X and Y.
3.4.1 Data Types of Semantic Values
In a simple program it may be sufficient to use the same data type for
the semantic values of all language constructs. This was true in the
RPN and infix calculator examples (*note Reverse Polish Notation
Calculator: RPN Calc.).
Bison normally uses the type 'int' for semantic values if your
program uses the same data type for all language constructs. To specify
some other type, define the '%define' variable 'api.value.type' like
%define api.value.type {double}
%define api.value.type {struct semantic_type}
The value of 'api.value.type' should be a type name that does not
contain parentheses or square brackets.
Alternatively, instead of relying of Bison's '%define' support, you
may rely on the C/C++ preprocessor and define 'YYSTYPE' as a macro, like
#define YYSTYPE double
This macro definition must go in the prologue of the grammar file (*note
Outline of a Bison Grammar: Grammar Outline.). If compatibility with
POSIX Yacc matters to you, use this. Note however that Bison cannot
know 'YYSTYPE''s value, not even whether it is defined, so there are
services it cannot provide. Besides this works only for languages that
have a preprocessor.
3.4.2 More Than One Value Type
In most programs, you will need different data types for different kinds
of tokens and groupings. For example, a numeric constant may need type
'int' or 'long int', while a string constant needs type 'char *', and an
identifier might need a pointer to an entry in the symbol table.
To use more than one data type for semantic values in one parser,
Bison requires you to do two things:
* Specify the entire collection of possible data types. There are
several options:
* let Bison compute the union type from the tags you assign to
* use the '%union' Bison declaration (*note The Union
Declaration: Union Decl.);
* define the '%define' variable 'api.value.type' to be a union
type whose members are the type tags (*note Providing a
Structured Semantic Value Type: Structured Value Type.);
* use a 'typedef' or a '#define' to define 'YYSTYPE' to be a
union type whose member names are the type tags.
* Choose one of those types for each symbol (terminal or nonterminal)
for which semantic values are used. This is done for tokens with
the '%token' Bison declaration (*note Token Type Names: Token
Decl.) and for groupings with the '%type' Bison declaration (*note
Nonterminal Symbols: Type Decl.).
3.4.3 Generating the Semantic Value Type
The special value 'union' of the '%define' variable 'api.value.type'
instructs Bison that the tags used with the '%token' and '%type'
directives are genuine types, not names of members of 'YYSTYPE'.
For example:
%define api.value.type union
%token INT "integer"
%token 'n'
%type expr
%token ID "identifier"
generates an appropriate value of 'YYSTYPE' to support each symbol type.
The name of the member of 'YYSTYPE' for tokens than have a declared
identifier ID (such as 'INT' and 'ID' above, but not ''n'') is 'ID'.
The other symbols have unspecified names on which you should not depend;
instead, relying on C casts to access the semantic value with the
appropriate type:
/* For an "integer". */
yylval.INT = 42;
return INT;
/* For an 'n', also declared as int. */
*((int*)&yylval) = 42;
return 'n';
/* For an "identifier". */
yylval.ID = "42";
return ID;
If the '%define' variable 'api.token.prefix' is defined (*note
api.token.prefix: %define Summary.), then it is also used to prefix the
union member names. For instance, with '%define api.token.prefix
/* For an "integer". */
yylval.TOK_INT = 42;
return TOK_INT;
This Bison extension cannot work if '%yacc' (or '-y'/'--yacc') is
enabled, as POSIX mandates that Yacc generate tokens as macros (e.g.,
'#define INT 258', or '#define TOK_INT 258').
This feature is new, and user feedback would be most welcome.
A similar feature is provided for C++ that in addition overcomes C++
limitations (that forbid non-trivial objects to be part of a 'union'):
'%define api.value.type variant', see *note C++ Variants::.
3.4.4 The Union Declaration
The '%union' declaration specifies the entire collection of possible
data types for semantic values. The keyword '%union' is followed by
braced code containing the same thing that goes inside a 'union' in C.
For example:
%union {
double val;
symrec *tptr;
This says that the two alternative types are 'double' and 'symrec *'.
They are given names 'val' and 'tptr'; these names are used in the
'%token' and '%type' declarations to pick one of the types for a
terminal or nonterminal symbol (*note Nonterminal Symbols: Type Decl.).
As an extension to POSIX, a tag is allowed after the '%union'. For
%union value {
double val;
symrec *tptr;
specifies the union tag 'value', so the corresponding C type is 'union
value'. If you do not specify a tag, it defaults to 'YYSTYPE'.
As another extension to POSIX, you may specify multiple '%union'
declarations; their contents are concatenated. However, only the first
'%union' declaration can specify a tag.
Note that, unlike making a 'union' declaration in C, you need not
write a semicolon after the closing brace.
3.4.5 Providing a Structured Semantic Value Type
Instead of '%union', you can define and use your own union type
'YYSTYPE' if your grammar contains at least one '' tag. For
example, you can put the following into a header file 'parser.h':
union YYSTYPE {
double val;
symrec *tptr;
and then your grammar can use the following instead of '%union':
#include "parser.h"
%define api.value.type {union YYSTYPE}
%type expr
%token ID
Actually, you may also provide a 'struct' rather that a 'union',
which may be handy if you want to track information for every symbol
(such as preceding comments).
The type you provide may even be structured and include pointers, in
which case the type tags you provide may be composite, with '.' and '->'
3.4.6 Actions
An action accompanies a syntactic rule and contains C code to be
executed each time an instance of that rule is recognized. The task of
most actions is to compute a semantic value for the grouping built by
the rule from the semantic values associated with tokens or smaller
An action consists of braced code containing C statements, and can be
placed at any position in the rule; it is executed at that position.
Most rules have just one action at the end of the rule, following all
the components. Actions in the middle of a rule are tricky and used
only for special purposes (*note Actions in Mid-Rule: Mid-Rule
The C code in an action can refer to the semantic values of the
components matched by the rule with the construct '$N', which stands for
the value of the Nth component. The semantic value for the grouping
being constructed is '$$'. In addition, the semantic values of symbols
can be accessed with the named references construct '$NAME' or
'$[NAME]'. Bison translates both of these constructs into expressions
of the appropriate type when it copies the actions into the parser
implementation file. '$$' (or '$NAME', when it stands for the current
grouping) is translated to a modifiable lvalue, so it can be assigned
Here is a typical example:
| exp '+' exp { $$ = $1 + $3; }
Or, in terms of named references:
| exp[left] '+' exp[right] { $result = $left + $right; }
This rule constructs an 'exp' from two smaller 'exp' groupings connected
by a plus-sign token. In the action, '$1' and '$3' ('$left' and
'$right') refer to the semantic values of the two component 'exp'
groupings, which are the first and third symbols on the right hand side
of the rule. The sum is stored into '$$' ('$result') so that it becomes
the semantic value of the addition-expression just recognized by the
rule. If there were a useful semantic value associated with the '+'
token, it could be referred to as '$2'.
*Note Named References::, for more information about using the named
references construct.
Note that the vertical-bar character '|' is really a rule separator,
and actions are attached to a single rule. This is a difference with
tools like Flex, for which '|' stands for either "or", or "the same
action as that of the next rule". In the following example, the action
is triggered only when 'b' is found:
a-or-b: 'a'|'b' { a_or_b_found = 1; };
If you don't specify an action for a rule, Bison supplies a default: '$$ = $1'.
Thus, the value of the first symbol in the rule becomes the value of the
whole rule. Of course, the default action is valid only if the two data
types match. There is no meaningful default action for an empty rule;
every empty rule must have an explicit action unless the rule's value
does not matter.
'$N' with N zero or negative is allowed for reference to tokens and
groupings on the stack _before_ those that match the current rule. This
is a very risky practice, and to use it reliably you must be certain of
the context in which the rule is applied. Here is a case in which you
can use this reliably:
expr bar '+' expr { ... }
| expr bar '-' expr { ... }
%empty { previous_expr = $0; }
As long as 'bar' is used only in the fashion shown here, '$0' always
refers to the 'expr' which precedes 'bar' in the definition of 'foo'.
It is also possible to access the semantic value of the lookahead
token, if any, from a semantic action. This semantic value is stored in
'yylval'. *Note Special Features for Use in Actions: Action Features.
3.4.7 Data Types of Values in Actions
If you have chosen a single data type for semantic values, the '$$' and
'$N' constructs always have that data type.
If you have used '%union' to specify a variety of data types, then
you must declare a choice among these types for each terminal or
nonterminal symbol that can have a semantic value. Then each time you
use '$$' or '$N', its data type is determined by which symbol it refers
to in the rule. In this example,
| exp '+' exp { $$ = $1 + $3; }
'$1' and '$3' refer to instances of 'exp', so they all have the data
type declared for the nonterminal symbol 'exp'. If '$2' were used, it
would have the data type declared for the terminal symbol ''+'',
whatever that might be.
Alternatively, you can specify the data type when you refer to the
value, by inserting '' after the '$' at the beginning of the
reference. For example, if you have defined types as shown here:
%union {
int itype;
double dtype;
then you can write '$1' to refer to the first subunit of the rule
as an integer, or '$1' to refer to it as a double.
3.4.8 Actions in Mid-Rule
Occasionally it is useful to put an action in the middle of a rule.
These actions are written just like usual end-of-rule actions, but they
are executed before the parser even recognizes the following components.
220.127.116.11 Using Mid-Rule Actions
A mid-rule action may refer to the components preceding it using '$N',
but it may not refer to subsequent components because it is run before
they are parsed.
The mid-rule action itself counts as one of the components of the
rule. This makes a difference when there is another action later in the
same rule (and usually there is another at the end): you have to count
the actions along with the symbols when working out which number N to
use in '$N'.
The mid-rule action can also have a semantic value. The action can
set its value with an assignment to '$$', and actions later in the rule
can refer to the value using '$N'. Since there is no symbol to name the
action, there is no way to declare a data type for the value in advance,
so you must use the '$<...>N' construct to specify a data type each time
you refer to this value.
There is no way to set the value of the entire rule with a mid-rule
action, because assignments to '$$' do not have that effect. The only
way to set the value for the entire rule is with an ordinary action at
the end of the rule.
Here is an example from a hypothetical compiler, handling a 'let'
statement that looks like 'let (VARIABLE) STATEMENT' and serves to
create a variable named VARIABLE temporarily for the duration of
STATEMENT. To parse this construct, we must put VARIABLE into the
symbol table while STATEMENT is parsed, then remove it afterward. Here
is how it is done:
"let" '(' var ')'
$$ = push_context ();
declare_variable ($3);
$$ = $6;
pop_context ($5);
As soon as 'let (VARIABLE)' has been recognized, the first action is
run. It saves a copy of the current semantic context (the list of
accessible variables) as its semantic value, using alternative 'context'
in the data-type union. Then it calls 'declare_variable' to add the new
variable to that list. Once the first action is finished, the embedded
statement 'stmt' can be parsed.
Note that the mid-rule action is component number 5, so the 'stmt' is
component number 6. Named references can be used to improve the
readability and maintainability (*note Named References::):
"let" '(' var ')'
$let = push_context ();
declare_variable ($3);
$$ = $6;
pop_context ($let);
After the embedded statement is parsed, its semantic value becomes
the value of the entire 'let'-statement. Then the semantic value from
the earlier action is used to restore the prior list of variables. This
removes the temporary 'let'-variable from the list so that it won't
appear to exist while the rest of the program is parsed.
In the above example, if the parser initiates error recovery (*note
Error Recovery::) while parsing the tokens in the embedded statement
'stmt', it might discard the previous semantic context '$5'
without restoring it. Thus, '$5' needs a destructor (*note
Freeing Discarded Symbols: Destructor Decl.). However, Bison currently
provides no means to declare a destructor specific to a particular
mid-rule action's semantic value.
One solution is to bury the mid-rule action inside a nonterminal
symbol and to declare a destructor for that symbol:
%type let
%destructor { pop_context ($$); } let
let stmt
$$ = $2;
pop_context ($let);
"let" '(' var ')'
$let = push_context ();
declare_variable ($3);
Note that the action is now at the end of its rule. Any mid-rule action
can be converted to an end-of-rule action in this way, and this is what
Bison actually does to implement mid-rule actions.
188.8.131.52 Mid-Rule Action Translation
As hinted earlier, mid-rule actions are actually transformed into
regular rules and actions. The various reports generated by Bison
(textual, graphical, etc., see *note Understanding Your Parser:
Understanding.) reveal this translation, best explained by means of an
example. The following rule:
exp: { a(); } "b" { c(); } { d(); } "e" { f(); };
is translated into:
$@1: %empty { a(); };
$@2: %empty { c(); };
$@3: %empty { d(); };
exp: $@1 "b" $@2 $@3 "e" { f(); };
with new nonterminal symbols '$@N', where N is a number.
A mid-rule action is expected to generate a value if it uses '$$', or
the (final) action uses '$N' where N denote the mid-rule action. In
that case its nonterminal is rather named '@N':
exp: { a(); } "b" { $$ = c(); } { d(); } "e" { f = $1; };
is translated into
@1: %empty { a(); };
@2: %empty { $$ = c(); };
$@3: %empty { d(); };
exp: @1 "b" @2 $@3 "e" { f = $1; }
There are probably two errors in the above example: the first
mid-rule action does not generate a value (it does not use '$$' although
the final action uses it), and the value of the second one is not used
(the final action does not use '$3'). Bison reports these errors when
the 'midrule-value' warnings are enabled (*note Invoking Bison:
$ bison -fcaret -Wmidrule-value mid.y
mid.y:2.6-13: warning: unset value: $$
exp: { a(); } "b" { $$ = c(); } { d(); } "e" { f = $1; };
mid.y:2.19-31: warning: unused value: $3
exp: { a(); } "b" { $$ = c(); } { d(); } "e" { f = $1; };
188.8.131.52 Conflicts due to Mid-Rule Actions
Taking action before a rule is completely recognized often leads to
conflicts since the parser must commit to a parse in order to execute
the action. For example, the following two rules, without mid-rule
actions, can coexist in a working parser because the parser can shift
the open-brace token and look at what follows before deciding whether
there is a declaration or not:
'{' declarations statements '}'
| '{' statements '}'
But when we add a mid-rule action as follows, the rules become
{ prepare_for_local_variables (); }
'{' declarations statements '}'
| '{' statements '}'
Now the parser is forced to decide whether to run the mid-rule action
when it has read no farther than the open-brace. In other words, it
must commit to using one rule or the other, without sufficient
information to do it correctly. (The open-brace token is what is called
the "lookahead" token at this time, since the parser is still deciding
what to do about it. *Note Lookahead Tokens: Lookahead.)
You might think that you could correct the problem by putting
identical actions into the two rules, like this:
{ prepare_for_local_variables (); }
'{' declarations statements '}'
| { prepare_for_local_variables (); }
'{' statements '}'
But this does not help, because Bison does not realize that the two
actions are identical. (Bison never tries to understand the C code in
an action.)
If the grammar is such that a declaration can be distinguished from a
statement by the first token (which is true in C), then one solution
which does work is to put the action after the open-brace, like this:
'{' { prepare_for_local_variables (); }
declarations statements '}'
| '{' statements '}'
Now the first token of the following declaration or statement, which
would in any case tell Bison which rule to use, can still do so.
Another solution is to bury the action inside a nonterminal symbol
which serves as a subroutine:
%empty { prepare_for_local_variables (); }
subroutine '{' declarations statements '}'
| subroutine '{' statements '}'
Now Bison can execute the action in the rule for 'subroutine' without
deciding which rule for 'compound' it will eventually use.
3.5 Tracking Locations
Though grammar rules and semantic actions are enough to write a fully
functional parser, it can be useful to process some additional
information, especially symbol locations.
The way locations are handled is defined by providing a data type,
and actions to take when rules are matched.
3.5.1 Data Type of Locations
Defining a data type for locations is much simpler than for semantic
values, since all tokens and groupings always use the same type.
You can specify the type of locations by defining a macro called
'YYLTYPE', just as you can specify the semantic value type by defining a
'YYSTYPE' macro (*note Value Type::). When 'YYLTYPE' is not defined,
Bison uses a default structure type with four members:
typedef struct YYLTYPE
int first_line;
int first_column;
int last_line;
int last_column;
When 'YYLTYPE' is not defined, at the beginning of the parsing, Bison
initializes all these fields to 1 for 'yylloc'. To initialize 'yylloc'
with a custom location type (or to chose a different initialization),
use the '%initial-action' directive. *Note Performing Actions before
Parsing: Initial Action Decl.
3.5.2 Actions and Locations
Actions are not only useful for defining language semantics, but also
for describing the behavior of the output parser with locations.
The most obvious way for building locations of syntactic groupings is
very similar to the way semantic values are computed. In a given rule,
several constructs can be used to access the locations of the elements
being matched. The location of the Nth component of the right hand side
is '@N', while the location of the left hand side grouping is '@$'.
In addition, the named references construct '@NAME' and '@[NAME]' may
also be used to address the symbol locations. *Note Named References::,
for more information about using the named references construct.
Here is a basic example using the default data type for locations:
| exp '/' exp
@$.first_column = @1.first_column;
@$.first_line = @1.first_line;
@$.last_column = @3.last_column;
@$.last_line = @3.last_line;
if ($3)
$$ = $1 / $3;
$$ = 1;
fprintf (stderr, "%d.%d-%d.%d: division by zero",
@3.first_line, @3.first_column,
@3.last_line, @3.last_column);
As for semantic values, there is a default action for locations that
is run each time a rule is matched. It sets the beginning of '@$' to
the beginning of the first symbol, and the end of '@$' to the end of the
last symbol.
With this default action, the location tracking can be fully
automatic. The example above simply rewrites this way:
| exp '/' exp
if ($3)
$$ = $1 / $3;
$$ = 1;
fprintf (stderr, "%d.%d-%d.%d: division by zero",
@3.first_line, @3.first_column,
@3.last_line, @3.last_column);
It is also possible to access the location of the lookahead token, if
any, from a semantic action. This location is stored in 'yylloc'.
*Note Special Features for Use in Actions: Action Features.
3.5.3 Default Action for Locations
Actually, actions are not the best place to compute locations. Since
locations are much more general than semantic values, there is room in
the output parser to redefine the default action to take for each rule.
The 'YYLLOC_DEFAULT' macro is invoked each time a rule is matched,
before the associated action is run. It is also invoked while
processing a syntax error, to compute the error's location. Before
reporting an unresolvable syntactic ambiguity, a GLR parser invokes
'YYLLOC_DEFAULT' recursively to compute the location of that ambiguity.
Most of the time, this macro is general enough to suppress location
dedicated code from semantic actions.
The 'YYLLOC_DEFAULT' macro takes three parameters. The first one is
the location of the grouping (the result of the computation). When a
rule is matched, the second parameter identifies locations of all right
hand side elements of the rule being matched, and the third parameter is
the size of the rule's right hand side. When a GLR parser reports an
ambiguity, which of multiple candidate right hand sides it passes to
'YYLLOC_DEFAULT' is undefined. When processing a syntax error, the
second parameter identifies locations of the symbols that were discarded
during error processing, and the third parameter is the number of
discarded symbols.
By default, 'YYLLOC_DEFAULT' is defined this way:
# define YYLLOC_DEFAULT(Cur, Rhs, N) \
do \
if (N) \
{ \
(Cur).first_line = YYRHSLOC(Rhs, 1).first_line; \
(Cur).first_column = YYRHSLOC(Rhs, 1).first_column; \
(Cur).last_line = YYRHSLOC(Rhs, N).last_line; \
(Cur).last_column = YYRHSLOC(Rhs, N).last_column; \
} \
else \
{ \
(Cur).first_line = (Cur).last_line = \
YYRHSLOC(Rhs, 0).last_line; \
(Cur).first_column = (Cur).last_column = \
YYRHSLOC(Rhs, 0).last_column; \
} \
while (0)
where 'YYRHSLOC (rhs, k)' is the location of the Kth symbol in RHS when
K is positive, and the location of the symbol just before the reduction
when K and N are both zero.
When defining 'YYLLOC_DEFAULT', you should consider that:
* All arguments are free of side-effects. However, only the first
one (the result) should be modified by 'YYLLOC_DEFAULT'.
* For consistency with semantic actions, valid indexes within the
right hand side range from 1 to N. When N is zero, only 0 is a
valid index, and it refers to the symbol just before the reduction.
During error processing N is always positive.
* Your macro should parenthesize its arguments, if need be, since the
actual arguments may not be surrounded by parentheses. Also, your
macro should expand to something that can be used as a single
statement when it is followed by a semicolon.
3.6 Named References
As described in the preceding sections, the traditional way to refer to
any semantic value or location is a "positional reference", which takes
the form '$N', '$$', '@N', and '@$'. However, such a reference is not
very descriptive. Moreover, if you later decide to insert or remove
symbols in the right-hand side of a grammar rule, the need to renumber
such references can be tedious and error-prone.
To avoid these issues, you can also refer to a semantic value or
location using a "named reference". First of all, original symbol names
may be used as named references. For example:
invocation: op '(' args ')'
{ $invocation = new_invocation ($op, $args, @invocation); }
Positional and named references can be mixed arbitrarily. For example:
invocation: op '(' args ')'
{ $$ = new_invocation ($op, $args, @$); }
However, sometimes regular symbol names are not sufficient due to
exp: exp '/' exp
{ $exp = $exp / $exp; } // $exp is ambiguous.
exp: exp '/' exp
{ $$ = $1 / $exp; } // One usage is ambiguous.
exp: exp '/' exp
{ $$ = $1 / $3; } // No error.
When ambiguity occurs, explicitly declared names may be used for values
and locations. Explicit names are declared as a bracketed name after a
symbol appearance in rule definitions. For example:
exp[result]: exp[left] '/' exp[right]
{ $result = $left / $right; }
In order to access a semantic value generated by a mid-rule action, an
explicit name may also be declared by putting a bracketed name after the
closing brace of the mid-rule action code:
exp[res]: exp[x] '+' {$left = $x;}[left] exp[right]
{ $res = $left + $right; }
In references, in order to specify names containing dots and dashes,
an explicit bracketed syntax '$[name]' and '@[name]' must be used:
if-stmt: "if" '(' expr ')' "then" then.stmt ';'
{ $[if-stmt] = new_if_stmt ($expr, $[then.stmt]); }
It often happens that named references are followed by a dot, dash or
other C punctuation marks and operators. By default, Bison will read
'$name.suffix' as a reference to symbol value '$name' followed by
'.suffix', i.e., an access to the 'suffix' field of the semantic value.
In order to force Bison to recognize 'name.suffix' in its entirety as
the name of a semantic value, the bracketed syntax '$[name.suffix]' must
be used.
The named references feature is experimental. More user feedback
will help to stabilize it.
3.7 Bison Declarations
The "Bison declarations" section of a Bison grammar defines the symbols
used in formulating the grammar and the data types of semantic values.
*Note Symbols::.
All token type names (but not single-character literal tokens such as
''+'' and ''*'') must be declared. Nonterminal symbols must be declared
if you need to specify which data type to use for the semantic value
(*note More Than One Value Type: Multiple Types.).
The first rule in the grammar file also specifies the start symbol,
by default. If you want some other symbol to be the start symbol, you
must declare it explicitly (*note Languages and Context-Free Grammars:
Language and Grammar.).
3.7.1 Require a Version of Bison
You may require the minimum version of Bison to process the grammar. If
the requirement is not met, 'bison' exits with an error (exit status
%require "VERSION"
3.7.2 Token Type Names
The basic way to declare a token type name (terminal symbol) is as
%token NAME
Bison will convert this into a '#define' directive in the parser, so
that the function 'yylex' (if it is in this file) can use the name NAME
to stand for this token type's code.
Alternatively, you can use '%left', '%right', '%precedence', or
'%nonassoc' instead of '%token', if you wish to specify associativity
and precedence. *Note Operator Precedence: Precedence Decl.
You can explicitly specify the numeric code for a token type by
appending a nonnegative decimal or hexadecimal integer value in the
field immediately following the token name:
%token NUM 300
%token XNUM 0x12d // a GNU extension
It is generally best, however, to let Bison choose the numeric codes for
all token types. Bison will automatically select codes that don't
conflict with each other or with normal characters.
In the event that the stack type is a union, you must augment the
'%token' or other token declaration to include the data type alternative
delimited by angle-brackets (*note More Than One Value Type: Multiple
For example:
%union { /* define stack type */
double val;
symrec *tptr;
%token NUM /* define token NUM and its type */
You can associate a literal string token with a token type name by
writing the literal string at the end of a '%token' declaration which
declares the name. For example:
%token arrow "=>"
For example, a grammar for the C language might specify these names with
equivalent literal string tokens:
%token OR "||"
%token LE 134 "<="
%left OR "<="
Once you equate the literal string and the token name, you can use them
interchangeably in further declarations or the grammar rules. The
'yylex' function can use the token name or the literal string to obtain
the token type code number (*note Calling Convention::). Syntax error
messages passed to 'yyerror' from the parser will reference the literal
string instead of the token name.
The token numbered as 0 corresponds to end of file; the following
line allows for nicer error messages referring to "end of file" instead
of "$end":
%token END 0 "end of file"
3.7.3 Operator Precedence
Use the '%left', '%right', '%nonassoc', or '%precedence' declaration to
declare a token and specify its precedence and associativity, all at
once. These are called "precedence declarations". *Note Operator
Precedence: Precedence, for general information on operator precedence.
The syntax of a precedence declaration is nearly the same as that of
'%token': either
%left SYMBOLS...
%left SYMBOLS...
And indeed any of these declarations serves the purposes of '%token'.
But in addition, they specify the associativity and relative precedence
for all the SYMBOLS:
* The associativity of an operator OP determines how repeated uses of
the operator nest: whether 'X OP Y OP Z' is parsed by grouping X
with Y first or by grouping Y with Z first. '%left' specifies
left-associativity (grouping X with Y first) and '%right' specifies
right-associativity (grouping Y with Z first). '%nonassoc'
specifies no associativity, which means that 'X OP Y OP Z' is
considered a syntax error.
'%precedence' gives only precedence to the SYMBOLS, and defines no
associativity at all. Use this to define precedence only, and
leave any potential conflict due to associativity enabled.
* The precedence of an operator determines how it nests with other
operators. All the tokens declared in a single precedence
declaration have equal precedence and nest together according to
their associativity. When two tokens declared in different
precedence declarations associate, the one declared later has the
higher precedence and is grouped first.
For backward compatibility, there is a confusing difference between
the argument lists of '%token' and precedence declarations. Only a
'%token' can associate a literal string with a token type name. A
precedence declaration always interprets a literal string as a reference
to a separate token. For example:
%left OR "<=" // Does not declare an alias.
%left OR 134 "<=" 135 // Declares 134 for OR and 135 for "<=".
3.7.4 Nonterminal Symbols
When you use '%union' to specify multiple value types, you must declare
the value type of each nonterminal symbol for which values are used.
This is done with a '%type' declaration, like this:
%type NONTERMINAL...
Here NONTERMINAL is the name of a nonterminal symbol, and TYPE is the
name given in the '%union' to the alternative that you want (*note The
Union Declaration: Union Decl.). You can give any number of nonterminal
symbols in the same '%type' declaration, if they have the same value
type. Use spaces to separate the symbol names.
You can also declare the value type of a terminal symbol. To do
this, use the same '' construction in a declaration for the
terminal symbol. All kinds of token declarations allow ''.
3.7.5 Performing Actions before Parsing
Sometimes your parser needs to perform some initializations before
parsing. The '%initial-action' directive allows for such arbitrary
-- Directive: %initial-action { CODE }
Declare that the braced CODE must be invoked before parsing each
time 'yyparse' is called. The CODE may use '$$' (or '$$') and
'@$' -- initial value and location of the lookahead -- and the
For instance, if your locations use a file name, you may use
%parse-param { char const *file_name };
@$.initialize (file_name);
3.7.6 Freeing Discarded Symbols
During error recovery (*note Error Recovery::), symbols already pushed
on the stack and tokens coming from the rest of the file are discarded
until the parser falls on its feet. If the parser runs out of memory,
or if it returns via 'YYABORT' or 'YYACCEPT', all the symbols on the
stack must be discarded. Even if the parser succeeds, it must discard
the start symbol.
When discarded symbols convey heap based information, this memory is
lost. While this behavior can be tolerable for batch parsers, such as
in traditional compilers, it is unacceptable for programs like shells or
protocol implementations that may parse and execute indefinitely.
The '%destructor' directive defines code that is called when a symbol
is automatically discarded.
-- Directive: %destructor { CODE } SYMBOLS
Invoke the braced CODE whenever the parser discards one of the
SYMBOLS. Within CODE, '$$' (or '$$') designates the semantic
value associated with the discarded symbol, and '@$' designates its
location. The additional parser parameters are also available
(*note The Parser Function 'yyparse': Parser Function.).
When a symbol is listed among SYMBOLS, its '%destructor' is called
a per-symbol '%destructor'. You may also define a per-type
'%destructor' by listing a semantic type tag among SYMBOLS. In
that case, the parser will invoke this CODE whenever it discards
any grammar symbol that has that semantic type tag unless that
symbol has its own per-symbol '%destructor'.
Finally, you can define two different kinds of default
'%destructor's. (These default forms are experimental. More user
feedback will help to determine whether they should become
permanent features.) You can place each of '<*>' and '<>' in the
SYMBOLS list of exactly one '%destructor' declaration in your
grammar file. The parser will invoke the CODE associated with one
of these whenever it discards any user-defined grammar symbol that
has no per-symbol and no per-type '%destructor'. The parser uses
the CODE for '<*>' in the case of such a grammar symbol for which
you have formally declared a semantic type tag ('%type' counts as
such a declaration, but '$$' does not). The parser uses the
CODE for '<>' in the case of such a grammar symbol that has no
declared semantic type tag.
For example:
%union { char *string; }
%token STRING1 STRING2
%type string1 string2
%union { char character; }
%token CHR
%type chr
%token TAGLESS
%destructor { }
%destructor { free ($$); } <*>
%destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
%destructor { printf ("Discarding tagless symbol.\n"); } <>
guarantees that, when the parser discards any user-defined symbol that
has a semantic type tag other than '', it passes its semantic
value to 'free' by default. However, when the parser discards a
'STRING1' or a 'string1', it also prints its line number to 'stdout'.
It performs only the second '%destructor' in this case, so it invokes
'free' only once. Finally, the parser merely prints a message whenever
it discards any symbol, such as 'TAGLESS', that has no semantic type
A Bison-generated parser invokes the default '%destructor's only for
user-defined as opposed to Bison-defined symbols. For example, the
parser will not invoke either kind of default '%destructor' for the
special Bison-defined symbols '$accept', '$undefined', or '$end' (*note
Bison Symbols: Table of Symbols.), none of which you can reference in
your grammar. It also will not invoke either for the 'error' token
(*note error: Table of Symbols.), which is always defined by Bison
regardless of whether you reference it in your grammar. However, it may
invoke one of them for the end token (token 0) if you redefine it from
'$end' to, for example, 'END':
%token END 0
Finally, Bison will never invoke a '%destructor' for an unreferenced
mid-rule semantic value (*note Actions in Mid-Rule: Mid-Rule Actions.).
That is, Bison does not consider a mid-rule to have a semantic value if
you do not reference '$$' in the mid-rule's action or '$N' (where N is
the right-hand side symbol position of the mid-rule) in any later action
in that rule. However, if you do reference either, the Bison-generated
parser will invoke the '<>' '%destructor' whenever it discards the
mid-rule symbol.
"Discarded symbols" are the following:
* stacked symbols popped during the first phase of error recovery,
* incoming terminals during the second phase of error recovery,
* the current lookahead and the entire stack (except the current
right-hand side symbols) when the parser returns immediately, and
* the current lookahead and the entire stack (including the current
right-hand side symbols) when the C++ parser ('lalr1.cc') catches
an exception in 'parse',
* the start symbol, when the parser succeeds.
The parser can "return immediately" because of an explicit call to
'YYABORT' or 'YYACCEPT', or failed error recovery, or memory exhaustion.
Right-hand side symbols of a rule that explicitly triggers a syntax
error via 'YYERROR' are not discarded automatically. As a rule of
thumb, destructors are invoked only when user actions cannot manage the
3.7.7 Printing Semantic Values
When run-time traces are enabled (*note Tracing Your Parser: Tracing.),
the parser reports its actions, such as reductions. When a symbol
involved in an action is reported, only its kind is displayed, as the
parser cannot know how semantic values should be formatted.
The '%printer' directive defines code that is called when a symbol is
reported. Its syntax is the same as '%destructor' (*note Freeing
Discarded Symbols: Destructor Decl.).
-- Directive: %printer { CODE } SYMBOLS
Invoke the braced CODE whenever the parser displays one of the
SYMBOLS. Within CODE, 'yyoutput' denotes the output stream (a
'FILE*' in C, and an 'std::ostream&' in C++), '$$' (or '$$')
designates the semantic value associated with the symbol, and '@$'
its location. The additional parser parameters are also available
(*note The Parser Function 'yyparse': Parser Function.).
The SYMBOLS are defined as for '%destructor' (*note Freeing
Discarded Symbols: Destructor Decl.): they can be per-type (e.g.,
''), per-symbol (e.g., 'exp', 'NUM', '"float"'), typed
per-default (i.e., '<*>', or untyped per-default (i.e., '<>').
For example:
%union { char *string; }
%token STRING1 STRING2
%type string1 string2
%union { char character; }
%token CHR
%type chr
%token TAGLESS
%printer { fprintf (yyoutput, "'%c'", $$); }
%printer { fprintf (yyoutput, "&%p", $$); } <*>
%printer { fprintf (yyoutput, "\"%s\"", $$); } STRING1 string1
%printer { fprintf (yyoutput, "<>"); } <>
guarantees that, when the parser print any symbol that has a semantic
type tag other than '', it display the address of the
semantic value by default. However, when the parser displays a
'STRING1' or a 'string1', it formats it as a string in double quotes.
It performs only the second '%printer' in this case, so it prints only
once. Finally, the parser print '<>' for any symbol, such as 'TAGLESS',
that has no semantic type tag. See also
3.7.8 Suppressing Conflict Warnings
Bison normally warns if there are any conflicts in the grammar (*note
Shift/Reduce Conflicts: Shift/Reduce.), but most real grammars have
harmless shift/reduce conflicts which are resolved in a predictable way
and would be difficult to eliminate. It is desirable to suppress the
warning about these conflicts unless the number of conflicts changes.
You can do this with the '%expect' declaration.
The declaration looks like this:
%expect N
Here N is a decimal integer. The declaration says there should be N
shift/reduce conflicts and no reduce/reduce conflicts. Bison reports an
error if the number of shift/reduce conflicts differs from N, or if
there are any reduce/reduce conflicts.
For deterministic parsers, reduce/reduce conflicts are more serious,
and should be eliminated entirely. Bison will always report
reduce/reduce conflicts for these parsers. With GLR parsers, however,
both kinds of conflicts are routine; otherwise, there would be no need
to use GLR parsing. Therefore, it is also possible to specify an
expected number of reduce/reduce conflicts in GLR parsers, using the
%expect-rr N
In general, using '%expect' involves these steps:
* Compile your grammar without '%expect'. Use the '-v' option to get
a verbose list of where the conflicts occur. Bison will also print
the number of conflicts.
* Check each of the conflicts to make sure that Bison's default
resolution is what you really want. If not, rewrite the grammar
and go back to the beginning.
* Add an '%expect' declaration, copying the number N from the number
which Bison printed. With GLR parsers, add an '%expect-rr'
declaration as well.
Now Bison will report an error if you introduce an unexpected
conflict, but will keep silent otherwise.
3.7.9 The Start-Symbol
Bison assumes by default that the start symbol for the grammar is the
first nonterminal specified in the grammar specification section. The
programmer may override this restriction with the '%start' declaration
as follows:
%start SYMBOL
3.7.10 A Pure (Reentrant) Parser
A "reentrant" program is one which does not alter in the course of
execution; in other words, it consists entirely of "pure" (read-only)
code. Reentrancy is important whenever asynchronous execution is
possible; for example, a nonreentrant program may not be safe to call
from a signal handler. In systems with multiple threads of control, a
nonreentrant program must be called only within interlocks.
Normally, Bison generates a parser which is not reentrant. This is
suitable for most uses, and it permits compatibility with Yacc. (The
standard Yacc interfaces are inherently nonreentrant, because they use
statically allocated variables for communication with 'yylex', including
'yylval' and 'yylloc'.)
Alternatively, you can generate a pure, reentrant parser. The Bison
declaration '%define api.pure' says that you want the parser to be
reentrant. It looks like this:
%define api.pure full
The result is that the communication variables 'yylval' and 'yylloc'
become local variables in 'yyparse', and a different calling convention
is used for the lexical analyzer function 'yylex'. *Note Calling
Conventions for Pure Parsers: Pure Calling, for the details of this.
The variable 'yynerrs' becomes local in 'yyparse' in pull mode but it
becomes a member of 'yypstate' in push mode. (*note The Error Reporting
Function 'yyerror': Error Reporting.). The convention for calling
'yyparse' itself is unchanged.
Whether the parser is pure has nothing to do with the grammar rules.
You can generate either a pure parser or a nonreentrant parser from any
valid grammar.
3.7.11 A Push Parser
(The current push parsing interface is experimental and may evolve.
More user feedback will help to stabilize it.)
A pull parser is called once and it takes control until all its input
is completely parsed. A push parser, on the other hand, is called each
time a new token is made available.
A push parser is typically useful when the parser is part of a main
event loop in the client's application. This is typically a requirement
of a GUI, when the main event loop needs to be triggered within a
certain time period.
Normally, Bison generates a pull parser. The following Bison
declaration says that you want the parser to be a push parser (*note
api.push-pull: %define Summary.):
%define api.push-pull push
In almost all cases, you want to ensure that your push parser is also
a pure parser (*note A Pure (Reentrant) Parser: Pure Decl.). The only
time you should create an impure push parser is to have backwards
compatibility with the impure Yacc pull mode interface. Unless you know
what you are doing, your declarations should look like this:
%define api.pure full
%define api.push-pull push
There is a major notable functional difference between the pure push
parser and the impure push parser. It is acceptable for a pure push
parser to have many parser instances, of the same type of parser, in
memory at the same time. An impure push parser should only use one
parser at a time.
When a push parser is selected, Bison will generate some new symbols
in the generated parser. 'yypstate' is a structure that the generated
parser uses to store the parser's state. 'yypstate_new' is the function
that will create a new parser instance. 'yypstate_delete' will free the
resources associated with the corresponding parser instance. Finally,
'yypush_parse' is the function that should be called whenever a token is
available to provide the parser. A trivial example of using a pure push
parser would look like this:
int status;
yypstate *ps = yypstate_new ();
do {
status = yypush_parse (ps, yylex (), NULL);
} while (status == YYPUSH_MORE);
yypstate_delete (ps);
If the user decided to use an impure push parser, a few things about
the generated parser will change. The 'yychar' variable becomes a
global variable instead of a variable in the 'yypush_parse' function.
For this reason, the signature of the 'yypush_parse' function is changed
to remove the token as a parameter. A nonreentrant push parser example
would thus look like this:
extern int yychar;
int status;
yypstate *ps = yypstate_new ();
do {
yychar = yylex ();
status = yypush_parse (ps);
} while (status == YYPUSH_MORE);
yypstate_delete (ps);
That's it. Notice the next token is put into the global variable
'yychar' for use by the next invocation of the 'yypush_parse' function.
Bison also supports both the push parser interface along with the
pull parser interface in the same generated parser. In order to get
this functionality, you should replace the '%define api.push-pull push'
declaration with the '%define api.push-pull both' declaration. Doing
this will create all of the symbols mentioned earlier along with the two
extra symbols, 'yyparse' and 'yypull_parse'. 'yyparse' can be used
exactly as it normally would be used. However, the user should note
that it is implemented in the generated parser by calling
'yypull_parse'. This makes the 'yyparse' function that is generated
with the '%define api.push-pull both' declaration slower than the normal
'yyparse' function. If the user calls the 'yypull_parse' function it
will parse the rest of the input stream. It is possible to
'yypush_parse' tokens to select a subgrammar and then 'yypull_parse' the
rest of the input stream. If you would like to switch back and forth
between between parsing styles, you would have to write your own
'yypull_parse' function that knows when to quit looking for input. An
example of using the 'yypull_parse' function would look like this:
yypstate *ps = yypstate_new ();
yypull_parse (ps); /* Will call the lexer */
yypstate_delete (ps);
Adding the '%define api.pure' declaration does exactly the same thing
to the generated parser with '%define api.push-pull both' as it did for
'%define api.push-pull push'.
3.7.12 Bison Declaration Summary
Here is a summary of the declarations used to define a grammar:
-- Directive: %union
Declare the collection of data types that semantic values may have
(*note The Union Declaration: Union Decl.).
-- Directive: %token
Declare a terminal symbol (token type name) with no precedence or
associativity specified (*note Token Type Names: Token Decl.).
-- Directive: %right
Declare a terminal symbol (token type name) that is
right-associative (*note Operator Precedence: Precedence Decl.).
-- Directive: %left
Declare a terminal symbol (token type name) that is
left-associative (*note Operator Precedence: Precedence Decl.).
-- Directive: %nonassoc
Declare a terminal symbol (token type name) that is nonassociative
(*note Operator Precedence: Precedence Decl.). Using it in a way
that would be associative is a syntax error.
-- Directive: %type
Declare the type of semantic values for a nonterminal symbol (*note
Nonterminal Symbols: Type Decl.).
-- Directive: %start
Specify the grammar's start symbol (*note The Start-Symbol: Start
-- Directive: %expect
Declare the expected number of shift-reduce conflicts (*note
Suppressing Conflict Warnings: Expect Decl.).
In order to change the behavior of 'bison', use the following
-- Directive: %code {CODE}
-- Directive: %code QUALIFIER {CODE}
Insert CODE verbatim into the output parser source at the default
location or at the location specified by QUALIFIER. *Note %code
-- Directive: %debug
Instrument the parser for traces. Obsoleted by '%define
parse.trace'. *Note Tracing Your Parser: Tracing.
-- Directive: %define VARIABLE
-- Directive: %define VARIABLE VALUE
-- Directive: %define VARIABLE {VALUE}
-- Directive: %define VARIABLE "VALUE"
Define a variable to adjust Bison's behavior. *Note %define
-- Directive: %defines
Write a parser header file containing macro definitions for the
token type names defined in the grammar as well as a few other
declarations. If the parser implementation file is named 'NAME.c'
then the parser header file is named 'NAME.h'.
For C parsers, the parser header file declares 'YYSTYPE' unless
'YYSTYPE' is already defined as a macro or you have used a ''
tag without using '%union'. Therefore, if you are using a '%union'
(*note More Than One Value Type: Multiple Types.) with components
that require other definitions, or if you have defined a 'YYSTYPE'
macro or type definition (*note Data Types of Semantic Values:
Value Type.), you need to arrange for these definitions to be
propagated to all modules, e.g., by putting them in a prerequisite
header that is included both by your parser and by any other module
that needs 'YYSTYPE'.
Unless your parser is pure, the parser header file declares
'yylval' as an external variable. *Note A Pure (Reentrant) Parser:
Pure Decl.
If you have also used locations, the parser header file declares
'YYLTYPE' and 'yylloc' using a protocol similar to that of the
'YYSTYPE' macro and 'yylval'. *Note Tracking Locations::.
This parser header file is normally essential if you wish to put
the definition of 'yylex' in a separate source file, because
'yylex' typically needs to be able to refer to the above-mentioned
declarations and to the token type codes. *Note Semantic Values of
Tokens: Token Values.
If you have declared '%code requires' or '%code provides', the
output header also contains their code. *Note %code Summary::.
The generated header is protected against multiple inclusions with
a C preprocessor guard: 'YY_PREFIX_FILE_INCLUDED', where PREFIX and
FILE are the prefix (*note Multiple Parsers in the Same Program:
Multiple Parsers.) and generated file name turned uppercase, with
each series of non alphanumerical characters converted to a single
For instance with '%define api.prefix {calc}' and '%defines
"lib/parse.h"', the header will be guarded as follows.
#ifndef YY_CALC_LIB_PARSE_H_INCLUDED
# define YY_CALC_LIB_PARSE_H_INCLUDED
#endif /* ! YY_CALC_LIB_PARSE_H_INCLUDED */
-- Directive: %defines DEFINES-FILE
Same as above, but save in the file 'DEFINES-FILE'.
-- Directive: %destructor
Specify how the parser should reclaim the memory associated to
discarded symbols. *Note Freeing Discarded Symbols: Destructor
-- Directive: %file-prefix "PREFIX"
Specify a prefix to use for all Bison output file names. The names
are chosen as if the grammar file were named 'PREFIX.y'.
-- Directive: %language "LANGUAGE"
Specify the programming language for the generated parser.
Currently supported languages include C, C++, and Java. LANGUAGE
is case-insensitive.
-- Directive: %locations
Generate the code processing the locations (*note Special Features
for Use in Actions: Action Features.). This mode is enabled as
soon as the grammar uses the special '@N' tokens, but if your
grammar does not use it, using '%locations' allows for more
accurate syntax error messages.
-- Directive: %name-prefix "PREFIX"
Rename the external symbols used in the parser so that they start
with PREFIX instead of 'yy'. The precise list of symbols renamed
in C parsers is 'yyparse', 'yylex', 'yyerror', 'yynerrs', 'yylval',
'yychar', 'yydebug', and (if locations are used) 'yylloc'. If you
use a push parser, 'yypush_parse', 'yypull_parse', 'yypstate',
'yypstate_new' and 'yypstate_delete' will also be renamed. For
example, if you use '%name-prefix "c_"', the names become
'c_parse', 'c_lex', and so on. For C++ parsers, see the '%define
api.namespace' documentation in this section. *Note Multiple
Parsers in the Same Program: Multiple Parsers.
-- Directive: %no-lines
Don't generate any '#line' preprocessor commands in the parser
implementation file. Ordinarily Bison writes these commands in the
parser implementation file so that the C compiler and debuggers
will associate errors and object code with your source file (the
grammar file). This directive causes them to associate errors with
the parser implementation file, treating it as an independent
source file in its own right.
-- Directive: %output "FILE"
Generate the parser implementation in 'FILE'.
-- Directive: %pure-parser
Deprecated version of '%define api.pure' (*note api.pure: %define
Summary.), for which Bison is more careful to warn about
unreasonable usage.
-- Directive: %require "VERSION"
Require version VERSION or higher of Bison. *Note Require a
Version of Bison: Require Decl.
-- Directive: %skeleton "FILE"
Specify the skeleton to use.
If FILE does not contain a '/', FILE is the name of a skeleton file
in the Bison installation directory. If it does, FILE is an
absolute file name or a file name relative to the directory of the
grammar file. This is similar to how most shells resolve commands.
-- Directive: %token-table
Generate an array of token names in the parser implementation file.
The name of the array is 'yytname'; 'yytname[I]' is the name of the
token whose internal Bison token code number is I. The first three
elements of 'yytname' correspond to the predefined tokens '"$end"',
'"error"', and '"$undefined"'; after these come the symbols defined
in the grammar file.
The name in the table includes all the characters needed to
represent the token in Bison. For single-character literals and
literal strings, this includes the surrounding quoting characters
and any escape sequences. For example, the Bison single-character
literal ''+'' corresponds to a three-character name, represented in
C as '"'+'"'; and the Bison two-character literal string '"\\/"'
corresponds to a five-character name, represented in C as
When you specify '%token-table', Bison also generates macro
definitions for macros 'YYNTOKENS', 'YYNNTS', and 'YYNRULES', and
The highest token number, plus one.
The number of nonterminal symbols.
The number of grammar rules,
The number of parser states (*note Parser States::).
-- Directive: %verbose
Write an extra output file containing verbose descriptions of the
parser states and what is done for each type of lookahead token in
that state. *Note Understanding Your Parser: Understanding, for
more information.
-- Directive: %yacc
Pretend the option '--yacc' was given, i.e., imitate Yacc,
including its naming conventions. *Note Bison Options::, for more.
3.7.13 %define Summary
There are many features of Bison's behavior that can be controlled by
assigning the feature a single value. For historical reasons, some such
features are assigned values by dedicated directives, such as '%start',
which assigns the start symbol. However, newer such features are
associated with variables, which are assigned by the '%define'
-- Directive: %define VARIABLE
-- Directive: %define VARIABLE VALUE
-- Directive: %define VARIABLE {VALUE}
-- Directive: %define VARIABLE "VALUE"
Define VARIABLE to VALUE.
The type of the values depend on the syntax. Braces denote value
in the target language (e.g., a namespace, a type, etc.). Keyword
values (no delimiters) denote finite choice (e.g., a variation of a
feature). String values denote remaining cases (e.g., a file
It is an error if a VARIABLE is defined by '%define' multiple
times, but see *note -D NAME[=VALUE]: Bison Options.
The rest of this section summarizes variables and values that
'%define' accepts.
Some VARIABLEs take Boolean values. In this case, Bison will
complain if the variable definition does not meet one of the following
four conditions:
1. 'VALUE' is 'true'
2. 'VALUE' is omitted (or '""' is specified). This is equivalent to
3. 'VALUE' is 'false'.
4. VARIABLE is never defined. In this case, Bison selects a default
What VARIABLEs are accepted, as well as their meanings and default
values, depend on the selected target language and/or the parser
skeleton (*note %language: Decl Summary, *note %skeleton: Decl
Summary.). Unaccepted VARIABLEs produce an error. Some of the accepted
VARIABLEs are described below.
-- Directive: %define api.namespace {NAMESPACE}
* Languages(s): C++
* Purpose: Specify the namespace for the parser class. For
example, if you specify:
%define api.namespace {foo::bar}
Bison uses 'foo::bar' verbatim in references such as:
However, to open a namespace, Bison removes any leading '::'
and then splits on any remaining occurrences:
namespace foo { namespace bar {
class position;
class location;
} }
* Accepted Values: Any absolute or relative C++ namespace
reference without a trailing '"::"'. For example, '"foo"' or
* Default Value: The value specified by '%name-prefix', which
defaults to 'yy'. This usage of '%name-prefix' is for
backward compatibility and can be confusing since
'%name-prefix' also specifies the textual prefix for the
lexical analyzer function. Thus, if you specify
'%name-prefix', it is best to also specify '%define
api.namespace' so that '%name-prefix' _only_ affects the
lexical analyzer function. For example, if you specify:
%define api.namespace {foo}
%name-prefix "bar::"
The parser namespace is 'foo' and 'yylex' is referenced as
-- Directive: %define api.location.type {TYPE}
* Language(s): C++, Java
* Purpose: Define the location type. *Note User Defined
Location Type::.
* Accepted Values: String
* Default Value: none
* History: Introduced in Bison 2.7 for C, C++ and Java.
Introduced under the name 'location_type' for C++ in Bison 2.5
and for Java in Bison 2.4.
-- Directive: %define api.prefix {PREFIX}
* Language(s): All
* Purpose: Rename exported symbols. *Note Multiple Parsers in
the Same Program: Multiple Parsers.
* Accepted Values: String
* Default Value: 'yy'
* History: introduced in Bison 2.6
-- Directive: %define api.pure PURITY
* Language(s): C
* Purpose: Request a pure (reentrant) parser program. *Note A
Pure (Reentrant) Parser: Pure Decl.
* Accepted Values: 'true', 'false', 'full'
The value may be omitted: this is equivalent to specifying
'true', as is the case for Boolean values.
When '%define api.pure full' is used, the parser is made
reentrant. This changes the signature for 'yylex' (*note Pure
Calling::), and also that of 'yyerror' when the tracking of
locations has been activated, as shown below.
The 'true' value is very similar to the 'full' value, the only
difference is in the signature of 'yyerror' on Yacc parsers
without '%parse-param', for historical reasons.
I.e., if '%locations %define api.pure' is passed then the
prototypes for 'yyerror' are:
void yyerror (char const *msg); // Yacc parsers.
void yyerror (YYLTYPE *locp, char const *msg); // GLR parsers.
But if '%locations %define api.pure %parse-param {int
*nastiness}' is used, then both parsers have the same
void yyerror (YYLTYPE *llocp, int *nastiness, char const *msg);
(*note The Error Reporting Function 'yyerror': Error
* Default Value: 'false'
* History: the 'full' value was introduced in Bison 2.7
-- Directive: %define api.push-pull KIND
* Language(s): C (deterministic parsers only)
* Purpose: Request a pull parser, a push parser, or both. *Note
A Push Parser: Push Decl. (The current push parsing interface
is experimental and may evolve. More user feedback will help
to stabilize it.)
* Accepted Values: 'pull', 'push', 'both'
* Default Value: 'pull'
-- Directive: %define api.token.constructor
* Language(s): C++
* Purpose: When variant-based semantic values are enabled (*note
C++ Variants::), request that symbols be handled as a whole
(type, value, and possibly location) in the scanner. *Note
Complete Symbols::, for details.
* Accepted Values: Boolean.
* Default Value: 'false'
* History: introduced in Bison 3.0
-- Directive: %define api.token.prefix {PREFIX}
* Languages(s): all
* Purpose: Add a prefix to the token names when generating their
definition in the target language. For instance
%token FILE for ERROR
%define api.token.prefix {TOK_}
start: FILE for ERROR;
generates the definition of the symbols 'TOK_FILE', 'TOK_for',
and 'TOK_ERROR' in the generated source files. In particular,
the scanner must use these prefixed token names, while the
grammar itself may still use the short names (as in the sample
rule given above). The generated informational files
('*.output', '*.xml', '*.dot') are not modified by this
Bison also prefixes the generated member names of the semantic
value union. *Note Generating the Semantic Value Type: Type
Generation, for more details.
See *note Calc++ Parser:: and *note Calc++ Scanner::, for a
complete example.
* Accepted Values: Any string. Should be a valid identifier
prefix in the target language, in other words, it should
typically be an identifier itself (sequence of letters,
underscores, and --not at the beginning-- digits).
* Default Value: empty
* History: introduced in Bison 3.0
-- Directive: %define api.value.type SUPPORT
-- Directive: %define api.value.type {TYPE}
* Language(s): all
* Purpose: The type for semantic values.
* Accepted Values:
This grammar has no semantic value at all. This is not
properly supported yet.
'union-directive' (C, C++)
The type is defined thanks to the '%union' directive.
You don't have to define 'api.value.type' in that case,
using '%union' suffices. *Note The Union Declaration:
Union Decl. For instance:
%define api.value.type union-directive
int ival;
char *sval;
%token INT "integer"
%token STR "string"
'union' (C, C++)
The symbols are defined with type names, from which Bison
will generate a 'union'. For instance:
%define api.value.type union
%token INT "integer"
%token STR "string"
This feature needs user feedback to stabilize. Note that
most C++ objects cannot be stored in a 'union'.
'variant' (C++)
This is similar to 'union', but special storage
techniques are used to allow any kind of C++ object to be
used. For instance:
%define api.value.type variant
%token INT "integer"
%token STR "string"
This feature needs user feedback to stabilize. *Note C++
Use this TYPE as semantic value.
%code requires
struct my_value
is_int, is_str
} kind;
int ival;
char *sval;
} u;
%define api.value.type {struct my_value}
%token INT "integer"
%token STR "string"
* Default Value:
- '%union' if '%union' is used, otherwise ...
- 'int' if type tags are used (i.e., '%token ...' or
'%token ...' is used), otherwise ...
- '""'
* History: introduced in Bison 3.0. Was introduced for Java
only in 2.3b as 'stype'.
-- Directive: %define location_type
Obsoleted by 'api.location.type' since Bison 2.7.
-- Directive: %define lr.default-reduction WHEN
* Language(s): all
* Purpose: Specify the kind of states that are permitted to
contain default reductions. *Note Default Reductions::. (The
ability to specify where default reductions should be used is
experimental. More user feedback will help to stabilize it.)
* Accepted Values: 'most', 'consistent', 'accepting'
* Default Value:
* 'accepting' if 'lr.type' is 'canonical-lr'.
* 'most' otherwise.
* History: introduced as 'lr.default-reductions' in 2.5, renamed
as 'lr.default-reduction' in 3.0.
-- Directive: %define lr.keep-unreachable-state
* Language(s): all
* Purpose: Request that Bison allow unreachable parser states to
remain in the parser tables. *Note Unreachable States::.
* Accepted Values: Boolean
* Default Value: 'false'
* History: introduced as 'lr.keep_unreachable_states' in 2.3b,
renamed as 'lr.keep-unreachable-states' in 2.5, and as
'lr.keep-unreachable-state' in 3.0.
-- Directive: %define lr.type TYPE
* Language(s): all
* Purpose: Specify the type of parser tables within the LR(1)
family. *Note LR Table Construction::. (This feature is
experimental. More user feedback will help to stabilize it.)
* Accepted Values: 'lalr', 'ielr', 'canonical-lr'
* Default Value: 'lalr'
-- Directive: %define namespace {NAMESPACE}
Obsoleted by 'api.namespace'
-- Directive: %define parse.assert
* Languages(s): C++
* Purpose: Issue runtime assertions to catch invalid uses. In
C++, when variants are used (*note C++ Variants::), symbols
must be constructed and destroyed properly. This option
checks these constraints.
* Accepted Values: Boolean
* Default Value: 'false'
-- Directive: %define parse.error VERBOSITY
* Languages(s): all
* Purpose: Control the kind of error messages passed to the
error reporting function. *Note The Error Reporting Function
'yyerror': Error Reporting.
* Accepted Values:
* 'simple' Error messages passed to 'yyerror' are simply
'"syntax error"'.
* 'verbose' Error messages report the unexpected token, and
possibly the expected ones. However, this report can
often be incorrect when LAC is not enabled (*note LAC::).
* Default Value: 'simple'
-- Directive: %define parse.lac WHEN
* Languages(s): C (deterministic parsers only)
* Purpose: Enable LAC (lookahead correction) to improve syntax
error handling. *Note LAC::.
* Accepted Values: 'none', 'full'
* Default Value: 'none'
-- Directive: %define parse.trace
* Languages(s): C, C++, Java
* Purpose: Require parser instrumentation for tracing. *Note
Tracing Your Parser: Tracing.
In C/C++, define the macro 'YYDEBUG' (or 'PREFIXDEBUG' with
'%define api.prefix {PREFIX}'), see *note Multiple Parsers in
the Same Program: Multiple Parsers.) to 1 in the parser
implementation file if it is not already defined, so that the
debugging facilities are compiled.
* Accepted Values: Boolean
* Default Value: 'false'
3.7.14 %code Summary
The '%code' directive inserts code verbatim into the output parser
source at any of a predefined set of locations. It thus serves as a
flexible and user-friendly alternative to the traditional Yacc prologue,
'%{CODE%}'. This section summarizes the functionality of '%code' for
the various target languages supported by Bison. For a detailed
discussion of how to use '%code' in place of '%{CODE%}' for C/C++ and
why it is advantageous to do so, *note Prologue Alternatives::.
-- Directive: %code {CODE}
This is the unqualified form of the '%code' directive. It inserts
CODE verbatim at a language-dependent default location in the
parser implementation.
For C/C++, the default location is the parser implementation file
after the usual contents of the parser header file. Thus, the
unqualified form replaces '%{CODE%}' for most purposes.
For Java, the default location is inside the parser class.
-- Directive: %code QUALIFIER {CODE}
This is the qualified form of the '%code' directive. QUALIFIER
identifies the purpose of CODE and thus the location(s) where Bison
should insert it. That is, if you need to specify
location-sensitive CODE that does not belong at the default
location selected by the unqualified '%code' form, use this form
For any particular qualifier or for the unqualified form, if there
are multiple occurrences of the '%code' directive, Bison concatenates
the specified code in the order in which it appears in the grammar file.
Not all qualifiers are accepted for all target languages. Unaccepted
qualifiers produce an error. Some of the accepted qualifiers are:
* Language(s): C, C++
* Purpose: This is the best place to write dependency code
required for 'YYSTYPE' and 'YYLTYPE'. In other words, it's
the best place to define types referenced in '%union'
directives. If you use '#define' to override Bison's default
'YYSTYPE' and 'YYLTYPE' definitions, then it is also the best
place. However you should rather '%define' 'api.value.type'
and 'api.location.type'.
* Location(s): The parser header file and the parser
implementation file before the Bison-generated 'YYSTYPE' and
'YYLTYPE' definitions.
* Language(s): C, C++
* Purpose: This is the best place to write additional
definitions and declarations that should be provided to other
* Location(s): The parser header file and the parser
implementation file after the Bison-generated 'YYSTYPE',
'YYLTYPE', and token definitions.
* Language(s): C, C++
* Purpose: The unqualified '%code' or '%code requires' should
usually be more appropriate than '%code top'. However,
occasionally it is necessary to insert code much nearer the
top of the parser implementation file. For example:
%code top {
#define _GNU_SOURCE
* Location(s): Near the top of the parser implementation file.
* Language(s): Java
* Purpose: This is the best place to write Java import
* Location(s): The parser Java file after any Java package
directive and before any class definitions.
Though we say the insertion locations are language-dependent, they
are technically skeleton-dependent. Writers of non-standard skeletons
however should choose their locations consistently with the behavior of
the standard Bison skeletons.
3.8 Multiple Parsers in the Same Program
Most programs that use Bison parse only one language and therefore
contain only one Bison parser. But what if you want to parse more than
one language with the same program? Then you need to avoid name
conflicts between different definitions of functions and variables such
as 'yyparse', 'yylval'. To use different parsers from the same
compilation unit, you also need to avoid conflicts on types and macros
(e.g., 'YYSTYPE') exported in the generated header.
The easy way to do this is to define the '%define' variable
'api.prefix'. With different 'api.prefix's it is guaranteed that
headers do not conflict when included together, and that compiled
objects can be linked together too. Specifying '%define api.prefix
{PREFIX}' (or passing the option '-Dapi.prefix={PREFIX}', see *note
Invoking Bison: Invocation.) renames the interface functions and
variables of the Bison parser to start with PREFIX instead of 'yy', and
all the macros to start by PREFIX (i.e., PREFIX upper-cased) instead of
The renamed symbols include 'yyparse', 'yylex', 'yyerror', 'yynerrs',
'yylval', 'yylloc', 'yychar' and 'yydebug'. If you use a push parser,
'yypush_parse', 'yypull_parse', 'yypstate', 'yypstate_new' and
'yypstate_delete' will also be renamed. The renamed macros include
'YYSTYPE', 'YYLTYPE', and 'YYDEBUG', which is treated specifically --
more about this below.
For example, if you use '%define api.prefix {c}', the names become
'cparse', 'clex', ..., 'CSTYPE', 'CLTYPE', and so on.
The '%define' variable 'api.prefix' works in two different ways. In
the implementation file, it works by adding macro definitions to the
beginning of the parser implementation file, defining 'yyparse' as
'PREFIXparse', and so on:
#define YYSTYPE CTYPE
#define yyparse cparse
#define yylval clval
YYSTYPE yylval;
int yyparse (void);
This effectively substitutes one name for the other in the entire
parser implementation file, thus the "original" names ('yylex',
'YYSTYPE', ...) are also usable in the parser implementation file.
However, in the parser header file, the symbols are defined renamed,
for instance:
extern CSTYPE clval;
int cparse (void);
The macro 'YYDEBUG' is commonly used to enable the tracing support in
parsers. To comply with this tradition, when 'api.prefix' is used,
'YYDEBUG' (not renamed) is used as a default value:
/* Debug traces. */
#ifndef CDEBUG
# if defined YYDEBUG
# if YYDEBUG
# define CDEBUG 1
# else
# define CDEBUG 0
# endif
# else
# define CDEBUG 0
# endif
#if CDEBUG
extern int cdebug;
Prior to Bison 2.6, a feature similar to 'api.prefix' was provided by
the obsolete directive '%name-prefix' (*note Bison Symbols: Table of
Symbols.) and the option '--name-prefix' (*note Bison Options::).
4 Parser C-Language Interface
The Bison parser is actually a C function named 'yyparse'. Here we
describe the interface conventions of 'yyparse' and the other functions
that it needs to use.
Keep in mind that the parser uses many C identifiers starting with
'yy' and 'YY' for internal purposes. If you use such an identifier
(aside from those in this manual) in an action or in epilogue in the
grammar file, you are likely to run into trouble.
4.1 The Parser Function 'yyparse'
You call the function 'yyparse' to cause parsing to occur. This
function reads tokens, executes actions, and ultimately returns when it
encounters end-of-input or an unrecoverable syntax error. You can also
write an action which directs 'yyparse' to return immediately without
reading further.
-- Function: int yyparse (void)
The value returned by 'yyparse' is 0 if parsing was successful
(return is due to end-of-input).
The value is 1 if parsing failed because of invalid input, i.e.,
input that contains a syntax error or that causes 'YYABORT' to be
The value is 2 if parsing failed due to memory exhaustion.
In an action, you can cause immediate return from 'yyparse' by using
these macros:
-- Macro: YYACCEPT
Return immediately with value 0 (to report success).
-- Macro: YYABORT
Return immediately with value 1 (to report failure).
If you use a reentrant parser, you can optionally pass additional
parameter information to it in a reentrant way. To do so, use the
declaration '%parse-param':
-- Directive: %parse-param {ARGUMENT-DECLARATION} ...
Declare that one or more ARGUMENT-DECLARATION are additional
'yyparse' arguments. The ARGUMENT-DECLARATION is used when
declaring functions or prototypes. The last identifier in
ARGUMENT-DECLARATION must be the argument name.
Here's an example. Write this in the parser:
%parse-param {int *nastiness} {int *randomness}
Then call the parser like this:
int nastiness, randomness;
... /* Store proper data in 'nastiness' and 'randomness'. */
value = yyparse (&nastiness, &randomness);
In the grammar actions, use expressions like this to refer to the data:
exp: ... { ...; *randomness += 1; ... }
Using the following:
%parse-param {int *randomness}
Results in these signatures:
void yyerror (int *randomness, const char *msg);
int yyparse (int *randomness);
Or, if both '%define api.pure full' (or just '%define api.pure') and
'%locations' are used:
void yyerror (YYLTYPE *llocp, int *randomness, const char *msg);
int yyparse (int *randomness);
4.2 The Push Parser Function 'yypush_parse'
(The current push parsing interface is experimental and may evolve.
More user feedback will help to stabilize it.)
You call the function 'yypush_parse' to parse a single token. This
function is available if either the '%define api.push-pull push' or
'%define api.push-pull both' declaration is used. *Note A Push Parser:
Push Decl.
-- Function: int yypush_parse (yypstate *YYPS)
The value returned by 'yypush_parse' is the same as for yyparse
with the following exception: it returns 'YYPUSH_MORE' if more
input is required to finish parsing the grammar.
4.3 The Pull Parser Function 'yypull_parse'
(The current push parsing interface is experimental and may evolve.
More user feedback will help to stabilize it.)
You call the function 'yypull_parse' to parse the rest of the input
stream. This function is available if the '%define api.push-pull both'
declaration is used. *Note A Push Parser: Push Decl.
-- Function: int yypull_parse (yypstate *YYPS)
The value returned by 'yypull_parse' is the same as for 'yyparse'.
4.4 The Parser Create Function 'yystate_new'
(The current push parsing interface is experimental and may evolve.
More user feedback will help to stabilize it.)
You call the function 'yypstate_new' to create a new parser instance.
This function is available if either the '%define api.push-pull push' or
'%define api.push-pull both' declaration is used. *Note A Push Parser:
Push Decl.
-- Function: yypstate* yypstate_new (void)
The function will return a valid parser instance if there was
memory available or 0 if no memory was available. In impure mode,
it will also return 0 if a parser instance is currently allocated.
4.5 The Parser Delete Function 'yystate_delete'
(The current push parsing interface is experimental and may evolve.
More user feedback will help to stabilize it.)
You call the function 'yypstate_delete' to delete a parser instance.
function is available if either the '%define api.push-pull push' or
'%define api.push-pull both' declaration is used. *Note A Push Parser:
Push Decl.
-- Function: void yypstate_delete (yypstate *YYPS)
This function will reclaim the memory associated with a parser
instance. After this call, you should no longer attempt to use the
parser instance.
4.6 The Lexical Analyzer Function 'yylex'
The "lexical analyzer" function, 'yylex', recognizes tokens from the
input stream and returns them to the parser. Bison does not create this
function automatically; you must write it so that 'yyparse' can call it.
The function is sometimes referred to as a lexical scanner.
In simple programs, 'yylex' is often defined at the end of the Bison
grammar file. If 'yylex' is defined in a separate source file, you need
to arrange for the token-type macro definitions to be available there.
To do this, use the '-d' option when you run Bison, so that it will
write these macro definitions into the separate parser header file,
'NAME.tab.h', which you can include in the other source files that need
it. *Note Invoking Bison: Invocation.
4.6.1 Calling Convention for 'yylex'
The value that 'yylex' returns must be the positive numeric code for the
type of token it has just found; a zero or negative value signifies
When a token is referred to in the grammar rules by a name, that name
in the parser implementation file becomes a C macro whose definition is
the proper numeric code for that token type. So 'yylex' can use the
name to indicate that type. *Note Symbols::.
When a token is referred to in the grammar rules by a character
literal, the numeric code for that character is also the code for the
token type. So 'yylex' can simply return that character code, possibly
converted to 'unsigned char' to avoid sign-extension. The null
character must not be used this way, because its code is zero and that
signifies end-of-input.
Here is an example showing these things:
yylex (void)
if (c == EOF) /* Detect end-of-input. */
return 0;
if (c == '+' || c == '-')
return c; /* Assume token type for '+' is '+'. */
return INT; /* Return the type of the token. */
This interface has been designed so that the output from the 'lex'
utility can be used without change as the definition of 'yylex'.
If the grammar uses literal string tokens, there are two ways that
'yylex' can determine the token type codes for them:
* If the grammar defines symbolic token names as aliases for the
literal string tokens, 'yylex' can use these symbolic names like
all others. In this case, the use of the literal string tokens in
the grammar file has no effect on 'yylex'.
* 'yylex' can find the multicharacter token in the 'yytname' table.
The index of the token in the table is the token type's code. The
name of a multicharacter token is recorded in 'yytname' with a
double-quote, the token's characters, and another double-quote.
The token's characters are escaped as necessary to be suitable as
input to Bison.
Here's code for looking up a multicharacter token in 'yytname',
assuming that the characters of the token are stored in
'token_buffer', and assuming that the token does not contain any
characters like '"' that require escaping.
for (i = 0; i < YYNTOKENS; i++)
if (yytname[i] != 0
&& yytname[i][0] == '"'
&& ! strncmp (yytname[i] + 1, token_buffer,
strlen (token_buffer))
&& yytname[i][strlen (token_buffer) + 1] == '"'
&& yytname[i][strlen (token_buffer) + 2] == 0)
The 'yytname' table is generated only if you use the '%token-table'
declaration. *Note Decl Summary::.
4.6.2 Semantic Values of Tokens
In an ordinary (nonreentrant) parser, the semantic value of the token
must be stored into the global variable 'yylval'. When you are using
just one data type for semantic values, 'yylval' has that type. Thus,
if the type is 'int' (the default), you might write this in 'yylex':
yylval = value; /* Put value onto Bison stack. */
return INT; /* Return the type of the token. */
When you are using multiple data types, 'yylval''s type is a union
made from the '%union' declaration (*note The Union Declaration: Union
Decl.). So when you store a token's value, you must use the proper
member of the union. If the '%union' declaration looks like this:
%union {
int intval;
double val;
symrec *tptr;
then the code in 'yylex' might look like this:
yylval.intval = value; /* Put value onto Bison stack. */
return INT; /* Return the type of the token. */
4.6.3 Textual Locations of Tokens
If you are using the '@N'-feature (*note Tracking Locations::) in
actions to keep track of the textual locations of tokens and groupings,
then you must provide this information in 'yylex'. The function
'yyparse' expects to find the textual location of a token just parsed in
the global variable 'yylloc'. So 'yylex' must store the proper data in
that variable.
By default, the value of 'yylloc' is a structure and you need only
initialize the members that are going to be used by the actions. The
four members are called 'first_line', 'first_column', 'last_line' and
'last_column'. Note that the use of this feature makes the parser
noticeably slower.
The data type of 'yylloc' has the name 'YYLTYPE'.
4.6.4 Calling Conventions for Pure Parsers
When you use the Bison declaration '%define api.pure full' to request a
pure, reentrant parser, the global communication variables 'yylval' and
'yylloc' cannot be used. (*Note A Pure (Reentrant) Parser: Pure Decl.)
In such parsers the two global variables are replaced by pointers passed
as arguments to 'yylex'. You must declare them as shown here, and pass
the information back by storing it through those pointers.
yylex (YYSTYPE *lvalp, YYLTYPE *llocp)
*lvalp = value; /* Put value onto Bison stack. */
return INT; /* Return the type of the token. */
If the grammar file does not use the '@' constructs to refer to
textual locations, then the type 'YYLTYPE' will not be defined. In this
case, omit the second argument; 'yylex' will be called with only one
If you wish to pass additional arguments to 'yylex', use '%lex-param'
just like '%parse-param' (*note Parser Function::). To pass additional
arguments to both 'yylex' and 'yyparse', use '%param'.
-- Directive: %lex-param {ARGUMENT-DECLARATION} ...
Specify that ARGUMENT-DECLARATION are additional 'yylex' argument
declarations. You may pass one or more such declarations, which is
equivalent to repeating '%lex-param'.
-- Directive: %param {ARGUMENT-DECLARATION} ...
Specify that ARGUMENT-DECLARATION are additional 'yylex'/'yyparse'
argument declaration. This is equivalent to '%lex-param
{ARGUMENT-DECLARATION} ... %parse-param {ARGUMENT-DECLARATION}
...'. You may pass one or more declarations, which is equivalent
to repeating '%param'.
For instance:
%lex-param {scanner_mode *mode}
%parse-param {parser_mode *mode}
%param {environment_type *env}
results in the following signatures:
int yylex (scanner_mode *mode, environment_type *env);
int yyparse (parser_mode *mode, environment_type *env);
If '%define api.pure full' is added:
int yylex (YYSTYPE *lvalp, scanner_mode *mode, environment_type *env);
int yyparse (parser_mode *mode, environment_type *env);
and finally, if both '%define api.pure full' and '%locations' are used:
int yylex (YYSTYPE *lvalp, YYLTYPE *llocp,
scanner_mode *mode, environment_type *env);
int yyparse (parser_mode *mode, environment_type *env);
4.7 The Error Reporting Function 'yyerror'
The Bison parser detects a "syntax error" (or "parse error") whenever it
reads a token which cannot satisfy any syntax rule. An action in the
grammar can also explicitly proclaim an error, using the macro 'YYERROR'
(*note Special Features for Use in Actions: Action Features.).
The Bison parser expects to report the error by calling an error
reporting function named 'yyerror', which you must supply. It is called
by 'yyparse' whenever a syntax error is found, and it receives one
argument. For a syntax error, the string is normally '"syntax error"'.
If you invoke '%define parse.error verbose' in the Bison declarations
section (*note The Bison Declarations Section: Bison Declarations.),
then Bison provides a more verbose and specific error message string
instead of just plain '"syntax error"'. However, that message sometimes
contains incorrect information if LAC is not enabled (*note LAC::).
The parser can detect one other kind of error: memory exhaustion.
This can happen when the input contains constructions that are very
deeply nested. It isn't likely you will encounter this, since the Bison
parser normally extends its stack automatically up to a very large
limit. But if memory is exhausted, 'yyparse' calls 'yyerror' in the
usual fashion, except that the argument string is '"memory exhausted"'.
In some cases diagnostics like '"syntax error"' are translated
automatically from English to some other language before they are passed
to 'yyerror'. *Note Internationalization::.
The following definition suffices in simple programs:
yyerror (char const *s)
fprintf (stderr, "%s\n", s);
After 'yyerror' returns to 'yyparse', the latter will attempt error
recovery if you have written suitable error recovery grammar rules
(*note Error Recovery::). If recovery is impossible, 'yyparse' will
immediately return 1.
Obviously, in location tracking pure parsers, 'yyerror' should have
an access to the current location. With '%define api.pure', this is
indeed the case for the GLR parsers, but not for the Yacc parser, for
historical reasons, and this is the why '%define api.pure full' should
be prefered over '%define api.pure'.
When '%locations %define api.pure full' is used, 'yyerror' has the
following signature:
void yyerror (YYLTYPE *locp, char const *msg);
The prototypes are only indications of how the code produced by Bison
uses 'yyerror'. Bison-generated code always ignores the returned value,
so 'yyerror' can return any type, including 'void'. Also, 'yyerror' can
be a variadic function; that is why the message is always passed last.
Traditionally 'yyerror' returns an 'int' that is always ignored, but
this is purely for historical reasons, and 'void' is preferable since it
more accurately describes the return type for 'yyerror'.
The variable 'yynerrs' contains the number of syntax errors reported
so far. Normally this variable is global; but if you request a pure
parser (*note A Pure (Reentrant) Parser: Pure Decl.) then it is a local
variable which only the actions can access.
4.8 Special Features for Use in Actions
Here is a table of Bison constructs, variables and macros that are
useful in actions.
-- Variable: $$
Acts like a variable that contains the semantic value for the
grouping made by the current rule. *Note Actions::.
-- Variable: $ N
Acts like a variable that contains the semantic value for the Nth
component of the current rule. *Note Actions::.
-- Variable: $< TYPEALT>$
Like '$$' but specifies alternative TYPEALT in the union specified
by the '%union' declaration. *Note Data Types of Values in
Actions: Action Types.
-- Variable: $< TYPEALT>N
Like '$N' but specifies alternative TYPEALT in the union specified
by the '%union' declaration. *Note Data Types of Values in
Actions: Action Types.
-- Macro: YYABORT ';'
Return immediately from 'yyparse', indicating failure. *Note The
Parser Function 'yyparse': Parser Function.
-- Macro: YYACCEPT ';'
Return immediately from 'yyparse', indicating success. *Note The
Parser Function 'yyparse': Parser Function.
-- Macro: YYBACKUP (TOKEN, VALUE)';'
Unshift a token. This macro is allowed only for rules that reduce
a single value, and only when there is no lookahead token. It is
also disallowed in GLR parsers. It installs a lookahead token with
token type TOKEN and semantic value VALUE; then it discards the
value that was going to be reduced by this rule.
If the macro is used when it is not valid, such as when there is a
lookahead token already, then it reports a syntax error with a
message 'cannot back up' and performs ordinary error recovery.
In either case, the rest of the action is not executed.
-- Macro: YYEMPTY
Value stored in 'yychar' when there is no lookahead token.
-- Macro: YYEOF
Value stored in 'yychar' when the lookahead is the end of the input
-- Macro: YYERROR ';'
Cause an immediate syntax error. This statement initiates error
recovery just as if the parser itself had detected an error;
however, it does not call 'yyerror', and does not print any
message. If you want to print an error message, call 'yyerror'
explicitly before the 'YYERROR;' statement. *Note Error
-- Macro: YYRECOVERING
The expression 'YYRECOVERING ()' yields 1 when the parser is
recovering from a syntax error, and 0 otherwise. *Note Error
-- Variable: yychar
Variable containing either the lookahead token, or 'YYEOF' when the
lookahead is the end of the input stream, or 'YYEMPTY' when no
lookahead has been performed so the next token is not yet known.
Do not modify 'yychar' in a deferred semantic action (*note GLR
Semantic Actions::). *Note Lookahead Tokens: Lookahead.
-- Macro: yyclearin ';'
Discard the current lookahead token. This is useful primarily in
error rules. Do not invoke 'yyclearin' in a deferred semantic
action (*note GLR Semantic Actions::). *Note Error Recovery::.
-- Macro: yyerrok ';'
Resume generating error messages immediately for subsequent syntax
errors. This is useful primarily in error rules. *Note Error
-- Variable: yylloc
Variable containing the lookahead token location when 'yychar' is
not set to 'YYEMPTY' or 'YYEOF'. Do not modify 'yylloc' in a
deferred semantic action (*note GLR Semantic Actions::). *Note
Actions and Locations: Actions and Locations.
-- Variable: yylval
Variable containing the lookahead token semantic value when
'yychar' is not set to 'YYEMPTY' or 'YYEOF'. Do not modify
'yylval' in a deferred semantic action (*note GLR Semantic
Actions::). *Note Actions: Actions.
-- Value: @ $
Acts like a structure variable containing information on the
textual location of the grouping made by the current rule. *Note
Tracking Locations::.
-- Value: @ N
Acts like a structure variable containing information on the
textual location of the Nth component of the current rule. *Note
Tracking Locations::.
4.9 Parser Internationalization
A Bison-generated parser can print diagnostics, including error and
tracing messages. By default, they appear in English. However, Bison
also supports outputting diagnostics in the user's native language. To
make this work, the user should set the usual environment variables.
*Note The User's View: (gettext)Users. For example, the shell command
'export LC_ALL=fr_CA.UTF-8' might set the user's locale to French
Canadian using the UTF-8 encoding. The exact set of available locales
depends on the user's installation.
The maintainer of a package that uses a Bison-generated parser
enables the internationalization of the parser's output through the
following steps. Here we assume a package that uses GNU Autoconf and
GNU Automake.
1. Into the directory containing the GNU Autoconf macros used by the
package --often called 'm4'-- copy the 'bison-i18n.m4' file
installed by Bison under 'share/aclocal/bison-i18n.m4' in Bison's
installation directory. For example:
cp /usr/local/share/aclocal/bison-i18n.m4 m4/bison-i18n.m4
2. In the top-level 'configure.ac', after the 'AM_GNU_GETTEXT'
invocation, add an invocation of 'BISON_I18N'. This macro is
defined in the file 'bison-i18n.m4' that you copied earlier. It
causes 'configure' to find the value of the 'BISON_LOCALEDIR'
variable, and it defines the source-language symbol 'YYENABLE_NLS'
to enable translations in the Bison-generated parser.
3. In the 'main' function of your program, designate the directory
containing Bison's runtime message catalog, through a call to
'bindtextdomain' with domain name 'bison-runtime'. For example:
bindtextdomain ("bison-runtime", BISON_LOCALEDIR);
Typically this appears after any other call 'bindtextdomain
(PACKAGE, LOCALEDIR)' that your package already has. Here we rely
on 'BISON_LOCALEDIR' to be defined as a string through the
4. In the 'Makefile.am' that controls the compilation of the 'main'
function, make 'BISON_LOCALEDIR' available as a C preprocessor
macro, either in 'DEFS' or in 'AM_CPPFLAGS'. For example:
5. Finally, invoke the command 'autoreconf' to generate the build
5 The Bison Parser Algorithm
As Bison reads tokens, it pushes them onto a stack along with their
semantic values. The stack is called the "parser stack". Pushing a
token is traditionally called "shifting".
For example, suppose the infix calculator has read '1 + 5 *', with a
'3' to come. The stack will have four elements, one for each token that
was shifted.
But the stack does not always have an element for each token read.
When the last N tokens and groupings shifted match the components of a
grammar rule, they can be combined according to that rule. This is
called "reduction". Those tokens and groupings are replaced on the
stack by a single grouping whose symbol is the result (left hand side)
of that rule. Running the rule's action is part of the process of
reduction, because this is what computes the semantic value of the
resulting grouping.
For example, if the infix calculator's parser stack contains this:
1 + 5 * 3
and the next input token is a newline character, then the last three
elements can be reduced to 15 via the rule:
expr: expr '*' expr;
Then the stack contains just these three elements:
1 + 15
At this point, another reduction can be made, resulting in the single
value 16. Then the newline token can be shifted.
The parser tries, by shifts and reductions, to reduce the entire
input down to a single grouping whose symbol is the grammar's
start-symbol (*note Languages and Context-Free Grammars: Language and
This kind of parser is known in the literature as a bottom-up parser.
5.1 Lookahead Tokens
The Bison parser does _not_ always reduce immediately as soon as the
last N tokens and groupings match a rule. This is because such a simple
strategy is inadequate to handle most languages. Instead, when a
reduction is possible, the parser sometimes "looks ahead" at the next
token in order to decide what to do.
When a token is read, it is not immediately shifted; first it becomes
the "lookahead token", which is not on the stack. Now the parser can
perform one or more reductions of tokens and groupings on the stack,
while the lookahead token remains off to the side. When no more
reductions should take place, the lookahead token is shifted onto the
stack. This does not mean that all possible reductions have been done;
depending on the token type of the lookahead token, some rules may
choose to delay their application.
Here is a simple case where lookahead is needed. These three rules
define expressions which contain binary addition operators and postfix
unary factorial operators ('!'), and allow parentheses for grouping.
term '+' expr
| term
'(' expr ')'
| term '!'
| "number"
Suppose that the tokens '1 + 2' have been read and shifted; what
should be done? If the following token is ')', then the first three
tokens must be reduced to form an 'expr'. This is the only valid
course, because shifting the ')' would produce a sequence of symbols
'term ')'', and no rule allows this.
If the following token is '!', then it must be shifted immediately so
that '2 !' can be reduced to make a 'term'. If instead the parser were
to reduce before shifting, '1 + 2' would become an 'expr'. It would
then be impossible to shift the '!' because doing so would produce on
the stack the sequence of symbols 'expr '!''. No rule allows that
The lookahead token is stored in the variable 'yychar'. Its semantic
value and location, if any, are stored in the variables 'yylval' and
'yylloc'. *Note Special Features for Use in Actions: Action Features.
5.2 Shift/Reduce Conflicts
Suppose we are parsing a language which has if-then and if-then-else
statements, with a pair of rules like this:
"if" expr "then" stmt
| "if" expr "then" stmt "else" stmt
Here '"if"', '"then"' and '"else"' are terminal symbols for specific
keyword tokens.
When the '"else"' token is read and becomes the lookahead token, the
contents of the stack (assuming the input is valid) are just right for
reduction by the first rule. But it is also legitimate to shift the
'"else"', because that would lead to eventual reduction by the second
This situation, where either a shift or a reduction would be valid,
is called a "shift/reduce conflict". Bison is designed to resolve these
conflicts by choosing to shift, unless otherwise directed by operator
precedence declarations. To see the reason for this, let's contrast it
with the other alternative.
Since the parser prefers to shift the '"else"', the result is to
attach the else-clause to the innermost if-statement, making these two
inputs equivalent:
if x then if y then win; else lose;
if x then do; if y then win; else lose; end;
But if the parser chose to reduce when possible rather than shift,
the result would be to attach the else-clause to the outermost
if-statement, making these two inputs equivalent:
if x then if y then win; else lose;
if x then do; if y then win; end; else lose;
The conflict exists because the grammar as written is ambiguous:
either parsing of the simple nested if-statement is legitimate. The
established convention is that these ambiguities are resolved by
attaching the else-clause to the innermost if-statement; this is what
Bison accomplishes by choosing to shift rather than reduce. (It would
ideally be cleaner to write an unambiguous grammar, but that is very
hard to do in this case.) This particular ambiguity was first
encountered in the specifications of Algol 60 and is called the
"dangling 'else'" ambiguity.
To avoid warnings from Bison about predictable, legitimate
shift/reduce conflicts, you can use the '%expect N' declaration. There
will be no warning as long as the number of shift/reduce conflicts is
exactly N, and Bison will report an error if there is a different
number. *Note Suppressing Conflict Warnings: Expect Decl. However, we
don't recommend the use of '%expect' (except '%expect 0'!), as an equal
number of conflicts does not mean that they are the _same_. When
possible, you should rather use precedence directives to _fix_ the
conflicts explicitly (*note Using Precedence For Non Operators: Non
The definition of 'if_stmt' above is solely to blame for the
conflict, but the conflict does not actually appear without additional
rules. Here is a complete Bison grammar file that actually manifests
the conflict:
| if_stmt
"if" expr "then" stmt
| "if" expr "then" stmt "else" stmt
5.3 Operator Precedence
Another situation where shift/reduce conflicts appear is in arithmetic
expressions. Here shifting is not always the preferred resolution; the
Bison declarations for operator precedence allow you to specify when to
shift and when to reduce.
5.3.1 When Precedence is Needed
Consider the following ambiguous grammar fragment (ambiguous because the
input '1 - 2 * 3' can be parsed in two different ways):
expr '-' expr
| expr '*' expr
| expr '<' expr
| '(' expr ')'
Suppose the parser has seen the tokens '1', '-' and '2'; should it
reduce them via the rule for the subtraction operator? It depends on
the next token. Of course, if the next token is ')', we must reduce;
shifting is invalid because no single rule can reduce the token sequence
'- 2 )' or anything starting with that. But if the next token is '*' or
'<', we have a choice: either shifting or reduction would allow the
parse to complete, but with different results.
To decide which one Bison should do, we must consider the results.
If the next operator token OP is shifted, then it must be reduced first
in order to permit another opportunity to reduce the difference. The
result is (in effect) '1 - (2 OP 3)'. On the other hand, if the
subtraction is reduced before shifting OP, the result is '(1 - 2) OP 3'.
Clearly, then, the choice of shift or reduce should depend on the
relative precedence of the operators '-' and OP: '*' should be shifted
first, but not '<'.
What about input such as '1 - 2 - 5'; should this be '(1 - 2) - 5' or
should it be '1 - (2 - 5)'? For most operators we prefer the former,
which is called "left association". The latter alternative, "right
association", is desirable for assignment operators. The choice of left
or right association is a matter of whether the parser chooses to shift
or reduce when the stack contains '1 - 2' and the lookahead token is
'-': shifting makes right-associativity.
5.3.2 Specifying Operator Precedence
Bison allows you to specify these choices with the operator precedence
declarations '%left' and '%right'. Each such declaration contains a
list of tokens, which are operators whose precedence and associativity
is being declared. The '%left' declaration makes all those operators
left-associative and the '%right' declaration makes them
right-associative. A third alternative is '%nonassoc', which declares
that it is a syntax error to find the same operator twice "in a row".
The last alternative, '%precedence', allows to define only precedence
and no associativity at all. As a result, any associativity-related
conflict that remains will be reported as an compile-time error. The
directive '%nonassoc' creates run-time error: using the operator in a
associative way is a syntax error. The directive '%precedence' creates
compile-time errors: an operator _can_ be involved in an
associativity-related conflict, contrary to what expected the grammar
The relative precedence of different operators is controlled by the
order in which they are declared. The first precedence/associativity
declaration in the file declares the operators whose precedence is
lowest, the next such declaration declares the operators whose
precedence is a little higher, and so on.
5.3.3 Specifying Precedence Only
Since POSIX Yacc defines only '%left', '%right', and '%nonassoc', which
all defines precedence and associativity, little attention is paid to
the fact that precedence cannot be defined without defining
associativity. Yet, sometimes, when trying to solve a conflict,
precedence suffices. In such a case, using '%left', '%right', or
'%nonassoc' might hide future (associativity related) conflicts that
would remain hidden.
The dangling 'else' ambiguity (*note Shift/Reduce Conflicts:
Shift/Reduce.) can be solved explicitly. This shift/reduce conflicts
occurs in the following situation, where the period denotes the current
parsing state:
if E1 then if E2 then S1 . else S2
The conflict involves the reduction of the rule 'IF expr THEN stmt',
which precedence is by default that of its last token ('THEN'), and the
shifting of the token 'ELSE'. The usual disambiguation (attach the
'else' to the closest 'if'), shifting must be preferred, i.e., the
precedence of 'ELSE' must be higher than that of 'THEN'. But neither is
expected to be involved in an associativity related conflict, which can
be specified as follows.
%precedence THEN
%precedence ELSE
The unary-minus is another typical example where associativity is
usually over-specified, see *note Infix Notation Calculator: 'calc':
Infix Calc. The '%left' directive is traditionally used to declare the
precedence of 'NEG', which is more than needed since it also defines its
associativity. While this is harmless in the traditional example, who
knows how 'NEG' might be used in future evolutions of the grammar...
5.3.4 Precedence Examples
In our example, we would want the following declarations:
%left '<'
%left '-'
%left '*'
In a more complete example, which supports other operators as well,
we would declare them in groups of equal precedence. For example, ''+''
is declared with ''-'':
%left '<' '>' '=' "!=" "<=" ">="
%left '+' '-'
%left '*' '/'
5.3.5 How Precedence Works
The first effect of the precedence declarations is to assign precedence
levels to the terminal symbols declared. The second effect is to assign
precedence levels to certain rules: each rule gets its precedence from
the last terminal symbol mentioned in the components. (You can also
specify explicitly the precedence of a rule. *Note Context-Dependent
Precedence: Contextual Precedence.)
Finally, the resolution of conflicts works by comparing the
precedence of the rule being considered with that of the lookahead
token. If the token's precedence is higher, the choice is to shift. If
the rule's precedence is higher, the choice is to reduce. If they have
equal precedence, the choice is made based on the associativity of that
precedence level. The verbose output file made by '-v' (*note Invoking
Bison: Invocation.) says how each conflict was resolved.
Not all rules and not all tokens have precedence. If either the rule
or the lookahead token has no precedence, then the default is to shift.
5.3.6 Using Precedence For Non Operators
Using properly precedence and associativity directives can help fixing
shift/reduce conflicts that do not involve arithmetics-like operators.
For instance, the "dangling 'else'" problem (*note Shift/Reduce
Conflicts: Shift/Reduce.) can be solved elegantly in two different ways.
In the present case, the conflict is between the token '"else"'
willing to be shifted, and the rule 'if_stmt: "if" expr "then" stmt',
asking for reduction. By default, the precedence of a rule is that of
its last token, here '"then"', so the conflict will be solved
appropriately by giving '"else"' a precedence higher than that of
'"then"', for instance as follows:
%precedence "then"
%precedence "else"
Alternatively, you may give both tokens the same precedence, in which
case associativity is used to solve the conflict. To preserve the shift
action, use right associativity:
%right "then" "else"
Neither solution is perfect however. Since Bison does not provide,
so far, "scoped" precedence, both force you to declare the precedence of
these keywords with respect to the other operators your grammar.
Therefore, instead of being warned about new conflicts you would be
unaware of (e.g., a shift/reduce conflict due to 'if test then 1 else 2
+ 3' being ambiguous: 'if test then 1 else (2 + 3)' or '(if test then 1
else 2) + 3'?), the conflict will be already "fixed".
5.4 Context-Dependent Precedence
Often the precedence of an operator depends on the context. This sounds
outlandish at first, but it is really very common. For example, a minus
sign typically has a very high precedence as a unary operator, and a
somewhat lower precedence (lower than multiplication) as a binary
The Bison precedence declarations can only be used once for a given
token; so a token has only one precedence declared in this way. For
context-dependent precedence, you need to use an additional mechanism:
the '%prec' modifier for rules.
The '%prec' modifier declares the precedence of a particular rule by
specifying a terminal symbol whose precedence should be used for that
rule. It's not necessary for that symbol to appear otherwise in the
rule. The modifier's syntax is:
%prec TERMINAL-SYMBOL
and it is written after the components of the rule. Its effect is to
assign the rule the precedence of TERMINAL-SYMBOL, overriding the
precedence that would be deduced for it in the ordinary way. The
altered rule precedence then affects how conflicts involving that rule
are resolved (*note Operator Precedence: Precedence.).
Here is how '%prec' solves the problem of unary minus. First,
declare a precedence for a fictitious terminal symbol named 'UMINUS'.
There are no tokens of this type, but the symbol serves to stand for its
%left '+' '-'
%left '*'
%left UMINUS
Now the precedence of 'UMINUS' can be used in specific rules:
| exp '-' exp
| '-' exp %prec UMINUS
5.5 Parser States
The function 'yyparse' is implemented using a finite-state machine. The
values pushed on the parser stack are not simply token type codes; they
represent the entire sequence of terminal and nonterminal symbols at or
near the top of the stack. The current state collects all the
information about previous input which is relevant to deciding what to
do next.
Each time a lookahead token is read, the current parser state
together with the type of lookahead token are looked up in a table.
This table entry can say, "Shift the lookahead token." In this case, it
also specifies the new parser state, which is pushed onto the top of the
parser stack. Or it can say, "Reduce using rule number N." This means
that a certain number of tokens or groupings are taken off the top of
the stack, and replaced by one grouping. In other words, that number of
states are popped from the stack, and one new state is pushed.
There is one other alternative: the table can say that the lookahead
token is erroneous in the current state. This causes error processing
to begin (*note Error Recovery::).
5.6 Reduce/Reduce Conflicts
A reduce/reduce conflict occurs if there are two or more rules that
apply to the same sequence of input. This usually indicates a serious
error in the grammar.
For example, here is an erroneous attempt to define a sequence of
zero or more 'word' groupings.
%empty { printf ("empty sequence\n"); }
| maybeword
| sequence word { printf ("added word %s\n", $2); }
%empty { printf ("empty maybeword\n"); }
| word { printf ("single word %s\n", $1); }
The error is an ambiguity: there is more than one way to parse a single
'word' into a 'sequence'. It could be reduced to a 'maybeword' and then
into a 'sequence' via the second rule. Alternatively, nothing-at-all
could be reduced into a 'sequence' via the first rule, and this could be
combined with the 'word' using the third rule for 'sequence'.
There is also more than one way to reduce nothing-at-all into a
'sequence'. This can be done directly via the first rule, or indirectly
via 'maybeword' and then the second rule.
You might think that this is a distinction without a difference,
because it does not change whether any particular input is valid or not.
But it does affect which actions are run. One parsing order runs the
second rule's action; the other runs the first rule's action and the
third rule's action. In this example, the output of the program
Bison resolves a reduce/reduce conflict by choosing to use the rule
that appears first in the grammar, but it is very risky to rely on this.
Every reduce/reduce conflict must be studied and usually eliminated.
Here is the proper way to define 'sequence':
%empty { printf ("empty sequence\n"); }
| sequence word { printf ("added word %s\n", $2); }
Here is another common error that yields a reduce/reduce conflict:
| sequence words
| sequence redirects
| words word
| redirects redirect
The intention here is to define a sequence which can contain either
'word' or 'redirect' groupings. The individual definitions of
'sequence', 'words' and 'redirects' are error-free, but the three
together make a subtle ambiguity: even an empty input can be parsed in
infinitely many ways!
Consider: nothing-at-all could be a 'words'. Or it could be two
'words' in a row, or three, or any number. It could equally well be a
'redirects', or two, or any number. Or it could be a 'words' followed
by three 'redirects' and another 'words'. And so on.
Here are two ways to correct these rules. First, to make it a single
level of sequence:
| sequence word
| sequence redirect
Second, to prevent either a 'words' or a 'redirects' from being
| sequence words
| sequence redirects
| words word
| redirects redirect
Yet this proposal introduces another kind of ambiguity! The input
'word word' can be parsed as a single 'words' composed of two 'word's,
or as two one-'word' 'words' (and likewise for 'redirect'/'redirects').
However this ambiguity is now a shift/reduce conflict, and therefore it
can now be addressed with precedence directives.
To simplify the matter, we will proceed with 'word' and 'redirect'
being tokens: '"word"' and '"redirect"'.
To prefer the longest 'words', the conflict between the token
'"word"' and the rule 'sequence: sequence words' must be resolved as a
shift. To this end, we use the same techniques as exposed above, see
*note Using Precedence For Non Operators: Non Operators. One solution
relies on precedences: use '%prec' to give a lower precedence to the
%precedence "word"
%precedence "sequence"
| sequence word %prec "sequence"
| sequence redirect %prec "sequence"
| words "word"
Another solution relies on associativity: provide both the token and
the rule with the same precedence, but make them right-associative:
%right "word" "redirect"
| sequence word %prec "word"
| sequence redirect %prec "redirect"
5.7 Mysterious Conflicts
Sometimes reduce/reduce conflicts can occur that don't look warranted.
Here is an example:
def: param_spec return_spec ',';
| name_list ':' type
| name ':' type
type: "id";
name: "id";
| name ',' name_list
It would seem that this grammar can be parsed with only a single
token of lookahead: when a 'param_spec' is being read, an '"id"' is a
'name' if a comma or colon follows, or a 'type' if another '"id"'
follows. In other words, this grammar is LR(1).
However, for historical reasons, Bison cannot by default handle all
LR(1) grammars. In this grammar, two contexts, that after an '"id"' at
the beginning of a 'param_spec' and likewise at the beginning of a
'return_spec', are similar enough that Bison assumes they are the same.
They appear similar because the same set of rules would be active--the
rule for reducing to a 'name' and that for reducing to a 'type'. Bison
is unable to determine at that stage of processing that the rules would
require different lookahead tokens in the two contexts, so it makes a
single parser state for them both. Combining the two contexts causes a
conflict later. In parser terminology, this occurrence means that the
grammar is not LALR(1).
For many practical grammars (specifically those that fall into the
non-LR(1) class), the limitations of LALR(1) result in difficulties
beyond just mysterious reduce/reduce conflicts. The best way to fix all
these problems is to select a different parser table construction
algorithm. Either IELR(1) or canonical LR(1) would suffice, but the
former is more efficient and easier to debug during development. *Note
LR Table Construction::, for details. (Bison's IELR(1) and canonical
LR(1) implementations are experimental. More user feedback will help to
stabilize them.)
If you instead wish to work around LALR(1)'s limitations, you can
often fix a mysterious conflict by identifying the two parser states
that are being confused, and adding something to make them look
distinct. In the above example, adding one rule to 'return_spec' as
follows makes the problem go away:
| name ':' type
| "id" "bogus" /* This rule is never used. */
This corrects the problem because it introduces the possibility of an
additional active rule in the context after the '"id"' at the beginning
of 'return_spec'. This rule is not active in the corresponding context
in a 'param_spec', so the two contexts receive distinct parser states.
As long as the token '"bogus"' is never generated by 'yylex', the added
rule cannot alter the way actual input is parsed.
In this particular example, there is another way to solve the
problem: rewrite the rule for 'return_spec' to use '"id"' directly
instead of via 'name'. This also causes the two confusing contexts to
have different sets of active rules, because the one for 'return_spec'
activates the altered rule for 'return_spec' rather than the one for
| name_list ':' type
| "id" ':' type
For a more detailed exposition of LALR(1) parsers and parser
generators, *note DeRemer 1982: Bibliography.
5.8 Tuning LR
The default behavior of Bison's LR-based parsers is chosen mostly for
historical reasons, but that behavior is often not robust. For example,
in the previous section, we discussed the mysterious conflicts that can
be produced by LALR(1), Bison's default parser table construction
algorithm. Another example is Bison's '%define parse.error verbose'
directive, which instructs the generated parser to produce verbose
syntax error messages, which can sometimes contain incorrect
In this section, we explore several modern features of Bison that
allow you to tune fundamental aspects of the generated LR-based parsers.
Some of these features easily eliminate shortcomings like those
mentioned above. Others can be helpful purely for understanding your
Most of the features discussed in this section are still
experimental. More user feedback will help to stabilize them.
5.8.1 LR Table Construction
For historical reasons, Bison constructs LALR(1) parser tables by
default. However, LALR does not possess the full language-recognition
power of LR. As a result, the behavior of parsers employing LALR parser
tables is often mysterious. We presented a simple example of this
effect in *note Mysterious Conflicts::.
As we also demonstrated in that example, the traditional approach to
eliminating such mysterious behavior is to restructure the grammar.
Unfortunately, doing so correctly is often difficult. Moreover, merely
discovering that LALR causes mysterious behavior in your parser can be
difficult as well.
Fortunately, Bison provides an easy way to eliminate the possibility
of such mysterious behavior altogether. You simply need to activate a
more powerful parser table construction algorithm by using the '%define
lr.type' directive.
-- Directive: %define lr.type TYPE
Specify the type of parser tables within the LR(1) family. The
accepted values for TYPE are:
* 'lalr' (default)
* 'ielr'
* 'canonical-lr'
(This feature is experimental. More user feedback will help to
stabilize it.)
For example, to activate IELR, you might add the following directive
to you grammar file:
%define lr.type ielr
For the example in *note Mysterious Conflicts::, the mysterious conflict
is then eliminated, so there is no need to invest time in comprehending
the conflict or restructuring the grammar to fix it. If, during future
development, the grammar evolves such that all mysterious behavior would
have disappeared using just LALR, you need not fear that continuing to
use IELR will result in unnecessarily large parser tables. That is,
IELR generates LALR tables when LALR (using a deterministic parsing
algorithm) is sufficient to support the full language-recognition power
of LR. Thus, by enabling IELR at the start of grammar development, you
can safely and completely eliminate the need to consider LALR's
While IELR is almost always preferable, there are circumstances where
LALR or the canonical LR parser tables described by Knuth (*note Knuth
1965: Bibliography.) can be useful. Here we summarize the relative
advantages of each parser table construction algorithm within Bison:
There are at least two scenarios where LALR can be worthwhile:
* GLR without static conflict resolution.
When employing GLR parsers (*note GLR Parsers::), if you do
not resolve any conflicts statically (for example, with
'%left' or '%precedence'), then the parser explores all
potential parses of any given input. In this case, the choice
of parser table construction algorithm is guaranteed not to
alter the language accepted by the parser. LALR parser tables
are the smallest parser tables Bison can currently construct,
so they may then be preferable. Nevertheless, once you begin
to resolve conflicts statically, GLR behaves more like a
deterministic parser in the syntactic contexts where those
conflicts appear, and so either IELR or canonical LR can then
be helpful to avoid LALR's mysterious behavior.
* Malformed grammars.
Occasionally during development, an especially malformed
grammar with a major recurring flaw may severely impede the
IELR or canonical LR parser table construction algorithm.
LALR can be a quick way to construct parser tables in order to
investigate such problems while ignoring the more subtle
differences from IELR and canonical LR.
IELR (Inadequacy Elimination LR) is a minimal LR algorithm. That
is, given any grammar (LR or non-LR), parsers using IELR or
canonical LR parser tables always accept exactly the same set of
sentences. However, like LALR, IELR merges parser states during
parser table construction so that the number of parser states is
often an order of magnitude less than for canonical LR. More
importantly, because canonical LR's extra parser states may contain
duplicate conflicts in the case of non-LR grammars, the number of
conflicts for IELR is often an order of magnitude less as well.
This effect can significantly reduce the complexity of developing a
* Canonical LR
While inefficient, canonical LR parser tables can be an interesting
means to explore a grammar because they possess a property that
IELR and LALR tables do not. That is, if '%nonassoc' is not used
and default reductions are left disabled (*note Default
Reductions::), then, for every left context of every canonical LR
state, the set of tokens accepted by that state is guaranteed to be
the exact set of tokens that is syntactically acceptable in that
left context. It might then seem that an advantage of canonical LR
parsers in production is that, under the above constraints, they
are guaranteed to detect a syntax error as soon as possible without
performing any unnecessary reductions. However, IELR parsers that
use LAC are also able to achieve this behavior without sacrificing
'%nonassoc' or default reductions. For details and a few caveats
of LAC, *note LAC::.
For a more detailed exposition of the mysterious behavior in LALR
parsers and the benefits of IELR, *note Denny 2008 March: Bibliography,
and *note Denny 2010 November: Bibliography.
5.8.2 Default Reductions
After parser table construction, Bison identifies the reduction with the
largest lookahead set in each parser state. To reduce the size of the
parser state, traditional Bison behavior is to remove that lookahead set
and to assign that reduction to be the default parser action. Such a
reduction is known as a "default reduction".
Default reductions affect more than the size of the parser tables.
They also affect the behavior of the parser:
* Delayed 'yylex' invocations.
A "consistent state" is a state that has only one possible parser
action. If that action is a reduction and is encoded as a default
reduction, then that consistent state is called a "defaulted
state". Upon reaching a defaulted state, a Bison-generated parser
does not bother to invoke 'yylex' to fetch the next token before
performing the reduction. In other words, whether default
reductions are enabled in consistent states determines how soon a
Bison-generated parser invokes 'yylex' for a token: immediately
when it _reaches_ that token in the input or when it eventually
_needs_ that token as a lookahead to determine the next parser
action. Traditionally, default reductions are enabled, and so the
parser exhibits the latter behavior.
The presence of defaulted states is an important consideration when
designing 'yylex' and the grammar file. That is, if the behavior
of 'yylex' can influence or be influenced by the semantic actions
associated with the reductions in defaulted states, then the delay
of the next 'yylex' invocation until after those reductions is
significant. For example, the semantic actions might pop a scope
stack that 'yylex' uses to determine what token to return. Thus,
the delay might be necessary to ensure that 'yylex' does not look
up the next token in a scope that should already be considered
* Delayed syntax error detection.
When the parser fetches a new token by invoking 'yylex', it checks
whether there is an action for that token in the current parser
state. The parser detects a syntax error if and only if either (1)
there is no action for that token or (2) the action for that token
is the error action (due to the use of '%nonassoc'). However, if
there is a default reduction in that state (which might or might
not be a defaulted state), then it is impossible for condition 1 to
exist. That is, all tokens have an action. Thus, the parser
sometimes fails to detect the syntax error until it reaches a later
While default reductions never cause the parser to accept
syntactically incorrect sentences, the delay of syntax error
detection can have unexpected effects on the behavior of the
parser. However, the delay can be caused anyway by parser state
merging and the use of '%nonassoc', and it can be fixed by another
Bison feature, LAC. We discuss the effects of delayed syntax error
detection and LAC more in the next section (*note LAC::).
For canonical LR, the only default reduction that Bison enables by
default is the accept action, which appears only in the accepting state,
which has no other action and is thus a defaulted state. However, the
default accept action does not delay any 'yylex' invocation or syntax
error detection because the accept action ends the parse.
For LALR and IELR, Bison enables default reductions in nearly all
states by default. There are only two exceptions. First, states that
have a shift action on the 'error' token do not have default reductions
because delayed syntax error detection could then prevent the 'error'
token from ever being shifted in that state. However, parser state
merging can cause the same effect anyway, and LAC fixes it in both
cases, so future versions of Bison might drop this exception when LAC is
activated. Second, GLR parsers do not record the default reduction as
the action on a lookahead token for which there is a conflict. The
correct action in this case is to split the parse instead.
To adjust which states have default reductions enabled, use the
'%define lr.default-reduction' directive.
-- Directive: %define lr.default-reduction WHERE
Specify the kind of states that are permitted to contain default
reductions. The accepted values of WHERE are:
* 'most' (default for LALR and IELR)
* 'consistent'
* 'accepting' (default for canonical LR)
(The ability to specify where default reductions are permitted is
experimental. More user feedback will help to stabilize it.)
Canonical LR, IELR, and LALR can suffer from a couple of problems upon
encountering a syntax error. First, the parser might perform additional
parser stack reductions before discovering the syntax error. Such
reductions can perform user semantic actions that are unexpected because
they are based on an invalid token, and they cause error recovery to
begin in a different syntactic context than the one in which the invalid
token was encountered. Second, when verbose error messages are enabled
(*note Error Reporting::), the expected token list in the syntax error
message can both contain invalid tokens and omit valid tokens.
The culprits for the above problems are '%nonassoc', default
reductions in inconsistent states (*note Default Reductions::), and
parser state merging. Because IELR and LALR merge parser states, they
suffer the most. Canonical LR can suffer only if '%nonassoc' is used or
if default reductions are enabled for inconsistent states.
LAC (Lookahead Correction) is a new mechanism within the parsing
algorithm that solves these problems for canonical LR, IELR, and LALR
without sacrificing '%nonassoc', default reductions, or state merging.
You can enable LAC with the '%define parse.lac' directive.
-- Directive: %define parse.lac VALUE
Enable LAC to improve syntax error handling.
* 'none' (default)
* 'full'
(This feature is experimental. More user feedback will help to
stabilize it. Moreover, it is currently only available for
deterministic parsers in C.)
Conceptually, the LAC mechanism is straight-forward. Whenever the
parser fetches a new token from the scanner so that it can determine the
next parser action, it immediately suspends normal parsing and performs
an exploratory parse using a temporary copy of the normal parser state
stack. During this exploratory parse, the parser does not perform user
semantic actions. If the exploratory parse reaches a shift action,
normal parsing then resumes on the normal parser stacks. If the
exploratory parse reaches an error instead, the parser reports a syntax
error. If verbose syntax error messages are enabled, the parser must
then discover the list of expected tokens, so it performs a separate
exploratory parse for each token in the grammar.
There is one subtlety about the use of LAC. That is, when in a
consistent parser state with a default reduction, the parser will not
attempt to fetch a token from the scanner because no lookahead is needed
to determine the next parser action. Thus, whether default reductions
are enabled in consistent states (*note Default Reductions::) affects
how soon the parser detects a syntax error: immediately when it
_reaches_ an erroneous token or when it eventually _needs_ that token as
a lookahead to determine the next parser action. The latter behavior is
probably more intuitive, so Bison currently provides no way to achieve
the former behavior while default reductions are enabled in consistent
Thus, when LAC is in use, for some fixed decision of whether to
enable default reductions in consistent states, canonical LR and IELR
behave almost exactly the same for both syntactically acceptable and
syntactically unacceptable input. While LALR still does not support the
full language-recognition power of canonical LR and IELR, LAC at least
enables LALR's syntax error handling to correctly reflect LALR's
language-recognition power.
There are a few caveats to consider when using LAC:
* Infinite parsing loops.
IELR plus LAC does have one shortcoming relative to canonical LR.
Some parsers generated by Bison can loop infinitely. LAC does not
fix infinite parsing loops that occur between encountering a syntax
error and detecting it, but enabling canonical LR or disabling
default reductions sometimes does.
* Verbose error message limitations.
Because of internationalization considerations, Bison-generated
parsers limit the size of the expected token list they are willing
to report in a verbose syntax error message. If the number of
expected tokens exceeds that limit, the list is simply dropped from
the message. Enabling LAC can increase the size of the list and
thus cause the parser to drop it. Of course, dropping the list is
better than reporting an incorrect list.
* Performance.
Because LAC requires many parse actions to be performed twice, it
can have a performance penalty. However, not all parse actions
must be performed twice. Specifically, during a series of default
reductions in consistent states and shift actions, the parser never
has to initiate an exploratory parse. Moreover, the most
time-consuming tasks in a parse are often the file I/O, the lexical
analysis performed by the scanner, and the user's semantic actions,
but none of these are performed during the exploratory parse.
Finally, the base of the temporary stack used during an exploratory
parse is a pointer into the normal parser state stack so that the
stack is never physically copied. In our experience, the
performance penalty of LAC has proved insignificant for practical
While the LAC algorithm shares techniques that have been recognized
in the parser community for years, for the publication that introduces
LAC, *note Denny 2010 May: Bibliography.
5.8.4 Unreachable States
If there exists no sequence of transitions from the parser's start state
to some state S, then Bison considers S to be an "unreachable state". A
state can become unreachable during conflict resolution if Bison
disables a shift action leading to it from a predecessor state.
By default, Bison removes unreachable states from the parser after
conflict resolution because they are useless in the generated parser.
However, keeping unreachable states is sometimes useful when trying to
understand the relationship between the parser and the grammar.
-- Directive: %define lr.keep-unreachable-state VALUE
Request that Bison allow unreachable states to remain in the parser
tables. VALUE must be a Boolean. The default is 'false'.
There are a few caveats to consider:
* Missing or extraneous warnings.
Unreachable states may contain conflicts and may use rules not used
in any other state. Thus, keeping unreachable states may induce
warnings that are irrelevant to your parser's behavior, and it may
eliminate warnings that are relevant. Of course, the change in
warnings may actually be relevant to a parser table analysis that
wants to keep unreachable states, so this behavior will likely
remain in future Bison releases.
* Other useless states.
While Bison is able to remove unreachable states, it is not
guaranteed to remove other kinds of useless states. Specifically,
when Bison disables reduce actions during conflict resolution, some
goto actions may become useless, and thus some additional states
may become useless. If Bison were to compute which goto actions
were useless and then disable those actions, it could identify such
states as unreachable and then remove those states. However, Bison
does not compute which goto actions are useless.
5.9 Generalized LR (GLR) Parsing
Bison produces _deterministic_ parsers that choose uniquely when to
reduce and which reduction to apply based on a summary of the preceding
input and on one extra token of lookahead. As a result, normal Bison
handles a proper subset of the family of context-free languages.
Ambiguous grammars, since they have strings with more than one possible
sequence of reductions cannot have deterministic parsers in this sense.
The same is true of languages that require more than one symbol of
lookahead, since the parser lacks the information necessary to make a
decision at the point it must be made in a shift-reduce parser.
Finally, as previously mentioned (*note Mysterious Conflicts::), there
are languages where Bison's default choice of how to summarize the input
seen so far loses necessary information.
When you use the '%glr-parser' declaration in your grammar file,
Bison generates a parser that uses a different algorithm, called
Generalized LR (or GLR). A Bison GLR parser uses the same basic
algorithm for parsing as an ordinary Bison parser, but behaves
differently in cases where there is a shift-reduce conflict that has not
been resolved by precedence rules (*note Precedence::) or a
reduce-reduce conflict. When a GLR parser encounters such a situation,
it effectively _splits_ into a several parsers, one for each possible
shift or reduction. These parsers then proceed as usual, consuming
tokens in lock-step. Some of the stacks may encounter other conflicts
and split further, with the result that instead of a sequence of states,
a Bison GLR parsing stack is what is in effect a tree of states.
In effect, each stack represents a guess as to what the proper parse
is. Additional input may indicate that a guess was wrong, in which case
the appropriate stack silently disappears. Otherwise, the semantics
actions generated in each stack are saved, rather than being executed
immediately. When a stack disappears, its saved semantic actions never
get executed. When a reduction causes two stacks to become equivalent,
their sets of semantic actions are both saved with the state that
results from the reduction. We say that two stacks are equivalent when
they both represent the same sequence of states, and each pair of
corresponding states represents a grammar symbol that produces the same
segment of the input token stream.
Whenever the parser makes a transition from having multiple states to
having one, it reverts to the normal deterministic parsing algorithm,
after resolving and executing the saved-up actions. At this transition,
some of the states on the stack will have semantic values that are sets
(actually multisets) of possible actions. The parser tries to pick one
of the actions by first finding one whose rule has the highest dynamic
precedence, as set by the '%dprec' declaration. Otherwise, if the
alternative actions are not ordered by precedence, but there the same
merging function is declared for both rules by the '%merge' declaration,
Bison resolves and evaluates both and then calls the merge function on
the result. Otherwise, it reports an ambiguity.
It is possible to use a data structure for the GLR parsing tree that
permits the processing of any LR(1) grammar in linear time (in the size
of the input), any unambiguous (not necessarily LR(1)) grammar in
quadratic worst-case time, and any general (possibly ambiguous)
context-free grammar in cubic worst-case time. However, Bison currently
uses a simpler data structure that requires time proportional to the
length of the input times the maximum number of stacks required for any
prefix of the input. Thus, really ambiguous or nondeterministic
grammars can require exponential time and space to process. Such badly
behaving examples, however, are not generally of practical interest.
Usually, nondeterminism in a grammar is local--the parser is "in doubt"
only for a few tokens at a time. Therefore, the current data structure
should generally be adequate. On LR(1) portions of a grammar, in
particular, it is only slightly slower than with the deterministic LR(1)
Bison parser.
For a more detailed exposition of GLR parsers, *note Scott 2000:
5.10 Memory Management, and How to Avoid Memory Exhaustion
The Bison parser stack can run out of memory if too many tokens are
shifted and not reduced. When this happens, the parser function
'yyparse' calls 'yyerror' and then returns 2.
Because Bison parsers have growing stacks, hitting the upper limit
usually results from using a right recursion instead of a left
recursion, see *note Recursive Rules: Recursion.
By defining the macro 'YYMAXDEPTH', you can control how deep the
parser stack can become before memory is exhausted. Define the macro
with a value that is an integer. This value is the maximum number of
tokens that can be shifted (and not reduced) before overflow.
The stack space allowed is not necessarily allocated. If you specify
a large value for 'YYMAXDEPTH', the parser normally allocates a small
stack at first, and then makes it bigger by stages as needed. This
increasing allocation happens automatically and silently. Therefore,
you do not need to make 'YYMAXDEPTH' painfully small merely to save
space for ordinary inputs that do not need much stack.
However, do not allow 'YYMAXDEPTH' to be a value so large that
arithmetic overflow could occur when calculating the size of the stack
space. Also, do not allow 'YYMAXDEPTH' to be less than 'YYINITDEPTH'.
The default value of 'YYMAXDEPTH', if you do not define it, is 10000.
You can control how much stack is allocated initially by defining the
macro 'YYINITDEPTH' to a positive integer. For the deterministic parser
in C, this value must be a compile-time constant unless you are assuming
C99 or some other target language or compiler that allows
variable-length arrays. The default is 200.
Do not allow 'YYINITDEPTH' to be greater than 'YYMAXDEPTH'.
You can generate a deterministic parser containing C++ user code from
the default (C) skeleton, as well as from the C++ skeleton (*note C++
Parsers::). However, if you do use the default skeleton and want to
allow the parsing stack to grow, be careful not to use semantic types or
location types that require non-trivial copy constructors. The C
skeleton bypasses these constructors when copying data to new, larger
6 Error Recovery
It is not usually acceptable to have a program terminate on a syntax
error. For example, a compiler should recover sufficiently to parse the
rest of the input file and check it for errors; a calculator should
accept another expression.
In a simple interactive command parser where each input is one line,
it may be sufficient to allow 'yyparse' to return 1 on error and have
the caller ignore the rest of the input line when that happens (and then
call 'yyparse' again). But this is inadequate for a compiler, because
it forgets all the syntactic context leading up to the error. A syntax
error deep within a function in the compiler input should not cause the
compiler to treat the following line like the beginning of a source
You can define how to recover from a syntax error by writing rules to
recognize the special token 'error'. This is a terminal symbol that is
always defined (you need not declare it) and reserved for error
handling. The Bison parser generates an 'error' token whenever a syntax
error happens; if you have provided a rule to recognize this token in
the current context, the parse can continue.
For example:
| stmts '\n'
| stmts exp '\n'
| stmts error '\n'
The fourth rule in this example says that an error followed by a
newline makes a valid addition to any 'stmts'.
What happens if a syntax error occurs in the middle of an 'exp'? The
error recovery rule, interpreted strictly, applies to the precise
sequence of a 'stmts', an 'error' and a newline. If an error occurs in
the middle of an 'exp', there will probably be some additional tokens
and subexpressions on the stack after the last 'stmts', and there will
be tokens to read before the next newline. So the rule is not
applicable in the ordinary way.
But Bison can force the situation to fit the rule, by discarding part
of the semantic context and part of the input. First it discards states
and objects from the stack until it gets back to a state in which the
'error' token is acceptable. (This means that the subexpressions
already parsed are discarded, back to the last complete 'stmts'.) At
this point the 'error' token can be shifted. Then, if the old lookahead
token is not acceptable to be shifted next, the parser reads tokens and
discards them until it finds a token which is acceptable. In this
example, Bison reads and discards input until the next newline so that
the fourth rule can apply. Note that discarded symbols are possible
sources of memory leaks, see *note Freeing Discarded Symbols: Destructor
Decl, for a means to reclaim this memory.
The choice of error rules in the grammar is a choice of strategies
for error recovery. A simple and useful strategy is simply to skip the
rest of the current input line or current statement if an error is
stmt: error ';' /* On error, skip until ';' is read. */
It is also useful to recover to the matching close-delimiter of an
opening-delimiter that has already been parsed. Otherwise the
close-delimiter will probably appear to be unmatched, and generate
another, spurious error message:
'(' expr ')'
| '(' error ')'
Error recovery strategies are necessarily guesses. When they guess
wrong, one syntax error often leads to another. In the above example,
the error recovery rule guesses that an error is due to bad input within
one 'stmt'. Suppose that instead a spurious semicolon is inserted in
the middle of a valid 'stmt'. After the error recovery rule recovers
from the first error, another syntax error will be found straightaway,
since the text following the spurious semicolon is also an invalid
To prevent an outpouring of error messages, the parser will output no
error message for another syntax error that happens shortly after the
first; only after three consecutive input tokens have been successfully
shifted will error messages resume.
Note that rules which accept the 'error' token may have actions, just
as any other rules can.
You can make error messages resume immediately by using the macro
'yyerrok' in an action. If you do this in the error rule's action, no
error messages will be suppressed. This macro requires no arguments;
'yyerrok;' is a valid C statement.
The previous lookahead token is reanalyzed immediately after an
error. If this is unacceptable, then the macro 'yyclearin' may be used
to clear this token. Write the statement 'yyclearin;' in the error
rule's action. *Note Special Features for Use in Actions: Action
For example, suppose that on a syntax error, an error handling
routine is called that advances the input stream to some point where
parsing should once again commence. The next symbol returned by the
lexical scanner is probably correct. The previous lookahead token ought
to be discarded with 'yyclearin;'.
The expression 'YYRECOVERING ()' yields 1 when the parser is
recovering from a syntax error, and 0 otherwise. Syntax error
diagnostics are suppressed while recovering from a syntax error.
7 Handling Context Dependencies
The Bison paradigm is to parse tokens first, then group them into larger
syntactic units. In many languages, the meaning of a token is affected
by its context. Although this violates the Bison paradigm, certain
techniques (known as "kludges") may enable you to write Bison parsers
for such languages.
(Actually, "kludge" means any technique that gets its job done but is
neither clean nor robust.)
7.1 Semantic Info in Token Types
The C language has a context dependency: the way an identifier is used
depends on what its current meaning is. For example, consider this:
foo (x);
This looks like a function call statement, but if 'foo' is a typedef
name, then this is actually a declaration of 'x'. How can a Bison
parser for C decide how to parse this input?
The method used in GNU C is to have two different token types,
'IDENTIFIER' and 'TYPENAME'. When 'yylex' finds an identifier, it looks
up the current declaration of the identifier in order to decide which
token type to return: 'TYPENAME' if the identifier is declared as a
typedef, 'IDENTIFIER' otherwise.
The grammar rules can then express the context dependency by the
choice of token type to recognize. 'IDENTIFIER' is accepted as an
expression, but 'TYPENAME' is not. 'TYPENAME' can start a declaration,
but 'IDENTIFIER' cannot. In contexts where the meaning of the
identifier is _not_ significant, such as in declarations that can shadow
a typedef name, either 'TYPENAME' or 'IDENTIFIER' is accepted--there is
one rule for each of the two token types.
This technique is simple to use if the decision of which kinds of
identifiers to allow is made at a place close to where the identifier is
parsed. But in C this is not always so: C allows a declaration to
redeclare a typedef name provided an explicit type has been specified
typedef int foo, bar;
int baz (void)
static bar (bar); /* redeclare 'bar' as static variable */
extern foo foo (foo); /* redeclare 'foo' as function */
return foo (bar);
Unfortunately, the name being declared is separated from the
declaration construct itself by a complicated syntactic structure--the
As a result, part of the Bison parser for C needs to be duplicated,
with all the nonterminal names changed: once for parsing a declaration
in which a typedef name can be redefined, and once for parsing a
declaration in which that can't be done. Here is a part of the
duplication, with actions omitted for brevity:
declarator maybeasm '=' init
| declarator maybeasm
notype_declarator maybeasm '=' init
| notype_declarator maybeasm
Here 'initdcl' can redeclare a typedef name, but 'notype_initdcl'
cannot. The distinction between 'declarator' and 'notype_declarator' is
the same sort of thing.
There is some similarity between this technique and a lexical tie-in
(described next), in that information which alters the lexical analysis
is changed during parsing by other parts of the program. The difference
is here the information is global, and is used for other purposes in the
program. A true lexical tie-in has a special-purpose flag controlled by
the syntactic context.
7.2 Lexical Tie-ins
One way to handle context-dependency is the "lexical tie-in": a flag
which is set by Bison actions, whose purpose is to alter the way tokens
are parsed.
For example, suppose we have a language vaguely like C, but with a
special construct 'hex (HEX-EXPR)'. After the keyword 'hex' comes an
expression in parentheses in which all integers are hexadecimal. In
particular, the token 'a1b' must be treated as an integer rather than as
an identifier if it appears in that context. Here is how you can do it:
int hexflag;
int yylex (void);
void yyerror (char const *);
| constant
| HEX '(' { hexflag = 1; }
expr ')' { hexflag = 0; $$ = $4; }
| expr '+' expr { $$ = make_sum ($1, $3); }
Here we assume that 'yylex' looks at the value of 'hexflag'; when it is
nonzero, all integers are parsed in hexadecimal, and tokens starting
with letters are parsed as integers if possible.
The declaration of 'hexflag' shown in the prologue of the grammar
file is needed to make it accessible to the actions (*note The Prologue:
Prologue.). You must also write the code in 'yylex' to obey the flag.
7.3 Lexical Tie-ins and Error Recovery
Lexical tie-ins make strict demands on any error recovery rules you
have. *Note Error Recovery::.
The reason for this is that the purpose of an error recovery rule is
to abort the parsing of one construct and resume in some larger
construct. For example, in C-like languages, a typical error recovery
rule is to skip tokens until the next semicolon, and then start a new
statement, like this:
expr ';'
| IF '(' expr ')' stmt { ... }
| error ';' { hexflag = 0; }
If there is a syntax error in the middle of a 'hex (EXPR)' construct,
this error rule will apply, and then the action for the completed 'hex
(EXPR)' will never run. So 'hexflag' would remain set for the entire
rest of the input, or until the next 'hex' keyword, causing identifiers
to be misinterpreted as integers.
To avoid this problem the error recovery rule itself clears
There may also be an error recovery rule that works within
expressions. For example, there could be a rule which applies within
parentheses and skips to the close-parenthesis:
| '(' expr ')' { $$ = $2; }
| '(' error ')'
If this rule acts within the 'hex' construct, it is not going to
abort that construct (since it applies to an inner level of parentheses
within the construct). Therefore, it should not clear the flag: the
rest of the 'hex' construct should be parsed with the flag still in
What if there is an error recovery rule which might abort out of the
'hex' construct or might not, depending on circumstances? There is no
way you can write the action to determine whether a 'hex' construct is
being aborted or not. So if you are using a lexical tie-in, you had
better make sure your error recovery rules are not of this kind. Each
rule must be such that you can be sure that it always will, or always
won't, have to clear the flag.
8 Debugging Your Parser
Developing a parser can be a challenge, especially if you don't
understand the algorithm (*note The Bison Parser Algorithm: Algorithm.).
This chapter explains how understand and debug a parser.
The first sections focus on the static part of the parser: its
structure. They explain how to generate and read the detailed
description of the automaton. There are several formats available:
- as text, see *note Understanding Your Parser: Understanding.;
- as a graph, see *note Visualizing Your Parser: Graphviz.;
- or as a markup report that can be turned, for instance, into HTML,
see *note Visualizing your parser in multiple formats: Xml.
The last section focuses on the dynamic part of the parser: how to
enable and understand the parser run-time traces (*note Tracing Your
Parser: Tracing.).
8.1 Understanding Your Parser
As documented elsewhere (*note The Bison Parser Algorithm: Algorithm.)
Bison parsers are "shift/reduce automata". In some cases (much more
frequent than one would hope), looking at this automaton is required to
tune or simply fix a parser.
The textual file is generated when the options '--report' or
'--verbose' are specified, see *note Invoking Bison: Invocation. Its
name is made by removing '.tab.c' or '.c' from the parser implementation
file name, and adding '.output' instead. Therefore, if the grammar file
is 'foo.y', then the parser implementation file is called 'foo.tab.c' by
default. As a consequence, the verbose output file is called
The following grammar file, 'calc.y', will be used in the sequel:
%token NUM STR
%left '+' '-'
%left '*'
exp '+' exp
| exp '-' exp
| exp '*' exp
| exp '/' exp
useless: STR;
'bison' reports:
calc.y: warning: 1 nonterminal useless in grammar
calc.y: warning: 1 rule useless in grammar
calc.y:12.1-7: warning: nonterminal useless in grammar: useless
calc.y:12.10-12: warning: rule useless in grammar: useless: STR
calc.y: conflicts: 7 shift/reduce
When given '--report=state', in addition to 'calc.tab.c', it creates
a file 'calc.output' with contents detailed below. The order of the
output and the exact presentation might vary, but the interpretation is
the same.
The first section reports useless tokens, nonterminals and rules.
Useless nonterminals and rules are removed in order to produce a smaller
parser, but useless tokens are preserved, since they might be used by
the scanner (note the difference between "useless" and "unused" below):
Nonterminals useless in grammar
Terminals unused in grammar
Rules useless in grammar
6 useless: STR
The next section lists states that still have conflicts.
State 8 conflicts: 1 shift/reduce
State 9 conflicts: 1 shift/reduce
State 10 conflicts: 1 shift/reduce
State 11 conflicts: 4 shift/reduce
Then Bison reproduces the exact grammar it used:
0 $accept: exp $end
1 exp: exp '+' exp
2 | exp '-' exp
3 | exp '*' exp
4 | exp '/' exp
and reports the uses of the symbols:
Terminals, with rules where they appear
$end (0) 0
'*' (42) 3
'+' (43) 1
'-' (45) 2
'/' (47) 4
error (256)
Nonterminals, with rules where they appear
$accept (9)
on left: 0
exp (10)
on left: 1 2 3 4 5, on right: 0 1 2 3 4
Bison then proceeds onto the automaton itself, describing each state
with its set of "items", also known as "pointed rules". Each item is a
production rule together with a point ('.') marking the location of the
input cursor.
State 0
0 $accept: . exp $end
NUM shift, and go to state 1
exp go to state 2
This reads as follows: "state 0 corresponds to being at the very
beginning of the parsing, in the initial rule, right before the start
symbol (here, 'exp'). When the parser returns to this state right after
having reduced a rule that produced an 'exp', the control flow jumps to
state 2. If there is no such transition on a nonterminal symbol, and
the lookahead is a 'NUM', then this token is shifted onto the parse
stack, and the control flow jumps to state 1. Any other lookahead
triggers a syntax error."
Even though the only active rule in state 0 seems to be rule 0, the
report lists 'NUM' as a lookahead token because 'NUM' can be at the
beginning of any rule deriving an 'exp'. By default Bison reports the
so-called "core" or "kernel" of the item set, but if you want to see
more detail you can invoke 'bison' with '--report=itemset' to list the
derived items as well:
State 0
0 $accept: . exp $end
1 exp: . exp '+' exp
2 | . exp '-' exp
3 | . exp '*' exp
4 | . exp '/' exp
NUM shift, and go to state 1
exp go to state 2
In the state 1...
State 1
5 exp: NUM .
$default reduce using rule 5 (exp)
the rule 5, 'exp: NUM;', is completed. Whatever the lookahead token
('$default'), the parser will reduce it. If it was coming from State 0,
then, after this reduction it will return to state 0, and will jump to
state 2 ('exp: go to state 2').
State 2
0 $accept: exp . $end
1 exp: exp . '+' exp
2 | exp . '-' exp
3 | exp . '*' exp
4 | exp . '/' exp
$end shift, and go to state 3
'+' shift, and go to state 4
'-' shift, and go to state 5
'*' shift, and go to state 6
'/' shift, and go to state 7
In state 2, the automaton can only shift a symbol. For instance,
because of the item 'exp: exp . '+' exp', if the lookahead is '+' it is
shifted onto the parse stack, and the automaton jumps to state 4,
corresponding to the item 'exp: exp '+' . exp'. Since there is no
default action, any lookahead not listed triggers a syntax error.
The state 3 is named the "final state", or the "accepting state":
State 3
0 $accept: exp $end .
$default accept
the initial rule is completed (the start symbol and the end-of-input
were read), the parsing exits successfully.
The interpretation of states 4 to 7 is straightforward, and is left
to the reader.
State 4
1 exp: exp '+' . exp
NUM shift, and go to state 1
exp go to state 8
State 5
2 exp: exp '-' . exp
NUM shift, and go to state 1
exp go to state 9
State 6
3 exp: exp '*' . exp
NUM shift, and go to state 1
exp go to state 10
State 7
4 exp: exp '/' . exp
NUM shift, and go to state 1
exp go to state 11
As was announced in beginning of the report, 'State 8 conflicts: 1
State 8
1 exp: exp . '+' exp
1 | exp '+' exp .
2 | exp . '-' exp
3 | exp . '*' exp
4 | exp . '/' exp
'*' shift, and go to state 6
'/' shift, and go to state 7
'/' [reduce using rule 1 (exp)]
$default reduce using rule 1 (exp)
Indeed, there are two actions associated to the lookahead '/': either
shifting (and going to state 7), or reducing rule 1. The conflict means
that either the grammar is ambiguous, or the parser lacks information to
make the right decision. Indeed the grammar is ambiguous, as, since we
did not specify the precedence of '/', the sentence 'NUM + NUM / NUM'
can be parsed as 'NUM + (NUM / NUM)', which corresponds to shifting '/',
or as '(NUM + NUM) / NUM', which corresponds to reducing rule 1.
Because in deterministic parsing a single decision can be made, Bison
arbitrarily chose to disable the reduction, see *note Shift/Reduce
Conflicts: Shift/Reduce. Discarded actions are reported between square
Note that all the previous states had a single possible action:
either shifting the next token and going to the corresponding state, or
reducing a single rule. In the other cases, i.e., when shifting _and_
reducing is possible or when _several_ reductions are possible, the
lookahead is required to select the action. State 8 is one such state:
if the lookahead is '*' or '/' then the action is shifting, otherwise
the action is reducing rule 1. In other words, the first two items,
corresponding to rule 1, are not eligible when the lookahead token is
'*', since we specified that '*' has higher precedence than '+'. More
generally, some items are eligible only with some set of possible
lookahead tokens. When run with '--report=lookahead', Bison specifies
these lookahead tokens:
State 8
1 exp: exp . '+' exp
1 | exp '+' exp . [$end, '+', '-', '/']
2 | exp . '-' exp
3 | exp . '*' exp
4 | exp . '/' exp
'*' shift, and go to state 6
'/' shift, and go to state 7
'/' [reduce using rule 1 (exp)]
$default reduce using rule 1 (exp)
Note however that while 'NUM + NUM / NUM' is ambiguous (which results
in the conflicts on '/'), 'NUM + NUM * NUM' is not: the conflict was
solved thanks to associativity and precedence directives. If invoked
with '--report=solved', Bison includes information about the solved
conflicts in the report:
Conflict between rule 1 and token '+' resolved as reduce (%left '+').
Conflict between rule 1 and token '-' resolved as reduce (%left '-').
Conflict between rule 1 and token '*' resolved as shift ('+' < '*').
The remaining states are similar:
State 9
1 exp: exp . '+' exp
2 | exp . '-' exp
2 | exp '-' exp .
3 | exp . '*' exp
4 | exp . '/' exp
'*' shift, and go to state 6
'/' shift, and go to state 7
'/' [reduce using rule 2 (exp)]
$default reduce using rule 2 (exp)
State 10
1 exp: exp . '+' exp
2 | exp . '-' exp
3 | exp . '*' exp
3 | exp '*' exp .
4 | exp . '/' exp
'/' shift, and go to state 7
'/' [reduce using rule 3 (exp)]
$default reduce using rule 3 (exp)
State 11
1 exp: exp . '+' exp
2 | exp . '-' exp
3 | exp . '*' exp
4 | exp . '/' exp
4 | exp '/' exp .
'+' shift, and go to state 4
'-' shift, and go to state 5
'*' shift, and go to state 6
'/' shift, and go to state 7
'+' [reduce using rule 4 (exp)]
'-' [reduce using rule 4 (exp)]
'*' [reduce using rule 4 (exp)]
'/' [reduce using rule 4 (exp)]
$default reduce using rule 4 (exp)
Observe that state 11 contains conflicts not only due to the lack of
precedence of '/' with respect to '+', '-', and '*', but also because
the associativity of '/' is not specified.
Bison may also produce an HTML version of this output, via an XML
file and XSLT processing (*note Visualizing your parser in multiple
formats: Xml.).
8.2 Visualizing Your Parser
As another means to gain better understanding of the shift/reduce
automaton corresponding to the Bison parser, a DOT file can be
generated. Note that debugging a real grammar with this is tedious at
best, and impractical most of the times, because the generated files are
huge (the generation of a PDF or PNG file from it will take very long,
and more often than not it will fail due to memory exhaustion). This
option was rather designed for beginners, to help them understand LR
This file is generated when the '--graph' option is specified (*note
Invoking Bison: Invocation.). Its name is made by removing '.tab.c' or
'.c' from the parser implementation file name, and adding '.dot'
instead. If the grammar file is 'foo.y', the Graphviz output file is
called 'foo.dot'. A DOT file may also be produced via an XML file and
XSLT processing (*note Visualizing your parser in multiple formats:
The following grammar file, 'rr.y', will be used in the sequel:
exp: a ";" | b ".";
a: "0";
b: "0";
The graphical output is very similar to the textual one, and as such
it is easier understood by making direct comparisons between them.
*Note Debugging Your Parser: Debugging, for a detailled analysis of the
textual report.
Graphical Representation of States
The items (pointed rules) for each state are grouped together in graph
nodes. Their numbering is the same as in the verbose file. See the
following points, about transitions, for examples
When invoked with '--report=lookaheads', the lookahead tokens, when
needed, are shown next to the relevant rule between square brackets as a
comma separated list. This is the case in the figure for the
representation of reductions, below.
The transitions are represented as directed edges between the current
and the target states.
Graphical Representation of Shifts
Shifts are shown as solid arrows, labelled with the lookahead token for
that shift. The following describes a reduction in the 'rr.output'
State 3
1 exp: a . ";"
";" shift, and go to state 6
A Graphviz rendering of this portion of the graph could be:
| State 3 |
| 1 exp: a . ";" |
| ";"
| State 6 |
| 1 exp: a ";" . |
Graphical Representation of Reductions
Reductions are shown as solid arrows, leading to a diamond-shaped node
bearing the number of the reduction rule. The arrow is labelled with
the appropriate comma separated lookahead tokens. If the reduction is
the default action for the given state, there is no such label.
This is how reductions are represented in the verbose file
State 1
3 a: "0" . [";"]
4 b: "0" . ["."]
"." reduce using rule 4 (b)
$default reduce using rule 3 (a)
A Graphviz rendering of this portion of the graph could be:
| State 1 |
| 3 a: "0" . [";"] |
| 4 b: "0" . ["."] |
/ \
/ \ ["."]
/ \
v v
/ \ / \
(green) \ 3 / \ 4 / (green)
\ / \ /
When unresolved conflicts are present, because in deterministic
parsing a single decision can be made, Bison can arbitrarily choose to
disable a reduction, see *note Shift/Reduce Conflicts: Shift/Reduce.
Discarded actions are distinguished by a red filling color on these
nodes, just like how they are reported between square brackets in the
verbose file.
The reduction corresponding to the rule number 0 is the acceptation
state. It is shown as a blue diamond, labelled "Acc".
Graphical representation of go tos
The 'go to' jump transitions are represented as dotted lines bearing the
name of the rule being jumped to.
8.3 Visualizing your parser in multiple formats
Bison supports two major report formats: textual output (*note
Understanding Your Parser: Understanding.) when invoked with option
'--verbose', and DOT (*note Visualizing Your Parser: Graphviz.) when
invoked with option '--graph'. However, another alternative is to
output an XML file that may then be, with 'xsltproc', rendered as either
a raw text format equivalent to the verbose file, or as an HTML version
of the same file, with clickable transitions, or even as a DOT. The
'.output' and DOT files obtained via XSLT have no difference whatsoever
with those obtained by invoking 'bison' with options '--verbose' or
The XML file is generated when the options '-x' or '--xml[=FILE]' are
specified, see *note Invoking Bison: Invocation. If not specified, its
name is made by removing '.tab.c' or '.c' from the parser implementation
file name, and adding '.xml' instead. For instance, if the grammar file
is 'foo.y', the default XML output file is 'foo.xml'.
Bison ships with a 'data/xslt' directory, containing XSL
Transformation files to apply to the XML file. Their names are
Used to output a copy of the DOT visualization of the automaton.
Used to output a copy of the '.output' file.
Used to output an xhtml enhancement of the '.output' file.
Sample usage (requires 'xsltproc'):
$ bison -x gr.y
$ bison --print-datadir
$ xsltproc /usr/local/share/bison/xslt/xml2xhtml.xsl gr.xml >gr.html
8.4 Tracing Your Parser
When a Bison grammar compiles properly but parses "incorrectly", the
'yydebug' parser-trace feature helps figuring out why.
8.4.1 Enabling Traces
There are several means to enable compilation of trace facilities:
the macro 'YYDEBUG'
Define the macro 'YYDEBUG' to a nonzero value when you compile the
parser. This is compliant with POSIX Yacc. You could use
'-DYYDEBUG=1' as a compiler option or you could put '#define
YYDEBUG 1' in the prologue of the grammar file (*note The Prologue:
If the '%define' variable 'api.prefix' is used (*note Multiple
Parsers in the Same Program: Multiple Parsers.), for instance
'%define api.prefix x', then if 'CDEBUG' is defined, its value
controls the tracing feature (enabled if and only if nonzero);
otherwise tracing is enabled if and only if 'YYDEBUG' is nonzero.
the option '-t' (POSIX Yacc compliant)
the option '--debug' (Bison extension)
Use the '-t' option when you run Bison (*note Invoking Bison:
Invocation.). With '%define api.prefix {c}', it defines 'CDEBUG'
to 1, otherwise it defines 'YYDEBUG' to 1.
the directive '%debug'
Add the '%debug' directive (*note Bison Declaration Summary: Decl
Summary.). This Bison extension is maintained for backward
compatibility with previous versions of Bison.
the variable 'parse.trace'
Add the '%define parse.trace' directive (*note parse.trace: %define
Summary.), or pass the '-Dparse.trace' option (*note Bison
Options::). This is a Bison extension, which is especially useful
for languages that don't use a preprocessor. Unless POSIX and Yacc
portability matter to you, this is the preferred solution.
We suggest that you always enable the trace option so that debugging
is always possible.
The trace facility outputs messages with macro calls of the form
'YYFPRINTF (stderr, FORMAT, ARGS)' where FORMAT and ARGS are the usual
'printf' format and variadic arguments. If you define 'YYDEBUG' to a
nonzero value but do not define 'YYFPRINTF', '' is
automatically included and 'YYFPRINTF' is defined to 'fprintf'.
Once you have compiled the program with trace facilities, the way to
request a trace is to store a nonzero value in the variable 'yydebug'.
You can do this by making the C code do it (in 'main', perhaps), or you
can alter the value with a C debugger.
Each step taken by the parser when 'yydebug' is nonzero produces a
line or two of trace information, written on 'stderr'. The trace
messages tell you these things:
* Each time the parser calls 'yylex', what kind of token was read.
* Each time a token is shifted, the depth and complete contents of
the state stack (*note Parser States::).
* Each time a rule is reduced, which rule it is, and the complete
contents of the state stack afterward.
To make sense of this information, it helps to refer to the automaton
description file (*note Understanding Your Parser: Understanding.).
This file shows the meaning of each state in terms of positions in
various rules, and also what each state will do with each possible input
token. As you read the successive trace messages, you can see that the
parser is functioning according to its specification in the listing
file. Eventually you will arrive at the place where something
undesirable happens, and you will see which parts of the grammar are to
The parser implementation file is a C/C++/Java program and you can
use debuggers on it, but it's not easy to interpret what it is doing.
The parser function is a finite-state machine interpreter, and aside
from the actions it executes the same code over and over. Only the
values of variables show where in the grammar it is working.
8.4.2 Enabling Debug Traces for 'mfcalc'
The debugging information normally gives the token type of each token
read, but not its semantic value. The '%printer' directive allows
specify how semantic values are reported, see *note Printing Semantic
Values: Printer Decl. For backward compatibility, Yacc like C parsers
may also use the 'YYPRINT' (*note The 'YYPRINT' Macro: The YYPRINT
Macro.), but its use is discouraged.
As a demonstration of '%printer', consider the multi-function
calculator, 'mfcalc' (*note Multi-function Calc::). To enable run-time
traces, and semantic value reports, insert the following directives in
its prologue:
/* Generate the parser description file. */
/* Enable run-time traces (yydebug). */
%define parse.trace
/* Formatting semantic values. */
%printer { fprintf (yyoutput, "%s", $$->name); } VAR;
%printer { fprintf (yyoutput, "%s()", $$->name); } FNCT;
%printer { fprintf (yyoutput, "%g", $$); } ;
The '%define' directive instructs Bison to generate run-time trace
support. Then, activation of these traces is controlled at run-time by
the 'yydebug' variable, which is disabled by default. Because these
traces will refer to the "states" of the parser, it is helpful to ask
for the creation of a description of that parser; this is the purpose of
(admittedly ill-named) '%verbose' directive.
The set of '%printer' directives demonstrates how to format the
semantic value in the traces. Note that the specification can be done
either on the symbol type (e.g., 'VAR' or 'FNCT'), or on the type tag:
since '' is the type for both 'NUM' and 'exp', this printer will
be used for them.
Here is a sample of the information provided by run-time traces. The
traces are sent onto standard error.
$ echo 'sin(1-1)' | ./mfcalc -p
Starting parse
Entering state 0
Reducing stack by rule 1 (line 34):
-> $$ = nterm input ()
Stack now 0
Entering state 1
This first batch shows a specific feature of this grammar: the first
rule (which is in line 34 of 'mfcalc.y' can be reduced without even
having to look for the first token. The resulting left-hand symbol
('$$') is a valueless ('()') 'input' non terminal ('nterm').
Then the parser calls the scanner.
Reading a token: Next token is token FNCT (sin())
Shifting token FNCT (sin())
Entering state 6
That token ('token') is a function ('FNCT') whose value is 'sin' as
formatted per our '%printer' specification: 'sin()'. The parser stores
('Shifting') that token, and others, until it can do something about it.
Reading a token: Next token is token '(' ()
Shifting token '(' ()
Entering state 14
Reading a token: Next token is token NUM (1.000000)
Shifting token NUM (1.000000)
Entering state 4
Reducing stack by rule 6 (line 44):
$1 = token NUM (1.000000)
-> $$ = nterm exp (1.000000)
Stack now 0 1 6 14
Entering state 24
The previous reduction demonstrates the '%printer' directive for
'': both the token 'NUM' and the resulting nonterminal 'exp'
have '1' as value.
Reading a token: Next token is token '-' ()
Shifting token '-' ()
Entering state 17
Reading a token: Next token is token NUM (1.000000)
Shifting token NUM (1.000000)
Entering state 4
Reducing stack by rule 6 (line 44):
$1 = token NUM (1.000000)
-> $$ = nterm exp (1.000000)
Stack now 0 1 6 14 24 17
Entering state 26
Reading a token: Next token is token ')' ()
Reducing stack by rule 11 (line 49):
$1 = nterm exp (1.000000)
$2 = token '-' ()
$3 = nterm exp (1.000000)
-> $$ = nterm exp (0.000000)
Stack now 0 1 6 14
Entering state 24
The rule for the subtraction was just reduced. The parser is about to
discover the end of the call to 'sin'.
Next token is token ')' ()
Shifting token ')' ()
Entering state 31
Reducing stack by rule 9 (line 47):
$1 = token FNCT (sin())
$2 = token '(' ()
$3 = nterm exp (0.000000)
$4 = token ')' ()
-> $$ = nterm exp (0.000000)
Stack now 0 1
Entering state 11
Finally, the end-of-line allow the parser to complete the computation,
and display its result.
Reading a token: Next token is token '\n' ()
Shifting token '\n' ()
Entering state 22
Reducing stack by rule 4 (line 40):
$1 = nterm exp (0.000000)
$2 = token '\n' ()
=> 0
-> $$ = nterm line ()
Stack now 0 1
Entering state 10
Reducing stack by rule 2 (line 35):
$1 = nterm input ()
$2 = nterm line ()
-> $$ = nterm input ()
Stack now 0
Entering state 1
The parser has returned into state 1, in which it is waiting for the
next expression to evaluate, or for the end-of-file token, which causes
the completion of the parsing.
Reading a token: Now at end of input.
Shifting token $end ()
Entering state 2
Stack now 0 1 2
Cleanup: popping token $end ()
Cleanup: popping nterm input ()
8.4.3 The 'YYPRINT' Macro
Before '%printer' support, semantic values could be displayed using the
'YYPRINT' macro, which works only for terminal symbols and only with the
'yacc.c' skeleton.
-- Macro: YYPRINT (STREAM, TOKEN, VALUE);
If you define 'YYPRINT', it should take three arguments. The
parser will pass a standard I/O stream, the numeric code for the
token type, and the token value (from 'yylval').
For 'yacc.c' only. Obsoleted by '%printer'.
Here is an example of 'YYPRINT' suitable for the multi-function
calculator (*note Declarations for 'mfcalc': Mfcalc Declarations.):
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(File, Type, Value) \
print_token_value (File, Type, Value)
... %% ... %% ...
static void
print_token_value (FILE *file, int type, YYSTYPE value)
if (type == VAR)
fprintf (file, "%s", value.tptr->name);
else if (type == NUM)
fprintf (file, "%d", value.val);
9 Invoking Bison
The usual way to invoke Bison is as follows:
bison INFILE
Here INFILE is the grammar file name, which usually ends in '.y'.
The parser implementation file's name is made by replacing the '.y' with
'.tab.c' and removing any leading directory. Thus, the 'bison foo.y'
file name yields 'foo.tab.c', and the 'bison hack/foo.y' file name
yields 'foo.tab.c'. It's also possible, in case you are writing C++
code instead of C in your grammar file, to name it 'foo.ypp' or
'foo.y++'. Then, the output files will take an extension like the given
one as input (respectively 'foo.tab.cpp' and 'foo.tab.c++'). This
feature takes effect with all options that manipulate file names like
'-o' or '-d'.
For example :
bison -d INFILE.YXX
will produce 'infile.tab.cxx' and 'infile.tab.hxx', and
bison -d -o OUTPUT.C++ INFILE.Y
will produce 'output.c++' and 'outfile.h++'.
For compatibility with POSIX, the standard Bison distribution also
contains a shell script called 'yacc' that invokes Bison with the '-y'
9.1 Bison Options
Bison supports both traditional single-letter options and mnemonic long
option names. Long option names are indicated with '--' instead of '-'.
Abbreviations for option names are allowed as long as they are unique.
When a long option takes an argument, like '--file-prefix', connect the
option name and the argument with '='.
Here is a list of options that can be used with Bison, alphabetized
by short option. It is followed by a cross key alphabetized by long
Operations modes:
Print a summary of the command-line options to Bison and exit.
Print the version number of Bison and exit.
Print the name of the directory containing locale-dependent data.
Print the name of the directory containing skeletons and XSLT.
Act more like the traditional Yacc command. This can cause
different diagnostics to be generated, and may change behavior in
other minor ways. Most importantly, imitate Yacc's output file
name conventions, so that the parser implementation file is called
'y.tab.c', and the other outputs are called 'y.output' and
'y.tab.h'. Also, if generating a deterministic parser in C,
generate '#define' statements in addition to an 'enum' to associate
token numbers with token names. Thus, the following shell script
can substitute for Yacc, and the Bison distribution contains such a
script for compatibility with POSIX:
#! /bin/sh
bison -y "$@"
The '-y'/'--yacc' option is intended for use with traditional Yacc
grammars. If your grammar uses a Bison extension like
'%glr-parser', Bison might not be Yacc-compatible even if this
option is specified.
Output warnings falling in CATEGORY. CATEGORY can be one of:
Warn about mid-rule values that are set but not used within
any of the actions of the parent rule. For example, warn
about unused '$2' in:
exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
Also warn about mid-rule values that are used but not set.
For example, warn about unset '$$' in the mid-rule action in:
exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };
These warnings are not enabled by default since they sometimes
prove to be false alarms in existing grammars employing the
Yacc constructs '$0' or '$-N' (where N is some positive
Incompatibilities with POSIX Yacc.
S/R and R/R conflicts. These warnings are enabled by default.
However, if the '%expect' or '%expect-rr' directive is
specified, an unexpected number of conflicts is an error, and
an expected number of conflicts is not reported, so '-W' and
'--warning' then have no effect on the conflict report.
Deprecated constructs whose support will be removed in future
versions of Bison.
Empty rules without '%empty'. *Note Empty Rules::. Disabled
by default, but enabled by uses of '%empty', unless
'-Wno-empty-rule' was specified.
Useless precedence and associativity directives. Disabled by
Consider for instance the following grammar:
%nonassoc "="
%left "+"
%left "*"
%precedence "("
| "var" "=" exp
exp "+" exp
| exp "*" "num"
| "(" exp ")"
| "num"
Bison reports:
warning: useless precedence and associativity for "="
%nonassoc "="
warning: useless associativity for "*", use %precedence
%left "*"
warning: useless precedence for "("
%precedence "("
One would get the exact same parser with the following
directives instead:
%left "+"
%precedence "*"
All warnings not categorized above. These warnings are
enabled by default.
This category is provided merely for the sake of completeness.
Future releases of Bison may move warnings from this category
to new, more specific categories.
All the warnings except 'yacc'.
Turn off all the warnings.
See '-Werror', below.
A category can be turned off by prefixing its name with 'no-'. For
instance, '-Wno-yacc' will hide the warnings about POSIX Yacc
Turn enabled warnings for every CATEGORY into errors, unless they
are explicitly disabled by '-Wno-error=CATEGORY'.
Enable warnings falling in CATEGORY, and treat them as errors.
CATEGORY is the same as for '--warnings', with the exception that
it may not be prefixed with 'no-' (see above).
Note that the precedence of the '=' and ',' operators is such that
the following commands are _not_ equivalent, as the first will not
treat S/R conflicts as errors.
$ bison -Werror=yacc,conflicts-sr input.y
$ bison -Werror=yacc,error=conflicts-sr input.y
Do not turn enabled warnings for every CATEGORY into errors, unless
they are explicitly enabled by '-Werror=CATEGORY'.
Deactivate the error treatment for this CATEGORY. However, the
warning itself won't be disabled, or enabled, by this option.
'-f [FEATURE]'
Activate miscellaneous FEATURE. FEATURE can be one of:
Show caret errors, in a manner similar to GCC's
'-fdiagnostics-show-caret', or Clang's '-fcaret-diagnotics'.
The location provided with the message is used to quote the
corresponding line of the source file, underlining the
important part of it with carets (^). Here is an example,
using the following file 'in.y':
%type exp
exp: exp '+' exp { $exp = $1 + $2; };
When invoked with '-fcaret' (or nothing), Bison will report:
in.y:3.20-23: error: ambiguous reference: '$exp'
exp: exp '+' exp { $exp = $1 + $2; };
in.y:3.1-3: refers to: $exp at $$
exp: exp '+' exp { $exp = $1 + $2; };
in.y:3.6-8: refers to: $exp at $1
exp: exp '+' exp { $exp = $1 + $2; };
in.y:3.14-16: refers to: $exp at $3
exp: exp '+' exp { $exp = $1 + $2; };
in.y:3.32-33: error: $2 of 'exp' has no declared type
exp: exp '+' exp { $exp = $1 + $2; };
Whereas, when invoked with '-fno-caret', Bison will only
in.y:3.20-23: error: ambiguous reference: '$exp'
in.y:3.1-3: refers to: $exp at $$
in.y:3.6-8: refers to: $exp at $1
in.y:3.14-16: refers to: $exp at $3
in.y:3.32-33: error: $2 of 'exp' has no declared type
This option is activated by default.
Tuning the parser:
In the parser implementation file, define the macro 'YYDEBUG' to 1
if it is not already defined, so that the debugging facilities are
compiled. *Note Tracing Your Parser: Tracing.
Each of these is equivalent to '%define NAME "VALUE"' (*note
%define Summary::) except that Bison processes multiple definitions
for the same NAME as follows:
* Bison quietly ignores all command-line definitions for NAME
except the last.
* If that command-line definition is specified by a '-D' or
'--define', Bison reports an error for any '%define'
definition for NAME.
* If that command-line definition is specified by a '-F' or
'--force-define' instead, Bison quietly ignores all '%define'
definitions for NAME.
* Otherwise, Bison reports an error if there are multiple
'%define' definitions for NAME.
You should avoid using '-F' and '--force-define' in your make files
unless you are confident that it is safe to quietly ignore any
conflicting '%define' that may be added to the grammar file.
Specify the programming language for the generated parser, as if
'%language' was specified (*note Bison Declaration Summary: Decl
Summary.). Currently supported languages include C, C++, and Java.
LANGUAGE is case-insensitive.
Pretend that '%locations' was specified. *Note Decl Summary::.
'-p PREFIX'
Pretend that '%name-prefix "PREFIX"' was specified (*note Decl
Summary::). Obsoleted by '-Dapi.prefix=PREFIX'. *Note Multiple
Parsers in the Same Program: Multiple Parsers.
Don't put any '#line' preprocessor commands in the parser
implementation file. Ordinarily Bison puts them in the parser
implementation file so that the C compiler and debuggers will
associate errors with your source file, the grammar file. This
option causes them to associate errors with the parser
implementation file, treating it as an independent source file in
its own right.
Specify the skeleton to use, similar to '%skeleton' (*note Bison
Declaration Summary: Decl Summary.).
If FILE does not contain a '/', FILE is the name of a skeleton file
in the Bison installation directory. If it does, FILE is an
absolute file name or a file name relative to the current working
directory. This is similar to how most shells resolve commands.
Pretend that '%token-table' was specified. *Note Decl Summary::.
Adjust the output:
Pretend that '%defines' was specified, i.e., write an extra output
file containing macro definitions for the token type names defined
in the grammar, as well as a few other declarations. *Note Decl
This is the same as '--defines' except '-d' does not accept a FILE
argument since POSIX Yacc requires that '-d' can be bundled with
other short options.
'-b FILE-PREFIX'
Pretend that '%file-prefix' was specified, i.e., specify prefix to
use for all Bison output file names. *Note Decl Summary::.
'-r THINGS'
Write an extra output file containing verbose description of the
comma separated list of THINGS among:
Description of the grammar, conflicts (resolved and
unresolved), and parser's automaton.
Implies 'state' and augments the description of the automaton
with the full set of items for each state, instead of its core
Implies 'state' and augments the description of the automaton
with each rule's lookahead set.
Implies 'state'. Explain how conflicts were solved thanks to
precedence and associativity directives.
Enable all the items.
Do not generate the report.
Specify the FILE for the verbose description.
Pretend that '%verbose' was specified, i.e., write an extra output
file containing verbose descriptions of the grammar and parser.
*Note Decl Summary::.
'-o FILE'
Specify the FILE for the parser implementation file.
The other output files' names are constructed from FILE as
described under the '-v' and '-d' options.
'-g [FILE]'
Output a graphical representation of the parser's automaton
computed by Bison, in Graphviz (http://www.graphviz.org/) DOT
(http://www.graphviz.org/doc/info/lang.html) format. 'FILE' is
optional. If omitted and the grammar file is 'foo.y', the output
file will be 'foo.dot'.
'-x [FILE]'
Output an XML report of the parser's automaton computed by Bison.
'FILE' is optional. If omitted and the grammar file is 'foo.y',
the output file will be 'foo.xml'. (The current XML schema is
experimental and may evolve. More user feedback will help to
stabilize it.)
9.2 Option Cross Key
Here is a list of options, alphabetized by long option, to help you find
the corresponding short option and directive.
Long Option Short Option Bison Directive
'--debug' '-t' '%debug'
'--define=NAME[=VALUE]' '-D NAME[=VALUE]' '%define NAME ["VALUE"]'
'--defines[=FILE]' '-d' '%defines ["FILE"]'
'--feature[=FEATURE]' '-f [FEATURE]'
'--file-prefix=PREFIX' '-b PREFIX' '%file-prefix "PREFIX"'
'--force-define=NAME[=VALUE]' '-F NAME[=VALUE]' '%define NAME ["VALUE"]'
'--graph[=FILE]' '-g [FILE]'
'--help' '-h'
'--language=LANGUAGE' '-L LANGUAGE' '%language "LANGUAGE"'
'--locations' '%locations'
'--name-prefix=PREFIX' '-p PREFIX' '%name-prefix "PREFIX"'
'--no-lines' '-l' '%no-lines'
'--output=FILE' '-o FILE' '%output "FILE"'
'--report=THINGS' '-r THINGS'
'--skeleton=FILE' '-S FILE' '%skeleton "FILE"'
'--token-table' '-k' '%token-table'
'--verbose' '-v' '%verbose'
'--version' '-V'
'--warnings[=CATEGORY]' '-W [CATEGORY]'
'--xml[=FILE]' '-x [FILE]'
'--yacc' '-y' '%yacc'
9.3 Yacc Library
The Yacc library contains default implementations of the 'yyerror' and
'main' functions. These default implementations are normally not
useful, but POSIX requires them. To use the Yacc library, link your
program with the '-ly' option. Note that Bison's implementation of the
Yacc library is distributed under the terms of the GNU General Public
License (*note Copying::).
If you use the Yacc library's 'yyerror' function, you should declare
'yyerror' as follows:
int yyerror (char const *);
Bison ignores the 'int' value returned by this 'yyerror'. If you use
the Yacc library's 'main' function, your 'yyparse' function should have
the following type signature:
int yyparse (void);
10 Parsers Written In Other Languages
10.1 C++ Parsers
10.1.1 C++ Bison Interface
The C++ deterministic parser is selected using the skeleton directive,
'%skeleton "lalr1.cc"', or the synonymous command-line option
'--skeleton=lalr1.cc'. *Note Decl Summary::.
When run, 'bison' will create several entities in the 'yy' namespace.
Use the '%define api.namespace' directive to change the namespace name,
see *note api.namespace: %define Summary. The various classes are
generated in the following files:
The definition of the classes 'position' and 'location', used for
location tracking when enabled. These files are not generated if
the '%define' variable 'api.location.type' is defined. *Note C++
Location Values::.
An auxiliary class 'stack' used by the parser.
(Assuming the extension of the grammar file was '.yy'.) The
declaration and implementation of the C++ parser class. The
basename and extension of these two files follow the same rules as
with regular C parsers (*note Invocation::).
The header is _mandatory_; you must either pass '-d'/'--defines' to
'bison', or use the '%defines' directive.
All these files are documented using Doxygen; run 'doxygen' for a
complete and accurate documentation.
10.1.2 C++ Semantic Values
Bison supports two different means to handle semantic values in C++.
One is alike the C interface, and relies on unions (*note C++ Unions::).
As C++ practitioners know, unions are inconvenient in C++, therefore
another approach is provided, based on variants (*note C++ Variants::).
10.1.2.1 C++ Unions
The '%union' directive works as for C, see *note The Union Declaration:
Union Decl. In particular it produces a genuine 'union', which have a
few specific features in C++.
- The type 'YYSTYPE' is defined but its use is discouraged: rather
you should refer to the parser's encapsulated type
- Non POD (Plain Old Data) types cannot be used. C++ forbids any
instance of classes with constructors in unions: only _pointers_ to
such objects are allowed.
Because objects have to be stored via pointers, memory is not
reclaimed automatically: using the '%destructor' directive is the only
means to avoid leaks. *Note Freeing Discarded Symbols: Destructor Decl.
10.1.2.2 C++ Variants
Bison provides a _variant_ based implementation of semantic values for
C++. This alleviates all the limitations reported in the previous
section, and in particular, object types can be used without pointers.
To enable variant-based semantic values, set '%define' variable
'variant' (*note variant: %define Summary.). Once this defined,
'%union' is ignored, and instead of using the name of the fields of the
'%union' to "type" the symbols, use genuine types.
For instance, instead of
int ival;
std::string* sval;
%token NUMBER;
%token STRING;
%token NUMBER;
%token STRING;
'STRING' is no longer a pointer, which should fairly simplify the
user actions in the grammar and in the scanner (in particular the memory
Since C++ features destructors, and since it is customary to
specialize 'operator<<' to support uniform printing of values, variants
also typically simplify Bison printers and destructors.
Variants are stricter than unions. When based on unions, you may
play any dirty game with 'yylval', say storing an 'int', reading a
'char*', and then storing a 'double' in it. This is no longer possible
with variants: they must be initialized, then assigned to, and
eventually, destroyed.
-- Method on semantic_type: T& build ()
Initialize, but leave empty. Returns the address where the actual
value may be stored. Requires that the variant was not initialized
-- Method on semantic_type: T& build (const T& T)
Initialize, and copy-construct from T.
*Warning*: We do not use Boost.Variant, for two reasons. First, it
appeared unacceptable to require Boost on the user's machine (i.e., the
machine on which the generated parser will be compiled, not the machine
on which 'bison' was run). Second, for each possible semantic value,
Boost.Variant not only stores the value, but also a tag specifying its
type. But the parser already "knows" the type of the semantic value, so
that would be duplicating the information.
Therefore we developed light-weight variants whose type tag is
external (so they are really like 'unions' for C++ actually). But our
code is much less mature that Boost.Variant. So there is a number of
limitations in (the current implementation of) variants:
* Alignment must be enforced: values should be aligned in memory
according to the most demanding type. Computing the smallest
alignment possible requires meta-programming techniques that are
not currently implemented in Bison, and therefore, since, as far as
we know, 'double' is the most demanding type on all platforms,
alignments are enforced for 'double' whatever types are actually
used. This may waste space in some cases.
* There might be portability issues we are not aware of.
As far as we know, these limitations _can_ be alleviated. All it
takes is some time and/or some talented C++ hacker willing to contribute
to Bison.
10.1.3 C++ Location Values
When the directive '%locations' is used, the C++ parser supports
location tracking, see *note Tracking Locations::.
By default, two auxiliary classes define a 'position', a single point
in a file, and a 'location', a range composed of a pair of 'position's
(possibly spanning several files). But if the '%define' variable
'api.location.type' is defined, then these classes will not be
generated, and the user defined type will be used.
In this section 'uint' is an abbreviation for 'unsigned int': in
genuine code only the latter is used.
10.1.3.1 C++ 'position'
-- Constructor on position: position (std::string* FILE = 0, uint LINE
= 1, uint COL = 1)
Create a 'position' denoting a given point. Note that 'file' is
not reclaimed when the 'position' is destroyed: memory managed must
be handled elsewhere.
-- Method on position: void initialize (std::string* FILE = 0, uint
LINE = 1, uint COL = 1)
Reset the position to the given values.
-- Instance Variable of position: std::string* file
The name of the file. It will always be handled as a pointer, the
parser will never duplicate nor deallocate it. As an experimental
feature you may change it to 'TYPE*' using '%define filename_type
-- Instance Variable of position: uint line
The line, starting at 1.
-- Method on position: void lines (int HEIGHT = 1)
If HEIGHT is not null, advance by HEIGHT lines, resetting the
column number. The resulting line number cannot be less than 1.
-- Instance Variable of position: uint column
The column, starting at 1.
-- Method on position: void columns (int WIDTH = 1)
Advance by WIDTH columns, without changing the line number. The
resulting column number cannot be less than 1.
-- Method on position: position& operator+= (int WIDTH)
-- Method on position: position operator+ (int WIDTH)
-- Method on position: position& operator-= (int WIDTH)
-- Method on position: position operator- (int WIDTH)
Various forms of syntactic sugar for 'columns'.
-- Method on position: bool operator== (const position& THAT)
-- Method on position: bool operator!= (const position& THAT)
Whether '*this' and 'that' denote equal/different positions.
-- Function: std::ostream& operator<< (std::ostream& O, const position&
Report P on O like this: 'FILE:LINE.COLUMN', or 'LINE.COLUMN' if
FILE is null.
10.1.3.2 C++ 'location'
-- Constructor on location: location (const position& BEGIN, const
position& END)
Create a 'Location' from the endpoints of the range.
-- Constructor on location: location (const position& POS = position())
-- Constructor on location: location (std::string* FILE, uint LINE,
uint COL)
Create a 'Location' denoting an empty range located at a given
-- Method on location: void initialize (std::string* FILE = 0, uint
LINE = 1, uint COL = 1)
Reset the location to an empty range at the given values.
-- Instance Variable of location: position begin
-- Instance Variable of location: position end
The first, inclusive, position of the range, and the first beyond.
-- Method on location: void columns (int WIDTH = 1)
-- Method on location: void lines (int HEIGHT = 1)
Forwarded to the 'end' position.
-- Method on location: location operator+ (const location& END)
-- Method on location: location operator+ (int WIDTH)
-- Method on location: location operator+= (int WIDTH)
-- Method on location: location operator- (int WIDTH)
-- Method on location: location operator-= (int WIDTH)
Various forms of syntactic sugar.
-- Method on location: void step ()
Move 'begin' onto 'end'.
-- Method on location: bool operator== (const location& THAT)
-- Method on location: bool operator!= (const location& THAT)
Whether '*this' and 'that' denote equal/different ranges of
-- Function: std::ostream& operator<< (std::ostream& O, const location&
Report P on O, taking care of special cases such as: no 'filename'
defined, or equal filename/line or column.
10.1.3.3 User Defined Location Type
Instead of using the built-in types you may use the '%define' variable
'api.location.type' to specify your own type:
%define api.location.type {LOCATIONTYPE}
The requirements over your LOCATIONTYPE are:
* it must be copyable;
* in order to compute the (default) value of '@$' in a reduction, the
parser basically runs
@$.begin = @$1.begin;
@$.end = @$N.end; // The location of last right-hand side symbol.
so there must be copyable 'begin' and 'end' members;
* alternatively you may redefine the computation of the default
location, in which case these members are not required (*note
Location Default Action::);
* if traces are enabled, then there must exist an 'std::ostream&
operator<< (std::ostream& o, const LOCATIONTYPE& s)' function.
In programs with several C++ parsers, you may also use the '%define'
variable 'api.location.type' to share a common set of built-in
definitions for 'position' and 'location'. For instance, one parser
'master/parser.yy' might use:
%define api.namespace {master::}
to generate the 'master/position.hh' and 'master/location.hh' files,
reused by other parsers as follows:
%define api.location.type {master::location}
%code requires { #include }
10.1.4 C++ Parser Interface
The output files 'OUTPUT.hh' and 'OUTPUT.cc' declare and define the
parser class in the namespace 'yy'. The class name defaults to
'parser', but may be changed using '%define parser_class_name {NAME}'.
The interface of this class is detailed below. It can be extended using
the '%parse-param' feature: its semantics is slightly changed since it
describes an additional member of the parser class, and an additional
argument for its constructor.
-- Type of parser: semantic_type
-- Type of parser: location_type
The types for semantic values and locations (if enabled).
-- Type of parser: token
A structure that contains (only) the 'yytokentype' enumeration,
which defines the tokens. To refer to the token 'FOO', use
'yy::parser::token::FOO'. The scanner can use 'typedef
yy::parser::token token;' to "import" the token enumeration (*note
Calc++ Scanner::).
-- Type of parser: syntax_error
This class derives from 'std::runtime_error'. Throw instances of
it from the scanner or from the user actions to raise parse errors.
This is equivalent with first invoking 'error' to report the
location and message of the syntax error, and then to invoke
'YYERROR' to enter the error-recovery mode. But contrary to
'YYERROR' which can only be invoked from user actions (i.e.,
written in the action itself), the exception can be thrown from
function invoked from the user action.
-- Method on parser: parser (TYPE1 ARG1, ...)
Build a new parser object. There are no arguments by default,
unless '%parse-param {TYPE1 ARG1}' was used.
-- Method on syntax_error: syntax_error (const location_type& L, const
std::string& M)
-- Method on syntax_error: syntax_error (const std::string& M)
Instantiate a syntax-error exception.
-- Method on parser: int parse ()
Run the syntactic analysis, and return 0 on success, 1 otherwise.
The whole function is wrapped in a 'try'/'catch' block, so that
when an exception is thrown, the '%destructor's are called to
release the lookahead symbol, and the symbols pushed on the stack.
-- Method on parser: std::ostream& debug_stream ()
-- Method on parser: void set_debug_stream (std::ostream& O)
Get or set the stream used for tracing the parsing. It defaults to
-- Method on parser: debug_level_type debug_level ()
-- Method on parser: void set_debug_level (debug_level L)
Get or set the tracing level. Currently its value is either 0, no
trace, or nonzero, full tracing.
-- Method on parser: void error (const location_type& L, const
std::string& M)
-- Method on parser: void error (const std::string& M)
The definition for this member function must be supplied by the
user: the parser uses it to report a parser error occurring at L,
described by M. If location tracking is not enabled, the second
signature is used.
10.1.5 C++ Scanner Interface
The parser invokes the scanner by calling 'yylex'. Contrary to C
parsers, C++ parsers are always pure: there is no point in using the
'%define api.pure' directive. The actual interface with 'yylex' depends
whether you use unions, or variants.
10.1.5.1 Split Symbols
The interface is as follows.
-- Method on parser: int yylex (semantic_type* YYLVAL, location_type*
-- Method on parser: int yylex (semantic_type* YYLVAL, TYPE1 ARG1, ...)
Return the next token. Its type is the return value, its semantic
value and location (if enabled) being YYLVAL and YYLLOC.
Invocations of '%lex-param {TYPE1 ARG1}' yield additional
Note that when using variants, the interface for 'yylex' is the same,
but 'yylval' is handled differently.
Regular union-based code in Lex scanner typically look like:
[0-9]+ {
yylval.ival = text_to_int (yytext);
return yy::parser::INTEGER;
[a-z]+ {
yylval.sval = new std::string (yytext);
return yy::parser::IDENTIFIER;
Using variants, 'yylval' is already constructed, but it is not
initialized. So the code would look like:
[0-9]+ {
yylval.build() = text_to_int (yytext);
return yy::parser::INTEGER;
[a-z]+ {
yylval.build = yytext;
return yy::parser::IDENTIFIER;
[0-9]+ {
yylval.build(text_to_int (yytext));
return yy::parser::INTEGER;
[a-z]+ {
return yy::parser::IDENTIFIER;
10.1.5.2 Complete Symbols
If you specified both '%define api.value.type variant' and '%define
api.token.constructor', the 'parser' class also defines the class
'parser::symbol_type' which defines a _complete_ symbol, aggregating its
type (i.e., the traditional value returned by 'yylex'), its semantic
value (i.e., the value passed in 'yylval', and possibly its location
-- Method on symbol_type: symbol_type (token_type TYPE, const
semantic_type& VALUE, const location_type& LOCATION)
Build a complete terminal symbol which token type is TYPE, and
which semantic value is VALUE. If location tracking is enabled,
also pass the LOCATION.
This interface is low-level and should not be used for two reasons.
First, it is inconvenient, as you still have to build the semantic
value, which is a variant, and second, because consistency is not
enforced: as with unions, it is still possible to give an integer as
semantic value for a string.
So for each token type, Bison generates named constructors as
-- Method on symbol_type: make_ TOKEN (const VALUE_TYPE& VALUE, const
location_type& LOCATION)
-- Method on symbol_type: make_ TOKEN (const location_type& LOCATION)
Build a complete terminal symbol for the token type TOKEN (not
including the 'api.token.prefix') whose possible semantic value is
VALUE of adequate VALUE_TYPE. If location tracking is enabled,
also pass the LOCATION.
For instance, given the following declarations:
%define api.token.prefix {TOK_}
%token IDENTIFIER;
%token INTEGER;
%token COLON;
Bison generates the following functions:
symbol_type make_IDENTIFIER(const std::string& v,
const location_type& l);
symbol_type make_INTEGER(const int& v,
const location_type& loc);
symbol_type make_COLON(const location_type& loc);
which should be used in a Lex-scanner as follows.
[0-9]+ return yy::parser::make_INTEGER(text_to_int (yytext), loc);
[a-z]+ return yy::parser::make_IDENTIFIER(yytext, loc);
":" return yy::parser::make_COLON(loc);
Tokens that do not have an identifier are not accessible: you cannot
simply use characters such as '':'', they must be declared with
10.1.6 A Complete C++ Example
This section demonstrates the use of a C++ parser with a simple but
complete example. This example should be available on your system,
ready to compile, in the directory ".../bison/examples/calc++". It
focuses on the use of Bison, therefore the design of the various C++
classes is very naive: no accessors, no encapsulation of members etc.
We will use a Lex scanner, and more precisely, a Flex scanner, to
demonstrate the various interactions. A hand-written scanner is
actually easier to interface with.
10.1.6.1 Calc++ -- C++ Calculator
Of course the grammar is dedicated to arithmetics, a single expression,
possibly preceded by variable assignments. An environment containing
possibly predefined variables such as 'one' and 'two', is exchanged with
the parser. An example of valid input follows.
three := 3
seven := one + two * three
seven * seven
10.1.6.2 Calc++ Parsing Driver
To support a pure interface with the parser (and the scanner) the
technique of the "parsing context" is convenient: a structure containing
all the data to exchange. Since, in addition to simply launch the
parsing, there are several auxiliary tasks to execute (open the file for
parsing, instantiate the parser etc.), we recommend transforming the
simple parsing context structure into a fully blown "parsing driver"
The declaration of this driver class, 'calc++-driver.hh', is as
follows. The first part includes the CPP guard and imports the required
standard library components, and the declaration of the parser class.
#ifndef CALCXX_DRIVER_HH
# define CALCXX_DRIVER_HH
# include
# include
# include "calc++-parser.hh"
Then comes the declaration of the scanning function. Flex expects the
signature of 'yylex' to be defined in the macro 'YY_DECL', and the C++
parser expects it to be declared. We can factor both as follows.
// Tell Flex the lexer's prototype ...
# define YY_DECL \
yy::calcxx_parser::symbol_type yylex (calcxx_driver& driver)
// ... and declare it for the parser's sake.
The 'calcxx_driver' class is then declared with its most obvious
// Conducting the whole scanning and parsing of Calc++.
class calcxx_driver
calcxx_driver ();
virtual ~calcxx_driver ();
std::map variables;
int result;
To encapsulate the coordination with the Flex scanner, it is useful to
have member functions to open and close the scanning phase.
// Handling the scanner.
void scan_begin ();
void scan_end ();
bool trace_scanning;
Similarly for the parser itself.
// Run the parser on file F.
// Return 0 on success.
int parse (const std::string& f);
// The name of the file being parsed.
// Used later to pass the file name to the location tracker.
std::string file;
// Whether parser traces should be generated.
bool trace_parsing;
To demonstrate pure handling of parse errors, instead of simply dumping
them on the standard error output, we will pass them to the compiler
driver using the following two member functions. Finally, we close the
class declaration and CPP guard.
// Error handling.
void error (const yy::location& l, const std::string& m);
void error (const std::string& m);
#endif // ! CALCXX_DRIVER_HH
The implementation of the driver is straightforward. The 'parse'
member function deserves some attention. The 'error' functions are
simple stubs, they should actually register the located error messages
and set error state.
#include "calc++-driver.hh"
#include "calc++-parser.hh"
calcxx_driver::calcxx_driver ()
: trace_scanning (false), trace_parsing (false)
variables["one"] = 1;
variables["two"] = 2;
calcxx_driver::~calcxx_driver ()
calcxx_driver::parse (const std::string &f)
file = f;
scan_begin ();
yy::calcxx_parser parser (*this);
parser.set_debug_level (trace_parsing);
int res = parser.parse ();
scan_end ();
return res;
calcxx_driver::error (const yy::location& l, const std::string& m)
std::cerr << l << ": " << m << std::endl;
calcxx_driver::error (const std::string& m)
std::cerr << m << std::endl;
10.1.6.3 Calc++ Parser
The grammar file 'calc++-parser.yy' starts by asking for the C++
deterministic parser skeleton, the creation of the parser header file,
and specifies the name of the parser class. Because the C++ skeleton
changed several times, it is safer to require the version you designed
the grammar for.
%skeleton "lalr1.cc" /* -*- C++ -*- */
%require "3.0.2"
%define parser_class_name {calcxx_parser}
This example will use genuine C++ objects as semantic values, therefore,
we require the variant-based interface. To make sure we properly use
it, we enable assertions. To fully benefit from type-safety and more
natural definition of "symbol", we enable 'api.token.constructor'.
%define api.token.constructor
%define api.value.type variant
%define parse.assert
Then come the declarations/inclusions needed by the semantic values.
Because the parser uses the parsing driver and reciprocally, both would
like to include the header of the other, which is, of course, insane.
This mutual dependency will be broken using forward declarations.
Because the driver's header needs detailed knowledge about the parser
class (in particular its inner types), it is the parser's header which
will use a forward declaration of the driver. *Note %code Summary::.
%code requires
# include
class calcxx_driver;
The driver is passed by reference to the parser and to the scanner.
This provides a simple but effective pure interface, not relying on
global variables.
// The parsing context.
%param { calcxx_driver& driver }
Then we request location tracking, and initialize the first location's
file name. Afterward new locations are computed relatively to the
previous locations: the file name will be propagated.
// Initialize the initial location.
@$.begin.filename = @$.end.filename = &driver.file;
Use the following two directives to enable parser tracing and verbose
error messages. However, verbose error messages can contain incorrect
information (*note LAC::).
%define parse.trace
%define parse.error verbose
The code between '%code {' and '}' is output in the '*.cc' file; it
needs detailed knowledge about the driver.
# include "calc++-driver.hh"
The token numbered as 0 corresponds to end of file; the following line
allows for nicer error messages referring to "end of file" instead of
"$end". Similarly user friendly names are provided for each symbol. To
avoid name clashes in the generated files (*note Calc++ Scanner::),
prefix tokens with 'TOK_' (*note api.token.prefix: %define Summary.).
%define api.token.prefix {TOK_}
END 0 "end of file"
Since we use variant-based semantic values, '%union' is not used, and
both '%type' and '%token' expect genuine types, as opposed to type tags.
%token IDENTIFIER "identifier"
%token NUMBER "number"
%type exp
No '%destructor' is needed to enable memory deallocation during error
recovery; the memory, for strings for instance, will be reclaimed by the
regular destructors. All the values are printed using their
'operator<<' (*note Printing Semantic Values: Printer Decl.).
%printer { yyoutput << $$; } <*>;
The grammar itself is straightforward (*note Location Tracking
Calculator: 'ltcalc': Location Tracking Calc.).
%start unit;
unit: assignments exp { driver.result = $2; };
%empty {}
| assignments assignment {};
"identifier" ":=" exp { driver.variables[$1] = $3; };
%left "+" "-";
%left "*" "/";
exp "+" exp { $$ = $1 + $3; }
| exp "-" exp { $$ = $1 - $3; }
| exp "*" exp { $$ = $1 * $3; }
| exp "/" exp { $$ = $1 / $3; }
| "(" exp ")" { std::swap ($$, $2); }
| "identifier" { $$ = driver.variables[$1]; }
| "number" { std::swap ($$, $1); };
Finally the 'error' member function registers the errors to the driver.
yy::calcxx_parser::error (const location_type& l,
const std::string& m)
driver.error (l, m);
10.1.6.4 Calc++ Scanner
The Flex scanner first includes the driver declaration, then the
parser's to get the set of defined tokens.
# include
# include
# include
# include
# include "calc++-driver.hh"
# include "calc++-parser.hh"
// Work around an incompatibility in flex (at least versions
// 2.5.31 through 2.5.33): it generates code that does
// not conform to C89. See Debian bug 333231
// .
# undef yywrap
# define yywrap() 1
// The location of the current token.
static yy::location loc;
Because there is no '#include'-like feature we don't need 'yywrap', we
don't need 'unput' either, and we parse an actual file, this is not an
interactive session with the user. Finally, we enable scanner tracing.
%option noyywrap nounput batch debug noinput
Abbreviations allow for more readable rules.
id [a-zA-Z][a-zA-Z_0-9]*
int [0-9]+
blank [ \t]
The following paragraph suffices to track locations accurately. Each
time 'yylex' is invoked, the begin position is moved onto the end
position. Then when a pattern is matched, its width is added to the end
column. When matching ends of lines, the end cursor is adjusted, and
each time blanks are matched, the begin cursor is moved onto the end
cursor to effectively ignore the blanks preceding tokens. Comments
would be treated equally.
// Code run each time a pattern is matched.
# define YY_USER_ACTION loc.columns (yyleng);
// Code run each time yylex is called.
loc.step ();
{blank}+ loc.step ();
[\n]+ loc.lines (yyleng); loc.step ();
The rules are simple. The driver is used to report errors.
"-" return yy::calcxx_parser::make_MINUS(loc);
"+" return yy::calcxx_parser::make_PLUS(loc);
"*" return yy::calcxx_parser::make_STAR(loc);
"/" return yy::calcxx_parser::make_SLASH(loc);
"(" return yy::calcxx_parser::make_LPAREN(loc);
")" return yy::calcxx_parser::make_RPAREN(loc);
":=" return yy::calcxx_parser::make_ASSIGN(loc);
{int} {
errno = 0;
long n = strtol (yytext, NULL, 10);
if (! (INT_MIN <= n && n <= INT_MAX && errno != ERANGE))
driver.error (loc, "integer is out of range");
return yy::calcxx_parser::make_NUMBER(n, loc);
{id} return yy::calcxx_parser::make_IDENTIFIER(yytext, loc);
. driver.error (loc, "invalid character");
<> return yy::calcxx_parser::make_END(loc);
Finally, because the scanner-related driver's member-functions depend on
the scanner's data, it is simpler to implement them in this file.
calcxx_driver::scan_begin ()
yy_flex_debug = trace_scanning;
if (file.empty () || file == "-")
yyin = stdin;
else if (!(yyin = fopen (file.c_str (), "r")))
error ("cannot open " + file + ": " + strerror(errno));
exit (EXIT_FAILURE);
calcxx_driver::scan_end ()
fclose (yyin);
10.1.6.5 Calc++ Top Level
The top level file, 'calc++.cc', poses no problem.
#include "calc++-driver.hh"
main (int argc, char *argv[])
int res = 0;
calcxx_driver driver;
for (int i = 1; i < argc; ++i)
if (argv[i] == std::string ("-p"))
driver.trace_parsing = true;
else if (argv[i] == std::string ("-s"))
driver.trace_scanning = true;
else if (!driver.parse (argv[i]))
std::cout << driver.result << std::endl;
res = 1;
return res;
10.2 Java Parsers
10.2.1 Java Bison Interface
(The current Java interface is experimental and may evolve. More user
feedback will help to stabilize it.)
The Java parser skeletons are selected using the '%language "Java"'
directive or the '-L java'/'--language=java' option.
When generating a Java parser, 'bison BASENAME.y' will create a
single Java source file named 'BASENAME.java' containing the parser
implementation. Using a grammar file without a '.y' suffix is currently
broken. The basename of the parser implementation file can be changed
by the '%file-prefix' directive or the '-p'/'--name-prefix' option. The
entire parser implementation file name can be changed by the '%output'
directive or the '-o'/'--output' option. The parser implementation file
contains a single class for the parser.
You can create documentation for generated parsers using Javadoc.
Contrary to C parsers, Java parsers do not use global variables; the
state of the parser is always local to an instance of the parser class.
Therefore, all Java parsers are "pure", and the '%pure-parser' and
'%define api.pure' directives do nothing when used in Java.
Push parsers are currently unsupported in Java and '%define
api.push-pull' have no effect.
GLR parsers are currently unsupported in Java. Do not use the
'glr-parser' directive.
No header file can be generated for Java parsers. Do not use the
'%defines' directive or the '-d'/'--defines' options.
Currently, support for tracing is always compiled in. Thus the
'%define parse.trace' and '%token-table' directives and the
'-t'/'--debug' and '-k'/'--token-table' options have no effect. This
may change in the future to eliminate unused code in the generated
parser, so use '%define parse.trace' explicitly if needed. Also, in the
future the '%token-table' directive might enable a public interface to
access the token names and codes.
Getting a "code too large" error from the Java compiler means the
code hit the 64KB bytecode per method limitation of the Java class file.
Try reducing the amount of code in actions and static initializers;
otherwise, report a bug so that the parser skeleton will be improved.
10.2.2 Java Semantic Values
There is no '%union' directive in Java parsers. Instead, the semantic
values' types (class names) should be specified in the '%type' or
'%token' directive:
%type expr assignment_expr term factor
%type number
By default, the semantic stack is declared to have 'Object' members,
which means that the class types you specify can be of any class. To
improve the type safety of the parser, you can declare the common
superclass of all the semantic values using the '%define api.value.type'
directive. For example, after the following declaration:
%define api.value.type {ASTNode}
any '%type' or '%token' specifying a semantic type which is not a
subclass of ASTNode, will cause a compile-time error.
Types used in the directives may be qualified with a package name.
Primitive data types are accepted for Java version 1.5 or later. Note
that in this case the autoboxing feature of Java 1.5 will be used.
Generic types may not be used; this is due to a limitation in the
implementation of Bison, and may change in future releases.
Java parsers do not support '%destructor', since the language adopts
garbage collection. The parser will try to hold references to semantic
values for as little time as needed.
Java parsers do not support '%printer', as 'toString()' can be used
to print the semantic values. This however may change (in a
backwards-compatible way) in future versions of Bison.
10.2.3 Java Location Values
When the directive '%locations' is used, the Java parser supports
location tracking, see *note Tracking Locations::. An auxiliary
user-defined class defines a "position", a single point in a file; Bison
itself defines a class representing a "location", a range composed of a
pair of positions (possibly spanning several files). The location class
is an inner class of the parser; the name is 'Location' by default, and
may also be renamed using '%define api.location.type {CLASS-NAME}'.
The location class treats the position as a completely opaque value.
By default, the class name is 'Position', but this can be changed with
'%define api.position.type {CLASS-NAME}'. This class must be supplied
by the user.
-- Instance Variable of Location: Position begin
-- Instance Variable of Location: Position end
The first, inclusive, position of the range, and the first beyond.
-- Constructor on Location: Location (Position LOC)
Create a 'Location' denoting an empty range located at a given
-- Constructor on Location: Location (Position BEGIN, Position END)
Create a 'Location' from the endpoints of the range.
-- Method on Location: String toString ()
Prints the range represented by the location. For this to work
properly, the position class should override the 'equals' and
'toString' methods appropriately.
10.2.4 Java Parser Interface
The name of the generated parser class defaults to 'YYParser'. The 'YY'
prefix may be changed using the '%name-prefix' directive or the
'-p'/'--name-prefix' option. Alternatively, use '%define
parser_class_name {NAME}' to give a custom name to the class. The
interface of this class is detailed below.
By default, the parser class has package visibility. A declaration
'%define public' will change to public visibility. Remember that,
according to the Java language specification, the name of the '.java'
file should match the name of the class in this case. Similarly, you
can use 'abstract', 'final' and 'strictfp' with the '%define'
declaration to add other modifiers to the parser class. A single
'%define annotations {ANNOTATIONS}' directive can be used to add any
number of annotations to the parser class.
The Java package name of the parser class can be specified using the
'%define package' directive. The superclass and the implemented
interfaces of the parser class can be specified with the '%define
extends' and '%define implements' directives.
The parser class defines an inner class, 'Location', that is used for
location tracking (see *note Java Location Values::), and a inner
interface, 'Lexer' (see *note Java Scanner Interface::). Other than
these inner class/interface, and the members described in the interface
below, all the other members and fields are preceded with a 'yy' or 'YY'
prefix to avoid clashes with user code.
The parser class can be extended using the '%parse-param' directive.
Each occurrence of the directive will add a 'protected final' field to
the parser class, and an argument to its constructor, which initialize
them automatically.
-- Constructor on YYParser: YYParser (LEX_PARAM, ..., PARSE_PARAM, ...)
Build a new parser object with embedded '%code lexer'. There are
no parameters, unless '%param's and/or '%parse-param's and/or
'%lex-param's are used.
Use '%code init' for code added to the start of the constructor
body. This is especially useful to initialize superclasses. Use
'%define init_throws' to specify any uncaught exceptions.
-- Constructor on YYParser: YYParser (Lexer LEXER, PARSE_PARAM, ...)
Build a new parser object using the specified scanner. There are
no additional parameters unless '%param's and/or '%parse-param's
are used.
If the scanner is defined by '%code lexer', this constructor is
declared 'protected' and is called automatically with a scanner
created with the correct '%param's and/or '%lex-param's.
Use '%code init' for code added to the start of the constructor
body. This is especially useful to initialize superclasses. Use
'%define init_throws' to specify any uncaught exceptions.
-- Method on YYParser: boolean parse ()
Run the syntactic analysis, and return 'true' on success, 'false'
-- Method on YYParser: boolean getErrorVerbose ()
-- Method on YYParser: void setErrorVerbose (boolean VERBOSE)
Get or set the option to produce verbose error messages. These are
only available with '%define parse.error verbose', which also turns
on verbose error messages.
-- Method on YYParser: void yyerror (String MSG)
-- Method on YYParser: void yyerror (Position POS, String MSG)
-- Method on YYParser: void yyerror (Location LOC, String MSG)
Print an error message using the 'yyerror' method of the scanner
instance in use. The 'Location' and 'Position' parameters are
available only if location tracking is active.
-- Method on YYParser: boolean recovering ()
During the syntactic analysis, return 'true' if recovering from a
syntax error. *Note Error Recovery::.
-- Method on YYParser: java.io.PrintStream getDebugStream ()
-- Method on YYParser: void setDebugStream (java.io.printStream O)
Get or set the stream used for tracing the parsing. It defaults to
-- Method on YYParser: int getDebugLevel ()
-- Method on YYParser: void setDebugLevel (int L)
Get or set the tracing level. Currently its value is either 0, no
trace, or nonzero, full tracing.
-- Constant of YYParser: String bisonVersion
-- Constant of YYParser: String bisonSkeleton
Identify the Bison version and skeleton used to generate this
10.2.5 Java Scanner Interface
There are two possible ways to interface a Bison-generated Java parser
with a scanner: the scanner may be defined by '%code lexer', or defined
elsewhere. In either case, the scanner has to implement the 'Lexer'
inner interface of the parser class. This interface also contain
constants for all user-defined token names and the predefined 'EOF'
In the first case, the body of the scanner class is placed in '%code
lexer' blocks. If you want to pass parameters from the parser
constructor to the scanner constructor, specify them with '%lex-param';
they are passed before '%parse-param's to the constructor.
In the second case, the scanner has to implement the 'Lexer'
interface, which is defined within the parser class (e.g.,
'YYParser.Lexer'). The constructor of the parser object will then
accept an object implementing the interface; '%lex-param' is not used in
this case.
In both cases, the scanner has to implement the following methods.
-- Method on Lexer: void yyerror (Location LOC, String MSG)
This method is defined by the user to emit an error message. The
first parameter is omitted if location tracking is not active. Its
type can be changed using '%define api.location.type {CLASS-NAME}'.
-- Method on Lexer: int yylex ()
Return the next token. Its type is the return value, its semantic
value and location are saved and returned by the their methods in
the interface.
Use '%define lex_throws' to specify any uncaught exceptions.
Default is 'java.io.IOException'.
-- Method on Lexer: Position getStartPos ()
-- Method on Lexer: Position getEndPos ()
Return respectively the first position of the last token that
'yylex' returned, and the first position beyond it. These methods
are not needed unless location tracking is active.
The return type can be changed using '%define api.position.type
-- Method on Lexer: Object getLVal ()
Return the semantic value of the last token that yylex returned.
The return type can be changed using '%define api.value.type
10.2.6 Special Features for Use in Java Actions
The following special constructs can be uses in Java actions. Other
analogous C action features are currently unavailable for Java.
Use '%define throws' to specify any uncaught exceptions from parser
actions, and initial actions specified by '%initial-action'.
-- Variable: $ N
The semantic value for the Nth component of the current rule. This
may not be assigned to. *Note Java Semantic Values::.
-- Variable: $< TYPEALT>N
Like '$N' but specifies a alternative type TYPEALT. *Note Java
Semantic Values::.
-- Variable: $$
The semantic value for the grouping made by the current rule. As a
value, this is in the base type ('Object' or as specified by
'%define api.value.type') as in not cast to the declared subtype
because casts are not allowed on the left-hand side of Java
assignments. Use an explicit Java cast if the correct subtype is
needed. *Note Java Semantic Values::.
-- Variable: $< TYPEALT>$
Same as '$$' since Java always allow assigning to the base type.
Perhaps we should use this and '$<>$' for the value and '$$' for
setting the value but there is currently no easy way to distinguish
these constructs. *Note Java Semantic Values::.
-- Variable: @ N
The location information of the Nth component of the current rule.
This may not be assigned to. *Note Java Location Values::.
-- Variable: @ $
The location information of the grouping made by the current rule.
*Note Java Location Values::.
-- Statement: return YYABORT ';'
Return immediately from the parser, indicating failure. *Note Java
Parser Interface::.
-- Statement: return YYACCEPT ';'
Return immediately from the parser, indicating success. *Note Java
Parser Interface::.
-- Statement: return YYERROR ';'
Start error recovery (without printing an error message). *Note
Error Recovery::.
-- Function: boolean recovering ()
Return whether error recovery is being done. In this state, the
parser reads token until it reaches a known state, and then
restarts normal operation. *Note Error Recovery::.
-- Function: void yyerror (String MSG)
-- Function: void yyerror (Position LOC, String MSG)
-- Function: void yyerror (Location LOC, String MSG)
Print an error message using the 'yyerror' method of the scanner
instance in use. The 'Location' and 'Position' parameters are
available only if location tracking is active.
10.2.7 Java Push Parser Interface
(The current push parsing interface is experimental and may evolve.
More user feedback will help to stabilize it.)
Normally, Bison generates a pull parser for Java. The following
Bison declaration says that you want the parser to be a push parser
(*note api.push-pull: %define Summary.):
%define api.push-pull push
Most of the discussion about the Java pull Parser Interface, (*note
Java Parser Interface::) applies to the push parser interface as well.
When generating a push parser, the method 'push_parse' is created
with the following signature (depending on if locations are enabled).
-- Method on YYParser: void push_parse (int TOKEN, Object YYLVAL)
-- Method on YYParser: void push_parse (int TOKEN, Object YYLVAL,
Location YYLOC)
-- Method on YYParser: void push_parse (int TOKEN, Object YYLVAL,
Position YYPOS)
The primary difference with respect to a pull parser is that the
parser method 'push_parse' is invoked repeatedly to parse each token.
This function is available if either the "%define api.push-pull push" or
"%define api.push-pull both" declaration is used (*note api.push-pull:
%define Summary.). The 'Location' and 'Position' parameters are
available only if location tracking is active.
The value returned by the 'push_parse' method is one of the following
four constants: 'YYABORT', 'YYACCEPT', 'YYERROR', or 'YYPUSH_MORE'.
This new value, 'YYPUSH_MORE', may be returned if more input is required
to finish parsing the grammar.
If api.push-pull is declared as 'both', then the generated parser
class will also implement the 'parse' method. This method's body is a
loop that repeatedly invokes the scanner and then passes the values
obtained from the scanner to the 'push_parse' method.
There is one additional complication. Technically, the push parser
does not need to know about the scanner (i.e. an object implementing
the 'YYParser.Lexer' interface), but it does need access to the
'yyerror' method. Currently, the 'yyerror' method is defined in the
'YYParser.Lexer' interface. Hence, an implementation of that interface
is still required in order to provide an implementation of 'yyerror'.
The current approach (and subject to change) is to require the
'YYParser' constructor to be given an object implementing the
'YYParser.Lexer' interface. This object need only implement the
'yyerror' method; the other methods can be stubbed since they will never
be invoked. The simplest way to do this is to add a trivial scanner
implementation to your grammar file using whatever implementation of
'yyerror' is desired. The following code sample shows a simple way to
accomplish this.
%code lexer
public Object getLVal () {return null;}
public int yylex () {return 0;}
public void yyerror (String s) {System.err.println(s);}
10.2.8 Differences between C/C++ and Java Grammars
The different structure of the Java language forces several differences
between C/C++ grammars, and grammars designed for Java parsers. This
section summarizes these differences.
* Java lacks a preprocessor, so the 'YYERROR', 'YYACCEPT', 'YYABORT'
symbols (*note Table of Symbols::) cannot obviously be macros.
Instead, they should be preceded by 'return' when they appear in an
action. The actual definition of these symbols is opaque to the
Bison grammar, and it might change in the future. The only
meaningful operation that you can do, is to return them. *Note
Java Action Features::.
Note that of these three symbols, only 'YYACCEPT' and 'YYABORT'
will cause a return from the 'yyparse' method(1).
* Java lacks unions, so '%union' has no effect. Instead, semantic
values have a common base type: 'Object' or as specified by
'%define api.value.type'. Angle brackets on '%token', 'type', '$N'
and '$$' specify subtypes rather than fields of an union. The type
of '$$', even with angle brackets, is the base type since Java
casts are not allow on the left-hand side of assignments. Also,
'$N' and '@N' are not allowed on the left-hand side of assignments.
*Note Java Semantic Values::, and *note Java Action Features::.
* The prologue declarations have a different meaning than in C/C++
'%code imports'
blocks are placed at the beginning of the Java source code.
They may include copyright notices. For a 'package'
declarations, it is suggested to use '%define package'
unqualified '%code'
blocks are placed inside the parser class.
'%code lexer'
blocks, if specified, should include the implementation of the
scanner. If there is no such block, the scanner can be any
class that implements the appropriate interface (*note Java
Scanner Interface::).
Other '%code' blocks are not supported in Java parsers. In
particular, '%{ ... %}' blocks should not be used and may give an
error in future versions of Bison.
The epilogue has the same meaning as in C/C++ code and it can be
used to define other classes used by the parser _outside_ the
parser class.
---------- Footnotes ----------
(1) Java parsers include the actions in a separate method than
'yyparse' in order to have an intuitive syntax that corresponds to these
C macros.
10.2.9 Java Declarations Summary
This summary only include declarations specific to Java or have special
meaning when used in a Java parser.
-- Directive: %language "Java"
Generate a Java class for the parser.
-- Directive: %lex-param {TYPE NAME}
A parameter for the lexer class defined by '%code lexer' _only_,
added as parameters to the lexer constructor and the parser
constructor that _creates_ a lexer. Default is none. *Note Java
Scanner Interface::.
-- Directive: %name-prefix "PREFIX"
The prefix of the parser class name 'PREFIXParser' if '%define
parser_class_name' is not used. Default is 'YY'. *Note Java Bison
-- Directive: %parse-param {TYPE NAME}
A parameter for the parser class added as parameters to
constructor(s) and as fields initialized by the constructor(s).
Default is none. *Note Java Parser Interface::.
-- Directive: %token TOKEN ...
Declare tokens. Note that the angle brackets enclose a Java
_type_. *Note Java Semantic Values::.
-- Directive: %type NONTERMINAL ...
Declare the type of nonterminals. Note that the angle brackets
enclose a Java _type_. *Note Java Semantic Values::.
-- Directive: %code { CODE ... }
Code appended to the inside of the parser class. *Note Java
-- Directive: %code imports { CODE ... }
Code inserted just after the 'package' declaration. *Note Java
-- Directive: %code init { CODE ... }
Code inserted at the beginning of the parser constructor body.
*Note Java Parser Interface::.
-- Directive: %code lexer { CODE ... }
Code added to the body of a inner lexer class within the parser
class. *Note Java Scanner Interface::.
-- Directive: %% CODE ...
Code (after the second '%%') appended to the end of the file,
_outside_ the parser class. *Note Java Differences::.
-- Directive: % { CODE ... %}
Not supported. Use '%code imports' instead. *Note Java
-- Directive: %define abstract
Whether the parser class is declared 'abstract'. Default is false.
*Note Java Bison Interface::.
-- Directive: %define annotations {ANNOTATIONS}
The Java annotations for the parser class. Default is none. *Note
Java Bison Interface::.
-- Directive: %define extends {SUPERCLASS}
The superclass of the parser class. Default is none. *Note Java
Bison Interface::.
-- Directive: %define final
Whether the parser class is declared 'final'. Default is false.
*Note Java Bison Interface::.
-- Directive: %define implements {INTERFACES}
The implemented interfaces of the parser class, a comma-separated
list. Default is none. *Note Java Bison Interface::.
-- Directive: %define init_throws {EXCEPTIONS}
The exceptions thrown by '%code init' from the parser class
constructor. Default is none. *Note Java Parser Interface::.
-- Directive: %define lex_throws {EXCEPTIONS}
The exceptions thrown by the 'yylex' method of the lexer, a
comma-separated list. Default is 'java.io.IOException'. *Note
Java Scanner Interface::.
-- Directive: %define api.location.type {CLASS}
The name of the class used for locations (a range between two
positions). This class is generated as an inner class of the
parser class by 'bison'. Default is 'Location'. Formerly named
'location_type'. *Note Java Location Values::.
-- Directive: %define package {PACKAGE}
The package to put the parser class in. Default is none. *Note
Java Bison Interface::.
-- Directive: %define parser_class_name {NAME}
The name of the parser class. Default is 'YYParser' or
'NAME-PREFIXParser'. *Note Java Bison Interface::.
-- Directive: %define api.position.type {CLASS}
The name of the class used for positions. This class must be
supplied by the user. Default is 'Position'. Formerly named
'position_type'. *Note Java Location Values::.
-- Directive: %define public
Whether the parser class is declared 'public'. Default is false.
*Note Java Bison Interface::.
-- Directive: %define api.value.type {CLASS}
The base type of semantic values. Default is 'Object'. *Note Java
Semantic Values::.
-- Directive: %define strictfp
Whether the parser class is declared 'strictfp'. Default is false.
*Note Java Bison Interface::.
-- Directive: %define throws {EXCEPTIONS}
The exceptions thrown by user-supplied parser actions and
'%initial-action', a comma-separated list. Default is none. *Note
Java Parser Interface::.
11 Frequently Asked Questions
Several questions about Bison come up occasionally. Here some of them
are addressed.
11.1 Memory Exhausted
My parser returns with error with a 'memory exhausted' message.
What can I do?
This question is already addressed elsewhere, see *note Recursive
Rules: Recursion.
11.2 How Can I Reset the Parser
The following phenomenon has several symptoms, resulting in the
following typical questions:
I invoke 'yyparse' several times, and on correct input it works
properly; but when a parse error is found, all the other calls fail
too. How can I reset the error flag of 'yyparse'?
My parser includes support for an '#include'-like feature, in which
case I run 'yyparse' from 'yyparse'. This fails although I did
specify '%define api.pure full'.
These problems typically come not from Bison itself, but from
Lex-generated scanners. Because these scanners use large buffers for
speed, they might not notice a change of input file. As a
demonstration, consider the following source file, 'first-line.l':
.*\n ECHO; return 1;
yyparse (char const *file)
yyin = fopen (file, "r");
if (!yyin)
perror ("fopen");
exit (EXIT_FAILURE);
/* One token only. */
yylex ();
if (fclose (yyin) != 0)
perror ("fclose");
exit (EXIT_FAILURE);
return 0;
main (void)
yyparse ("input");
yyparse ("input");
return 0;
If the file 'input' contains
input:1: Hello,
input:2: World!
then instead of getting the first line twice, you get:
$ flex -ofirst-line.c first-line.l
$ gcc -ofirst-line first-line.c -ll
$ ./first-line
input:1: Hello,
input:2: World!
Therefore, whenever you change 'yyin', you must tell the
Lex-generated scanner to discard its current buffer and switch to the
new one. This depends upon your implementation of Lex; see its
documentation for more. For Flex, it suffices to call 'YY_FLUSH_BUFFER'
after each change to 'yyin'. If your Flex-generated scanner needs to
read from several input streams to handle features like include files,
you might consider using Flex functions like 'yy_switch_to_buffer' that
manipulate multiple input buffers.
If your Flex-generated scanner uses start conditions (*note Start
conditions: (flex)Start conditions.), you might also want to reset the
scanner's state, i.e., go back to the initial start condition, through a
call to 'BEGIN (0)'.
11.3 Strings are Destroyed
My parser seems to destroy old strings, or maybe it loses track of
them. Instead of reporting '"foo", "bar"', it reports '"bar",
"bar"', or even '"foo\nbar", "bar"'.
This error is probably the single most frequent "bug report" sent to
Bison lists, but is only concerned with a misunderstanding of the role
of the scanner. Consider the following Lex code:
char *yylval = NULL;
.* yylval = yytext; return 1;
\n /* IGNORE */
main ()
/* Similar to using $1, $2 in a Bison action. */
char *fst = (yylex (), yylval);
char *snd = (yylex (), yylval);
printf ("\"%s\", \"%s\"\n", fst, snd);
return 0;
If you compile and run this code, you get:
$ flex -osplit-lines.c split-lines.l
$ gcc -osplit-lines split-lines.c -ll
$ printf 'one\ntwo\n' | ./split-lines
two", "two"
this is because 'yytext' is a buffer provided for _reading_ in the
action, but if you want to keep it, you have to duplicate it (e.g.,
using 'strdup'). Note that the output may depend on how your
implementation of Lex handles 'yytext'. For instance, when given the
Lex compatibility option '-l' (which triggers the option '%array') Flex
generates a different behavior:
$ flex -l -osplit-lines.c split-lines.l
$ gcc -osplit-lines split-lines.c -ll
$ printf 'one\ntwo\n' | ./split-lines
"two", "two"
11.4 Implementing Gotos/Loops
My simple calculator supports variables, assignments, and
functions, but how can I implement gotos, or loops?
Although very pedagogical, the examples included in the document blur
the distinction to make between the parser--whose job is to recover the
structure of a text and to transmit it to subsequent modules of the
program--and the processing (such as the execution) of this structure.
This works well with so called straight line programs, i.e., precisely
those that have a straightforward execution model: execute simple
instructions one after the others.
If you want a richer model, you will probably need to use the parser
to construct a tree that does represent the structure it has recovered;
this tree is usually called the "abstract syntax tree", or "AST" for
short. Then, walking through this tree, traversing it in various ways,
will enable treatments such as its execution or its translation, which
will result in an interpreter or a compiler.
This topic is way beyond the scope of this manual, and the reader is
invited to consult the dedicated literature.
11.5 Multiple start-symbols
I have several closely related grammars, and I would like to share
their implementations. In fact, I could use a single grammar but
with multiple entry points.
Bison does not support multiple start-symbols, but there is a very
simple means to simulate them. If 'foo' and 'bar' are the two pseudo
start-symbols, then introduce two new tokens, say 'START_FOO' and
'START_BAR', and use them as switches from the real start-symbol:
%token START_FOO START_BAR;
%start start;
START_FOO foo
| START_BAR bar;
These tokens prevents the introduction of new conflicts. As far as
the parser goes, that is all that is needed.
Now the difficult part is ensuring that the scanner will send these
tokens first. If your scanner is hand-written, that should be
straightforward. If your scanner is generated by Lex, them there is
simple means to do it: recall that anything between '%{ ... %}' after
the first '%%' is copied verbatim in the top of the generated 'yylex'
function. Make sure a variable 'start_token' is available in the
scanner (e.g., a global variable or using '%lex-param' etc.), and use
the following:
/* Prologue. */
if (start_token)
int t = start_token;
start_token = 0;
return t;
/* The rules. */
11.6 Secure? Conform?
Is Bison secure? Does it conform to POSIX?
If you're looking for a guarantee or certification, we don't provide
it. However, Bison is intended to be a reliable program that conforms
to the POSIX specification for Yacc. If you run into problems, please
send us a bug report.
11.7 I can't build Bison
I can't build Bison because 'make' complains that 'msgfmt' is not
found. What should I do?
Like most GNU packages with internationalization support, that
feature is turned on by default. If you have problems building in the
'po' subdirectory, it indicates that your system's internationalization
support is lacking. You can re-configure Bison with '--disable-nls' to
turn off this support, or you can install GNU gettext from
and re-configure Bison. See the file
'ABOUT-NLS' for more information.
11.8 Where can I find help?
I'm having trouble using Bison. Where can I find help?
First, read this fine manual. Beyond that, you can send mail to
. This mailing list is intended to be populated
with people who are willing to answer questions about using and
installing Bison. Please keep in mind that (most of) the people on the
list have aspects of their lives which are not related to Bison (!), so
you may not receive an answer to your question right away. This can be
frustrating, but please try not to honk them off; remember that any help
they provide is purely voluntary and out of the kindness of their
11.9 Bug Reports
I found a bug. What should I include in the bug report?
Before you send a bug report, make sure you are using the latest
version. Check or one of its
mirrors. Be sure to include the version number in your bug report. If
the bug is present in the latest version but not in a previous version,
try to determine the most recent version which did not contain the bug.
If the bug is parser-related, you should include the smallest grammar
you can which demonstrates the bug. The grammar file should also be
complete (i.e., I should be able to run it through Bison without having
to edit or add anything). The smaller and simpler the grammar, the
easier it will be to fix the bug.
Include information about your compilation environment, including
your operating system's name and version and your compiler's name and
version. If you have trouble compiling, you should also include a
transcript of the build session, starting with the invocation of
'configure'. Depending on the nature of the bug, you may be asked to
send additional files as well (such as 'config.h' or 'config.cache').
Patches are most welcome, but not required. That is, do not hesitate
to send a bug report just because you cannot provide a fix.
Send bug reports to .
11.10 More Languages
Will Bison ever have C++ and Java support? How about INSERT YOUR
C++ and Java support is there now, and is documented. We'd love to
add other languages; contributions are welcome.
11.11 Beta Testing
What is involved in being a beta tester?
It's not terribly involved. Basically, you would download a test
release, compile it, and use it to build and run a parser or two. After
that, you would submit either a bug report or a message saying that
everything is okay. It is important to report successes as well as
failures because test releases eventually become mainstream releases,
but only if they are adequately tested. If no one tests, development is
essentially halted.
Beta testers are particularly needed for operating systems to which
the developers do not have easy access. They currently have easy access
to recent GNU/Linux and Solaris versions. Reports about other operating
systems are especially welcome.
11.12 Mailing Lists
How do I join the help-bison and bug-bison mailing lists?
See .
Appendix A Bison Symbols
-- Variable: @ $
In an action, the location of the left-hand side of the rule.
*Note Tracking Locations::.
-- Variable: @ N
-- Symbol: @ N
In an action, the location of the N-th symbol of the right-hand
side of the rule. *Note Tracking Locations::.
In a grammar, the Bison-generated nonterminal symbol for a mid-rule
action with a semantical value. *Note Mid-Rule Action
-- Variable: @ NAME
-- Variable: @ [NAME]
In an action, the location of a symbol addressed by NAME. *Note
Tracking Locations::.
-- Symbol: $ @N
In a grammar, the Bison-generated nonterminal symbol for a mid-rule
action with no semantical value. *Note Mid-Rule Action
-- Variable: $$
In an action, the semantic value of the left-hand side of the rule.
*Note Actions::.
-- Variable: $ N
In an action, the semantic value of the N-th symbol of the
right-hand side of the rule. *Note Actions::.
-- Variable: $ NAME
-- Variable: $[ NAME]
In an action, the semantic value of a symbol addressed by NAME.
*Note Actions::.
-- Delimiter: %%
Delimiter used to separate the grammar rule section from the Bison
declarations section or the epilogue. *Note The Overall Layout of
a Bison Grammar: Grammar Layout.
-- Delimiter: % {CODE%}
All code listed between '%{' and '%}' is copied verbatim to the
parser implementation file. Such code forms the prologue of the
grammar file. *Note Outline of a Bison Grammar: Grammar Outline.
-- Directive: %? {EXPRESSION}
Predicate actions. This is a type of action clause that may appear
in rules. The expression is evaluated, and if false, causes a
syntax error. In GLR parsers during nondeterministic operation,
this silently causes an alternative parse to die. During
deterministic operation, it is the same as the effect of YYERROR.
*Note Semantic Predicates::.
This feature is experimental. More user feedback will help to
determine whether it should become a permanent feature.
-- Construct: /* ... */
-- Construct: // ...
Comments, as in C/C++.
-- Delimiter: :
Separates a rule's result from its components. *Note Syntax of
Grammar Rules: Rules.
-- Delimiter: ;
Terminates a rule. *Note Syntax of Grammar Rules: Rules.
-- Delimiter: |
Separates alternate rules for the same result nonterminal. *Note
Syntax of Grammar Rules: Rules.
-- Directive: <*>
Used to define a default tagged '%destructor' or default tagged
This feature is experimental. More user feedback will help to
determine whether it should become a permanent feature.
*Note Freeing Discarded Symbols: Destructor Decl.
-- Directive: <>
Used to define a default tagless '%destructor' or default tagless
This feature is experimental. More user feedback will help to
determine whether it should become a permanent feature.
*Note Freeing Discarded Symbols: Destructor Decl.
-- Symbol: $accept
The predefined nonterminal whose only rule is '$accept: START
$end', where START is the start symbol. *Note The Start-Symbol:
Start Decl. It cannot be used in the grammar.
-- Directive: %code {CODE}
-- Directive: %code QUALIFIER {CODE}
Insert CODE verbatim into the output parser source at the default
location or at the location specified by QUALIFIER. *Note %code
-- Directive: %debug
Equip the parser for debugging. *Note Decl Summary::.
-- Directive: %define VARIABLE
-- Directive: %define VARIABLE VALUE
-- Directive: %define VARIABLE {VALUE}
-- Directive: %define VARIABLE "VALUE"
Define a variable to adjust Bison's behavior. *Note %define
-- Directive: %defines
Bison declaration to create a parser header file, which is usually
meant for the scanner. *Note Decl Summary::.
-- Directive: %defines DEFINES-FILE
Same as above, but save in the file DEFINES-FILE. *Note Decl
-- Directive: %destructor
Specify how the parser should reclaim the memory associated to
discarded symbols. *Note Freeing Discarded Symbols: Destructor
-- Directive: %dprec
Bison declaration to assign a precedence to a rule that is used at
parse time to resolve reduce/reduce conflicts. *Note Writing GLR
Parsers: GLR Parsers.
-- Directive: %empty
Bison declaration to declare make explicit that a rule has an empty
right-hand side. *Note Empty Rules::.
-- Symbol: $end
The predefined token marking the end of the token stream. It
cannot be used in the grammar.
-- Symbol: error
A token name reserved for error recovery. This token may be used
in grammar rules so as to allow the Bison parser to recognize an
error in the grammar without halting the process. In effect, a
sentence containing an error may be recognized as valid. On a
syntax error, the token 'error' becomes the current lookahead
token. Actions corresponding to 'error' are then executed, and the
lookahead token is reset to the token that originally caused the
violation. *Note Error Recovery::.
-- Directive: %error-verbose
An obsolete directive standing for '%define parse.error verbose'
(*note The Error Reporting Function 'yyerror': Error Reporting.).
-- Directive: %file-prefix "PREFIX"
Bison declaration to set the prefix of the output files. *Note
Decl Summary::.
-- Directive: %glr-parser
Bison declaration to produce a GLR parser. *Note Writing GLR
Parsers: GLR Parsers.
-- Directive: %initial-action
Run user code before parsing. *Note Performing Actions before
Parsing: Initial Action Decl.
-- Directive: %language
Specify the programming language for the generated parser. *Note
Decl Summary::.
-- Directive: %left
Bison declaration to assign precedence and left associativity to
token(s). *Note Operator Precedence: Precedence Decl.
-- Directive: %lex-param {ARGUMENT-DECLARATION} ...
Bison declaration to specifying additional arguments that 'yylex'
should accept. *Note Calling Conventions for Pure Parsers: Pure
-- Directive: %merge
Bison declaration to assign a merging function to a rule. If there
is a reduce/reduce conflict with a rule having the same merging
function, the function is applied to the two semantic values to get
a single result. *Note Writing GLR Parsers: GLR Parsers.
-- Directive: %name-prefix "PREFIX"
Obsoleted by the '%define' variable 'api.prefix' (*note Multiple
Parsers in the Same Program: Multiple Parsers.).
Rename the external symbols (variables and functions) used in the
parser so that they start with PREFIX instead of 'yy'. Contrary to
'api.prefix', do no rename types and macros.
The precise list of symbols renamed in C parsers is 'yyparse',
'yylex', 'yyerror', 'yynerrs', 'yylval', 'yychar', 'yydebug', and
(if locations are used) 'yylloc'. If you use a push parser,
'yypush_parse', 'yypull_parse', 'yypstate', 'yypstate_new' and
'yypstate_delete' will also be renamed. For example, if you use
'%name-prefix "c_"', the names become 'c_parse', 'c_lex', and so
on. For C++ parsers, see the '%define api.namespace' documentation
in this section.
-- Directive: %no-lines
Bison declaration to avoid generating '#line' directives in the
parser implementation file. *Note Decl Summary::.
-- Directive: %nonassoc
Bison declaration to assign precedence and nonassociativity to
token(s). *Note Operator Precedence: Precedence Decl.
-- Directive: %output "FILE"
Bison declaration to set the name of the parser implementation
file. *Note Decl Summary::.
-- Directive: %param {ARGUMENT-DECLARATION} ...
Bison declaration to specify additional arguments that both 'yylex'
and 'yyparse' should accept. *Note The Parser Function 'yyparse':
Parser Function.
-- Directive: %parse-param {ARGUMENT-DECLARATION} ...
Bison declaration to specify additional arguments that 'yyparse'
should accept. *Note The Parser Function 'yyparse': Parser
-- Directive: %prec
Bison declaration to assign a precedence to a specific rule. *Note
Context-Dependent Precedence: Contextual Precedence.
-- Directive: %precedence
Bison declaration to assign precedence to token(s), but no
associativity *Note Operator Precedence: Precedence Decl.
-- Directive: %pure-parser
Deprecated version of '%define api.pure' (*note api.pure: %define
Summary.), for which Bison is more careful to warn about
unreasonable usage.
-- Directive: %require "VERSION"
Require version VERSION or higher of Bison. *Note Require a
Version of Bison: Require Decl.
-- Directive: %right
Bison declaration to assign precedence and right associativity to
token(s). *Note Operator Precedence: Precedence Decl.
-- Directive: %skeleton
Specify the skeleton to use; usually for development. *Note Decl
-- Directive: %start
Bison declaration to specify the start symbol. *Note The
Start-Symbol: Start Decl.
-- Directive: %token
Bison declaration to declare token(s) without specifying
precedence. *Note Token Type Names: Token Decl.
-- Directive: %token-table
Bison declaration to include a token name table in the parser
implementation file. *Note Decl Summary::.
-- Directive: %type
Bison declaration to declare nonterminals. *Note Nonterminal
Symbols: Type Decl.
-- Symbol: $undefined
The predefined token onto which all undefined values returned by
'yylex' are mapped. It cannot be used in the grammar, rather, use
-- Directive: %union
Bison declaration to specify several possible data types for
semantic values. *Note The Union Declaration: Union Decl.
-- Macro: YYABORT
Macro to pretend that an unrecoverable syntax error has occurred,
by making 'yyparse' return 1 immediately. The error reporting
function 'yyerror' is not called. *Note The Parser Function
'yyparse': Parser Function.
For Java parsers, this functionality is invoked using 'return
YYABORT;' instead.
-- Macro: YYACCEPT
Macro to pretend that a complete utterance of the language has been
read, by making 'yyparse' return 0 immediately. *Note The Parser
Function 'yyparse': Parser Function.
For Java parsers, this functionality is invoked using 'return
YYACCEPT;' instead.
-- Macro: YYBACKUP
Macro to discard a value from the parser stack and fake a lookahead
token. *Note Special Features for Use in Actions: Action Features.
-- Variable: yychar
External integer variable that contains the integer value of the
lookahead token. (In a pure parser, it is a local variable within
'yyparse'.) Error-recovery rule actions may examine this variable.
*Note Special Features for Use in Actions: Action Features.
-- Variable: yyclearin
Macro used in error-recovery rule actions. It clears the previous
lookahead token. *Note Error Recovery::.
-- Macro: YYDEBUG
Macro to define to equip the parser with tracing code. *Note
Tracing Your Parser: Tracing.
-- Variable: yydebug
External integer variable set to zero by default. If 'yydebug' is
given a nonzero value, the parser will output information on input
symbols and parser action. *Note Tracing Your Parser: Tracing.
-- Macro: yyerrok
Macro to cause parser to recover immediately to its normal mode
after a syntax error. *Note Error Recovery::.
-- Macro: YYERROR
Cause an immediate syntax error. This statement initiates error
recovery just as if the parser itself had detected an error;
however, it does not call 'yyerror', and does not print any
message. If you want to print an error message, call 'yyerror'
explicitly before the 'YYERROR;' statement. *Note Error
For Java parsers, this functionality is invoked using 'return
YYERROR;' instead.
-- Function: yyerror
User-supplied function to be called by 'yyparse' on error. *Note
The Error Reporting Function 'yyerror': Error Reporting.
-- Macro: YYERROR_VERBOSE
An obsolete macro used in the 'yacc.c' skeleton, that you define
with '#define' in the prologue to request verbose, specific error
message strings when 'yyerror' is called. It doesn't matter what
definition you use for 'YYERROR_VERBOSE', just whether you define
it. Using '%define parse.error verbose' is preferred (*note The
Error Reporting Function 'yyerror': Error Reporting.).
-- Macro: YYFPRINTF
Macro used to output run-time traces. *Note Enabling Traces::.
-- Macro: YYINITDEPTH
Macro for specifying the initial size of the parser stack. *Note
Memory Management::.
-- Function: yylex
User-supplied lexical analyzer function, called with no arguments
to get the next token. *Note The Lexical Analyzer Function
'yylex': Lexical.
-- Variable: yylloc
External variable in which 'yylex' should place the line and column
numbers associated with a token. (In a pure parser, it is a local
variable within 'yyparse', and its address is passed to 'yylex'.)
You can ignore this variable if you don't use the '@' feature in
the grammar actions. *Note Textual Locations of Tokens: Token
Locations. In semantic actions, it stores the location of the
lookahead token. *Note Actions and Locations: Actions and
-- Type: YYLTYPE
Data type of 'yylloc'; by default, a structure with four members.
*Note Data Types of Locations: Location Type.
-- Variable: yylval
External variable in which 'yylex' should place the semantic value
associated with a token. (In a pure parser, it is a local variable
within 'yyparse', and its address is passed to 'yylex'.) *Note
Semantic Values of Tokens: Token Values. In semantic actions, it
stores the semantic value of the lookahead token. *Note Actions:
-- Macro: YYMAXDEPTH
Macro for specifying the maximum size of the parser stack. *Note
Memory Management::.
-- Variable: yynerrs
Global variable which Bison increments each time it reports a
syntax error. (In a pure parser, it is a local variable within
'yyparse'. In a pure push parser, it is a member of 'yypstate'.)
*Note The Error Reporting Function 'yyerror': Error Reporting.
-- Function: yyparse
The parser function produced by Bison; call this function to start
parsing. *Note The Parser Function 'yyparse': Parser Function.
-- Macro: YYPRINT
Macro used to output token semantic values. For 'yacc.c' only.
Obsoleted by '%printer'. *Note The 'YYPRINT' Macro: The YYPRINT
-- Function: yypstate_delete
The function to delete a parser instance, produced by Bison in push
mode; call this function to delete the memory associated with a
parser. *Note The Parser Delete Function 'yypstate_delete': Parser
Delete Function. (The current push parsing interface is
experimental and may evolve. More user feedback will help to
stabilize it.)
-- Function: yypstate_new
The function to create a parser instance, produced by Bison in push
mode; call this function to create a new parser. *Note The Parser
Create Function 'yypstate_new': Parser Create Function. (The
current push parsing interface is experimental and may evolve.
More user feedback will help to stabilize it.)
-- Function: yypull_parse
The parser function produced by Bison in push mode; call this
function to parse the rest of the input stream. *Note The Pull
Parser Function 'yypull_parse': Pull Parser Function. (The current
push parsing interface is experimental and may evolve. More user
feedback will help to stabilize it.)
-- Function: yypush_parse
The parser function produced by Bison in push mode; call this
function to parse a single token. *Note The Push Parser Function
'yypush_parse': Push Parser Function. (The current push parsing
interface is experimental and may evolve. More user feedback will
help to stabilize it.)
-- Macro: YYRECOVERING
The expression 'YYRECOVERING ()' yields 1 when the parser is
recovering from a syntax error, and 0 otherwise. *Note Special
Features for Use in Actions: Action Features.
-- Macro: YYSTACK_USE_ALLOCA
Macro used to control the use of 'alloca' when the deterministic
parser in C needs to extend its stacks. If defined to 0, the
parser will use 'malloc' to extend its stacks. If defined to 1,
the parser will use 'alloca'. Values other than 0 and 1 are
reserved for future Bison extensions. If not defined,
'YYSTACK_USE_ALLOCA' defaults to 0.
In the all-too-common case where your code may run on a host with a
limited stack and with unreliable stack-overflow checking, you
should set 'YYMAXDEPTH' to a value that cannot possibly result in
unchecked stack overflow on any of your target hosts when 'alloca'
is called. You can inspect the code that Bison generates in order
to determine the proper numeric values. This will require some
expertise in low-level implementation details.
-- Type: YYSTYPE
Deprecated in favor of the '%define' variable 'api.value.type'.
Data type of semantic values; 'int' by default. *Note Data Types
of Semantic Values: Value Type.
Appendix B Glossary
Accepting state
A state whose only action is the accept action. The accepting
state is thus a consistent state. *Note Understanding Your Parser:
Backus-Naur Form (BNF; also called "Backus Normal Form")
Formal method of specifying context-free grammars originally
proposed by John Backus, and slightly improved by Peter Naur in his
1960-01-02 committee document contributing to what became the Algol
60 report. *Note Languages and Context-Free Grammars: Language and
Consistent state
A state containing only one possible action. *Note Default
Context-free grammars
Grammars specified as rules that can be applied regardless of
context. Thus, if there is a rule which says that an integer can
be used as an expression, integers are allowed _anywhere_ an
expression is permitted. *Note Languages and Context-Free
Grammars: Language and Grammar.
Default reduction
The reduction that a parser should perform if the current parser
state contains no other action for the lookahead token. In
permitted parser states, Bison declares the reduction with the
largest lookahead set to be the default reduction and removes that
lookahead set. *Note Default Reductions::.
Defaulted state
A consistent state with a default reduction. *Note Default
Dynamic allocation
Allocation of memory that occurs during execution, rather than at
compile time or on entry to a function.
Empty string
Analogous to the empty set in set theory, the empty string is a
character string of length zero.
Finite-state stack machine
A "machine" that has discrete states in which it is said to exist
at each instant in time. As input to the machine is processed, the
machine moves from state to state as specified by the logic of the
machine. In the case of the parser, the input is the language
being parsed, and the states correspond to various stages in the
grammar rules. *Note The Bison Parser Algorithm: Algorithm.
Generalized LR (GLR)
A parsing algorithm that can handle all context-free grammars,
including those that are not LR(1). It resolves situations that
Bison's deterministic parsing algorithm cannot by effectively
splitting off multiple parsers, trying all possible parsers, and
discarding those that fail in the light of additional right
context. *Note Generalized LR Parsing: Generalized LR Parsing.
A language construct that is (in general) grammatically divisible;
for example, 'expression' or 'declaration' in C. *Note Languages
and Context-Free Grammars: Language and Grammar.
IELR(1) (Inadequacy Elimination LR(1))
A minimal LR(1) parser table construction algorithm. That is,
given any context-free grammar, IELR(1) generates parser tables
with the full language-recognition power of canonical LR(1) but
with nearly the same number of parser states as LALR(1). This
reduction in parser states is often an order of magnitude. More
importantly, because canonical LR(1)'s extra parser states may
contain duplicate conflicts in the case of non-LR(1) grammars, the
number of conflicts for IELR(1) is often an order of magnitude less
as well. This can significantly reduce the complexity of
developing a grammar. *Note LR Table Construction::.
Infix operator
An arithmetic operator that is placed between the operands on which
it performs some operation.
Input stream
A continuous flow of data between devices or programs.
LAC (Lookahead Correction)
A parsing mechanism that fixes the problem of delayed syntax error
detection, which is caused by LR state merging, default reductions,
and the use of '%nonassoc'. Delayed syntax error detection results
in unexpected semantic actions, initiation of error recovery in the
wrong syntactic context, and an incorrect list of expected tokens
in a verbose syntax error message. *Note LAC::.
Language construct
One of the typical usage schemas of the language. For example, one
of the constructs of the C language is the 'if' statement. *Note
Languages and Context-Free Grammars: Language and Grammar.
Left associativity
Operators having left associativity are analyzed from left to
right: 'a+b+c' first computes 'a+b' and then combines with 'c'.
*Note Operator Precedence: Precedence.
Left recursion
A rule whose result symbol is also its first component symbol; for
example, 'expseq1 : expseq1 ',' exp;'. *Note Recursive Rules:
Left-to-right parsing
Parsing a sentence of a language by analyzing it token by token
from left to right. *Note The Bison Parser Algorithm: Algorithm.
Lexical analyzer (scanner)
A function that reads an input stream and returns tokens one by
one. *Note The Lexical Analyzer Function 'yylex': Lexical.
Lexical tie-in
A flag, set by actions in the grammar rules, which alters the way
tokens are parsed. *Note Lexical Tie-ins::.
Literal string token
A token which consists of two or more fixed characters. *Note
Lookahead token
A token already read but not yet shifted. *Note Lookahead Tokens:
The class of context-free grammars that Bison (like most other
parser generators) can handle by default; a subset of LR(1). *Note
Mysterious Conflicts::.
The class of context-free grammars in which at most one token of
lookahead is needed to disambiguate the parsing of any piece of
Nonterminal symbol
A grammar symbol standing for a grammatical construct that can be
expressed through rules in terms of smaller constructs; in other
words, a construct that is not a token. *Note Symbols::.
A function that recognizes valid sentences of a language by
analyzing the syntax structure of a set of tokens passed to it from
a lexical analyzer.
Postfix operator
An arithmetic operator that is placed after the operands upon which
it performs some operation.
Replacing a string of nonterminals and/or terminals with a single
nonterminal, according to a grammar rule. *Note The Bison Parser
Algorithm: Algorithm.
A reentrant subprogram is a subprogram which can be in invoked any
number of times in parallel, without interference between the
various invocations. *Note A Pure (Reentrant) Parser: Pure Decl.
Reverse polish notation
A language in which all operators are postfix operators.
Right recursion
A rule whose result symbol is also its last component symbol; for
example, 'expseq1: exp ',' expseq1;'. *Note Recursive Rules:
In computer languages, the semantics are specified by the actions
taken for each instance of the language, i.e., the meaning of each
statement. *Note Defining Language Semantics: Semantics.
A parser is said to shift when it makes the choice of analyzing
further input from the stream rather than reducing immediately some
already-recognized rule. *Note The Bison Parser Algorithm:
Single-character literal
A single character that is recognized and interpreted as is. *Note
From Formal Rules to Bison Input: Grammar in Bison.
Start symbol
The nonterminal symbol that stands for a complete valid utterance
in the language being parsed. The start symbol is usually listed
as the first nonterminal symbol in a language specification. *Note
The Start-Symbol: Start Decl.
Symbol table
A data structure where symbol names and associated data are stored
during parsing to allow for recognition and use of existing
information in repeated uses of a symbol. *Note Multi-function
Syntax error
An error encountered during parsing of an input stream due to
invalid syntax. *Note Error Recovery::.
A basic, grammatically indivisible unit of a language. The symbol
that describes a token in the grammar is a terminal symbol. The
input of the Bison parser is a stream of tokens which comes from
the lexical analyzer. *Note Symbols::.
Terminal symbol
A grammar symbol that has no rules in the grammar and therefore is
grammatically indivisible. The piece of text it represents is a
token. *Note Languages and Context-Free Grammars: Language and
Unreachable state
A parser state to which there does not exist a sequence of
transitions from the parser's start state. A state can become
unreachable during conflict resolution. *Note Unreachable
Appendix C Copying This Manual
Version 1.3, 3 November 2008
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[Denny 2008]
Joel E. Denny and Brian A. Malloy, IELR(1): Practical LR(1) Parser
Tables for Non-LR(1) Grammars with Conflict Resolution, in
'Proceedings of the 2008 ACM Symposium on Applied Computing'
(SAC'08), ACM, New York, NY, USA, pp. 240-245.
[Denny 2010 May]
Joel E. Denny, PSLR(1): Pseudo-Scannerless Minimal LR(1) for the
Deterministic Parsing of Composite Languages, Ph.D. Dissertation,
Clemson University, Clemson, SC, USA (May 2010).
[Denny 2010 November]
Joel E. Denny and Brian A. Malloy, The IELR(1) Algorithm for
Generating Minimal LR(1) Parser Tables for Non-LR(1) Grammars with
Conflict Resolution, in 'Science of Computer Programming', Vol. 75,
Issue 11 (November 2010), pp. 943-979.
[DeRemer 1982]
Frank DeRemer and Thomas Pennello, Efficient Computation of LALR(1)
Look-Ahead Sets, in 'ACM Transactions on Programming Languages and
Systems', Vol. 4, No. 4 (October 1982), pp. 615-649.
[Knuth 1965]
Donald E. Knuth, On the Translation of Languages from Left to
Right, in 'Information and Control', Vol. 8, Issue 6 (December
1965), pp. 607-639.
[Scott 2000]
Elizabeth Scott, Adrian Johnstone, and Shamsa Sadaf Hussain,
'Tomita-Style Generalised LR Parsers', Royal Holloway, University
of London, Department of Computer Science, TR-00-12 (December
Index of Terms
* Menu:
* $: Java Action Features.
(line 9987)
* $ <1>: Action Features. (line 5992)
* $ <2>: Table of Symbols. (line 10619)
* $ <3>: Table of Symbols. (line 10628)
* $ <4>: Table of Symbols. (line 10632)
* $$: Actions. (line 3689)
* $$ <1>: Java Action Features.
(line 9995)
* $$ <2>: Action Features. (line 5988)
* $$ <3>: Table of Symbols. (line 10624)
* $<: Java Action Features.
(line 9991)
* $< <1>: Java Action Features.
(line 10003)
* $< <2>: Action Features. (line 5996)
* $< <3>: Action Features. (line 6001)
* $@N: Mid-Rule Action Translation.
(line 3925)
* $accept: Table of Symbols. (line 10691)
* $end: Table of Symbols. (line 10734)
* $N: Actions. (line 3689)
* $NAME: Actions. (line 3689)
* $undefined: Table of Symbols. (line 10863)
* $[: Table of Symbols. (line 10633)
* $[NAME]: Actions. (line 3689)
* %: Java Declarations Summary.
(line 10215)
* % <1>: Table of Symbols. (line 10642)
* %%: Java Declarations Summary.
(line 10211)
* %% <1>: Table of Symbols. (line 10637)
* %?: Semantic Predicates.
(line 1592)
* %? <1>: Table of Symbols. (line 10647)
* %code: Prologue Alternatives.
(line 2929)
* %code <1>: Decl Summary. (line 4840)
* %code <2>: Decl Summary. (line 4841)
* %code <3>: Decl Summary. (line 4842)
* %code <4>: %code Summary. (line 5401)
* %code <5>: %code Summary. (line 5409)
* %code <6>: %code Summary. (line 5420)
* %code <7>: Calc++ Parser. (line 9516)
* %code <8>: Java Declarations Summary.
(line 10195)
* %code <9>: Table of Symbols. (line 10696)
* %code <10>: Table of Symbols. (line 10697)
* %code imports: %code Summary. (line 5480)
* %code imports <1>: Java Declarations Summary.
(line 10199)
* %code init: Java Declarations Summary.
(line 10203)
* %code lexer: Java Declarations Summary.
(line 10207)
* %code provides: Prologue Alternatives.
(line 2929)
* %code provides <1>: Decl Summary. (line 4888)
* %code provides <2>: %code Summary. (line 5452)
* %code requires: Prologue Alternatives.
(line 2929)
* %code requires <1>: Decl Summary. (line 4888)
* %code requires <2>: %code Summary. (line 5436)
* %code requires <3>: Calc++ Parser. (line 9477)
* %code top: Prologue Alternatives.
(line 2929)
* %code top <1>: %code Summary. (line 5464)
* %debug: Decl Summary. (line 4846)
* %debug <1>: Enabling Traces. (line 8142)
* %debug <2>: Table of Symbols. (line 10702)
* %define: Decl Summary. (line 4850)
* %define <1>: Decl Summary. (line 4851)
* %define <2>: Decl Summary. (line 4852)
* %define <3>: Decl Summary. (line 4853)
* %define <4>: %define Summary. (line 5020)
* %define <5>: %define Summary. (line 5021)
* %define <6>: %define Summary. (line 5022)
* %define <7>: %define Summary. (line 5023)
* %define <8>: %define Summary. (line 5342)
* %define <9>: Table of Symbols. (line 10705)
* %define <10>: Table of Symbols. (line 10706)
* %define <11>: Table of Symbols. (line 10707)
* %define <12>: Table of Symbols. (line 10708)
* %define abstract: Java Declarations Summary.
(line 10219)
* %define annotations: Java Declarations Summary.
(line 10223)
* %define api.location.type: %define Summary. (line 5097)
* %define api.location.type <1>: User Defined Location Type.
(line 9093)
* %define api.location.type <2>: Java Declarations Summary.
(line 10248)
* %define api.namespace: %define Summary. (line 5058)
* %define api.namespace <1>: C++ Bison Interface.
(line 8855)
* %define api.position.type: Java Declarations Summary.
(line 10262)
* %define api.prefix: %define Summary. (line 5112)
* %define api.pure: Pure Decl. (line 4669)
* %define api.pure <1>: %define Summary. (line 5125)
* %define api.push-pull: Push Decl. (line 4704)
* %define api.push-pull <1>: %define Summary. (line 5165)
* %define api.push-pull <2>: Java Push Parser Interface.
(line 10044)
* %define api.token.constructor: %define Summary. (line 5178)
* %define api.token.constructor <1>: Calc++ Parser. (line 9468)
* %define api.token.prefix: %define Summary. (line 5192)
* %define api.value.type: %define Summary. (line 5227)
* %define api.value.type <1>: %define Summary. (line 5228)
* %define api.value.type <2>: Java Declarations Summary.
(line 10271)
* %define api.value.type union: Type Generation. (line 3570)
* %define api.value.type variant: Calc++ Parser. (line 9468)
* %define extends: Java Declarations Summary.
(line 10227)
* %define final: Java Declarations Summary.
(line 10231)
* %define implements: Java Declarations Summary.
(line 10235)
* %define init_throws: Java Declarations Summary.
(line 10239)
* %define lex_throws: Java Declarations Summary.
(line 10243)
* %define location_type: %define Summary. (line 5300)
* %define lr.default-reduction: %define Summary. (line 5303)
* %define lr.default-reduction <1>: Default Reductions.
(line 6946)
* %define lr.default-reduction <2>: Default Reductions.
(line 7022)
* %define lr.keep-unreachable-state: %define Summary. (line 5319)
* %define lr.keep-unreachable-state <1>: Unreachable States.
(line 7139)
* %define lr.keep-unreachable-state <2>: Unreachable States.
(line 7149)
* %define lr.type: %define Summary. (line 5330)
* %define lr.type <1>: LR Table Construction.
(line 6830)
* %define lr.type <2>: LR Table Construction.
(line 6847)
* %define package: Java Declarations Summary.
(line 10254)
* %define parse.assert: %define Summary. (line 5345)
* %define parse.error: %define Summary. (line 5358)
* %define parse.error <1>: Error Reporting. (line 5927)
* %define parse.lac: %define Summary. (line 5372)
* %define parse.lac <1>: LAC. (line 7035)
* %define parse.lac <2>: LAC. (line 7056)
* %define parse.trace: %define Summary. (line 5381)
* %define parse.trace <1>: Enabling Traces. (line 8147)
* %define parser_class_name: Java Declarations Summary.
(line 10258)
* %define public: Java Declarations Summary.
(line 10267)
* %define strictfp: Java Declarations Summary.
(line 10275)
* %define throws: Java Declarations Summary.
(line 10279)
* %defines: Decl Summary. (line 4857)
* %defines <1>: Decl Summary. (line 4905)
* %defines <2>: Table of Symbols. (line 10712)
* %defines <3>: Table of Symbols. (line 10716)
* %destructor: Using Mid-Rule Actions.
(line 3887)
* %destructor <1>: Destructor Decl. (line 4453)
* %destructor <2>: Destructor Decl. (line 4468)
* %destructor <3>: Destructor Decl. (line 4469)
* %destructor <4>: Decl Summary. (line 4908)
* %destructor <5>: Table of Symbols. (line 10720)
* %dprec: Merging GLR Parses.
(line 1408)
* %dprec <1>: Table of Symbols. (line 10725)
* %empty: Empty Rules. (line 3408)
* %empty <1>: Table of Symbols. (line 10730)
* %error-verbose: Table of Symbols. (line 10748)
* %expect: Expect Decl. (line 4613)
* %expect <1>: Decl Summary. (line 4832)
* %expect-rr: Simple GLR Parsers.
(line 1258)
* %expect-rr <1>: Expect Decl. (line 4613)
* %file-prefix: Decl Summary. (line 4913)
* %file-prefix <1>: Table of Symbols. (line 10752)
* %glr-parser: GLR Parsers. (line 1217)
* %glr-parser <1>: Simple GLR Parsers.
(line 1258)
* %glr-parser <2>: Table of Symbols. (line 10756)
* %initial-action: Initial Action Decl.
(line 4432)
* %initial-action <1>: Initial Action Decl.
(line 4436)
* %initial-action <2>: Initial Action Decl.
(line 4437)
* %initial-action <3>: Table of Symbols. (line 10760)
* %language: Decl Summary. (line 4917)
* %language <1>: Table of Symbols. (line 10764)
* %language "Java": Java Declarations Summary.
(line 10168)
* %left: Decl Summary. (line 4815)
* %left <1>: Using Precedence. (line 6371)
* %left <2>: Table of Symbols. (line 10768)
* %lex-param: Pure Calling. (line 5879)
* %lex-param <1>: Pure Calling. (line 5880)
* %lex-param <2>: Java Declarations Summary.
(line 10171)
* %lex-param <3>: Table of Symbols. (line 10772)
* %locations: Decl Summary. (line 4922)
* %merge: Merging GLR Parses.
(line 1408)
* %merge <1>: Table of Symbols. (line 10777)
* %name-prefix: Decl Summary. (line 4929)
* %name-prefix <1>: Java Declarations Summary.
(line 10177)
* %name-prefix <2>: Table of Symbols. (line 10783)
* %no-lines: Decl Summary. (line 4941)
* %no-lines <1>: Table of Symbols. (line 10800)
* %nonassoc: Decl Summary. (line 4819)
* %nonassoc <1>: Using Precedence. (line 6371)
* %nonassoc <2>: LR Table Construction.
(line 6924)
* %nonassoc <3>: Default Reductions.
(line 6946)
* %nonassoc <4>: Table of Symbols. (line 10804)
* %output: Decl Summary. (line 4950)
* %output <1>: Table of Symbols. (line 10808)
* %param: Pure Calling. (line 5884)
* %param <1>: Pure Calling. (line 5885)
* %param <2>: Table of Symbols. (line 10812)
* %parse-param: Parser Function. (line 5617)
* %parse-param <1>: Parser Function. (line 5618)
* %parse-param <2>: Java Declarations Summary.
(line 10182)
* %parse-param <3>: Table of Symbols. (line 10817)
* %prec: Contextual Precedence.
(line 6502)
* %prec <1>: Table of Symbols. (line 10822)
* %precedence: Using Precedence. (line 6371)
* %precedence <1>: Precedence Only. (line 6396)
* %precedence <2>: Table of Symbols. (line 10826)
* %printer: Printer Decl. (line 4565)
* %printer <1>: Printer Decl. (line 4574)
* %printer <2>: Printer Decl. (line 4575)
* %pure-parser: Decl Summary. (line 4953)
* %pure-parser <1>: Table of Symbols. (line 10830)
* %require: Require Decl. (line 4289)
* %require <1>: Decl Summary. (line 4958)
* %require <2>: Table of Symbols. (line 10835)
* %right: Decl Summary. (line 4811)
* %right <1>: Using Precedence. (line 6371)
* %right <2>: Table of Symbols. (line 10839)
* %skeleton: Decl Summary. (line 4962)
* %skeleton <1>: Table of Symbols. (line 10843)
* %start: Start Decl. (line 4659)
* %start <1>: Decl Summary. (line 4828)
* %start <2>: Table of Symbols. (line 10847)
* %token: Token Decl. (line 4298)
* %token <1>: Decl Summary. (line 4807)
* %token <2>: Java Declarations Summary.
(line 10187)
* %token <3>: Table of Symbols. (line 10851)
* %token-table: Decl Summary. (line 4970)
* %token-table <1>: Table of Symbols. (line 10855)
* %type: Type Decl. (line 4413)
* %type <1>: Decl Summary. (line 4824)
* %type <2>: Java Declarations Summary.
(line 10191)
* %type <3>: Table of Symbols. (line 10859)
* %union: Union Decl. (line 3623)
* %union <1>: Structured Value Type.
(line 3660)
* %union <2>: Decl Summary. (line 4803)
* %union <3>: Table of Symbols. (line 10868)
* %verbose: Decl Summary. (line 5000)
* %yacc: Decl Summary. (line 5006)
* /*: Table of Symbols. (line 10658)
* /* ... */: Grammar Outline. (line 2862)
* //: Table of Symbols. (line 10659)
* // ...: Grammar Outline. (line 2862)
* :: Table of Symbols. (line 10662)
* ;: Table of Symbols. (line 10666)
* <*>: Destructor Decl. (line 4453)
* <*> <1>: Printer Decl. (line 4565)
* <*> <2>: Table of Symbols. (line 10673)
* <>: Destructor Decl. (line 4453)
* <> <1>: Printer Decl. (line 4565)
* <> <2>: Table of Symbols. (line 10682)
* @: Java Action Features.
(line 10009)
* @ <1>: Java Action Features.
(line 10013)
* @ <2>: Action Features. (line 6076)
* @ <3>: Action Features. (line 6081)
* @ <4>: Table of Symbols. (line 10601)
* @ <5>: Table of Symbols. (line 10605)
* @ <6>: Table of Symbols. (line 10606)
* @ <7>: Table of Symbols. (line 10614)
* @ <8>: Table of Symbols. (line 10615)
* @$: Actions and Locations.
(line 4081)
* @N: Mid-Rule Action Translation.
(line 3925)
* @N <1>: Actions and Locations.
(line 4081)
* @N <2>: Action Features. (line 6082)
* @NAME: Actions and Locations.
(line 4081)
* @[NAME]: Actions and Locations.
(line 4081)
* |: Rules Syntax. (line 3394)
* | <1>: Table of Symbols. (line 10669)
* abstract syntax tree: Implementing Gotos/Loops.
(line 10444)
* accepting state: Understanding. (line 7776)
* action: Actions. (line 3689)
* action data types: Action Types. (line 3777)
* action features summary: Action Features. (line 5985)
* actions in mid-rule: Mid-Rule Actions. (line 3810)
* actions in mid-rule <1>: Destructor Decl. (line 4533)
* actions, location: Actions and Locations.
(line 4081)
* actions, semantic: Semantic Actions. (line 1192)
* additional C code section: Epilogue. (line 3194)
* algorithm of parser: Algorithm. (line 6144)
* ambiguous grammars: Language and Grammar.
(line 1031)
* ambiguous grammars <1>: Generalized LR Parsing.
(line 7179)
* associativity: Why Precedence. (line 6359)
* AST: Implementing Gotos/Loops.
(line 10444)
* Backus-Naur form: Language and Grammar.
(line 1014)
* begin of location: C++ location. (line 9062)
* begin of Location: Java Location Values.
(line 9818)
* Bison declaration summary: Decl Summary. (line 4801)
* Bison declarations: Declarations. (line 4272)
* Bison declarations (introduction): Bison Declarations.
(line 3176)
* Bison grammar: Grammar in Bison. (line 1113)
* Bison invocation: Invocation. (line 8377)
* Bison parser: Bison Parser. (line 1693)
* Bison parser algorithm: Algorithm. (line 6144)
* Bison symbols, table of: Table of Symbols. (line 10601)
* Bison utility: Bison Parser. (line 1693)
* bison-i18n.m4: Internationalization.
(line 6103)
* bison-po: Internationalization.
(line 6089)
* bisonSkeleton of YYParser: Java Parser Interface.
(line 9923)
* bisonVersion of YYParser: Java Parser Interface.
(line 9922)
* BISON_I18N: Internationalization.
(line 6110)
* BISON_LOCALEDIR: Internationalization.
(line 6110)
* BNF: Language and Grammar.
(line 1014)
* braced code: Rules Syntax. (line 3375)
* build on semantic_type: C++ Variants. (line 8948)
* build on semantic_type <1>: C++ Variants. (line 8953)
* C code, section for additional: Epilogue. (line 3194)
* C-language interface: Interface. (line 5576)
* 'calc': Infix Calc. (line 2208)
* calculator, infix notation: Infix Calc. (line 2208)
* calculator, location tracking: Location Tracking Calc.
(line 2328)
* calculator, multi-function: Multi-function Calc.
(line 2498)
* calculator, simple: RPN Calc. (line 1829)
* canonical LR: Mysterious Conflicts.
(line 6756)
* canonical LR <1>: LR Table Construction.
(line 6830)
* character token: Symbols. (line 3246)
* column of position: C++ position. (line 9023)
* columns on location: C++ location. (line 9066)
* columns on position: C++ position. (line 9026)
* comment: Grammar Outline. (line 2862)
* compiling the parser: Rpcalc Compile. (line 2174)
* conflicts: GLR Parsers. (line 1217)
* conflicts <1>: Simple GLR Parsers.
(line 1258)
* conflicts <2>: Merging GLR Parses.
(line 1408)
* conflicts <3>: Shift/Reduce. (line 6238)
* conflicts, reduce/reduce: Reduce/Reduce. (line 6570)
* conflicts, suppressing warnings of: Expect Decl. (line 4613)
* consistent states: Default Reductions.
(line 6957)
* context-dependent precedence: Contextual Precedence.
(line 6502)
* context-free grammar: Language and Grammar.
(line 1004)
* controlling function: Rpcalc Main. (line 2111)
* core, item set: Understanding. (line 7723)
* dangling 'else': Shift/Reduce. (line 6238)
* data type of locations: Location Type. (line 4056)
* data types in actions: Action Types. (line 3777)
* data types of semantic values: Value Type. (line 3503)
* debugging: Tracing. (line 8114)
* debug_level on parser: C++ Parser Interface.
(line 9183)
* debug_stream on parser: C++ Parser Interface.
(line 9178)
* declaration summary: Decl Summary. (line 4801)
* declarations: Prologue. (line 2884)
* declarations section: Prologue. (line 2884)
* declarations, Bison: Declarations. (line 4272)
* declarations, Bison (introduction): Bison Declarations.
(line 3176)
* declaring literal string tokens: Token Decl. (line 4298)
* declaring operator precedence: Precedence Decl. (line 4364)
* declaring the start symbol: Start Decl. (line 4659)
* declaring token type names: Token Decl. (line 4298)
* declaring value types: Type Generation. (line 3570)
* declaring value types <1>: Union Decl. (line 3623)
* declaring value types <2>: Structured Value Type.
(line 3660)
* declaring value types, nonterminals: Type Decl. (line 4413)
* default action: Actions. (line 3745)
* default data type: Value Type. (line 3503)
* default location type: Location Type. (line 4056)
* default reductions: Default Reductions.
(line 6946)
* default stack limit: Memory Management. (line 7275)
* default start symbol: Start Decl. (line 4659)
* defaulted states: Default Reductions.
(line 6957)
* deferred semantic actions: GLR Semantic Actions.
(line 1547)
* defining language semantics: Semantics. (line 3489)
* delayed syntax error detection: LR Table Construction.
(line 6924)
* delayed syntax error detection <1>: Default Reductions.
(line 6983)
* delayed yylex invocations: Default Reductions.
(line 6957)
* discarded symbols: Destructor Decl. (line 4543)
* discarded symbols, mid-rule actions: Using Mid-Rule Actions.
(line 3887)
* dot: Graphviz. (line 7951)
* 'else', dangling: Shift/Reduce. (line 6238)
* empty rule: Empty Rules. (line 3408)
* end of location: C++ location. (line 9063)
* end of Location: Java Location Values.
(line 9819)
* epilogue: Epilogue. (line 3194)
* error: Error Recovery. (line 7310)
* error <1>: Table of Symbols. (line 10738)
* error on parser: C++ Parser Interface.
(line 9188)
* error on parser <1>: C++ Parser Interface.
(line 9190)
* error recovery: Error Recovery. (line 7296)
* error recovery, mid-rule actions: Using Mid-Rule Actions.
(line 3887)
* error recovery, simple: Simple Error Recovery.
(line 2291)
* error reporting function: Error Reporting. (line 5916)
* error reporting routine: Rpcalc Error. (line 2124)
* examples, simple: Examples. (line 1816)
* exceptions: C++ Parser Interface.
(line 9174)
* exercises: Exercises. (line 2841)
* file format: Grammar Layout. (line 1778)
* file of position: C++ position. (line 9010)
* finite-state machine: Parser States. (line 6547)
* formal grammar: Grammar in Bison. (line 1113)
* format of grammar file: Grammar Layout. (line 1778)
* freeing discarded symbols: Destructor Decl. (line 4453)
* frequently asked questions: FAQ. (line 10287)
* generalized LR (GLR) parsing: Language and Grammar.
(line 1031)
* generalized LR (GLR) parsing <1>: GLR Parsers. (line 1217)
* generalized LR (GLR) parsing <2>: Generalized LR Parsing.
(line 7179)
* generalized LR (GLR) parsing, ambiguous grammars: Merging GLR Parses.
(line 1408)
* generalized LR (GLR) parsing, unambiguous grammars: Simple GLR Parsers.
(line 1258)
* getDebugLevel on YYParser: Java Parser Interface.
(line 9917)
* getDebugStream on YYParser: Java Parser Interface.
(line 9912)
* getEndPos on Lexer: Java Scanner Interface.
(line 9964)
* getErrorVerbose on YYParser: Java Parser Interface.
(line 9895)
* getLVal on Lexer: Java Scanner Interface.
(line 9972)
* getStartPos on Lexer: Java Scanner Interface.
(line 9963)
* gettext: Internationalization.
(line 6089)
* glossary: Glossary. (line 11050)
* GLR parsers and 'inline': Compiler Requirements.
(line 1643)
* GLR parsers and 'yychar': GLR Semantic Actions.
(line 1551)
* GLR parsers and 'yyclearin': GLR Semantic Actions.
(line 1559)
* GLR parsers and 'YYERROR': GLR Semantic Actions.
(line 1572)
* GLR parsers and 'yylloc': GLR Semantic Actions.
(line 1551)
* GLR parsers and 'YYLLOC_DEFAULT': Location Default Action.
(line 4145)
* GLR parsers and 'yylval': GLR Semantic Actions.
(line 1551)
* GLR parsing: Language and Grammar.
(line 1031)
* GLR parsing <1>: GLR Parsers. (line 1217)
* GLR parsing <2>: Generalized LR Parsing.
(line 7179)
* GLR parsing, ambiguous grammars: Merging GLR Parses.
(line 1408)
* GLR parsing, unambiguous grammars: Simple GLR Parsers.
(line 1258)
* GLR with LALR: LR Table Construction.
(line 6886)
* grammar file: Grammar Layout. (line 1778)
* grammar rule syntax: Rules Syntax. (line 3352)
* grammar rules section: Grammar Rules. (line 3184)
* grammar, Bison: Grammar in Bison. (line 1113)
* grammar, context-free: Language and Grammar.
(line 1004)
* grouping, syntactic: Language and Grammar.
(line 1044)
* Header guard: Decl Summary. (line 4891)
* i18n: Internationalization.
(line 6089)
* IELR: Mysterious Conflicts.
(line 6756)
* IELR <1>: LR Table Construction.
(line 6830)
* IELR grammars: Language and Grammar.
(line 1020)
* infix notation calculator: Infix Calc. (line 2208)
* initialize on location: C++ location. (line 9058)
* initialize on position: C++ position. (line 9006)
* 'inline': Compiler Requirements.
(line 1643)
* interface: Interface. (line 5576)
* internationalization: Internationalization.
(line 6089)
* introduction: Introduction. (line 216)
* invoking Bison: Invocation. (line 8377)
* item: Understanding. (line 7701)
* item set core: Understanding. (line 7723)
* item set core <1>: Understanding. (line 7723)
* kernel, item set: Understanding. (line 7723)
* LAC: LR Table Construction.
(line 6924)
* LAC <1>: Default Reductions.
(line 6994)
* LAC <2>: LAC. (line 7035)
* LALR: Mysterious Conflicts.
(line 6744)
* LALR <1>: LR Table Construction.
(line 6830)
* LALR grammars: Language and Grammar.
(line 1020)
* language semantics, defining: Semantics. (line 3489)
* layout of Bison grammar: Grammar Layout. (line 1778)
* left recursion: Recursion. (line 3450)
* lexical analyzer: Lexical. (line 5717)
* lexical analyzer, purpose: Bison Parser. (line 1693)
* lexical analyzer, writing: Rpcalc Lexer. (line 2046)
* lexical tie-in: Lexical Tie-ins. (line 7491)
* line of position: C++ position. (line 9016)
* lines on location: C++ location. (line 9067)
* lines on position: C++ position. (line 9019)
* literal string token: Symbols. (line 3268)
* literal token: Symbols. (line 3246)
* location: Locations. (line 1665)
* location <1>: Tracking Locations.
(line 4046)
* location actions: Actions and Locations.
(line 4081)
* location on location: C++ location. (line 9048)
* location on location <1>: C++ location. (line 9052)
* location on location <2>: C++ location. (line 9053)
* Location on Location: Java Location Values.
(line 9822)
* Location on Location <1>: Java Location Values.
(line 9826)
* location tracking calculator: Location Tracking Calc.
(line 2328)
* location, textual: Locations. (line 1665)
* location, textual <1>: Tracking Locations.
(line 4046)
* location_type: C++ Parser Interface.
(line 9142)
* lookahead correction: LAC. (line 7035)
* lookahead token: Lookahead. (line 6187)
* LR: Mysterious Conflicts.
(line 6744)
* LR grammars: Language and Grammar.
(line 1020)
* 'ltcalc': Location Tracking Calc.
(line 2328)
* main function in simple example: Rpcalc Main. (line 2111)
* make_ on symbol_type: Complete Symbols. (line 9279)
* make_ on symbol_type <1>: Complete Symbols. (line 9281)
* memory exhaustion: Memory Management. (line 7251)
* memory management: Memory Management. (line 7251)
* 'mfcalc': Multi-function Calc.
(line 2498)
* mid-rule actions: Mid-Rule Actions. (line 3810)
* mid-rule actions <1>: Destructor Decl. (line 4533)
* multi-function calculator: Multi-function Calc.
(line 2498)
* multicharacter literal: Symbols. (line 3268)
* mutual recursion: Recursion. (line 3467)
* Mysterious Conflict: LR Table Construction.
(line 6830)
* Mysterious Conflicts: Mysterious Conflicts.
(line 6716)
* named references: Named References. (line 4210)
* NLS: Internationalization.
(line 6089)
* nondeterministic parsing: Language and Grammar.
(line 1031)
* nondeterministic parsing <1>: Generalized LR Parsing.
(line 7179)
* nonterminal symbol: Symbols. (line 3215)
* nonterminal, useless: Understanding. (line 7648)
* operator precedence: Precedence. (line 6321)
* operator precedence, declaring: Precedence Decl. (line 4364)
* operator!= on location: C++ location. (line 9081)
* operator!= on position: C++ position. (line 9037)
* operator+ on location: C++ location. (line 9070)
* operator+ on location <1>: C++ location. (line 9071)
* operator+ on position: C++ position. (line 9031)
* operator+= on location: C++ location. (line 9072)
* operator+= on position: C++ position. (line 9030)
* operator- on location: C++ location. (line 9073)
* operator- on position: C++ position. (line 9033)
* operator-= on location: C++ location. (line 9074)
* operator-= on position: C++ position. (line 9032)
* operator<<: C++ position. (line 9040)
* operator<< <1>: C++ location. (line 9085)
* operator== on location: C++ location. (line 9080)
* operator== on position: C++ position. (line 9036)
* options for invoking Bison: Invocation. (line 8377)
* overflow of parser stack: Memory Management. (line 7251)
* parse error: Error Reporting. (line 5916)
* parse on parser: C++ Parser Interface.
(line 9171)
* parse on YYParser: Java Parser Interface.
(line 9891)
* parser: Bison Parser. (line 1693)
* parser on parser: C++ Parser Interface.
(line 9162)
* parser stack: Algorithm. (line 6144)
* parser stack overflow: Memory Management. (line 7251)
* parser state: Parser States. (line 6547)
* pointed rule: Understanding. (line 7701)
* polish notation calculator: RPN Calc. (line 1829)
* position on position: C++ position. (line 9000)
* precedence declarations: Precedence Decl. (line 4364)
* precedence of operators: Precedence. (line 6321)
* precedence, context-dependent: Contextual Precedence.
(line 6502)
* precedence, unary operator: Contextual Precedence.
(line 6502)
* preventing warnings about conflicts: Expect Decl. (line 4613)
* printing semantic values: Printer Decl. (line 4565)
* Prologue: Prologue. (line 2884)
* Prologue <1>: %code Summary. (line 5401)
* Prologue Alternatives: Prologue Alternatives.
(line 2929)
* pure parser: Pure Decl. (line 4669)
* push parser: Push Decl. (line 4704)
* push parser <1>: Push Decl. (line 4704)
* push_parse on YYParser: Java Push Parser Interface.
(line 10059)
* push_parse on YYParser <1>: Java Push Parser Interface.
(line 10060)
* push_parse on YYParser <2>: Java Push Parser Interface.
(line 10062)
* questions: FAQ. (line 10287)
* recovering: Java Action Features.
(line 10029)
* recovering on YYParser: Java Parser Interface.
(line 9908)
* recovery from errors: Error Recovery. (line 7296)
* recursive rule: Recursion. (line 3439)
* reduce/reduce conflict: Reduce/Reduce. (line 6570)
* reduce/reduce conflicts: GLR Parsers. (line 1217)
* reduce/reduce conflicts <1>: Simple GLR Parsers.
(line 1258)
* reduce/reduce conflicts <2>: Merging GLR Parses.
(line 1408)
* reduction: Algorithm. (line 6144)
* reentrant parser: Pure Decl. (line 4669)
* requiring a version of Bison: Require Decl. (line 4289)
* reverse polish notation: RPN Calc. (line 1829)
* right recursion: Recursion. (line 3450)
* 'rpcalc': RPN Calc. (line 1829)
* rule syntax: Rules Syntax. (line 3352)
* rule, empty: Empty Rules. (line 3408)
* rule, pointed: Understanding. (line 7701)
* rule, recursive: Recursion. (line 3439)
* rule, useless: Understanding. (line 7648)
* rules section for grammar: Grammar Rules. (line 3184)
* running Bison (introduction): Rpcalc Generate. (line 2149)
* semantic actions: Semantic Actions. (line 1192)
* Semantic predicates in GLR parsers: Semantic Predicates.
(line 1592)
* semantic value: Semantic Values. (line 1152)
* semantic value type: Value Type. (line 3503)
* semantic_type: C++ Parser Interface.
(line 9141)
* setDebugLevel on YYParser: Java Parser Interface.
(line 9918)
* setDebugStream on YYParser: Java Parser Interface.
(line 9913)
* setErrorVerbose on YYParser: Java Parser Interface.
(line 9896)
* set_debug_level on parser: C++ Parser Interface.
(line 9184)
* set_debug_stream on parser: C++ Parser Interface.
(line 9179)
* shift/reduce conflicts: GLR Parsers. (line 1217)
* shift/reduce conflicts <1>: Simple GLR Parsers.
(line 1258)
* shift/reduce conflicts <2>: Shift/Reduce. (line 6238)
* shifting: Algorithm. (line 6144)
* simple examples: Examples. (line 1816)
* single-character literal: Symbols. (line 3246)
* stack overflow: Memory Management. (line 7251)
* stack, parser: Algorithm. (line 6144)
* stages in using Bison: Stages. (line 1746)
* start symbol: Language and Grammar.
(line 1092)
* start symbol, declaring: Start Decl. (line 4659)
* state (of parser): Parser States. (line 6547)
* step on location: C++ location. (line 9077)
* string token: Symbols. (line 3268)
* summary, action features: Action Features. (line 5985)
* summary, Bison declaration: Decl Summary. (line 4801)
* suppressing conflict warnings: Expect Decl. (line 4613)
* symbol: Symbols. (line 3215)
* symbol table example: Mfcalc Symbol Table.
(line 2618)
* symbols (abstract): Language and Grammar.
(line 1044)
* symbols in Bison, table of: Table of Symbols. (line 10601)
* symbol_type on symbol_type: Complete Symbols. (line 9264)
* syntactic grouping: Language and Grammar.
(line 1044)
* syntax error: Error Reporting. (line 5916)
* syntax of grammar rules: Rules Syntax. (line 3352)
* syntax_error: C++ Parser Interface.
(line 9152)
* syntax_error on syntax_error: C++ Parser Interface.
(line 9166)
* syntax_error on syntax_error <1>: C++ Parser Interface.
(line 9168)
* terminal symbol: Symbols. (line 3215)
* textual location: Locations. (line 1665)
* textual location <1>: Tracking Locations.
(line 4046)
* token: Language and Grammar.
(line 1044)
* token <1>: C++ Parser Interface.
(line 9145)
* token type: Symbols. (line 3215)
* token type names, declaring: Token Decl. (line 4298)
* token, useless: Understanding. (line 7648)
* toString on Location: Java Location Values.
(line 9829)
* tracing the parser: Tracing. (line 8114)
* uint: C++ Location Values.
(line 8994)
* unary operator precedence: Contextual Precedence.
(line 6502)
* unreachable states: Unreachable States.
(line 7139)
* useless nonterminal: Understanding. (line 7648)
* useless rule: Understanding. (line 7648)
* useless token: Understanding. (line 7648)
* using Bison: Stages. (line 1746)
* value type, semantic: Value Type. (line 3503)
* value types, declaring: Type Generation. (line 3570)
* value types, declaring <1>: Union Decl. (line 3623)
* value types, declaring <2>: Structured Value Type.
(line 3660)
* value types, nonterminals, declaring: Type Decl. (line 4413)
* value, semantic: Semantic Values. (line 1152)
* version requirement: Require Decl. (line 4289)
* warnings, preventing: Expect Decl. (line 4613)
* writing a lexical analyzer: Rpcalc Lexer. (line 2046)
* xml: Xml. (line 8077)
* YYABORT: Parser Function. (line 5610)
* YYABORT <1>: Parser Function. (line 5611)
* YYABORT <2>: Action Features. (line 6006)
* YYABORT <3>: Java Action Features.
(line 10017)
* YYABORT <4>: Table of Symbols. (line 10872)
* YYACCEPT: Parser Function. (line 5607)
* YYACCEPT <1>: Parser Function. (line 5608)
* YYACCEPT <2>: Action Features. (line 6010)
* YYACCEPT <3>: Java Action Features.
(line 10021)
* YYACCEPT <4>: Table of Symbols. (line 10881)
* YYBACKUP: Action Features. (line 6014)
* YYBACKUP <1>: Action Features. (line 6015)
* YYBACKUP <2>: Table of Symbols. (line 10889)
* yychar: GLR Semantic Actions.
(line 1551)
* yychar <1>: Lookahead. (line 6231)
* yychar <2>: Action Features. (line 6047)
* yychar <3>: Table of Symbols. (line 10893)
* yyclearin: GLR Semantic Actions.
(line 1559)
* yyclearin <1>: Action Features. (line 6054)
* yyclearin <2>: Error Recovery. (line 7389)
* yyclearin <3>: Table of Symbols. (line 10899)
* yydebug: Tracing. (line 8114)
* YYDEBUG: Enabling Traces. (line 8123)
* YYDEBUG <1>: Table of Symbols. (line 10903)
* yydebug <1>: Table of Symbols. (line 10907)
* YYEMPTY: Action Features. (line 6027)
* YYENABLE_NLS: Internationalization.
(line 6110)
* YYEOF: Action Features. (line 6030)
* yyerrok: Action Features. (line 6059)
* yyerrok <1>: Error Recovery. (line 7384)
* yyerrok <2>: Table of Symbols. (line 10912)
* YYERROR: GLR Semantic Actions.
(line 1572)
* yyerror: Error Reporting. (line 5916)
* YYERROR <1>: Action Features. (line 6034)
* YYERROR <2>: Java Action Features.
(line 10025)
* yyerror <1>: Java Action Features.
(line 10034)
* yyerror <2>: Java Action Features.
(line 10035)
* yyerror <3>: Java Action Features.
(line 10036)
* YYERROR <3>: Table of Symbols. (line 10916)
* yyerror <4>: Table of Symbols. (line 10927)
* yyerror on Lexer: Java Scanner Interface.
(line 9950)
* yyerror on YYParser: Java Parser Interface.
(line 9901)
* yyerror on YYParser <1>: Java Parser Interface.
(line 9902)
* yyerror on YYParser <2>: Java Parser Interface.
(line 9903)
* YYERROR_VERBOSE: Table of Symbols. (line 10931)
* YYFPRINTF: Enabling Traces. (line 8156)
* YYFPRINTF <1>: Table of Symbols. (line 10939)
* YYINITDEPTH: Memory Management. (line 7277)
* YYINITDEPTH <1>: Table of Symbols. (line 10942)
* yylex: Lexical. (line 5717)
* yylex <1>: Table of Symbols. (line 10946)
* yylex on Lexer: Java Scanner Interface.
(line 9955)
* yylex on parser: Split Symbols. (line 9209)
* yylex on parser <1>: Split Symbols. (line 9211)
* yylloc: GLR Semantic Actions.
(line 1551)
* yylloc <1>: Actions and Locations.
(line 4138)
* yylloc <2>: Token Locations. (line 5836)
* yylloc <3>: Lookahead. (line 6231)
* yylloc <4>: Action Features. (line 6064)
* yylloc <5>: Table of Symbols. (line 10951)
* YYLLOC_DEFAULT: Location Default Action.
(line 4145)
* YYLTYPE: Token Locations. (line 5849)
* YYLTYPE <1>: Table of Symbols. (line 10961)
* yylval: GLR Semantic Actions.
(line 1551)
* yylval <1>: Actions. (line 3770)
* yylval <2>: Token Values. (line 5805)
* yylval <3>: Lookahead. (line 6231)
* yylval <4>: Action Features. (line 6070)
* yylval <5>: Table of Symbols. (line 10965)
* YYMAXDEPTH: Memory Management. (line 7259)
* YYMAXDEPTH <1>: Table of Symbols. (line 10973)
* yynerrs: Error Reporting. (line 5977)
* yynerrs <1>: Table of Symbols. (line 10977)
* yyoutput: Printer Decl. (line 4575)
* yyparse: Parser Function. (line 5588)
* yyparse <1>: Parser Function. (line 5594)
* yyparse <2>: Table of Symbols. (line 10983)
* YYParser on YYParser: Java Parser Interface.
(line 9869)
* YYParser on YYParser <1>: Java Parser Interface.
(line 9878)
* YYPRINT: The YYPRINT Macro. (line 8343)
* YYPRINT <1>: The YYPRINT Macro. (line 8347)
* YYPRINT <2>: The YYPRINT Macro. (line 8348)
* YYPRINT <3>: Table of Symbols. (line 10987)
* yypstate_delete: Parser Delete Function.
(line 5701)
* yypstate_delete <1>: Parser Delete Function.
(line 5709)
* yypstate_delete <2>: Table of Symbols. (line 10992)
* yypstate_new: Parser Create Function.
(line 5685)
* yypstate_new <1>: Parser Create Function.
(line 5693)
* yypstate_new <2>: Table of Symbols. (line 11000)
* yypull_parse: Pull Parser Function.
(line 5672)
* yypull_parse <1>: Pull Parser Function.
(line 5679)
* yypull_parse <2>: Table of Symbols. (line 11007)
* yypush_parse: Push Parser Function.
(line 5656)
* yypush_parse <1>: Push Parser Function.
(line 5664)
* yypush_parse <2>: Table of Symbols. (line 11014)
* YYRECOVERING: Error Recovery. (line 7401)
* YYRECOVERING <1>: Action Features. (line 6042)
* YYRECOVERING <2>: Action Features. (line 6043)
* YYRECOVERING <3>: Table of Symbols. (line 11021)
* YYSTACK_USE_ALLOCA: Table of Symbols. (line 11026)
* YYSTYPE: Table of Symbols. (line 11042)