Maintaining Programs
********************

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

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

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.

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.

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.

A change log entry starts with a header line that contains the current date, your name, and your email address (taken from the variable 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 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:

     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.  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.

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

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.

Versions of Emacs before 20.1 used a different format for the time of the change log entry:

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

The 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.  See Log Buffer.

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 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.  Use of --no-globals and --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 --declarations option to etags.

  * 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 --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, \newcomand, \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 makefiles, targets are tags.

  * 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.  When using the --members option, vars are tags too.

  * 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 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 --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 (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 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.

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:

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

or else:

     --regex=@REGEXFILE

where TAGREGEXP is used to find the tags.  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 all the gcc character escape sequences are supported.  Here is the list of the character escape sequences:

\a
     BEL (bell).

\b
     BS (back space).

\d
     DEL (delete).

\e
     ESC (delete).

\f
     FF (form feed).

\n
     NL (new line).

\r
     CR (carriage return).

\t
     TAB (horizontal tab).

\v
     VT (vertical tab).

The syntax of regular expressions in etags is the same as in Emacs.

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 sometimes 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.

A --regex option can be restricted to match only files of a given language using the optional prefix {LANGUAGE}.  This is particularly useful when storing many predefined regular expressions for etags in a file.

The MODIFIERS are a sequence of 0 or more characters that modify the way etags does the matching.  Without modifiers, each regexp 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.

m
     do not match line by line; rather, match the whole file, so that multi-line matches are possible.

s
     implies m, and causes dots in TAGREGEXP to match newlines as well.

A REGEXFILE is the name of a file where regular expressions are stored, one per line.  Lines beginning with space or tab are ignored, and can be used for adding comments.

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 recognized 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.

  * 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.  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.

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.

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.  This works much like running a compilation; finding the source locations of the grep matches works like finding the compilation errors. See Compilation.

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 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.  See Symbol Completion.

Merging Files with Emerge
=========================

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 Comparing Files, for commands to compare in a more manual fashion, and Ediff (The Ediff Manual).

Overview of Emerge
------------------

To start Emerge, run one of these four commands:

M-x emerge-files
     Merge two specified files.

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

M-x emerge-buffers
     Merge two buffers.

M-x emerge-buffers-with-ancestor
     Merge two buffers with reference to a common ancestor in a third buffer.

The Emerge commands compare two files or buffers, and display the comparison in three buffers: one for each input text (the A buffer and the B buffer), and one (the 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 (see 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 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 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 selected difference.  This difference is marked off in the three buffers like this:

     vvvvvvvvvvvvvvvvvvvv
     TEXT THAT DIFFERS
     ^^^^^^^^^^^^^^^^^^^^

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 C-x C-w.  If you give a numeric argument to emerge-files or 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 C-], the Emerge command does not save the output buffer, but you can save it yourself if you wish.

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 C-c C-c, and the normal Emacs commands are also available.  This allows editing the merge buffer, but slows down Emerge operations.

Use e to switch to Edit mode, and C-c C-c f to switch to Fast mode.  The mode line indicates Edit and Fast modes with E and 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 a and 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 A.

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

Use the command s a (emerge-auto-advance-mode) to set or clear Auto Advance mode.  Use s s (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.

State of a Difference
---------------------

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

A
     The difference is showing the A version.  The a command always produces this state; the mode line indicates it with A.

B
     The difference is showing the B version.  The b command always produces this state; the mode line indicates it with B.

default-A
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 d a chooses default-A as the default state, and 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 d a and d b between sections.

prefer-A
prefer-B
     The difference is showing the A or B state because it is 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 A* and B*.

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

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

     The mode line displays this state as comb.

Merge Commands
--------------

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

p
     Select the previous difference.

n
     Select the next difference.

a
     Choose the A version of this difference.

b
     Choose the B version of this difference.

C-u N j
     Select difference number N.

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

q
     Quit---finish the merge.

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

f
     Go into Fast mode.  (In Edit mode, this is actually C-c C-c f.)

e
     Go into Edit mode.

l
     Recenter (like C-l) all three windows.

-
     Specify part of a prefix numeric argument.

DIGIT
     Also specify part of a prefix numeric argument.

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

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

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

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

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

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

m
     Put point and mark around the difference.

^
     Scroll all three windows down (like M-v).

v
     Scroll all three windows up (like C-v).

<
     Scroll all three windows left (like C-x <).

>
     Scroll all three windows right (like C-x >).

|
     Reset horizontal scroll on all three windows.

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

x c
     Combine the two versions of this difference (see Combining in Emerge).

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

x j
     Join this difference with the following one. (C-u x j joins this difference with the previous one.)

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.

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.

Exiting Emerge
--------------

The q command (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.

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 t for successful completion, or nil if you abort.

Combining the Two Versions
--------------------------

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

     #ifdef NEW
     VERSION FROM A BUFFER
     #else /* not NEW */
     VERSION FROM B BUFFER
     #endif /* not NEW */

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

     "#ifdef NEW\n%a#else /* not NEW */\n%b#endif /* not NEW */\n"

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 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.

After setting up the merge, Emerge runs the hook emerge-startup-hook (see Hooks).