NAME

ctags — create a tags file (DEVELOPMENT)

SYNOPSIS

[CD SD] ctags [-a] [-f tagsfile] pathname...

ctags -x pathname...

DESCRIPTION

The ctags utility shall write a tagsfile or an index of objects from C-language source files specified by the pathname operands. The tagsfile shall list the locators of C-language objects within the source files. A locator consists of a name, pathname, and either a search pattern or a line number that can be used in searching for the object definition. The objects that shall be recognized are specified in the EXTENDED DESCRIPTION section.

OPTIONS

The ctags utility shall conform to XBD 12.2 Utility Syntax Guidelines.

The following options shall be supported:

-a

Append to tagsfile.

-f tagsfile

Write the object locator lists into tagsfile instead of the default file named tags in the current directory.

-x

Produce a list of object names, the line number, and filename in which each is defined, as well as the text of that line, and write this to the standard output. A tagsfile shall not be created when -x is specified.

OPERANDS

The following pathname operands are supported:

file.c

Files with basenames ending with the .c suffix shall be treated as C-language source code. Such files that are not valid input to c17 produce unspecified results.

file.h

Files with basenames ending with the .h suffix shall be treated as C-language source code. Such files that are not valid input to c17 produce unspecified results.

The handling of other files is implementation-defined.

STDIN

See the INPUT FILES section.

INPUT FILES

The input files shall be text files containing C-language source code.

ENVIRONMENT VARIABLES

The following environment variables shall affect the execution of ctags:

LANG

Provide a default value for the internationalization variables that are unset or null. (See XBD 8.2 Internationalization Variables for the precedence of internationalization variables used to determine the values of locale categories.)

LC_ALL

If set to a non-empty string value, override the values of all the other internationalization variables.

LC_COLLATE

Determine the order in which output is sorted for the -x option. The POSIX locale determines the order in which the tagsfile is written.

LC_CTYPE

Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as opposed to multi-byte characters in arguments and input files). If the locale is not compatible with the C locale described by the ISO C standard, the results are unspecified.

LC_MESSAGES

Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.

NLSPATH

^[XSI] Determine the location of messages objects and message catalogs.

ASYNCHRONOUS EVENTS

Default.

STDOUT

The list of object name information produced by the -x option shall be written to standard output in the following format:

"%s %d %s %s", <object-name>, <line-number>, <filename>, <text>

where <text> is the text of line <line-number> of file <filename>.

STDERR

The standard error shall be used only for diagnostic messages.

OUTPUT FILES

When the -x option is not specified, the format of the output file shall be:

"%s\t%s\t/%s/\n", <identifier>, <filename>, <pattern>

where <pattern> is a search pattern that could be used by an editor to find the defining instance of <identifier> in <filename> (where defining instance is indicated by the declarations listed in the EXTENDED DESCRIPTION).

An optional <circumflex> ('^') can be added as a prefix to <pattern>, and an optional <dollar-sign> can be appended to <pattern> to indicate that the pattern is anchored to the beginning (end) of a line of text. Any <slash> or <backslash> characters in <pattern> shall be preceded by a <backslash> character. The anchoring <circumflex>, <dollar-sign>, and escaping <backslash> characters shall not be considered part of the search pattern. All other characters in the search pattern shall be considered literal characters.

An alternative format is:

"%s\t%s\t?%s?\n", <identifier>, <filename>, <pattern>

which is identical to the first format except that <slash> characters in <pattern> shall not be preceded by escaping <backslash> characters, and <question-mark> characters in <pattern> shall be preceded by <backslash> characters.

A second alternative format is:

"%s\t%s\t%d\n", <identifier>, <filename>, <lineno>

where <lineno> is a decimal line number that could be used by an editor to find <identifier> in <filename>.

Neither alternative format shall be produced by ctags when it is used as described by POSIX.1-2024, but the standard utilities that process tags files shall be able to process those formats as well as the first format.

In any of these formats, the file shall be sorted by identifier, based on the collation sequence in the POSIX locale.

EXTENDED DESCRIPTION

The ctags utility shall attempt to produce an output line for each of the following objects:

• Function definitions

• Type definitions

• Macros with arguments

It may also produce output for any of the following objects:

• Function prototypes

• Structures

• Unions

• Global variable definitions

• Enumeration types

• Macros without arguments

• #define statements

• #line statements

Any #if and #ifdef statements shall produce no output. The tag main is treated specially in C programs. The tag formed shall be created by prefixing M to the name of the file, with the trailing .c, and leading pathname components (if any) removed.

It is implementation-defined what other objects (including duplicate identifiers) produce output.

On systems that do not support the C-Language Development Utilities option, if ctags is supported it produces unspecified results for C-language source code files. It should write to standard error a message identifying this condition and cause a non-zero exit status to be produced.

EXIT STATUS

The following exit values shall be returned:

 0

Successful completion.

>0

An error occurred.

CONSEQUENCES OF ERRORS

Default.

APPLICATION USAGE

The output with -x is meant to be a simple index that can be written out as an off-line readable function index. If the input files to ctags (such as .c files) were not created using the same locale as that in effect when ctags -x is run, results might not be as expected.

The description of C-language processing says "attempts to" because the C language can be greatly confused, especially through the use of #defines, and this utility would be of no use if the real C preprocessor were run to identify them. The output from ctags may be fooled and incorrect for various constructs.

EXAMPLES

None.

RATIONALE

The option list was significantly reduced from that provided by historical implementations. The -F option was omitted as redundant, since it is the default. The -B option was omitted as being of very limited usefulness. The -t option was omitted since the recognition of typedefs is now required for C source files. The -u option was omitted because the update function was judged to be not only inefficient, but also rarely needed.

An early proposal included a -w option to suppress warning diagnostics. Since the types of such diagnostics could not be described, the option was omitted as being not useful.

The text for LC_CTYPE about compatibility with the C locale acknowledges that the ISO C standard imposes requirements on the locale used to process C source. This could easily be a superset of that known as "the C locale" by way of implementation extensions, or one of a few alternative locales for systems supporting different codesets.

The collation sequence of the tags file is not affected by LC_COLLATE because it is typically not used by human readers, but only by programs such as vi to locate the tag within the source files. Using the POSIX locale eliminates some of the problems of coordinating locales between the ctags file creator and the vi file reader.

Historically, the tags file has been used only by ex and vi. However, the format of the tags file has been published to encourage other programs to use the tags in new ways. The format allows either patterns or line numbers to find the identifiers because the historical vi recognizes either. The ctags utility does not produce the format using line numbers because it is not useful following any source file changes that add or delete lines. The documented search patterns match historical practice. It should be noted that literal leading <circumflex> or trailing <dollar-sign> characters in the search pattern will only behave correctly if anchored to the beginning of the line or end of the line by an additional <circumflex> or <dollar-sign> character.

Historical implementations also understand the objects used by the languages FORTRAN, Pascal, and sometimes LISP, and they understand the C source output by lex and yacc. The ctags utility is not required to accommodate these languages, although implementors are encouraged to do so.

The following historical option was not specified, as vgrind is not included in this volume of POSIX.1-2024:

-v

If the -v flag is given, an index of the form expected by vgrind is produced on the standard output. This listing contains the function name, filename, and page number (assuming 64-line pages). Since the output is sorted into lexicographic order, it may be desired to run the output through sort -f. Sample use:

ctags -v files | sort -f > index
vgrind -x index

The special treatment of the tag main makes the use of ctags practical in directories with more than one program.

FUTURE DIRECTIONS

If this utility is directed to display a pathname that contains any bytes that have the encoded value of a <newline> character when <newline> is a terminator or separator in the output format being used, implementations are encouraged to treat this as an error. A future version of this standard may require implementations to treat this as an error.

If this utility is directed to create a new directory entry that contains any bytes that have the encoded value of a <newline> character, implementations are encouraged to treat this as an error. A future version of this standard may require implementations to treat this as an error.

SEE ALSO

c17, vi

XBD 8. Environment Variables, 12.2 Utility Syntax Guidelines

CHANGE HISTORY

First released in Issue 4.

Issue 5

The FUTURE DIRECTIONS section is added.

Issue 6

This utility is marked as part of the User Portability Utilities option.

The OUTPUT FILES section is changed to align with the IEEE P1003.2b draft standard.

The normative text is reworded to avoid use of the term "must" for application requirements.

IEEE PASC Interpretation 1003.2 #168 is applied, changing "create" to "write" in the DESCRIPTION.

Issue 7

The ctags utility is no longer dependent on support for the User Portability Utilities option.

SD5-XCU-ERN-97 is applied, updating the SYNOPSIS.

Issue 8

Austin Group Defect 251 is applied, encouraging implementations to behave as follows:

1. Report an error if a utility is directed to display a pathname that contains any bytes that have the encoded value of a <newline> character when <newline> is a terminator or separator in the output format being used.

2. Disallow the creation of filenames containing any bytes that have the encoded value of a <newline> character.

Austin Group Defect 1122 is applied, changing the description of NLSPATH .

Austin Group Defect 1312 is applied, inserting a missing line break in the example commands in the RATIONALE section.

Austin Group Defect 1330 is applied, removing obsolescent interfaces.