24.16. Tags Tables

A tags table is a description of how a multi-file program is broken up into files. It lists the names of the component files and the names and positions of the functions (or other named subunits) in each file. Grouping the related files makes it possible to search or replace through all the files with one command. Recording the function names and positions makes possible the M-. command which finds the definition of a function by looking up which of the files it is in.

Tags tables are stored in files called tags table files. The conventional name for a tags table file is TAGS.

Each entry in the tags table records the name of one tag, the name of the file that the tag is defined in (implicitly), and the position in that file of the tag's definition.

Just what names from the described files are recorded in the tags table depends on the programming language of the described file. They normally include all file names, functions and subroutines, and may also include global variables, data types, and anything else convenient. Each name recorded is called a tag.

See also the Ebrowse facility, which is tailored for C++. .

24.16.1. Source File Tag Syntax

Here is how tag syntax is defined for the most popular languages:

Several other languages are also supported:

You can also generate tags based on regexp matching (Section 24.16.3) to handle other formats and languages.

24.16.2. Creating Tags Tables

The etags program is used to create a tags table file. It knows the syntax of several languages, as described in Section 24.16.1. Here is how to run etags:

etags inputfiles

The etags program reads the specified files, and writes a tags table named TAGS in the current working directory.

If the specified files don't exist, etags looks for compressed versions of them and uncompresses them to read them. Under MS-DOS, etags also looks for file names like mycode.cgz if it is given mycode.c on the command line and mycode.c does not exist.

etags recognizes the language used in an input file based on its file name and contents. You can specify the language with the -language=name option, described below.

If the tags table data become outdated due to changes in the files described in the table, the way to update the tags table is the same way it was made in the first place. But it is not necessary to do this very often.

If the tags table fails to record a tag, or records it for the wrong file, then Emacs cannot possibly find its definition. However, if the position recorded in the tags table becomes a little bit wrong (due to some editing in the file that the tag definition is in), the only consequence is a slight delay in finding the tag. Even if the stored position is very wrong, Emacs will still find the tag, but it must search the entire file for it.

So you should update a tags table when you define new tags that you want to have listed, or when you move tag definitions from one file to another, or when changes become substantial. Normally there is no need to update the tags table after each edit, or even every day.

One tags table can virtually include another. Specify the included tags file name with the -include=file option when creating the file that is to include it. The latter file then acts as if it covered all the source files specified in the included file, as well as the files it directly contains.

If you specify the source files with relative file names when you run etags, the tags file will contain file names relative to the directory where the tags file was initially written. This way, you can move an entire directory tree containing both the tags file and the source files, and the tags file will still refer correctly to the source files.

If you specify absolute file names as arguments to etags, then the tags file will contain absolute file names. This way, the tags file will still refer to the same files even if you move it, as long as the source files remain in the same place. Absolute file names start with /, or with device:/ on MS-DOS and MS-Windows.

When you want to make a tags table from a great number of files, you may have problems listing them on the command line, because some systems have a limit on its length. The simplest way to circumvent this limit is to tell etags to read the file names from its standard input, by typing a dash in place of the file names, like this:

find . -name "*.[chCH]" -print | etags -

Use the option -language=name to specify the language explicitly. You can intermix these options with file names; each one applies to the file names that follow it. Specify -language=auto to tell etags to resume guessing the language from the file names and file contents. Specify -language=none to turn off language-specific processing entirely; then etags recognizes tags by regexp matching alone (Section 24.16.3).

etags -help prints the list of the languages etags knows, and the file name rules for guessing the language. It also prints a list of all the available etags options, together with a short explanation.

24.16.3. Etags Regexps

The -regex option provides a general way of recognizing tags based on regexp matching. You can freely intermix it with file names. Each -regex option adds to the preceding ones, and applies only to the following files. The syntax is:


where tagregexp is used to match the lines to tag. It is always anchored, that is, it behaves as if preceded by ^. If you want to account for indentation, just match any initial number of blanks by beginning your regular expression with [ \t]*. In the regular expressions, \ quotes the next character, and \t stands for the tab character. Note that etags does not handle the other C escape sequences for special characters.

The syntax of regular expressions in etags is the same as in Emacs, augmented with the interval operator, which works as in grep and ed. The syntax of an interval operator is \{m,n\}, and its meaning is to match the preceding expression at least m times and up to n times.

You should not match more characters with tagregexp than that needed to recognize what you want to tag. If the match is such that more characters than needed are unavoidably matched by tagregexp (as will usually be the case), you should add a nameregexp, to pick out just the tag. This will enable Emacs to find tags more accurately and to do completion on tag names more reliably. You can find some examples below.

The option -ignore-case-regex (or -c) works like -regex, except that matching ignores case. This is appropriate for certain programming languages.

The -R option deletes all the regexps defined with -regex options. It applies to the file names following it, as you can see from the following example:

etags --regex=/reg1/ voo.doo --regex=/reg2/ \
    bar.ber -R --lang=lisp los.er

Here etags chooses the parsing language for voo.doo and bar.ber according to their contents. etags also uses reg1 to recognize additional tags in voo.doo, and both reg1 and reg2 to recognize additional tags in bar.ber. etags uses the Lisp tags rules, and no regexp matching, to recognize tags in los.er.

You can specify a regular expression for a particular language, by writing {lang} in front of it. Then etags will use the regular expression only for files of that language. (etags -help prints the list of languages recognised by etags.) The following example tags the DEFVAR macros in the Emacs source files, for the C language only:

--regex='{c}/[ \t]*DEFVAR_[A-Z_ \t(]+"\([^"]+\)"/'

This feature is particularly useful when you store a list of regular expressions in a file. The following option syntax instructs etags to read two files of regular expressions. The regular expressions contained in the second file are matched without regard to case.

--regex=@first-file --ignore-case-regex=@second-file

A regex file contains one regular expressions per line. Empty lines, and lines beginning with space or tab are ignored. When the first character in a line is @, etags assumes that the rest of the line is the name of a file of regular expressions; thus, one such file can include another file. All the other lines are taken to be regular expressions. If the first non-whitespace text on the line is -, that line is a comment.

For example, one can create a file called emacs.tags with the following contents:

        -- This is for GNU Emacs C source files
{c}/[ \t]*DEFVAR_[A-Z_ \t(]+"\([^"]+\)"/\1/

and then use it like this:

etags --regex=@emacs.tags *.[ch] */*.[ch]

Here are some more examples. The regexps are quoted to protect them from shell interpretation.

24.16.4. Selecting a Tags Table

Emacs has at any time one selected tags table, and all the commands for working with tags tables use the selected one. To select a tags table, type M-x visit-tags-table, which reads the tags table file name as an argument. The name TAGS in the default directory is used as the default file name.

All this command does is store the file name in the variable tags-file-name. Emacs does not actually read in the tags table contents until you try to use them. Setting this variable yourself is just as good as using visit-tags-table. The variable's initial value is nil; that value tells all the commands for working with tags tables that they must ask for a tags table file name to use.

Using visit-tags-table when a tags table is already loaded gives you a choice: you can add the new tags table to the current list of tags tables, or start a new list. The tags commands use all the tags tables in the current list. If you start a new list, the new tags table is used instead of others. If you add the new table to the current list, it is used as well as the others. When the tags commands scan the list of tags tables, they don't always start at the beginning of the list; they start with the first tags table (if any) that describes the current file, proceed from there to the end of the list, and then scan from the beginning of the list until they have covered all the tables in the list.

You can specify a precise list of tags tables by setting the variable tags-table-list to a list of strings, like this:

(setq tags-table-list
      '("~/emacs" "/usr/local/lib/emacs/src"))

This tells the tags commands to look at the TAGS files in your ~/emacs directory and in the /usr/local/lib/emacs/src directory. The order depends on which file you are in and which tags table mentions that file, as explained above.

Do not set both tags-file-name and tags-table-list.

24.16.5. Finding a Tag

The most important thing that a tags table enables you to do is to find the definition of a specific tag.

M-. tag RET

Find first definition of tag (find-tag).

C-u M-.

Find next alternate definition of last tag specified.

C-u - M-.

Go back to previous tag found.

C-M-. pattern RET

Find a tag whose name matches pattern (find-tag-regexp).

C-u C-M-.

Find the next tag whose name matches the last pattern used.

C-x 4 . tag RET

Find first definition of tag, but display it in another window (find-tag-other-window).

C-x 5 . tag RET

Find first definition of tag, and create a new frame to select the buffer (find-tag-other-frame).


Pop back to where you previously invoked M-. and friends.

M-. (find-tag) is the command to find the definition of a specified tag. It searches through the tags table for that tag, as a string, and then uses the tags table info to determine the file that the definition is in and the approximate character position in the file of the definition. Then find-tag visits that file, moves point to the approximate character position, and searches ever-increasing distances away to find the tag definition.

If an empty argument is given (just type RET), the sexp in the buffer before or around point is used as the tag argument. Section 24.2, for info on sexps.

You don't need to give M-. the full name of the tag; a part will do. This is because M-. finds tags in the table which contain tag as a substring. However, it prefers an exact match to a substring match. To find other tags that match the same substring, give find-tag a numeric argument, as in C-u M-.; this does not read a tag name, but continues searching the tags table's text for another tag containing the same substring last used. If you have a real META key, M-0 M-. is an easier alternative to C-u M-..

Like most commands that can switch buffers, find-tag has a variant that displays the new buffer in another window, and one that makes a new frame for it. The former is C-x 4 ., which invokes the command find-tag-other-window. The latter is C-x 5 ., which invokes find-tag-other-frame.

To move back to places you've found tags recently, use C-u - M-.; more generally, M-. with a negative numeric argument. This command can take you to another buffer. C-x 4 . with a negative argument finds the previous tag location in another window.

As well as going back to places you've found tags recently, you can go back to places from where you found them. Use M-*, which invokes the command pop-tag-mark, for this. Typically you would find and study the definition of something with M-. and then return to where you were with M-*.

Both C-u - M-. and M-* allow you to retrace your steps to a depth determined by the variable find-tag-marker-ring-length.

The command C-M-. (find-tag-regexp) visits the tags that match a specified regular expression. It is just like M-. except that it does regexp matching instead of substring matching.

24.16.6. Searching and Replacing with Tags Tables

The commands in this section visit and search all the files listed in the selected tags table, one by one. For these commands, the tags table serves only to specify a sequence of files to search.

M-x tags-search RET regexp RET

Search for regexp through the files in the selected tags table.

M-x tags-query-replace RET regexp RET replacement RET

Perform a query-replace-regexp on each file in the selected tags table.


Restart one of the commands above, from the current location of point (tags-loop-continue).

M-x tags-search reads a regexp using the minibuffer, then searches for matches in all the files in the selected tags table, one file at a time. It displays the name of the file being searched so you can follow its progress. As soon as it finds an occurrence, tags-search returns.

Having found one match, you probably want to find all the rest. To find one more match, type M-, (tags-loop-continue) to resume the tags-search. This searches the rest of the current buffer, followed by the remaining files of the tags table.

M-x tags-query-replace performs a single query-replace-regexp through all the files in the tags table. It reads a regexp to search for and a string to replace with, just like ordinary M-x query-replace-regexp. It searches much like M-x tags-search, but repeatedly, processing matches according to your input. Section 14.7, for more information on query replace.

You can control the case-sensitivity of tags search commands by customizing the value of the variable tags-case-fold-search. The default is to use the same setting as the value of case-fold-search (Section 14.6).

It is possible to get through all the files in the tags table with a single invocation of M-x tags-query-replace. But often it is useful to exit temporarily, which you can do with any input event that has no special query replace meaning. You can resume the query replace subsequently by typing M-,; this command resumes the last tags search or replace command that you did.

The commands in this section carry out much broader searches than the find-tag family. The find-tag commands search only for definitions of tags that match your substring or regexp. The commands tags-search and tags-query-replace find every occurrence of the regexp, as ordinary search commands and replace commands do in the current buffer.

These commands create buffers only temporarily for the files that they have to search (those which are not already visited in Emacs buffers). Buffers in which no match is found are quickly killed; the others continue to exist.

It may have struck you that tags-search is a lot like grep. You can also run grep itself as an inferior of Emacs and have Emacs show you the matching lines one by one. This works much like running a compilation; finding the source locations of the grep matches works like finding the compilation errors. Section 25.1.

24.16.7. Tags Table Inquiries

M-x list-tags RET file RET

Display a list of the tags defined in the program file file.

M-x tags-apropos RET regexp RET

Display a list of all tags matching regexp.

M-x list-tags reads the name of one of the files described by the selected tags table, and displays a list of all the tags defined in that file. The "file name" argument is really just a string to compare against the file names recorded in the tags table; it is read as a string rather than as a file name. Therefore, completion and defaulting are not available, and you must enter the file name the same way it appears in the tags table. Do not include a directory as part of the file name unless the file name recorded in the tags table includes a directory.

M-x tags-apropos is like apropos for tags (Section 10.4). It finds all the tags in the selected tags table whose entries match regexp, and displays them. If the variable tags-apropos-verbose is non-nil, it displays the names of the tags files together with the tag names.

You can customize the appearance of the output with the face tags-tag-face. You can display additional output with M-x tags-apropos by customizing the variable tags-apropos-additional-actions--see its documentation for details.

You can also use the collection of tag names to complete a symbol name in the buffer. Section 24.9.