Maintaining Large Programs
**************************

This chapter describes Emacs features for maintaining large programs.  The version control features (see Version Control) are also particularly useful for this purpose.

Change Logs
===========

A change log file contains a chronological record of when and why you have changed a program, consisting of a sequence of entries describing individual changes.  Normally it is kept in a file called ChangeLog in the same directory as the file you are editing, or one of its parent directories.  A single ChangeLog file can record changes for all the files in its directory and all its subdirectories.

The Emacs command C-x 4 a adds a new entry to the change log file for the file you are editing (add-change-log-entry-other-window).  If that file is actually a backup file, it makes an entry appropriate for the file's parent---that is useful for making log entries for functions that have been deleted in the current version.

C-x 4 a visits the change log file and creates a new entry unless the most recent entry is for today's date and your name.  It also creates a new item for the current file.  For many languages, it can even guess the name of the function or other object that was changed.

When the variable add-log-keep-changes-together is non-nil, C-x 4 a adds to any existing item for the file rather than starting a new item.

If add-log-always-start-new-record is non-nil, C-x 4 a always makes a new entry, even if the last entry was made by you and on the same date.

If the value of the variable change-log-version-info-enabled is non-nil, C-x 4 a adds the file's version number to the change log entry.  It finds the version number by searching the first ten percent of the file, using regular expressions from the variable change-log-version-number-regexp-list.

The change log file is visited in Change Log mode.  In this major mode, each bunch of grouped items counts as one paragraph, and each entry is considered a page.  This facilitates editing the entries. C-j and auto-fill indent each new line like the previous line; this is convenient for entering the contents of an entry.

You can use the command M-x change-log-merge to merge other log files into a buffer in Change Log Mode, preserving the date ordering of entries.

Version control systems are another way to keep track of changes in your program and keep a change log.  See Log Buffer.

Format of ChangeLog
===================

A change log entry starts with a header line that contains the current date, your name, and your email address (taken from the variable add-log-mailing-address).  Aside from these header lines, every line in the change log starts with a space or a tab.  The bulk of the entry consists of items, each of which starts with a line starting with whitespace and a star.  Here are two entries, both dated in May 1993, with two items and one item respectively.

     1993-05-25  Richard Stallman  <rms@gnu.org>

             * man.el: Rename symbols `man-*' to `Man-*'.
             (manual-entry): Make prompt string clearer.

             * simple.el (blink-matching-paren-distance):
             Change default to 12,000.

     1993-05-24  Richard Stallman  <rms@gnu.org>

             * vc.el (minor-mode-map-alist): Don't use it if it's void.
             (vc-cancel-version): Doc fix.

One entry can describe several changes; each change should have its own item, or its own line in an item.  Normally there should be a blank line between items.  When items are related (parts of the same change, in different places), group them by leaving no blank line between them.

You should put a copyright notice and permission notice at the end of the change log file.  Here is an example:

     Copyright 1997, 1998 Free Software Foundation, Inc.
     Copying and distribution of this file, with or without modification, are
     permitted provided the copyright notice and this notice are preserved.

Of course, you should substitute the proper years and copyright holder.

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.  When a file parsed by etags is generated from a different source file, like a C file generated from a Cweb source file, the tags of the parsed file reference the source file.

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++. See Ebrowse (Ebrowse User's Manual).

Source File Tag Syntax
----------------------

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

  * In C code, any C function or typedef is a tag, and so are definitions of struct, union and enum. #define macro definitions, #undef and enum constants are also tags, unless you specify --no-defines when making the tags table. Similarly, global variables are tags, unless you specify --no-globals, and so are struct members, unless you specify --no-members.  Use of --no-globals, --no-defines and --no-members can make the tags table file much smaller.

     You can tag function declarations and external variables in addition to function definitions by giving the --declarations option to etags.

  * In C++ code, in addition to all the tag constructs of C code, member functions are also recognized; member variables are also recognized, unless you use the --no-members option.  Tags for variables and functions in classes are named CLASS::VARIABLE and CLASS::FUNCTION.  operator definitions have tag names like operator+.

  * In Java code, tags include all the constructs recognized in C++, plus the interface, extends and implements constructs. Tags for variables and functions in classes are named CLASS.VARIABLE and CLASS.FUNCTION.

  * In LaTeX text, the argument of any of the commands \chapter, \section, \subsection, \subsubsection, \eqno, \label, \ref, \cite, \bibitem, \part, \appendix, \entry, \index, \def, \newcommand, \renewcommand, \newenvironment or \renewenvironment is a tag.

     Other commands can make tags as well, if you specify them in the environment variable TEXTAGS before invoking etags.  The value of this environment variable should be a colon-separated list of command names.  For example,

          TEXTAGS="mycommand:myothercommand"
          export TEXTAGS

     specifies (using Bourne shell syntax) that the commands \mycommand and \myothercommand also define tags.

  * In Lisp code, any function defined with defun, any variable defined with defvar or defconst, and in general the first argument of any expression that starts with (def in column zero is a tag.

  * In Scheme code, tags include anything defined with def or with a construct whose name starts with def.  They also include variables set with set! at top level in the file.

Several other languages are also supported:

  * In Ada code, functions, procedures, packages, tasks and types are tags.  Use the --packages-only option to create tags for packages only.

     In Ada, the same name can be used for different kinds of entity (e.g., for a procedure and for a function).  Also, for things like packages, procedures and functions, there is the spec (i.e. the interface) and the body (i.e. the implementation).  To make it easier to pick the definition you want, Ada tag name have suffixes indicating the type of entity:

     /b
          package body.

     /f
          function.

     /k
          task.

     /p
          procedure.

     /s
          package spec.

     /t
          type.

     Thus, M-x find-tag RET bidule/b RET will go directly to the body of the package bidule, while M-x find-tag RET bidule RET will just search for any tag bidule.

  * In assembler code, labels appearing at the beginning of a line, followed by a colon, are tags.

  * In Bison or Yacc input files, each rule defines as a tag the nonterminal it constructs.  The portions of the file that contain C code are parsed as C code.

  * In Cobol code, tags are paragraph names; that is, any word starting in column 8 and followed by a period.

  * In Erlang code, the tags are the functions, records and macros defined in the file.

  * In Fortran code, functions, subroutines and block data are tags.

  * In HTML input files, the tags are the title and the h1, h2, h3 headers.  Also, tags are name= in anchors and all occurrences of id=.

  * In Lua input files, all functions are tags.

  * In makefiles, targets are tags; additionally, variables are tags unless you specify --no-globals.

  * In Objective C code, tags include Objective C definitions for classes, class categories, methods and protocols.  Tags for variables and functions in classes are named CLASS::VARIABLE and CLASS::FUNCTION.

  * In Pascal code, the tags are the functions and procedures defined in the file.

  * In Perl code, the tags are the packages, subroutines and variables defined by the package, sub, my and local keywords.  Use --globals if you want to tag global variables. Tags for subroutines are named PACKAGE::SUB.  The name for subroutines defined in the default package is main::SUB.

  * In PHP code, tags are functions, classes and defines.  Vars are tags too, unless you use the --no-members option.

  * In PostScript code, the tags are the functions.

  * In Prolog code, tags are predicates and rules at the beginning of line.

  * In Python code, def or class at the beginning of a line generate a tag.

You can also generate tags based on regexp matching (see Etags Regexps) to handle other formats and languages.

Creating Tags Tables
--------------------

The etags program is used to create a tags table file.  It knows the syntax of several languages, as described in 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.

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 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.

Etags Regexps
-------------

The --regex option provides a general way of recognizing tags based on regexp matching.  You can freely intermix this option with file names, and each one applies to the source files that follow it. If you specify multiple --regex options, all of them are used in parallel.  The syntax is:

     --regex=[{LANGUAGE}]/TAGREGEXP/[NAMEREGEXP/]MODIFIERS

The essential part of the option value is TAGREGEXP, the regexp for matching tags.  It is always used anchored, that is, it only matches at the beginning of a line.  If you want to allow indented tags, use a regexp that matches initial whitespace; start it with [ \t]*.

In these regular expressions, \ quotes the next character, and all the GCC character escape sequences are supported (\a for bell, \b for back space, \d for delete, \e for escape, \f for formfeed, \n for newline, \r for carriage return, \t for tab, and \v for vertical tab).

Ideally, TAGREGEXP should not match more characters than are needed to recognize what you want to tag.  If the syntax requires you to write TAGREGEXP so it matches more characters beyond the tag itself, 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 MODIFIERS are a sequence of zero or more characters that modify the way etags does the matching.  A regexp with no modifiers is applied sequentially to each line of the input file, in a case-sensitive way.  The modifiers and their meanings are:

i
     Ignore case when matching this regexp.

m
     Match this regular expression against the whole file, so that multi-line matches are possible.

s
     Match this regular expression against the whole file, and allow . in TAGREGEXP to match newlines.

The -R option cancels all the regexps defined by preceding --regex options.  It too applies to the file names following it.  Here's an example:

     etags --regex=/REG1/i voo.doo --regex=/REG2/m \
         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.  REG1 is checked against each line of voo.doo and bar.ber, in a case-insensitive way, while REG2 is checked against the whole bar.ber file, permitting multi-line matches, in a case-sensitive way.  etags uses only the Lisp tags rules, with no user-specified regexp matching, to recognize tags in los.er.

You can restrict a --regex option to match only files of a given language by using the optional prefix {LANGUAGE}. (etags --help prints the list of languages recognized by etags.)  This is particularly useful when storing many predefined regular expressions for etags in a file.  The following example tags the DEFVAR macros in the Emacs source files, for the C language only:

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

When you have complex regular expressions, you can store the list of them 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=@CASE-SENSITIVE-FILE --ignore-case-regex=@IGNORE-CASE-FILE

A regex file for etags contains one regular expression 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 another 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, we 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.

  * Tag Octave files:

          etags --language=none \
                --regex='/[ \t]*function.*=[ \t]*\([^ \t]*\)[ \t]*(/\1/' \
                --regex='/###key \(.*\)/\1/' \
                --regex='/[ \t]*global[ \t].*/' \
                *.m

     Note that tags are not generated for scripts, so that you have to add a line by yourself of the form ###key SCRIPTNAME if you want to jump to it.

  * Tag Tcl files:

          etags --language=none --regex='/proc[ \t]+\([^ \t]+\)/\1/' *.tcl

  * Tag VHDL files:

          etags --language=none \
            --regex='/[ \t]*\(ARCHITECTURE\|CONFIGURATION\) +[^ ]* +OF/' \
            --regex='/[ \t]*\(ATTRIBUTE\|ENTITY\|FUNCTION\|PACKAGE\
            \( BODY\)?\|PROCEDURE\|PROCESS\|TYPE\)[ \t]+\([^ \t(]+\)/\3/'

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, with TAGS in the default directory as the default.

Emacs does not actually read in the tags table contents until you try to use them; all visit-tags-table does is store the file name in the variable tags-file-name, and setting the variable yourself is just as good.  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.

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.

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).

M-*
     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 balanced expression in the buffer before or around point is used as the TAG argument.  See Expressions.

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.

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.  These commands scan the list of tags tables starting 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.

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.

M-,
     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.  See Replace, 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 (see Search Case).

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. See Grep Searching.

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 (see Apropos).  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 by setting the variable tags-tag-face to a 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.  See Symbol Completion.

     [file emerge-xtra.texi not included]