Ctags

Ctags is a programming tool that generates an index (or tag) file of names found in source and header files of various programming languages to aid code comprehension. Depending on the language, functions, variables, class members, macros and so on may be indexed. These tags allow definitions to be quickly and easily located by a text editor, a code search engine, or other utility. Alternatively, there is also an output mode that generates a cross reference file, listing information about various names found in a set of language files in human-readable form.
The original Ctags was introduced in BSD Unix 3.0 and was written by Ken Arnold, with Fortran support by Jim Kleckner and Pascal support by Bill Joy. It is part of the initial release of Single Unix Specification and XPG4 of 1992.
Editors that support ctags.
"Tag index files" are supported by many source code editors, including:
Variants of ctags.
There are a few other implementations of the "ctags" program:
Etags.
GNU Emacs comes with two ctags utilities, etags and ctags, which are compiled from the same source code. Etags generates a tag table file for Emacs, while the ctags command is used to create a similar table in a format understood by vi. They have different sets of command line options: For those options which only make sense for vi style tag files produced by the ctags command, etags could not recognize them and would ignore them.
Exuberant Ctags.
Exuberant Ctags, written and maintained by Darren Hiebert until 2009, was initially distributed with Vim, but became a separate project upon the release of Vim 6. It includes support for Emacs and compatibility.
Exuberant Ctags includes support for over 40 programming languages with the ability to add support for even more using regular expressions.
Universal Ctags.
Universal Ctags is a fork of Exuberant Ctags, with the objective of continuing its development. A few parsers are rewritten to better support the languages.
Language-specific.
 creates ctags compatible tag files for Haskell source files. It includes support for creating Emacs etags files.
 is a ctags-compatible code indexing solution for JavaScript. It is specialized for JavaScript and uses the CommonJS packaging system. It outperforms "Exuberant Ctags" for JavaScript code, finding more tags than the latter.
Tags file formats.
There are multiple tag file formats. Some of them are described below. In the following, represents the byte with hexadecimal representation . Every line ends with a line feed (LF, = ).
Ctags and descendants.
The original "ctags" and the Exuberant/Universal descendants have similar file formats:
Ctags.
This is the format used by "vi" and various clones. The tags file is normally named "tags".
The tags file is a list of lines, each line in the format:
The fields are specified as follows:
The tags file is sorted on the } field which allows for fast searching of the tags file.
Extended Ctags.
This is the format used by Vim's Exuberant Ctags and Universal Ctags. These programs can generate an original "ctags" file format or an extended format that attempts to retain backward compatibility.
The extended tags file is a list of lines, each line in the format:
 {tagname}\t{tagfile}\t{tagaddress}[;"\t{tagfield...}]
The fields up to and including } are the same as for ctags above.
Optional additional fields are indicated by square brackets ("[...]") and include:
This format is compatible with non-POSIX "vi" as the additional data is interpreted as a comment. POSIX implementations of "vi" must be changed to support it, however.
Etags.
This is the format used by Emacs "etags". The tags file is normally named "TAGS".
The etags files consists of multiple sections—one section per input source file. Sections are plain-text with several non-printable ascii characters used for special purposes. These characters are represented as bracketed hexadecimal codes below.
A section starts with a two line header (the first two bytes make up a magic number):
 \x0c
The header is followed by tag definitions, one definition per line, with the format:
 can be omitted if the name of the tag can be deduced from the text at the tag definition.
Example.
Given a single line test.c source code:
 #define CCC(x)
The TAGS (etags) file would look like this:
 \x0c
 test.c,21
 #define CCC(\x7fCCC\x011,0
The tags (ctags) file may look like:
or more flexibly using a search: