emacs/man/maintaining.texi

@c This is part of the Emacs manual.
@c Copyright (C) 1985,86,87,93,94,95,97,99,00,2001 Free Software Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Maintaining, Abbrevs, Building, Top
@chapter Maintaining Programs
@cindex Lisp editing
@cindex C editing
@cindex program editing

  This chapter describes Emacs features for maintaining programs.  The
version control features (@pxref{Version Control}) are also
particularly useful for this purpose.

@menu
* Change Log::	        Maintaining a change history for your program.
* Tags::	        Go direct to any function in your program in one
			  command.  Tags remembers which file it is in.
* Emerge::	        A convenient way of merging two versions of a program.
@end menu

@node Change Log
@section Change Logs

@cindex change log
@kindex C-x 4 a
@findex add-change-log-entry-other-window
  The Emacs command @kbd{C-x 4 a} adds a new entry to the change log
file for the file you are editing
(@code{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.

  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
@file{ChangeLog} in the same directory as the file you are editing, or
one of its parent directories.  A single @file{ChangeLog} file can
record changes for all the files in its directory and all its
subdirectories.

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

@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.
@end example

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

  A change log entry starts with a header line that contains the
current date, your name, and your email address (taken from the
variable @code{user-mail-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 @dfn{items}, each of which starts with a line
starting with whitespace and a star.  Here are two entries, both dated
in May 1993, each with two items:

@iftex
@medbreak
@end iftex
@smallexample
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.
@end smallexample

  One entry can describe several changes; each change should have its
own 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.  The second entry above
contains two items grouped in this way.

  @kbd{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.

@vindex add-log-keep-changes-together
  When the option @code{add-log-keep-changes-together} is
non-@code{nil}, @kbd{C-x 4 a} adds to any existing entry for the file
rather than starting a new entry.

@vindex change-log-version-info-enabled
@vindex change-log-version-number-regexp-list
@cindex file version in change log entries
  If the value of the variable @code{change-log-version-info-enabled}
is non-@code{nil}, @kbd{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
@code{change-log-version-number-regexp-list}.

@cindex Change Log mode
@findex change-log-mode
  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.
@kbd{C-j} and auto-fill indent each new line like the previous line;
this is convenient for entering the contents of an entry.

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

@findex change-log-redate
@cindex converting change log date style
  Versions of Emacs before 20.1 used a different format for the time of
the change log entry:

@smallexample
Fri May 25 11:23:23 1993 Richard Stallman  <rms@@gnu.org>
@end smallexample

@noindent
The @kbd{M-x change-log-redate} command converts all the old-style
date entries in the change log file visited in the current buffer to
the new format, to make the file uniform in style.  This is handy when
entries are contributed by many different people, some of whom use old
versions of Emacs.

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

@ignore
@c This is commented out because the command is specific
@c to maintenance of Emacs itself.

@node Authors
@section @file{AUTHORS} files
@cindex @file{AUTHORS} file

  Programs which have many contributors usually include a file named
@file{AUTHORS} in their distribution, which lists the individual
contributions.  Emacs has a special command for maintaining the
@file{AUTHORS} file that is part of the Emacs distribution.

@findex authors
  The @kbd{M-x authors} command prompts for the name of the root of the
Emacs source directory.  It then scans @file{ChangeLog} files and Lisp
source files under that directory for information about authors of
individual packages, and people who made changes in source files, and
puts the information it gleans into a buffer named @samp{*Authors*}.
You can then edit the contents of that buffer and merge it with the
existing @file{AUTHORS} file.

  Do not assume that this command finds all the contributors; don't
assume that a person not listed in the output was not a contributor.
If you merged in someone's contribution and did not put his name
in the change log, he won't show up in @kbd{M-x authors} either.
@end ignore

@node Tags
@section Tags Tables
@cindex tags table

  A @dfn{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 @kbd{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 @dfn{tags table files}.  The
conventional name for a tags table file is @file{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 @code{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 @dfn{tag}.

@cindex C++ class browser, tags
@cindex tags, C++
@cindex class browser, C++
@cindex Ebrowse
  See also the Ebrowse facility, which is tailored for C++.
@xref{Top,, Ebrowse, ebrowse, Ebrowse User's Manual}.

@menu
* Tag Syntax::		Tag syntax for various types of code and text files.
* Create Tags Table::	Creating a tags table with @code{etags}.
* Etags Regexps::       Create arbitrary tags using regular expressions.
* Select Tags Table::	How to visit a tags table.
* Find Tag::		Commands to find the definition of a specific tag.
* Tags Search::		Using a tags table for searching and replacing.
* List Tags::		Listing and finding tags defined in a file.
@end menu

@node Tag Syntax
@subsection Source File Tag Syntax

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

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

You can tag function declarations and external variables in addition
to function definitions by giving the @samp{--declarations} option to
@code{etags}.

@item
In C++ code, in addition to all the tag constructs of C code, member
functions are also recognized, and optionally member variables if you
use the @samp{--members} option.  Tags for variables and functions in
classes are named @samp{@var{class}::@var{variable}} and
@samp{@var{class}::@var{function}}.  @code{operator} definitions have
tag names like @samp{operator+}.

@item
In Java code, tags include all the constructs recognized in C++, plus
the @code{interface}, @code{extends} and @code{implements} constructs.
Tags for variables and functions in classes are named
@samp{@var{class}.@var{variable}} and @samp{@var{class}.@var{function}}.

@item
In La@TeX{} text, the argument of any of the commands @code{\chapter},
@code{\section}, @code{\subsection}, @code{\subsubsection},
@code{\eqno}, @code{\label}, @code{\ref}, @code{\cite}, @code{\bibitem},
@code{\part}, @code{\appendix}, @code{\entry}, or @code{\index}, is a
tag.@refill

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

@example
TEXTAGS="def:newcommand:newenvironment"
export TEXTAGS
@end example

@noindent
specifies (using Bourne shell syntax) that the commands @samp{\def},
@samp{\newcommand} and @samp{\newenvironment} also define tags.

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

@item
In Scheme code, tags include anything defined with @code{def} or with a
construct whose name starts with @samp{def}.  They also include variables
set with @code{set!} at top level in the file.
@end itemize

  Several other languages are also supported:

@itemize @bullet

@item
In Ada code, functions, procedures, packages, tasks, and types are
tags.  Use the @samp{--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:

@table @samp
@item /b
package body.
@item /f
function.
@item /k
task.
@item /p
procedure.
@item /s
package spec.
@item /t
type.
@end table

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

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

@item
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.

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

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

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

@item
In makefiles, targets are tags.

@item
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 @samp{@var{class}::@var{variable}} and
@samp{@var{class}::@var{function}}.

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

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

@item
In PHP code, tags are functions, classes and defines.  When using the
@samp{--members} option, vars are tags too.

@item
In PostScript code, the tags are the functions.

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

@item
In Python code, @code{def} or @code{class} at the beginning of a line
generate a tag.
@end itemize

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

@node Create Tags Table
@subsection Creating Tags Tables
@cindex @code{etags} program

  The @code{etags} program is used to create a tags table file.  It knows
the syntax of several languages, as described in
@iftex
the previous section.
@end iftex
@ifinfo
@ref{Tag Syntax}.
@end ifinfo
Here is how to run @code{etags}:

@example
etags @var{inputfiles}@dots{}
@end example

@noindent
The @code{etags} program reads the specified files, and writes a tags
table named @file{TAGS} in the current working directory.

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

  @code{etags} recognizes the language used in an input file based on
its file name and contents.  You can specify the language with the
@samp{--language=@var{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 only 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.  Even that
delay is hardly noticeable with today's computers.

  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 @samp{--include=@var{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
@code{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 @code{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
@samp{/}, or with @samp{@var{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 @code{etags} to read the file names from its standard input,
by typing a dash in place of the file names, like this:

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

  Use the option @samp{--language=@var{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
@samp{--language=auto} to tell @code{etags} to resume guessing the
language from the file names and file contents.  Specify
@samp{--language=none} to turn off language-specific processing
entirely; then @code{etags} recognizes tags by regexp matching alone
(@pxref{Etags Regexps}).

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

@node Etags Regexps
@subsection Etags Regexps

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

@smallexample
--regex=/@var{tagregexp}[/@var{nameregexp}]/
@end smallexample

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

  The syntax of regular expressions in @code{etags} is the same as in
Emacs.  However, non-greedy operators and shy groups are not
available.

  You should not match more characters with @var{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 @var{tagregexp}
(as will usually be the case), you should add a @var{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 @samp{--ignore-case-regex} (or @samp{-c}) works like
@samp{--regex}, except that matching ignores case.  This is
appropriate for certain programming languages.

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

@smallexample
etags --regex=/@var{reg1}/ voo.doo --regex=/@var{reg2}/ \
    bar.ber -R --lang=lisp los.er
@end smallexample

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

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

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

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

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

@noindent
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 @samp{@@}, @code{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 @samp{--}, that line is a comment.

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

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

@noindent
and then use it like this:

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

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

@itemize @bullet

@item
Tag Octave files:

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

@noindent
Note that tags are not generated for scripts, so that you have to add
a line by yourself of the form @samp{###key @var{scriptname}} if you
want to jump to it.

@item
Tag Tcl files:

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

@item
Tag VHDL files:

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

@node Select Tags Table
@subsection Selecting a Tags Table

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

  All this command does is store the file name in the variable
@code{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 @code{visit-tags-table}.  The variable's initial value is
@code{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 @code{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 @emph{instead} of others.  If you add the new table to the
current list, it is used @emph{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.

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

@c keep this on two lines for formatting in smallbook
@example
@group
(setq tags-table-list
      '("~/emacs" "/usr/local/lib/emacs/src"))
@end group
@end example

@noindent
This tells the tags commands to look at the @file{TAGS} files in your
@file{~/emacs} directory and in the @file{/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 @code{tags-file-name} and @code{tags-table-list}.

@node Find Tag
@subsection Finding a Tag

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

@table @kbd
@item M-.@: @var{tag} @key{RET}
Find first definition of @var{tag} (@code{find-tag}).
@item C-u M-.
Find next alternate definition of last tag specified.
@item C-u - M-.
Go back to previous tag found.
@item C-M-. @var{pattern} @key{RET}
Find a tag whose name matches @var{pattern} (@code{find-tag-regexp}).
@item C-u C-M-.
Find the next tag whose name matches the last pattern used.
@item C-x 4 .@: @var{tag} @key{RET}
Find first definition of @var{tag}, but display it in another window
(@code{find-tag-other-window}).
@item C-x 5 .@: @var{tag} @key{RET}
Find first definition of @var{tag}, and create a new frame to select the
buffer (@code{find-tag-other-frame}).
@item M-*
Pop back to where you previously invoked @kbd{M-.} and friends.
@end table

@kindex M-.
@findex find-tag
  @kbd{M-.}@: (@code{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 @code{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 @key{RET}), the balanced
expression in the buffer before or around point is used as the
@var{tag} argument.  @xref{Expressions}.

  You don't need to give @kbd{M-.} the full name of the tag; a part
will do.  This is because @kbd{M-.} finds tags in the table which
contain @var{tag} as a substring.  However, it prefers an exact match
to a substring match.  To find other tags that match the same
substring, give @code{find-tag} a numeric argument, as in @kbd{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 @key{META} key, @kbd{M-0 M-.}@: is an easier
alternative to @kbd{C-u M-.}.

@kindex C-x 4 .
@findex find-tag-other-window
@kindex C-x 5 .
@findex find-tag-other-frame
  Like most commands that can switch buffers, @code{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 @kbd{C-x 4 .}, which invokes
the command @code{find-tag-other-window}.  The latter is @kbd{C-x 5 .},
which invokes @code{find-tag-other-frame}.

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

@kindex M-*
@findex pop-tag-mark
@vindex find-tag-marker-ring-length
  As well as going back to places you've found tags recently, you can go
back to places @emph{from where} you found them.  Use @kbd{M-*}, which
invokes the command @code{pop-tag-mark}, for this.  Typically you would
find and study the definition of something with @kbd{M-.} and then
return to where you were with @kbd{M-*}.

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

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

@node Tags Search
@subsection Searching and Replacing with Tags Tables
@cindex search and replace in multiple files
@cindex multiple-file search and replace

  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.

@table @kbd
@item M-x tags-search @key{RET} @var{regexp} @key{RET}
Search for @var{regexp} through the files in the selected tags
table.
@item M-x tags-query-replace @key{RET} @var{regexp} @key{RET} @var{replacement} @key{RET}
Perform a @code{query-replace-regexp} on each file in the selected tags table.
@item M-,
Restart one of the commands above, from the current location of point
(@code{tags-loop-continue}).
@end table

@findex tags-search
  @kbd{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,
@code{tags-search} returns.

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

@findex tags-query-replace
  @kbd{M-x tags-query-replace} performs a single
@code{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 @kbd{M-x query-replace-regexp}.  It searches much like @kbd{M-x
tags-search}, but repeatedly, processing matches according to your
input.  @xref{Replace}, for more information on query replace.

@vindex tags-case-fold-search
@cindex case-sensitivity and tags search
  You can control the case-sensitivity of tags search commands by
customizing the value of the variable @code{tags-case-fold-search}.  The
default is to use the same setting as the value of
@code{case-fold-search} (@pxref{Search Case}).

  It is possible to get through all the files in the tags table with a
single invocation of @kbd{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 @kbd{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
@code{find-tag} family.  The @code{find-tag} commands search only for
definitions of tags that match your substring or regexp.  The commands
@code{tags-search} and @code{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 @code{tags-search} is a lot like
@code{grep}.  You can also run @code{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
@code{grep} matches works like finding the compilation errors.
@xref{Compilation}.

@node List Tags
@subsection Tags Table Inquiries

@table @kbd
@item M-x list-tags @key{RET} @var{file} @key{RET}
Display a list of the tags defined in the program file @var{file}.
@item M-x tags-apropos @key{RET} @var{regexp} @key{RET}
Display a list of all tags matching @var{regexp}.
@end table

@findex list-tags
  @kbd{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.

@findex tags-apropos
@vindex tags-apropos-verbose
  @kbd{M-x tags-apropos} is like @code{apropos} for tags
(@pxref{Apropos}).  It finds all the tags in the selected tags table
whose entries match @var{regexp}, and displays them.  If the variable
@code{tags-apropos-verbose} is non-@code{nil}, it displays the names
of the tags files together with the tag names.

@vindex tags-tag-face
@vindex tags-apropos-additional-actions
You can customize the appearance of the output with the face
@code{tags-tag-face}.  You can display additional output with @kbd{M-x
tags-apropos} by customizing the variable
@code{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.  @xref{Symbol Completion}.

@node Emerge
@section Merging Files with Emerge
@cindex Emerge
@cindex merging files

It's not unusual for programmers to get their signals crossed and modify
the same program in two different directions.  To recover from this
confusion, you need to merge the two versions.  Emerge makes this
easier.  See also @ref{Comparing Files}, for commands to compare
in a more manual fashion, and @ref{Top, Ediff,, ediff, The Ediff Manual}.

@menu
* Overview of Emerge::	How to start Emerge.  Basic concepts.
* Submodes of Emerge::	Fast mode vs. Edit mode.
			  Skip Prefers mode and Auto Advance mode.
* State of Difference::	You do the merge by specifying state A or B
			  for each difference.
* Merge Commands::	Commands for selecting a difference,
			  changing states of differences, etc.
* Exiting Emerge::	What to do when you've finished the merge.
* Combining in Emerge::	    How to keep both alternatives for a difference.
* Fine Points of Emerge::   Misc.
@end menu

@node Overview of Emerge
@subsection Overview of Emerge

To start Emerge, run one of these four commands:

@table @kbd
@item M-x emerge-files
@findex emerge-files
Merge two specified files.

@item M-x emerge-files-with-ancestor
@findex emerge-files-with-ancestor
Merge two specified files, with reference to a common ancestor.

@item M-x emerge-buffers
@findex emerge-buffers
Merge two buffers.

@item M-x emerge-buffers-with-ancestor
@findex emerge-buffers-with-ancestor
Merge two buffers with reference to a common ancestor in a third
buffer.
@end table

@cindex merge buffer (Emerge)
@cindex A and B buffers (Emerge)
  The Emerge commands compare two files or buffers, and display the
comparison in three buffers: one for each input text (the @dfn{A buffer}
and the @dfn{B buffer}), and one (the @dfn{merge buffer}) where merging
takes place.  The merge buffer shows the full merged text, not just the
differences.  Wherever the two input texts differ, you can choose which
one of them to include in the merge buffer.

  The Emerge commands that take input from existing buffers use only the
accessible portions of those buffers, if they are narrowed
(@pxref{Narrowing}).

  If a common ancestor version is available, from which the two texts to
be merged were both derived, Emerge can use it to guess which
alternative is right.  Wherever one current version agrees with the
ancestor, Emerge presumes that the other current version is a deliberate
change which should be kept in the merged version.  Use the
@samp{with-ancestor} commands if you want to specify a common ancestor
text.  These commands read three file or buffer names---variant A,
variant B, and the common ancestor.

  After the comparison is done and the buffers are prepared, the
interactive merging starts.  You control the merging by typing special
@dfn{merge commands} in the merge buffer.  The merge buffer shows you a
full merged text, not just differences.  For each run of differences
between the input texts, you can choose which one of them to keep, or
edit them both together.

  The merge buffer uses a special major mode, Emerge mode, with commands
for making these choices.  But you can also edit the buffer with
ordinary Emacs commands.

  At any given time, the attention of Emerge is focused on one
particular difference, called the @dfn{selected} difference.  This
difference is marked off in the three buffers like this:

@example
vvvvvvvvvvvvvvvvvvvv
@var{text that differs}
^^^^^^^^^^^^^^^^^^^^
@end example

@noindent
Emerge numbers all the differences sequentially and the mode
line always shows the number of the selected difference.

  Normally, the merge buffer starts out with the A version of the text.
But when the A version of a difference agrees with the common ancestor,
then the B version is initially preferred for that difference.

  Emerge leaves the merged text in the merge buffer when you exit.  At
that point, you can save it in a file with @kbd{C-x C-w}.  If you give a
numeric argument to @code{emerge-files} or
@code{emerge-files-with-ancestor}, it reads the name of the output file
using the minibuffer.  (This is the last file name those commands read.)
Then exiting from Emerge saves the merged text in the output file.

  Normally, Emerge commands save the output buffer in its file when you
exit.  If you abort Emerge with @kbd{C-]}, the Emerge command does not
save the output buffer, but you can save it yourself if you wish.

@node Submodes of Emerge
@subsection Submodes of Emerge

  You can choose between two modes for giving merge commands: Fast mode
and Edit mode.  In Fast mode, basic merge commands are single
characters, but ordinary Emacs commands are disabled.  This is
convenient if you use only merge commands.  In Edit mode, all merge
commands start with the prefix key @kbd{C-c C-c}, and the normal Emacs
commands are also available.  This allows editing the merge buffer, but
slows down Emerge operations.

  Use @kbd{e} to switch to Edit mode, and @kbd{C-c C-c f} to switch to
Fast mode.  The mode line indicates Edit and Fast modes with @samp{E}
and @samp{F}.

  Emerge has two additional submodes that affect how particular merge
commands work: Auto Advance mode and Skip Prefers mode.

  If Auto Advance mode is in effect, the @kbd{a} and @kbd{b} commands
advance to the next difference.  This lets you go through the merge
faster as long as you simply choose one of the alternatives from the
input.  The mode line indicates Auto Advance mode with @samp{A}.

  If Skip Prefers mode is in effect, the @kbd{n} and @kbd{p} commands
skip over differences in states prefer-A and prefer-B (@pxref{State of
Difference}).  Thus you see only differences for which neither version
is presumed ``correct.''  The mode line indicates Skip Prefers mode with
@samp{S}.

@findex emerge-auto-advance-mode
@findex emerge-skip-prefers-mode
  Use the command @kbd{s a} (@code{emerge-auto-advance-mode}) to set or
clear Auto Advance mode.  Use @kbd{s s}
(@code{emerge-skip-prefers-mode}) to set or clear Skip Prefers mode.
These commands turn on the mode with a positive argument, turns it off
with a negative or zero argument, and toggle the mode with no argument.

@node State of Difference
@subsection State of a Difference

  In the merge buffer, a difference is marked with lines of @samp{v} and
@samp{^} characters.  Each difference has one of these seven states:

@table @asis
@item A
The difference is showing the A version.  The @kbd{a} command always
produces this state; the mode line indicates it with @samp{A}.

@item B
The difference is showing the B version.  The @kbd{b} command always
produces this state; the mode line indicates it with @samp{B}.

@item default-A
@itemx default-B
The difference is showing the A or the B state by default, because you
haven't made a choice.  All differences start in the default-A state
(and thus the merge buffer is a copy of the A buffer), except those for
which one alternative is ``preferred'' (see below).

When you select a difference, its state changes from default-A or
default-B to plain A or B.  Thus, the selected difference never has
state default-A or default-B, and these states are never displayed in
the mode line.

The command @kbd{d a} chooses default-A as the default state, and @kbd{d
b} chooses default-B.  This chosen default applies to all differences
which you haven't ever selected and for which no alternative is preferred.
If you are moving through the merge sequentially, the differences you
haven't selected are those following the selected one.  Thus, while
moving sequentially, you can effectively make the A version the default
for some sections of the merge buffer and the B version the default for
others by using @kbd{d a} and @kbd{d b} between sections.

@item prefer-A
@itemx prefer-B
The difference is showing the A or B state because it is
@dfn{preferred}.  This means that you haven't made an explicit choice,
but one alternative seems likely to be right because the other
alternative agrees with the common ancestor.  Thus, where the A buffer
agrees with the common ancestor, the B version is preferred, because
chances are it is the one that was actually changed.

These two states are displayed in the mode line as @samp{A*} and @samp{B*}.

@item combined
The difference is showing a combination of the A and B states, as a
result of the @kbd{x c} or @kbd{x C} commands.

Once a difference is in this state, the @kbd{a} and @kbd{b} commands
don't do anything to it unless you give them a numeric argument.

The mode line displays this state as @samp{comb}.
@end table

@node Merge Commands
@subsection Merge Commands

  Here are the Merge commands for Fast mode; in Edit mode, precede them
with @kbd{C-c C-c}:

@table @kbd
@item p
Select the previous difference.

@item n
Select the next difference.

@item a
Choose the A version of this difference.

@item b
Choose the B version of this difference.

@item C-u @var{n} j
Select difference number @var{n}.

@item .
Select the difference containing point.  You can use this command in the
merge buffer or in the A or B buffer.

@item q
Quit---finish the merge.

@item C-]
Abort---exit merging and do not save the output.

@item f
Go into Fast mode.  (In Edit mode, this is actually @kbd{C-c C-c f}.)

@item e
Go into Edit mode.

@item l
Recenter (like @kbd{C-l}) all three windows.

@item -
Specify part of a prefix numeric argument.

@item @var{digit}
Also specify part of a prefix numeric argument.

@item d a
Choose the A version as the default from here down in
the merge buffer.

@item d b
Choose the B version as the default from here down in
the merge buffer.

@item c a
Copy the A version of this difference into the kill ring.

@item c b
Copy the B version of this difference into the kill ring.

@item i a
Insert the A version of this difference at point.

@item i b
Insert the B version of this difference at point.

@item m
Put point and mark around the difference.

@item ^
Scroll all three windows down (like @kbd{M-v}).

@item v
Scroll all three windows up (like @kbd{C-v}).

@item <
Scroll all three windows left (like @kbd{C-x <}).

@item >
Scroll all three windows right (like @kbd{C-x >}).

@item |
Reset horizontal scroll on all three windows.

@item x 1
Shrink the merge window to one line.  (Use @kbd{C-u l} to restore it
to full size.)

@item x c
Combine the two versions of this difference (@pxref{Combining in
Emerge}).

@item x f
Show the names of the files/buffers Emerge is operating on, in a Help
window.  (Use @kbd{C-u l} to restore windows.)

@item x j
Join this difference with the following one.
(@kbd{C-u x j} joins this difference with the previous one.)

@item x s
Split this difference into two differences.  Before you use this
command, position point in each of the three buffers at the place where
you want to split the difference.

@item x t
Trim identical lines off the top and bottom of the difference.
Such lines occur when the A and B versions are
identical but differ from the ancestor version.
@end table

@node Exiting Emerge
@subsection Exiting Emerge

  The @kbd{q} command (@code{emerge-quit}) finishes the merge, storing
the results into the output file if you specified one.  It restores the
A and B buffers to their proper contents, or kills them if they were
created by Emerge and you haven't changed them.  It also disables the
Emerge commands in the merge buffer, since executing them later could
damage the contents of the various buffers.

  @kbd{C-]} aborts the merge.  This means exiting without writing the
output file.  If you didn't specify an output file, then there is no
real difference between aborting and finishing the merge.

  If the Emerge command was called from another Lisp program, then its
return value is @code{t} for successful completion, or @code{nil} if you
abort.

@node Combining in Emerge
@subsection Combining the Two Versions

  Sometimes you want to keep @emph{both} alternatives for a particular
difference.  To do this, use @kbd{x c}, which edits the merge buffer
like this:

@example
@group
#ifdef NEW
@var{version from A buffer}
#else /* not NEW */
@var{version from B buffer}
#endif /* not NEW */
@end group
@end example

@noindent
@vindex emerge-combine-versions-template
While this example shows C preprocessor conditionals delimiting the two
alternative versions, you can specify the strings to use by setting
the variable @code{emerge-combine-versions-template} to a string of your
choice.  In the string, @samp{%a} says where to put version A, and
@samp{%b} says where to put version B.  The default setting, which
produces the results shown above, looks like this:

@example
@group
"#ifdef NEW\n%a#else /* not NEW */\n%b#endif /* not NEW */\n"
@end group
@end example

@node Fine Points of Emerge
@subsection Fine Points of Emerge

  During the merge, you mustn't try to edit the A and B buffers yourself.
Emerge modifies them temporarily, but ultimately puts them back the way
they were.

  You can have any number of merges going at once---just don't use any one
buffer as input to more than one merge at once, since the temporary
changes made in these buffers would get in each other's way.

  Starting Emerge can take a long time because it needs to compare the
files fully.  Emacs can't do anything else until @code{diff} finishes.
Perhaps in the future someone will change Emerge to do the comparison in
the background when the input files are large---then you could keep on
doing other things with Emacs until Emerge is ready to accept
commands.

@vindex emerge-startup-hook
  After setting up the merge, Emerge runs the hook
@code{emerge-startup-hook} (@pxref{Hooks}).