ctags-client-tools

Hints for developing a tool using ctags command and tags output

Version:

6.0.0

Manual group:

Universal Ctags

Manual section:

7

SYNOPSIS

ctags [options] [file(s)]
etags [options] [file(s)]

DESCRIPTION

Client tool means a tool running the ctags command and/or reading a tags file generated by ctags command. This man page gathers hints for people who develop client tools.

PSEUDO-TAGS

Pseudo-tags, stored in a tag file, indicate how ctags generated the tags file: whether the tags file is sorted or not, which version of tags file format is used, the name of tags generator, and so on. The opposite term for pseudo-tags is regular-tags. A regular-tag is for a language object in an input file. A pseudo-tag is for the tags file itself. Client tools may use pseudo-tags as reference for processing regular-tags.

A pseudo-tag is stored in a tags file in the same format as regular-tags as described in tags(5), except that pseudo-tag names are prefixed with “!_”. For the general information about pseudo-tags, see “TAG FILE INFORMATION” in tags(5).

An example of a pseudo tag:

!_TAG_PROGRAM_NAME      Universal Ctags /Derived from Exuberant Ctags/

The value, “Universal Ctags”, associated with the pseudo tag TAG_PROGRAM_NAME, is used in the field for input file. The description, “Derived from Exuberant Ctags”, is used in the field for pattern.

Universal Ctags extends the naming scheme of the classical pseudo-tags available in Exuberant Ctags for emitting language specific information as pseudo tags:

!_{pseudo-tag-name}!{language-name}     {associated-value}      /{description}/

The language-name is appended to the pseudo-tag name with a separator, “!”.

An example of pseudo tag with a language suffix:

!_TAG_KIND_DESCRIPTION!C        f,function      /function definitions/

This pseudo-tag says “the function kind of C language is enabled when generating this tags file.” --pseudo-tags is the option for enabling/disabling individual pseudo-tags. When enabling/disabling a pseudo tag with the option, specify the tag name only TAG_KIND_DESCRIPTION, without the prefix (“!_”) or the suffix (“!C”).

Options for Pseudo-tags

--extras=+p (or --extras=+{pseudo})

Forces writing pseudo-tags.

ctags emits pseudo-tags by default when writing tags to a regular file (e.g. “tags’.) However, when specifying -o - or -f - for writing tags to standard output, ctags doesn’t emit pseudo-tags. --extras=+p or --extras=+{pseudo} will force pseudo-tags to be written.

--list-pseudo-tags

Lists available types of pseudo-tags and shows whether they are enabled or disabled.

Running ctags with --list-pseudo-tags option lists available pseudo-tags. Some of pseudo-tags newly introduced in Universal Ctags project are disabled by default. Use --pseudo-tags=... to enable them.

--pseudo-tags=[+|-]names|*

Specifies a list of pseudo-tag types to include in the output.

The parameters are a set of pseudo tag names. Valid pseudo tag names can be listed with --list-pseudo-tags. Surround each name in the set with braces, like “{TAG_PROGRAM_AUTHOR}”. You don’t have to include the “!_” pseudo tag prefix when specifying a name in the option argument for --pseudo-tags= option.

pseudo-tags don’t have a notation using one-letter flags.

If a name is preceded by either the ‘+’ or ‘-’ characters, that tags’s effect has been added or removed. Otherwise the names replace any current settings. All entries are included if ‘*’ is given.

--fields=+E (or --fields=+{extras})

Attach “extras:pseudo” field to pseudo-tags.

An example of pseudo tags with the field:

!_TAG_PROGRAM_NAME      Universal Ctags /Derived from Exuberant Ctags/  extras:pseudo

If the name of a regular tag in a tag file starts with “!_”, a client tool cannot distinguish whether the tag is a regular-tag or pseudo-tag. The fields attached with this option help the tool distinguish them.

List of notable pseudo-tags

Running ctags with --list-pseudo-tags option lists available types of pseudo-tags with short descriptions. This subsection shows hints for using notable ones.

TAG_EXTRA_DESCRIPTION (new in Universal Ctags)

Indicates the names and descriptions of enabled extras:

!_TAG_EXTRA_DESCRIPTION       {extra-name}    /description/
!_TAG_EXTRA_DESCRIPTION!{language-name}       {extra-name}    /description/

If your tool relies on some extra tags (extras), refer to the pseudo-tags of this type. A tool can reject the tags file that doesn’t include expected extras, and raise an error in an early stage of processing.

An example of the pseudo-tags:

$ ctags --extras=+p --pseudo-tags='{TAG_EXTRA_DESCRIPTION}' -o - input.c
!_TAG_EXTRA_DESCRIPTION       anonymous       /Include tags for non-named objects like lambda/
!_TAG_EXTRA_DESCRIPTION       fileScope       /Include tags of file scope/
!_TAG_EXTRA_DESCRIPTION       pseudo  /Include pseudo tags/
!_TAG_EXTRA_DESCRIPTION       subparser       /Include tags generated by subparsers/
...

A client tool can know “{anonymous}”, “{fileScope}”, “{pseudo}”, and “{subparser}” extras are enabled from the output.

Universal Ctags version 6.0 will turn on this pseudo tag by default.

TAG_FIELD_DESCRIPTION (new in Universal Ctags)

Indicates the names and descriptions of enabled fields:

!_TAG_FIELD_DESCRIPTION       {field-name}    /description/
!_TAG_FIELD_DESCRIPTION!{language-name}       {field-name}    /description/

If your tool relies on some fields, refer to the pseudo-tags of this type. A tool can reject a tags file that doesn’t include expected fields, and raise an error in an early stage of processing.

An example of the pseudo-tags:

$ ctags --fields-C=+'{macrodef}' --extras=+p --pseudo-tags='{TAG_FIELD_DESCRIPTION}' -o - input.c
!_TAG_FIELD_DESCRIPTION       file    /File-restricted scoping/
!_TAG_FIELD_DESCRIPTION       input   /input file/
!_TAG_FIELD_DESCRIPTION       name    /tag name/
!_TAG_FIELD_DESCRIPTION       pattern /pattern/
!_TAG_FIELD_DESCRIPTION       typeref /Type and name of a variable or typedef/
!_TAG_FIELD_DESCRIPTION!C     macrodef        /macro definition/
...

A client tool can know “{file}”, “{input}”, “{name}”, “{pattern}”, and “{typeref}” fields are enabled from the output. The fields are common in languages. In addition to the common fields, the tool can known “{macrodef}” field of C language is also enabled.

Universal Ctags version 6.0 will turn on this pseudo tag by default.

TAG_FILE_ENCODING (new in Universal Ctags)

TBW

TAG_FILE_FORMAT

See also tags(5).

TAG_FILE_SORTED

See also tags(5).

TAG_KIND_DESCRIPTION (new in Universal Ctags)

Indicates the names and descriptions of enabled kinds:

!_TAG_KIND_DESCRIPTION!{language-name}        {kind-letter},{kind-name}       /description/

If your tool relies on some kinds, refer to the pseudo-tags of this type. A tool can reject the tags file that doesn’t include expected kinds, and raise an error in an early stage of processing.

Kinds are language specific, so a language name is always appended to the tag name as suffix.

An example of the pseudo-tags:

$ ctags --extras=+p --kinds-C=vfm --pseudo-tags='{TAG_KIND_DESCRIPTION}' -o - input.c
!_TAG_KIND_DESCRIPTION!C      f,function      /function definitions/
!_TAG_KIND_DESCRIPTION!C      m,member        /struct, and union members/
!_TAG_KIND_DESCRIPTION!C      v,variable      /variable definitions/
...

A client tool can know “{function}”, “{member}”, and “{variable}” kinds of C language are enabled from the output.

Universal Ctags version 6.0 will turn on this pseudo tag by default.

TAG_KIND_SEPARATOR (new in Universal Ctags)

TBW

TAG_OUTPUT_EXCMD (new in Universal Ctags)

Indicates the specified type of EX command with --excmd option.

TAG_OUTPUT_FILESEP (new in Universal Ctags)

Indicates filename separators (“slash” or “backslsh”) used in input fields.

Universal Ctags running on MS Windows replaces backslashes with slashes when emitting input fields by default. This pseudo tag is for notifying this replacement to client tools.

See also the description for --use-slash-as-filename-separator option in ctags(1).

TAG_OUTPUT_MODE (new in Universal Ctags)

Indicates whether using Universal Ctags extended escape sequences (“u-ctags”) or not (“e-ctags”).

To reduce illegal characters like <Tab> in tags files, Universal Ctags extends the escape sequences originally used in Exuberant Ctags, and applies the escaping rules to more fields.

See tags(5) about the escaping rules.

--output-format option is for choosing the output mode within the tags output format. See ctags(1) about the option.

In “e-ctags” mode, for not violating the tags file format described in tags(5), Universal Ctags skips emitting tag entries including illegal characters like <Tab>.

In input fields ({tagfile} in tags(5)), we have one more condition for applying the escaping rules: \ characters are not used as filename separators. UNIX-like systems use / for the purpose. On MS Windows, Universal Ctags converts \ in filenames to / by default. So, generally this condition is satisfied. The condition is not satisfied only when you specify --use-slash-as-filename-separator=no on MS Windows.

TAG_OUTPUT_VERSION (new in Universal Ctags 6.0)

Indicates the language-common interface version of the output:

!_TAG_OUTPUT_VERSION  {current}.{age} /.../

The public interface includes common fields, common extras, pseudo tags.

The maintainer of Universal Ctags may update the numbers, “{current}” and “{age}” in the same manner as explained in TAG_PARSER_VERSION.

TAG_PARSER_VERSION (new in Universal Ctags 6.0)

Indicates the interface version of the parser:

!_TAG_PARSER_VERSION!{language-name}  {current}.{age} /.../

The public interfaces include kinds, roles, language specific fields, and language specific extras.

The maintainer of the parser for “${language-name}” may update the numbers, “{current}” and “{age}” in the following rules:

  • If kinds, roles, language specific fields, and/or language specific extras have been added, removed or changed since last release, increment “{current}”.

  • If they have been added since last release, increment “{age}”.

  • If they have been removed since last release, set “{age}” to 0.

This concept is baesd on the versioning in libtool (7.2 Libtool’s versioning system.) In Universal Ctags, we simplified the concept with removing “revision” in the versioning in libtool.

Manual pages for languages may document changes that increase the number of “{current}”.

TAG_PATTERN_LENGTH_LIMIT (new in Universal Ctags)

TBW

TAG_PROC_CWD (new in Universal Ctags)

Indicates the working directory of ctags during processing.

This pseudo-tag helps a client tool solve the absolute paths for the input files for tag entries even when they are tagged with relative paths.

An example of the pseudo-tags:

$ cat tags
!_TAG_PROC_CWD        /tmp/   //
main  input.c /^int main (void) { return 0; }$/;"     f       typeref:typename:int
...

From the regular tag for “main”, the client tool can know the “main” is at “input.c”. However, it is a relative path. So if the directory where ctags run and the directory where the client tool runs are different, the client tool cannot find “input.c” from the file system. In that case, TAG_PROC_CWD gives the tool a hint; “input.c” may be at “/tmp”.

TAG_PROGRAM_NAME

Indicates the name of program generating this tags file.

TAG_PROGRAM_VERSION

Indicates the version of program generating this tags file.

TAG_ROLE_DESCRIPTION (new in Universal Ctags)

Indicates the names and descriptions of enabled roles:

!_TAG_ROLE_DESCRIPTION!{language-name}!{kind-name}    {role-name}     /description/

If your tool relies on some roles, refer to the pseudo-tags of this type. Note that a role owned by a disabled kind is not listed even if the role itself is enabled.

REDUNDANT-KINDS

TBW

MULTIPLE-LANGUAGES FOR AN INPUT FILE

Universal ctags can run multiple parsers. That means a parser, which supports multiple parsers, may output tags for different languages. language/l field can be used to show the language for each tag.

$ cat /tmp/foo.html
<html>
<script>var x = 1</script>
<h1>title</h1>
</html>
$ ./ctags -o - --extras=+g /tmp/foo.html
title   /tmp/foo.html   /^  <h1>title<\/h1>$/;" h
x       /tmp/foo.html   /var x = 1/;"   v
$ ./ctags -o - --extras=+g --fields=+l /tmp/foo.html
title   /tmp/foo.html   /^  <h1>title<\/h1>$/;" h       language:HTML
x       /tmp/foo.html   /var x = 1/;"   v       language:JavaScript

UTILIZING READTAGS

See readtags(1) to know how to use readtags. This section is for discussing some notable topics for client tools.

Build Filter/Sorter Expressions

Certain escape sequences in expressions are recognized by readtags. For example, when searching for a tag that matches a\?b, if using a filter expression like '(eq? $name "a\?b")', since \? is translated into a single ? by readtags, it actually searches for a?b.

Another problem is: If the client tools talks to readtags not by subprocess directly, but through a shell, then if a single quote appear in filter expressions (which is also wrapped by single quotes), it terminates the expression, producing broken expressions, and may even cause unintended shell injection. Single quotes can be escaped using '"'"'.

So, client tools need to:

  • Replace \ by \\

  • Replace ' by '"'"', if it talks to readtags through a shell.

inside the expressions. If the expression also contains strings, " in the strings needs to be replaced by \".

Another thing to notice is that missing fields are represented by #f, and applying string operators to them will produce an error. You should always check if a field is missing before applying string operators. See the “Filtering” section in readtags(1) to know how to do this. Run “readtags -H filter” to see which operators take string arguments.

Build Filter/Sorter Expressions using Lisp Languages

Client tools written in Lisp could build the expression using lists. prin1 (in Common Lisp style Lisps) and write (in Scheme style Lisps) can translate the list into a string that can be directly used. For example, in EmacsLisp:

(let ((name "hi"))
  (prin1 `(eq? $name ,name)))
=> "(eq\\? $name "hi")"

The “?” is escaped, and readtags can handle it.

Escape sequences produced by write in Scheme style Lisps are exactly those supported by readtags, so any legal readtags expressions can be used. Common Lisp style Lisps may produce escape sequences that are unrecgonized by readtags, like \#, so symbols that contain “#” can’t be used. Readtags provides some aliases for these Lisps, so they should:

  • Use true for #t.

  • Use false for #f.

  • Use nil or () for ().

  • Use (string->regexp "PATTERN") for #/PATTERN/. Use (string->regexp "PATTERN" :case-fold true) for #/PATTERN/i. Notice that string->regexp doesn’t require escaping “/” in the pattern.

Notice that if the client tool talks to readtags through a shell, then in the produced string, ' still needs to be replaced by '"'"' to prevent broken expressions and shell injection.

Parse Readtags Output

In the output of readtags, tabs can appear in all field values (e.g., the tag name itself could contain tabs), which makes it hard to split the line into fields. Client tools should use the -E option, which keeps the escape sequences in the tags file, so the only field that could contain tabs is the pattern field.

The pattern field could:

  • Use a line number. It will look like number;" (e.g. 10;").

  • Use a search pattern. It will look like /pattern/;" or ?pattern?;". Notice that the search pattern could contain tabs.

  • Combine these two, like number;/pattern/;" or number;?pattern?;".

These are true for tags files using extended format, which is the default one. The legacy format (i.e. --format=1) doesn’t include the semicolons. It’s old and barely used, so we won’t discuss it here.

Client tools could split the line using the following steps:

  • Find the first 2 tabs in the line, so we get the name and input field.

  • From the 2nd tab:

    • If a / follows, then the pattern delimiter is /.

    • If a ? follows, then the pattern delimiter is ?.

    • If a number follows, then:

      • If a ;/ follows the number, then the delimiter is /.

      • If a ;? follows the number, then the delimiter is ?.

      • If a ;" follows the number, then the field uses only line number, and there’s no pattern delimiter (since there’s no regex pattern). In this case the pattern field ends at the 3rd tab.

  • After the opening delimiter, find the next unescaped pattern delimiter, and that’s the closing delimiter. It will be followed by ;" and then a tab. That’s the end of the pattern field. By “unescaped pattern delimiter”, we mean there’s an even number (including 0) of backslashes before it.

  • From here, split the rest of the line into fields by tabs.

Then, the escape sequences in fields other than the pattern field should be translated. See “Proposal” in tags(5) to know about all the escape sequences.

Make Use of the Pattern Field

The pattern field specifies how to find a tag in its source file. The code generating this field seems to have a long history, so there are some pitfalls and it’s a bit hard to handle. A client tool could simply require the line: field and jump to the line it specifies, to avoid using the pattern field. But anyway, we’ll discuss how to make the best use of it here.

You should take the words here merely as suggestions, and not standards. A client tool could definitely develop better (or simpler) ways to use the pattern field.

From the last section, we know the pattern field could contain a line number and a search pattern. When it only contains the line number, handling it is easy: you simply go to that line.

The search pattern resembles an EX command, but as we’ll see later, it’s actually not a valid one, so some manual work are required to process it.

The search pattern could look like /pat/, called “forward search pattern”, or ?pat?, called “backward search pattern”. Using a search pattern means even if the source file is updated, as long as the part containing the tag doesn’t change, we could still locate the tag correctly by searching.

When the pattern field only contains the search pattern, you just search for it. The search direction (forward/backward) doesn’t matter, as it’s decided solely by whether the -B option is enabled, and not the actual context. You could always start the search from say the beginning of the file.

When both the search pattern and the line number are presented, you could make good use of the line number, by going to the line first, then searching for the nearest occurrence of the pattern. A way to do this is to search both forward and backward for the pattern, and when there is a occurrence on both sides, go to the nearer one.

What’s good about this is when there are multiple identical lines in the source file (e.g. the COMMON block in Fortran), this could help us find the correct one, even after the source file is updated and the tag position is shifted by a few lines.

Now let’s discuss how to search for the pattern. After you trim the / or ? around it, the pattern resembles a regex pattern. It should be a regex pattern, as required by being a valid EX command, but it’s actually not, as you’ll see below.

It could begin with a ^, which means the pattern starts from the beginning of a line. It could also end with an unescaped $ which means the pattern ends at the end of a line. Let’s keep this information, and trim them too.

Now the remaining part is the actual string containing the tag. Some characters are escaped:

  • \.

  • $, but only at the end of the string.

  • /, but only in forward search patterns.

  • ?, but only in backward search patterns.

You need to unescape these to get the literal string. Now you could convert this literal string to a regexp that matches it (by escaping, like re.escape in Python or regexp-quote in Elisp), and assemble it with ^ or $ if the pattern originally has it, and finally search for the tag using this regexp.

Remark: About a Previous Format of the Pattern Field

In some earlier versions of Universal Ctags, the line number in the pattern field is the actual line number minus one, for forward search patterns; or plus one, for backward search patterns. The idea is to resemble an EX command: you go to the line, then search forward/backward for the pattern, and you can always find the correct one. But this denies the purpose of using a search pattern: to tolerate file updates. For example, the tag is at line 50, according to this scheme, the pattern field should be:

49;/pat/;"

Then let’s assume that some code above are removed, and the tag is now at line 45. Now you can’t find it if you search forward from line 49.

Due to this reason, Universal Ctags turns to use the actual line number. A client tool could distinguish them by the TAG_OUTPUT_EXCMD pseudo tag, it’s “combine” for the old scheme, and “combineV2” for the present scheme. But probably there’s no need to treat them differently, since “search for the nearest occurrence from the line” gives good result on both schemes.

JSON OUTPUT

See ctags-json-output(5).

CHANGES

Version 6.0

  • ctags enables TAG_KIND_DESCRIPTION, TAG_ROLE_DESCRIPTION, TAG_FIELD_DESCRIPTION, and TAG_EXTRA_DESCRIPTION pseudo tags by default.

  • TAG_PARSER_VERSION is introduced.

SEE ALSO

ctags(1), ctags-lang-python(7), ctags-incompatibilities(7), tags(5), ctags-json-output(5), readtags(1), 7.2 Libtool’s versioning system <https://www.gnu.org/software/libtool/manual/libtool.html#Libtool-versioning>