Hints for developing a tool using ctags command and tags output

Manual group:Universal Ctags
Manual section:7


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


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, 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, “2”, 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.


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.


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 normal 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.

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.

TAG_FILE_ENCODING (new in Universal Ctags)
See also tags(5).
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.

TAG_KIND_SEPARATOR (new in Universal Ctags)
TAG_OUTPUT_EXCMD (new in Universal Ctags)
Indicates the specified type of EX command with --excmd option.
TAG_OUTPUT_FILESEP (new in Universal Ctags)
TAG_OUTPUT_MODE (new in Universal Ctags)
TAG_PATTERN_LENGTH_LIMIT (new in Universal Ctags)
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_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.






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 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 '"'"'

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

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. Scheme style Lisps should do proper escaping so the expression readtags gets is just the expression passed into write. Common Lisp style Lisps may produce unrecognized escape sequences by readtags, like \#. Readtags provides some aliases for these Lisps:

  • 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 even when the client tool uses this method, ' still needs to be replaced by '"'"' to prevent broken expressions and shell injection.

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.

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 occurence of the pattern. A way to do this is to search both forward and backward for the pattern, and when there is a occurence 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:


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 occurence from the line” gives good result on both schemes.