POSIX terminal interface

The POSIX terminal interface is the generalized abstraction, comprising both an Application Programming Interface for programs, and a set of behavioural expectations for users of a terminal, as defined by the POSIX standard and the Single Unix Specification. It is a historical development from the terminal interfaces of BSD version 4 and Seventh Edition Unix.
General underlying concepts.
Hardware.
A multiplicity of I/O devices are regarded as "terminals" in Unix systems. These include:
Terminal intelligence and capabilities.
Intelligence: terminals are dumb, not intelligent.
Unlike its mainframe and minicomputer contemporaries, the original Unix system was developed solely for "dumb" terminals, and that remains the case today. A terminal is a character-oriented device, comprising streams of characters received from and sent to the device. Although the streams of characters are structured, incorporating control characters, escape codes, and special characters, the I/O protocol is not structured as would be the I/O protocol of "smart", or "intelligent", terminals. There are no field format specifications. There's no block transmission of entire screens (input forms) of input data.
By contrast mainframes and minicomputers in closed architectures commonly use Block-oriented terminals.
Capabilities: terminfo, termcap, curses, et al..
The "capabilities" of a terminal comprise various dumb terminal features that are above and beyond what is available from a pure teletypewriter, which programs can make use of. They (mainly) comprise escape codes that can be sent to or received from the terminal. The escape codes sent to the terminal perform various functions that a CRT terminal (or software terminal emulator) is capable of that a teletypewriter is not, such as moving the terminal's cursor to positions on the screen, clearing and scrolling all or parts of the screen, turning on and off attached printer devices, programmable function keys, changing display colours and attributes (such as reverse video), and setting display title strings. The escape codes received from the terminal signify things such as function key, arrow key, and other special keystrokes (home key, end key, help key, PgUp key, PgDn key, insert key, delete key, and so forth).
These capabilities are encoded in databases that are configured by a system administrator and accessed from programs via the terminfo library (which supersedes the older termcap library), upon which in turn are built libraries such as the curses and ncurses libraries. Application programs use the terminal capabilities to provide textual user interfaces with windows, dialogue boxes, buttons, labels, input fields, menus, and so forth.
Controlling environment variables: codice_1 et al..
The particular set of capabilities for the terminal that a (terminal-aware) program's input and output uses is obtained from the database rather than hardwired into programs and libraries, and is controlled by the codice_1 environment variable (and, optionally for the termcap and terminfo libraries, the codice_3 and codice_4 environment variables, respectively). This variable is set by whatever "terminal monitor" program spawns the programs that then use that terminal for its input and output, or sometimes explicitly. For example:
Job control.
Terminals provide job control facilities. Interactively, the user at the terminal can send control characters that suspend the currently running job, reverting to the interactive job control shell that spawned the job, and can run commands that place jobs in the "background" or that switch another, background, job into the foreground (unsuspending it if necessary).
Line disciplines.
Strictly speaking, in Unices a terminal device comprises the underlying "tty device driver", responsible for the physical control of the device hardware via I/O instructions and handling device interrupt requests for character input and output, and the "line discipline". A line discipline is independent of the actual device hardware, and the same line discipline can be used for a terminal concentrator device responsible for multiple controlling terminals as for a pseudoterminal. In fact, the line discipline (or, in the case of BSD, AIX, and other systems, line "disciplines") are the same across all terminal devices. It is the line discipline that is responsible for local echo, line editing, processing of input modes, processing of output modes, and character mapping. All these things are independent of the actual hardware, dealing as they do in the simple abstractions provided by tty device drivers: transmit a character, receive a character, set various hardware states.
In Seventh Edition Unix, BSD systems and derivatives including macOS, and Linux, each terminal device can be switched amongst multiple line disciplines. In the AT&T STREAMS system, line disciplines are STREAMS modules that may be pushed onto and popped off a STREAMS I/O stack.
History.
The POSIX terminal interface is derived from the terminal interfaces of various Unix systems.
Early Unices: Seventh Edition Unix.
The terminal interface provided by Unix 32V and Seventh Edition Unix, and also presented by BSD version 4 as the "old terminal driver", was a simple one, largely geared towards teletypewriters as terminals. Input was entered a line at a time, with the terminal driver in the operating system (and not the terminals themselves) providing simple line editing capabilities. A buffer was maintained by the kernel in which editing took place. Applications reading terminal input would receive the contents of the buffer only when the key was pressed on the terminal to end line editing. The key sent from the terminal to the system would erase ("kill") the entire current contents of the editing buffer, and would be normally displayed as an " symbol followed by a newline sequence to move the print position to a fresh blank line. The key sent from the terminal to the system would erase the last character from the end of the editing buffer, and would be normally displayed as an " symbol, which users would have to recognize as denoting a "rubout" of the preceding character (teletypewriters not being physically capable of erasing characters once they have been printed on the paper).
From a programming point of view, a terminal device had transmit and receive baud rates, "erase" and "kill" characters (that performed line editing, as explained), "interrupt" and "quit" characters (generating signals to all of the processes for which the terminal was a controlling terminal), "start" and "stop" characters (used for modem flow control), an "end of file" character (acting like a carriage return except discarded from the buffer by the codice_11 system call and therefore potentially causing a zero-length result to be returned) and various "basic mode flags" determining whether local echo was emulated by the kernel's terminal driver, whether modem flow control was enabled, the lengths of various output delays, mapping for the carriage return character, and the three input modes.
The three input modes were:
In line mode the line discipline performs all line editing functions and recognizes the "interrupt" and "quit" control characters and transforms them into signals sent to processes. Applications programs reading from the terminal receive entire lines, after line editing has been completed by the user pressing return.
cbreak mode is one of two character-at-a-time modes. (Stephen R. Bourne jokingly referred to it as a "half-cooked" and therefore "rare" mode.) The line discipline performs no line editing, and the control sequences for line editing functions are treated as normal character input. Applications programs reading from the terminal receive characters immediately, as soon as they are available in the input queue to be read. However, the "interrupt" and "quit" control characters, as well as modem flow control characters, are still handled specially and stripped from the input stream.
The programmatic interface for querying and modifying all of these modes and control characters was the codice_12 system call. (This replaced the codice_13 and codice_14 system calls of Sixth Edition Unix.) Although the "erase" and "kill" characters were modifiable from their defaults of and , for many years they were the pre-set defaults in the terminal device drivers, and on many Unix systems, which only altered terminal device settings as part of the login process, in system login scripts that ran "after" the user had entered username and password, any mistakes at the login and password prompts had to be corrected using the historical editing key characters inherited from teletypewriter terminals.
BSD: the advent of job control.
With the BSD Unices came job control, and a "new terminal driver" with extended capabilities. These extensions comprised additional (again programmatically modifiable) special characters: 
The programmatic interface for querying and modifying all of these extra modes and control characters was still the codice_12 system call, which its creators described as a "rather cluttered interface". All of the original Seventh Edition Unix functionality was retained, and the new functionality was added via additional codice_12 operation codes, resulting in a programmatic interface that had clearly grown, and that presented some duplication of functionality.
System III and System V.
System III introduced a new programming interface that combined Seventh Edition's separate codice_12 operations to get and set flags and to get and set control characters into calls that used a codice_24 structure to hold both flags and control characters and that could get them in a single operation and set them in another single operation. It also split some of the flags from the Seventh Edition interface into multiple separate flags, and added some additional capabilities, although it did not support job control or the cooked-mode enhancements of 4BSD. For example, it replaced the "cooked", "cbreak", and "raw" modes of Seventh Edition with different abstractions. The recognition of signal-generating characters is independent of input mode, and there are only the two input modes: canonical and non-canonical. (This allows a terminal input mode not present in Seventh Edition and BSD: canonical mode with signal generation disabled.)
System III's successors, including System V, used the same interface.
POSIX: Consolidation and abstraction.
One of the major problems that the POSIX standard addressed with its definition of a general terminal interface was the plethora of programmatic interfaces. Although by the time of the standard the behaviour of terminals was fairly uniform from system to system, most Unices having adopted the notions of line disciplines and the BSD job control capabilities, the programmatic interface to terminals via the codice_12 system call was a mess. Different Unices supplied different codice_12 operations, with different (symbolic) names, and different flags. Portable source code had to contain a significant amount of conditional compilation to accommodate the differences across software platforms, even though they were all notionally Unix.
The POSIX standard replaces the codice_12 system entirely, with a set of library functions (which, of course, may be "implemented" under the covers via platform-specific codice_12 operations) with standardized names and parameters. The codice_24 data structure of System V Unix was used as a template for the POSIX codice_30 data structure, whose fields were largely unchanged except that they now used alias data types for specifying the fields, allowing them to be easily ported across multiple processor architectures by implementors, rather than explicitly requiring the codice_31 and codice_32 data types of the C and C++ programming languages (which might be inconvenient sizes on some processor architectures).
POSIX also introduced support for job control, with the codice_30 structure containing suspend and delayed-suspend characters in addition to the control characters supported by System III and System V. It did not add any of the cooked-mode extensions from BSD, although SunOS 4.x, System V Release 4, Solaris, HP-UX, AIX, newer BSDs, macOS, and Linux have implemented them as extensions to codice_30.
What the standard defines.
Controlling terminals and process groups.
Each process in the system has either a single "controlling terminal", or no controlling terminal at all. A process inherits its controlling terminal from its parent, and the only operations upon a process are acquiring a controlling terminal, by a process that has no controlling terminal, and relinquishing it, by a process that has a controlling terminal.
No portable way of acquiring a controlling terminal is defined, the method being implementation defined. The standard defines the codice_35 flag for the codice_36 system call, which is the way of "preventing" what is otherwise the conventional way of acquiring a controlling terminal (a process with no controlling terminal codice_36s a terminal device file that isn't already the controlling terminal for some other process, without specifying the codice_35 flag) but leaves its conventional semantics optional.
Each process also is a member of a process group. Each terminal device records a process group that is termed its "foreground process group". The process groups control terminal access and signal delivery. Signals generated at the terminal are sent to all processes that are members of the terminal's foreground process group. codice_11 and codice_40 I/O operations on a terminal by a process that is not a member of the terminal's foreground process group will and may optionally (respectively) cause signals (codice_41 and codice_42 respectively) to be sent to the invoking process. Various terminal-mode-altering library functions have the same behaviour as codice_40, except that they always generate the signals, even if that functionality is turned off for codice_40 itself.
The codice_30 data structure.
The data structure used by all of the terminal library calls is the codice_30 structure, whose C and C++ programming language definition is as follows:struct termios {
 tcflag_t c_iflag ; // Input modes
 tcflag_t c_oflag ; // Output modes
 tcflag_t c_cflag ; // Control modes
 tcflag_t c_lflag ; // Local modes
 cc_t c_cc[NCCS] ; // Control characters
The order of the fields within the codice_30 structure is not defined, and implementations are allowed to add non-standard fields. Indeed, implementations have to add non-standard fields for recording input and output baud rates. These are recorded in the structure, in an implementation-defined form, and accessed via accessor functions, rather than by direct manipulation of the field values, as is the case for the standardized structure fields.
The data type aliases codice_48 and codice_49, as well as the symbolic constant codice_50 and symbolic constants for the various mode flags, control character names, and baud rates, are all defined in a standard header codice_51. (This is not to be confused with the similarly named header codice_52 from System III and System V, which defines a similar codice_24 structure and a lot of similarly named symbolic constants. This interface is specific to System III and System V, and code that uses it will not necessarily be portable to other systems.)
The structure's fields are (in summary, for details see the main article):
The library functions are (in summary, for details see the main article):
Special characters.
The codice_78 array member of the codice_30 data structure specifies all of the (programmatically modifiable) special characters. The indexes into the array are symbolic constants, one for each special character type, as in the table at right. (Two further entries in the array are relevant to non-canonical mode input processing and are discussed below.)
Non-programmatically modifiable special characters are "linefeed" (ASCII codice_80) and "carriage return" (ASCII codice_81).
Input processing.
Input processing determines the behaviour of the codice_11 system call on a terminal device and the line editing and signal-generation characteristics of the line discipline. Unlike the case of Seventh Edition Unix and BSD version 4, and like the case of System III and System V, line editing operates in one of just two modes: canonical mode and non-canonical mode. The basic difference between them is when, from the point of view of the blocking/non-blocking requirements of the codice_11 system call (specified with the codice_84 flag on the file descriptor via codice_36 or codice_86), data "are available for reading".
Canonical mode processing.
In canonical mode, data are accumulated in a line editing buffer, and do not become "available for reading" until line editing has been terminated by the user (at the terminal) sending a "line delimiter character". Line delimiter characters are special characters, and they are "end of file", "end of line", and "linefeed" (ASCII codice_80). The former two are settable programmatically, whilst the latter is fixed. The latter two are included in the line editing buffer, whilst the former one is not.
More strictly, zero or more lines are accumulated in the line editing buffer, separated by line delimiters (which may or may not be discarded once codice_11 comes around to reading them), and line editing operates upon the part of the line editing buffer that follows the last (if any) line delimiter in the buffer. So, for example, the "erase" character (whatever that has been programmed to be) will erase the last character in the line buffer only up to (but not including) a preceding line delimiter.
Non-canonical mode processing.
In non-canonical mode, data are accumulated in a buffer (which may or may not be the line editing buffer — some implementations having separate "processed input" and "raw input" queues) and become "available for reading" according to the values of two input control parameters, the codice_89 and codice_90 members of the codice_30 data structure. Both are unsigned quantities (because codice_49 is required to be an alias for an unsigned type). The former specifies a minimum number of characters, and the latter specifies a timeout in tenths of a second. There are four possibilities:
Output processing.
Output processing is largely unchanged from its System III/System V roots. Output mode control flags determine various options: