33.3.2 Creating Tags Tables

The etags program is used to create a tags table file. It knows the syntax of several languages, as described in Source File Tag Syntax. 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If the tags table data become outdated due to changes in the files described in the table, the way to update the tags table is the same way it was made in the first place. If the tags table fails to record a tag, or records it for the wrong file, then Emacs cannot possibly find its definition until you update the tags table. However, if the position recorded in the tags table becomes a little bit wrong (due to other editing), the worst consequence is a slight delay in finding the tag. Even if the stored position is very far wrong, Emacs will still find the tag, after searching most of the file for it. That delay is hardly noticeable with today's computers.

Thus, there is no need to update the tags table after each edit. 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.

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 the tags file is in ‘/dev’, however, the file names are made relative to the current working directory. This is useful, for example, when writing the tags to ‘/dev/stdout’.

When using a a relative file name, it should not be a symbolic link pointing to a tags file in a different directory, because this would generally render the file names invalid.

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 (see section Etags Regexps).

The option ‘--parse-stdin=file’ is mostly useful when calling etags from programs. It can be used (only once) in place of a file name on the command line. Etags will read from standard input and mark the produced tags as belonging to the file file.

‘etags --help’ outputs 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. If followed by one or more ‘--language=lang’ options, it outputs detailed information about how tags are generated for lang.