Another general feature of Emacs that applies to programmers is the etags facility. etags works with code in many other languages as well, including Fortran, Java, Perl, Pascal, LATEX,, Lisp, and many assembly languages. If you work on large, multifile projects, you will find etags to be an enormous help.
etags is basically a multifile search facility that knows about C and Perl function definitions as well as searching in general. With it, you can find a function anywhere in an entire directory without having to remember in which file the function is defined, and you can do searches and query-replaces that span multiple files. etags uses tag tables, which contain lists of function names for each file in a directory along with information on where the functions' definitions are located within the files. Many of the commands associated with etags involve regular expressions (see Chapter 11) in search strings.
To use etags, you must first invoke the separate etags program in your current directory to create the tag table. Its arguments are the files for which you want tag information. The usual way to invoke it is etags *.[ch], that is, building a tag table from all files ending in .c or .h. (That's for you C programmers; other languages would use their appropriate extensions, of course.) You can run etags from shell mode or with the command M-! (for shell-command). The output of etags is the file TAGS, which is the tag table. When you are writing code, you can update your tag table to reflect new files and function definitions by invoking etags again.
After you have created the tag table, you need to make it known to Emacs. To do this, type M-x visit-tags-table Enter. This prompts you for the name of the tag table file; the default is TAGS in the current directory, as you would expect. After you execute this step, you can use the various Emacs tags commands.
The most important tag command is M-. (for find-tag). This command prompts you for a string to use in searching the tag table for a function whose name contains the string. Supply the search string, and Emacs visits the file containing the matching function name in the current window and goes to the first line of the function's definition. A variation of M-. is C-x 4 . (for find-tag-other-window), which uses another window instead of replacing the text in your current window.
A nice feature of M-. is that it picks up the word the cursor is on and uses it as the default search string. For example, if your cursor is anywhere on the string my_function, M-. uses my_function as the default. Thus, when you are looking at a C statement that calls a function, you can type M-. to see the code for that function.
If you have multiple functions with the same name, M-. finds the function in the file whose name comes first in alphabetical order. To find the others, you can use the command M-, (for tags-loop-continue) to find the next one (or complain if there are no more). This feature is especially useful if your directory contains more than one program, that is, if there is more than one function called main. M-, also has other uses, as we will see.
You can use the tag table to search for more than just function definitions. The command M-x tags-search Enter prompts for a regular expression; it searches through all files listed in the tag table (such as, all .c and .h files) for any occurrence of the regular expression, whether it is a function name or not. This capability is similar to the grep facility discussed earlier in this chapter. After you have invoked tags-search, you can find additional matches by typing M-,.
There is also an analogous query-replace capability. The command M-x tags-query-replace Enter does a regular expression query-replace (see Chapter 3) on all files listed in the tag table. As with the regular query-replace-regexp command, if you precede tags-query-replace with a prefix argument (i.e., C-u M-x tags-query-replace Enter), Emacs replaces only matches that are whole words. This feature is useful, for example, if you want to replace occurrences of printf without disturbing occurrences of fprintf. If you exit a tags-query-replace with Esc or C-g, you can resume it later by typing M-,.
The command M-x tags-apropos rounds out the search facilities of etags. If you give it a regular expression argument, it opens a
*Tags List* buffer that contains a list of all tags in the tag table (including names of files as well as functions) that match the regular expression. For example, if you want to find out the names of output routines in a multiple-file C program, you could invoke tags-apropos with the argument print or write.
Finally, you can type M-x list-tags Enter to list all the tags in the table—that is, all the functions—for a given C file. Supply the filename at the prompt, and you get a
*Tags List* buffer showing the names of functions defined in that file along with their return types (if any). Note that if you move your cursor to this list, you can use M-. to look at the actual code for the function. M-. picks up the word the cursor is on as the default function name, so you can just move the cursor to the name of the function you want to see and press M-. followed by Enter to see it.
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