Introduction

The file format for the "tags" file, as used by Vi and many of its descendants, has limited capabilities.

This additional functionality is desired:

1. Static or local tags. The scope of these tags is the file where they are defined. The same tag can appear in several files, without really being a duplicate.
2. Duplicate tags. Allow the same tag to occur more then once. They can be located in a different file and/or have a different command.
3. Support for C++. A tag is not only specified by its name, but also by the context (the class name).
4. Future extension. When even more additional functionality is desired, it must be possible to add this later, without breaking programs that don't support it.

From proposal to standard

To make this proposal into a standard for tags files, it needs to be supported by most people working on versions of Vi, ctags, etc.. Currently this standard is supported by:

Darren Hiebert <dhiebert at users.sourceforge.net>

Exuberant Ctags

Bram Moolenaar <Bram at vim.org>

Vim (Vi IMproved)

These have been or will be asked to support this standard:

Nvi

Keith Bostic <bostic at bsdi.com>

Vile

Tom E. Dickey <dickey at clark.net>

NEdit

Mark Edel <edel at ltx.com>

CRiSP

Paul Fox <fox at crisp.demon.co.uk>

Lemmy

James Iuliano <jai at accessone.com>

Zeus

Jussi Jumppanen <jussij at ca.com.au>

Elvis

Steve Kirkendall <kirkenda at cs.pdx.edu>

FTE

Marko Macek <Marko.Macek at snet.fri.uni-lj.si>

Backwards compatibility

A tags file that is generated in the new format should still be usable by Vi. This makes it possible to distribute tags files that are usable by all versions and descendants of Vi.

This restricts the format to what Vi can handle. The format is:

1. The tags file is a list of lines, each line in the format:

   {tagname}<Tab>{tagfile}<Tab>{tagaddress}
   

   {tagname}

   Any identifier, not containing white space..

   EXCEPTION: Universal Ctags violates this item of the proposal; tagname may contain spaces. However, tabs are not allowed.

   <Tab>

   Exactly one TAB character (although many versions of Vi can handle any amount of white space).

   {tagfile}

   The name of the file where {tagname} is defined, relative to the current directory (or location of the tags file?).

   {tagaddress}

   Any Ex command. When executed, it behaves like 'magic' was not set.

2. The tags file is sorted on {tagname}. This allows for a binary search in the file.

3. Duplicate tags are allowed, but which one is actually used is unpredictable (because of the binary search).

The best way to add extra text to the line for the new functionality, without breaking it for Vi, is to put a comment in the {tagaddress}. This gives the freedom to use any text, and should work in any traditional Vi implementation.

For example, when the old tags file contains:

main    main.c  /^main(argc, argv)$/
DEBUG   defines.c       89

The new lines can be:

main    main.c  /^main(argc, argv)$/;"any additional text
DEBUG   defines.c       89;"any additional text

Note that the ';' is required to put the cursor in the right line, and then the '"' is recognized as the start of a comment.

For Posix compliant Vi versions this will NOT work, since only a line number or a search command is recognized. I hope Posix can be adjusted. Nvi suffers from this.

Security

Vi allows the use of any Ex command in a tags file. This has the potential of a trojan horse security leak.

The proposal is to allow only Ex commands that position the cursor in a single file. Other commands, like editing another file, quitting the editor, changing a file or writing a file, are not allowed. It is therefore logical to call the command a tagaddress.

Specifically, these two Ex commands are allowed:

• A decimal line number:

  89
  

• A search command. It is a regular expression pattern, as used by Vi, enclosed in // or ??:

  /^int c;$/
  ?main()?
  

There are two combinations possible:

• Concatenation of the above, with ';' in between. The meaning is that the first line number or search command is used, the cursor is positioned in that line, and then the second search command is used (a line number would not be useful). This can be done multiple times. This is useful when the information in a single line is not unique, and the search needs to start in a specified line.

  /struct xyz {/;/int count;/
  389;/struct foo/;/char *s;/
  

• A trailing comment can be added, starting with ';"' (two characters: semi-colon and double-quote). This is used below.

  89;" foo bar
  

This might be extended in the future. What is currently missing is a way to position the cursor in a certain column.

Goals

Now the usage of the comment text has to be defined. The following is aimed at:

1. Keep the text short, because:
   • The line length that Vi can handle is limited to 512 characters.
   • Tags files can contain thousands of tags. I have seen tags files of several Mbytes.
   • More text makes searching slower.
2. Keep the text readable, because:
   • It is often necessary to check the output of a new ctags program.
   • Be able to edit the file by hand.
   • Make it easier to write a program to produce or parse the file.
3. Don't use special characters, because:
   • It should be possible to treat a tags file like any normal text file.

Proposal

Use a comment after the {tagaddress} field. The format would be:

{tagname}<Tab>{tagfile}<Tab>{tagaddress}[;"<Tab>{tagfield}..]

{tagname}

Any identifier, not containing white space..

EXCEPTION: Universal Ctags violates this item of the proposal; name may contain spaces. However, tabs are not allowed. Conversion, for some characters including <Tab> in the "value", explained in the last of this section is applied.

<Tab>

Exactly one TAB character (although many versions of Vi can handle any amount of white space).

{tagfile}

The name of the file where {tagname} is defined, relative to the current directory (or location of the tags file?).

{tagaddress}

Any Ex command. When executed, it behaves like 'magic' was not set. It may be restricted to a line number or a search pattern (Posix).

Optionally:

;"

semicolon + doublequote: Ends the tagaddress in way that looks like the start of a comment to Vi.

{tagfield}

See below.

A tagfield has a name, a colon, and a value: "name:value".

• The name consist only out of alphabetical characters. Upper and lower case are allowed. Lower case is recommended. Case matters ("kind:" and "Kind: are different tagfields).

  EXCEPTION: Universal Ctags allows users to use a numerical character in the name other than its initial letter.

• The value may be empty. It cannot contain a <Tab>.

  • When a value contains a \t, this stands for a <Tab>.
  • When a value contains a \r, this stands for a <CR>.
  • When a value contains a \n, this stands for a <NL>.
  • When a value contains a \\, this stands for a single \ character.

  Other use of the backslash character is reserved for future expansion. Warning: When a tagfield value holds an MS-DOS file name, the backslashes must be doubled!

  EXCEPTION: Universal Ctags introduces more conversion rules.

  • When a value contains a \a, this stands for a <BEL> (0x07).
  • When a value contains a \b, this stands for a <BS> (0x08).
  • When a value contains a \v, this stands for a <VT> (0x0b).
  • When a value contains a \f, this stands for a <FF> (0x0c).
  • The characters in range 0x01 to 0x1F included, and 0x7F are converted to \x prefixed hexadecimal number if the characters are not handled in the above "value" rules.
  • The leading space (0x20) and ! (0x21) in {tagname} are converted to \x prefixed hexadecimal number (\x20 and \x21) if the tag is not a pseudo-tag. As described later, a pseudo-tag starts with !. These rules are for distinguishing pseudo-tags and non pseudo-tags (regular tags) when tags lines in a tag file are sorted.

Proposed tagfield names:

Note that these are mostly for C and C++. When tags programs are written for other languages, this list should be extended to include the used field names. This will help users to be independent of the tags program used.

Examples:

asdf    sub.cc  /^asdf()$/;"    new_field:some\svalue   file:
foo_t   sub.h   /^typedef foo_t$/;"     kind:t
func3   sub.p   /^func3()$/;"   function:/func1/func2   file:
getflag sub.c   /^getflag(arg)$/;"      kind:f  file:
inc     sub.cc  /^inc()$/;"     file: class:PipeBuf

The name of the "kind:" field can be omitted. This is to reduce the size of the tags file by about 15%. A program reading the tags file can recognize the "kind:" field by the missing ':'. Examples:

foo_t   sub.h   /^typedef foo_t$/;"     t
getflag sub.c   /^getflag(arg)$/;"      f       file:

Additional remarks:

• When a tagfield appears twice in a tag line, only the last one is used.

Note about line separators:

Vi traditionally runs on Unix systems, where the line separator is a single linefeed character <NL>. On MS-DOS and compatible systems <CR><NL> is the standard line separator. To increase portability, this line separator is also supported.

On the Macintosh a single <CR> is used for line separator. Supporting this on Unix systems causes problems, because most fgets() implementation don't see the <CR> as a line separator. Therefore the support for a <CR> as line separator is limited to the Macintosh.

Summary:

The characters <CR> and <LF> cannot be used inside a tag line. This is not mentioned elsewhere (because it's obvious).

Note about white space:

Vi allowed any white space to separate the tagname from the tagfile, and the filename from the tagaddress. This would need to be allowed for backwards compatibility. However, all known programs that generate tags use a single <Tab> to separate fields.

There is a problem for using file names with embedded white space in the tagfile field. To work around this, the same special characters could be used as in the new fields, for example \s. But, unfortunately, in MS-DOS the backslash character is used to separate file names. The file name c:\vim\sap contains \s, but this is not a <Space>. The number of backslashes could be doubled, but that will add a lot of characters, and make parsing the tags file slower and clumsy.

To avoid these problems, we will only allow a <Tab> to separate fields, and not support a file name or tagname that contains a <Tab> character. This means that we are not 100% Vi compatible. However, there is no known tags program that uses something else than a <Tab> to separate the fields. Only when a user typed the tags file himself, or made his own program to generate a tags file, we could run into problems. To solve this, the tags file should be filtered, to replace the arbitrary white space with a single <Tab>. This Vi command can be used:

:%s/^\([^ ^I]*\)[ ^I]*\([^ ^I]*\)[ ^I]*/\1^I\2^I/

(replace ^I with a real <Tab>).

TAG FILE INFORMATION:

Pseudo-tag lines can be used to encode information into the tag file regarding details about its content (e.g. have the tags been sorted?, are the optional tagfields present?), and regarding the program used to generate the tag file. This information can be used both to optimize use of the tag file (e.g. enable/disable binary searching) and provide general information (what version of the generator was used).

The names of the tags used in these lines may be suitably chosen to ensure that when sorted, they will always be located near the first lines of the tag file. The use of "!_TAG_" is recommended. Note that a rare tag like "!" can sort to before these lines. The program reading the tags file should be smart enough to skip over these tags.

The lines described below have been chosen to convey a select set of information.

Tag lines providing information about the content of the tag file:

!_TAG_FILE_FORMAT   {version-number}        /optional comment/
!_TAG_FILE_SORTED   {0|1}                   /0=unsorted, 1=sorted/

The {version-number} used in the tag file format line reserves the value of "1" for tag files complying with the original UNIX vi/ctags format, and reserves the value "2" for tag files complying with this proposal. This value may be used to determine if the extended features described in this proposal are present.

Tag lines providing information about the program used to generate the tag file, and provided solely for documentation purposes:

!_TAG_PROGRAM_AUTHOR        {author-name}   /{email-address}/
!_TAG_PROGRAM_NAME  {program-name}  /optional comment/
!_TAG_PROGRAM_URL   {URL}   /optional comment/
!_TAG_PROGRAM_VERSION       {version-id}    /optional comment/

EXCEPTION: Universal Ctags introduces more kinds of pseudo-tags. See ctags-client-tools(7) about them.

Exceptions

1. {tagname} in tags file generated by Universal Ctags may contain spaces and several escape sequences. Parsers for documents like Tex and reStructuredText, or liberal languages such as JavaScript need these exceptions. See {tagname} of Proposal section for more detail about the conversion.
2. "name" part of {tagfield} in a tag generated by Universal Ctags may contain numeric characters, but the first character of the "name" must be alphabetic.

Compatible output and weakness

Default behavior (--output-format=u-ctags option) has the exceptions. In other hand, with --output-format=e-ctags option ctags has no exception; Universal Ctags command may use the same file format as Exuberant Ctags. However, --output-format=e-ctags throws away a tag entry which name includes a space or a tab character. TAG_OUTPUT_MODE pseudo-tag tells which format is used when ctags generating tags file.