.. _ctags-client-tools(7):

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ctags-client-tools

==============================================================

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Hints for developing a tool using @CTAGS_NAME_EXECUTABLE@ command and tags output

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:Version: @VERSION@

:Manual group: Universal Ctags

:Manual section: 7

SYNOPSIS

--------

| **@CTAGS_NAME_EXECUTABLE@** [options] [file(s)]

| **@ETAGS_NAME_EXECUTABLE@** [options] [file(s)]

DESCRIPTION

-----------

**Client tool** means a tool running the @CTAGS_NAME_EXECUTABLE@ command

and/or reading a tags file generated by @CTAGS_NAME_EXECUTABLE@ command.

This man page gathers hints for people who develop client tools.

PSEUDO-TAGS

-----------

**Pseudo-tags**, stored in a tag file, indicate how

@CTAGS_NAME_EXECUTABLE@ 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_NAME_EXECUTABLE@ 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_NAME_EXECUTABLE@ 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_NAME_EXECUTABLE@ 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 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_NAME_EXECUTABLE@ --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_NAME_EXECUTABLE@ --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)

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_NAME_EXECUTABLE@ --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)

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)

TBW

``TAG_OUTPUT_MODE`` (new in Universal Ctags)

TBW

``TAG_PATTERN_LENGTH_LIMIT`` (new in Universal Ctags)

TBW

``TAG_PROC_CWD`` (new in Universal Ctags)

Indicates the working directory of @CTAGS_NAME_EXECUTABLE@ 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_NAME_EXECUTABLE@ 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``

TBW

``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.

.. code-block:: console

$ 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 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:

.. code-block:: 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 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

-----------

Universal Ctags supports `JSON <https://www.json.org/>`_ (strictly

speaking `JSON Lines <https://jsonlines.org/>`_) output format if the

ctags executable is built with ``libjansson``. JSON output goes to

standard output by default.

Format

~~~~~~

Each JSON line represents a tag.

.. code-block:: console

$ ctags --extras=+p --output-format=json --fields=-s input.py

{"_type": "ptag", "name": "JSON_OUTPUT_VERSION", "path": "0.0", "pattern": "in development"}

{"_type": "ptag", "name": "TAG_FILE_SORTED", "path": "1", "pattern": "0=unsorted, 1=sorted, 2=foldcase"}

...

{"_type": "tag", "name": "Klass", "path": "/tmp/input.py", "pattern": "/^class Klass:$/", "language": "Python", "kind": "class"}

{"_type": "tag", "name": "method", "path": "/tmp/input.py", "pattern": "/^ def method(self):$/", "language": "Python", "kind": "member", "scope": "Klass", "scopeKind": "class"}

...

A key not starting with ``_`` is mapped to a field of ctags.

"``--output-format=json --list-fields``" options list the fields.

A key starting with ``_`` represents meta information of the JSON

line. Currently only ``_type`` key is used. If the value for the key

is ``tag``, the JSON line represents a normal tag. If the value is

``ptag``, the line represents a pseudo-tag.

The output format can be changed in the

future. ``JSON_OUTPUT_VERSION`` pseudo-tag provides a change

client-tools to handle the changes. Current version is "0.0". A

client-tool can extract the version with ``path`` key from the

pseudo-tag.

The JSON output format is newly designed and has no limitation found

in the default tags file format.

* The values for ``kind`` key are represented in long-name flags.

No one-letter is here.

* Scope names and scope kinds have distinguished keys: ``scope`` and ``scopeKind``.

They are combined in the default tags file format.

Data type used in a field

~~~~~~~~~~~~~~~~~~~~~~~~~

Values for the most of all keys are represented in JSON string type.

However, some of them are represented in string, integer, and/or boolean type.

"``--output-format=json --list-fields``" options show What kind of data type

used in a field of JSON.

.. code-block:: console

$ ctags --output-format=json --list-fields

#LETTER NAME ENABLED LANGUAGE JSTYPE FIXED DESCRIPTION

F input yes NONE s-- no input file

...

P pattern yes NONE s-b no pattern

...

f file yes NONE --b no File-restricted scoping

...

e end no NONE -i- no end lines of various items

...

``JSTYPE`` column shows the data types.

'``s``'

string

'``i``'

integer

'``b``'

boolean (true or false)

For an example, the value for ``pattern`` field of ctags takes a string or a boolean value.

SEE ALSO

--------

ctags(1), ctags-lang-python(7), ctags-incompatibilities(7), tags(5), readtags(1)