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@c This is part of the Emacs manual.
@c Copyright (C) 1985, 1986, 1987, 1993, 1994, 1995, 1997, 1999, 2000,
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@c   2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008
@c   Free Software Foundation, Inc.
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@c See file emacs.texi for copying conditions.
@node Maintaining, Abbrevs, Building, Top
@chapter Maintaining Large Programs

  This chapter describes Emacs features for maintaining large
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programs.
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@menu
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* Version Control::     Using version control systems.
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* Change Log::	        Maintaining a change history for your program.
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* Tags::	        Go directly to any function in your program in one
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			  command.  Tags remembers which file it is in.
@ifnottex
* Emerge::              A convenient way of merging two versions of a program.
@end ifnottex
@end menu

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@node Version Control
@section Version Control
@cindex version control

  A @dfn{version control system} is a package that can record multiple
versions of a source file, storing information such as the creation
time of each version, who created it, and a description of what was
changed in that version.

  The Emacs version control interface is called VC.  Its commands work
with several different version control systems; currently, it supports
GNU Arch, Bazaar, CVS, Git, Mercurial, Monotone, RCS, SCCS/CSSC, and
Subversion.  Of these, the GNU project distributes CVS, GNU Arch, RCS,
and Bazaar.

  VC is enabled automatically whenever you visit a file that is
governed by a version control system.  To disable VC entirely, set the
customizable variable @code{vc-handled-backends} to @code{nil}
@iftex
(@pxref{Customizing VC,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
@ifnottex
(@pxref{Customizing VC}).
@end ifnottex

@menu
* Introduction to VC::  How version control works in general.
* VC Mode Line::        How the mode line shows version control status.
* Basic VC Editing::    How to edit a file under version control.
* Old Revisions::       Examining and comparing old versions.
* Secondary VC Commands::    The commands used a little less frequently.
* VC Directory Mode::   Listing files managed by version control.
* Branches::            Multiple lines of development.
@ifnottex
* Remote Repositories:: Efficient access to remote CVS servers.
* Revision Tags::       Symbolic names for revisions
* Miscellaneous VC::    Various other commands and features of VC.
* Customizing VC::      Variables that change VC's behavior.
@end ifnottex
@end menu

@node Introduction to VC
@subsection Introduction to Version Control

  VC allows you to use a version control system from within Emacs,
integrating the version control operations smoothly with editing.
Though VC cannot completely bridge the gaps between version control
systems with widely differing capabilities, it does provide a uniform
interface to many version control operations. Regardless of which
version control system is in use, you will be able to do basic
operations in much the same way.

  This section provides a general overview of version control, and
describes the version control systems that VC supports.  You can skip
this section if you are already familiar with the version control system
you want to use.

@menu
* Why Version Control?::    Understanding the problems it addresses
* Version Control Systems:: Supported version control back-end systems.
* VCS Concepts::            Words and concepts related to version control.
* Types of Log File::       The VCS log in contrast to the ChangeLog.
@end menu

@node Why Version Control?
@subsubsection Understanding the problems it addresses

  Version control systems provide you with three important
capabilities:

@itemize @bullet
@item
@dfn{Reversibility}: the ability to back up to a previous state if you
discover that some modification you did was a mistake or a bad idea.

@item
@dfn{Concurrency}: the ability to have many people modifying the same
collection of files knowing that conflicting modifications can be
detected and resolved.

@item
@dfn{History}: the ability to attach historical data to your data,
such as explanatory comments about the intention behind each change to
it.  Even for a programmer working solo, change histories are an
important aid to memory; for a multi-person project, they are a
vitally important form of communication among developers.
@end itemize

@node Version Control Systems
@subsubsection Supported Version Control Systems

@cindex back end (version control)
  VC currently works with many different version control systems or
@dfn{back ends}:

@itemize @bullet

@cindex SCCS
@item
SCCS was the first version control system ever built, and was long ago
superseded by more advanced ones.  VC compensates for certain features
missing in SCCS (e.g., tag names for releases) by implementing them
itself.  Other VC features, such as multiple branches, are simply
unavailable.  Since SCCS is non-free, we recommend avoiding it.

@cindex CSSC
@item
CSSC is a free replacement for SCCS.  You should use CSSC only if, for
some reason, you cannot use a more recent and better-designed version
control system.

@cindex RCS
@item
RCS is the free version control system around which VC was initially
built.  Almost everything you can do with RCS can be done through VC.
However, you cannot use RCS over the network, and it only works at the
level of individual files rather than projects.

@cindex CVS
@item
CVS is the free version control system that was, until recently (circa
2008), used by the majority of free software projects.  Nowadays, it
is slowly being superseded by newer systems.  CVS allows concurrent
multi-user development either locally or over the network.  It lacks
support for atomic commits or file moving/renaming.  VC supports all
basic editing operations under CVS.  For some less common tasks, you
still need to call CVS from the command line.  Note also that before
using CVS you must set up a repository, which is a subject too complex
to treat here.

@cindex SVN
@cindex Subversion
@item
Subversion (SVN) is a free version control system designed to be
similar to CVS but without its problems.  It supports atomic commits
of filesets, and versioning of directories, symbolic links, meta-data,
renames, copies, and deletes.

@cindex GNU Arch
@cindex Arch
@item
GNU Arch is a version control system designed for distributed work.
It differs in many ways from older systems like CVS and RCS.  It
provides different methods for interoperating between users, support
for offline operations, and good branching and merging features.  It
also supports atomic commits of filesets and file moving/renaming.  VC
does not support all operations provided by GNU Arch, so you must
sometimes invoke it from the command line.

@cindex git
@item
Git is a distributed version control system invented by Linus Torvalds to support
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development of Linux (his kernel).  It supports atomic commits of filesets and
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file moving/renaming.  One significant feature of git is that it
largely abolishes the notion of a single centralized repository;
instead, each working copy of a git project is its own repository and
coordination is done through repository-sync operations.  VC supports
most git operations, with the exception of news merges and repository
syncing; these must be done from the command line.

@cindex hg
@cindex Mercurial
@item
Mercurial (hg) is a distributed version control system broadly
resembling GNU Arch and git, with atomic fileset commits and file
moving/renaming.  Like git, it is fully decentralized.  VC supports
most Mercurial commands, with the exception of repository sync
operations; this needs to be done from the command line.

@cindex bzr
@cindex Bazaar
@item
Bazaar (bzr) is a distributed version control system that supports both
repository-based and distributed versioning, with atomic fileset
commits and file moving/renaming.  VC supports most basic editing
operations under Bazaar.
@end itemize

  Previous versions of VC supported a version control system known as
Meta-CVS.  This support has been dropped because of limited interest
from users and developers.

@node VCS Concepts
@subsubsection Concepts of Version Control

@cindex repository
@cindex registered file
   When a file is under version control, we say that it is
@dfn{registered} in the version control system.  The system has a
@dfn{repository} which stores both the file's present state and its
change history---enough to reconstruct the current version or any
earlier version.  The repository also contains other information, such
as @dfn{log entries} that describe the changes made to each file.

@cindex work file
@cindex checking out files
  A file @dfn{checked out} of a repository is called the @dfn{work
file}.  You edit the work file and make changes in it, as you would
with an ordinary file.  After you are done with a set of changes, you
@dfn{check in} or @dfn{commit} the file; this records the changes in
the repository, along with a log entry for those changes.

@cindex revision
@cindex revision ID
  A copy of a file stored in a repository is called a @dfn{revision}.
The history of a file is a sequence of revisions.  Each revisions is
named by a @dfn{revision ID}.  The format of the revision ID depends
on the version control system; in the simplest case, it is just an
integer.

  To go beyond these basic concepts, you will need to understand three
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aspects in which version control systems differ.
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They can be locking-based or merging-based; they can be file-based or
changeset-based; and they can be centralized or decentralized.  VC
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handles all these modes of operation, but it cannot hide the differences.
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@cindex locking versus merging
  A version control system typically has some mechanism to coordinate
between users who want to change the same file.  There are two ways to
do this: merging and locking.

  In a version control system that uses merging, each user may check
out and modify a work file at any time.  The system lets you
@dfn{merge} your work file, which may contain changes that have not
been checked in, with the latest changes that others have checked into
the repository.

  Older version control systems use a @dfn{locking} scheme instead.
Here, work files are normally read-only.  To edit a file, you ask the
version control system to make it writable for you by @dfn{locking}
it; only one user can lock a given file at any given time.  This
procedure is analogous to, but different from, the locking that Emacs
uses to detect simultaneous editing of ordinary files
(@pxref{Interlocking}).  When you check in your changes, that unlocks
the file, and the work file becomes read-only again.  Other users may
then lock the file to make their own changes.

  Both locking and merging systems can have problems when multiple
users try to modify the same file at the same time.  Locking systems
have @dfn{lock conflicts}; a user may try to check a file out and be
unable to because it is locked.  In merging systems, @dfn{merge
conflicts} happen when you check in a change to a file that conflicts
with a change checked in by someone else after your checkout.  Both
kinds of conflict have to be resolved by human judgment and
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communication.  Experience has shown that merging is superior to
locking, both in convenience to developers and in minimizing the
number and severity of conflicts that actually occur.
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  SCCS always uses locking.  RCS is lock-based by default but can be
told to operate in a merging style.  CVS and Subversion are
merge-based by default but can be told to operate in a locking mode.
Distributed version control systems, such as GNU Arch, git, and
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Mercurial, are exclusively merging-based.
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  VC mode supports both locking and merging version control.  The
terms ``checkin'' and ``checkout'' come from locking-based version
control systems; newer version control systems have slightly different
operations usually called ``commit'' and ``update'', but VC hides the
differences between them as much as possible.
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@cindex files versus changesets.
  On SCCS, RCS, CVS, and other early version control systems, version
control operations are @dfn{file-based}: each file has its own comment
and revision history separate from that of all other files in the
system.  Later systems, beginning with Subversion, are
@dfn{changeset-based}: a checkin may include changes to several files,
and the entire set of changes is treated as a unit by the system.  Any
comment associated with the change does not belong to a single file,
but to the changeset itself.

  Changeset-based version control is more flexible and powerful than
file-based version control; usually, when a change to multiple files
has to be reversed, it's good to be able to easily identify and remove
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all of it.
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@cindex centralized vs. decentralized version control
  Early version control systems were designed around a
@dfn{centralized} model in which each project has only one repository
used by all developers.  SCCS, RCS, CVS, and Subversion share this
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kind of model.  One of its drawbacks is that the repository is a choke
point for reliability and efficiency.
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  GNU Arch pioneered the concept of @dfn{decentralized} version
control, later implemented in git, Mercurial, and Bazaar.  A project
may have several different repositories, and these systems support a
sort of super-merge between repositories that tries to reconcile their
change histories.  At the limit, each developer has his/her own
repository, and repository merges replace checkin/commit operations.
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  VC's job is to help you manage the traffic between your personal
workfiles and a repository.  Whether that repository is a single
master or one of a network of peer repositories is not something VC
has to care about.  Thus, the difference between a centralized and a
decentralized version control system is invisible to VC mode.

@node Types of Log File
@subsubsection Types of Log File
@cindex types of log file
@cindex log File, types of
@cindex version control log

  Projects that use a version control system can have two types of log
for changes.  One is the log maintained by the version control system:
each time you check in a change, you fill out a @dfn{log entry} for
the change (@pxref{Log Buffer}).  This is called the @dfn{version
control log}.

  The other kind of log is the file @file{ChangeLog} (@pxref{Change
Log}).  It provides a chronological record of all changes to a large
portion of a program---typically one directory and its subdirectories.
A small program would use one @file{ChangeLog} file; a large program
may have a @file{ChangeLog} file in each major directory.
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@xref{Change Log}.  Programmers have used change logs since long
before version control systems.

  Changeset-based version systems typically maintain a changeset-based
modification log for the entire system, which makes change log files
somewhat redundant.  One advantage that they retain is that it is
sometimes useful to be able to view the transaction history of a
single directory separately from those of other directories.
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  A project maintained with version control can use just the version
control log, or it can use both kinds of logs.  It can handle some
files one way and some files the other way.  Each project has its
policy, which you should follow.

  When the policy is to use both, you typically want to write an entry
for each change just once, then put it into both logs.  You can write
the entry in @file{ChangeLog}, then copy it to the log buffer with
@kbd{C-c C-a} when checking in the change (@pxref{Log Buffer}).  Or
you can write the entry in the log buffer while checking in the
change, and later use the @kbd{C-x v a} command to copy it to
@file{ChangeLog}
@iftex
(@pxref{Change Logs and VC,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
@ifnottex
(@pxref{Change Logs and VC}).
@end ifnottex

@node VC Mode Line
@subsection Version Control and the Mode Line

  When you visit a file that is under version control, Emacs indicates
this on the mode line.  For example, @samp{RCS-1.3} says that RCS is
used for that file, and the current version is 1.3.

  The character between the back-end name and the revision ID
indicates the version control status of the file.  @samp{-} means that
the work file is not locked (if locking is in use), or not modified (if
locking is not in use).  @samp{:} indicates that the file is locked, or
that it is modified.  If the file is locked by some other user (for
instance, @samp{jim}), that is displayed as @samp{RCS:jim:1.3}.

  On a graphical display, you can move the mouse over this mode line
indicator to pop up a ``tool-tip'', which displays a more verbose
description of the version control status.  Pressing @kbd{Mouse-1}
over the indicator pops up a menu of VC commands.  This menu is
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identical to the @samp{Tools / Version Control} menu item.
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@vindex auto-revert-check-vc-info
  When Auto Revert mode (@pxref{Reverting}) reverts a buffer that is
under version control, it updates the version control information in
the mode line.  However, Auto Revert mode may not properly update this
information if the version control status changes without changes to
the work file, from outside the current Emacs session.  If you set
@code{auto-revert-check-vc-info} to @code{t}, Auto Revert mode updates
the version control status information every
@code{auto-revert-interval} seconds, even if the work file itself is
unchanged.  The resulting CPU usage depends on the version control
system, but is usually not excessive.

@node Basic VC Editing
@subsection Basic Editing under Version Control

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@cindex filesets, VC
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   Most VC commands operate on @dfn{VC filesets}.  A VC fileset is a
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collection of one or more files that a VC operation acts on.  When you
type VC commands in a buffer visiting a version-controlled file, the
VC fileset is simply that one file.  When you type them in a VC
Directory buffer, and some files in it are marked, the VC fileset
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consists of the marked files (@pxref{VC Directory Mode}).

  The principal VC command is an all-purpose command, @kbd{C-x v v}
(@code{vc-next-action}), that performs either locking, merging or a
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check-in (depending on the situation) on the current VC fileset.  You
can use @kbd{C-x v v} in a file-visiting buffer or in a VC Directory
buffer.
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@table @kbd
@itemx C-x v v
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Perform the appropriate next version control operation on the VC fileset.
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@end table

@findex vc-next-action
@kindex C-x v v
  The precise action of @kbd{C-x v v} depends on the state of the VC
fileset, and whether the version control system uses locking or
merging.  This is described in detail in the subsequent sections.

  VC filesets are the way that VC mode bridges the gap between
file-based and changeset-based version control systems.  They are,
essentially, a way to pass multiple file arguments as a group to
version control commands.  For example, on Subversion, a checkin with
a multi-file VC fileset becomes a joint commit, as though you had
typed @command{svn commit} with those file arguments at the shell
command line.  All files in a VC fileset must be under the same
version control system; if they are not, Emacs signals an error when
you attempt to execute a command on the fileset.

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  Support for VC filesets and changeset-based version control systems
is the main improvement to VC in Emacs 23.  When you mark multi-file
VC in a VC Directory buffer, VC operations treat them as a VC fileset,
and operate on them all at once if the version control system is
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changeset-based.  @xref{VC Directory Mode}.
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  VC filesets are distinct from the ``named filesets'' used for
viewing and visiting files in functional groups (@pxref{Filesets}).
Unlike named filesets, VC filesets are not named and don't persist
across sessions.

@menu
* VC With A Merging VCS::  Without locking: default mode for CVS.
* VC With A Locking VCS::  RCS in its default mode, SCCS, and optionally CVS.
* Advanced C-x v v::       Advanced features available with a prefix argument.
* Log Buffer::             Features available in log entry buffers.
@end menu

@node VC With A Merging VCS
@subsubsection Basic Version Control with Merging

  When your version control system is merging-based (the default for
CVS and all newer version control systems), work files are always
writable; you need not do anything special to begin editing a file.
The status indicator on the mode line is @samp{-} if the file is
unmodified; it flips to @samp{:} as soon as you save any changes
(@pxref{VC Mode Line}).

  Here is what @kbd{C-x v v} does when using a merging-based system:

@itemize @bullet
@item
If the work file is the same as in the repository, it does nothing.

@item
If you have not changed the work file, but some other user has checked
in changes to the repository, @kbd{C-x v v} merges those changes into
the work file.

@item
If you have made modifications to the work file, @kbd{C-x v v}
attempts to check in your changes.  To do this, Emacs first reads the
log entry for the new revision (@pxref{Log Buffer}).  If some other
user has checked in changes to the repository since you last checked
it out, the checkin fails.  In that case, type @kbd{C-x v v} again to
merge those changes into your own work file; this puts the work file
into a ``conflicted'' state.  Type @kbd{C-x v v} to clear the
``conflicted'' state; VC then regards the file as up-to-date and
modified, and you can try to check it in again.

To pick up any recent changes from the repository @emph{without}
trying to commit your own changes, type @kbd{C-x v m @key{RET}}.
@xref{Merging}.
@end itemize

  These rules also apply when you use RCS in its ``non-locking'' mode,
except that changes will not be automatically merged from the
repository.  Nothing informs you if another user has checked in
changes in the same file since you began editing it; when you check in
your revision, his changes are removed (however, they remain in the
repository and are thus not irrevocably lost).  Therefore, you must
verify that the current revision is unchanged before checking in your
changes.  In addition, locking is possible with RCS even in this mode:
@kbd{C-x v v} with an unmodified file locks the file, just as it does
with RCS in its normal locking mode (@pxref{VC With A Locking VCS}).

@node VC With A Locking VCS
@subsubsection Basic Version Control with Locking

  Under a locking-based version control system (such as SCCS, and RCS
in its default mode), @kbd{C-x v v} does the following:

 @itemize @bullet
@item
If the file is not locked, @kbd{C-x v v} locks it, and makes it
writable so that you can change it.

@item
If the file is locked by you, and contains changes, @kbd{C-x v v}
checks in the changes.  In order to do this, it first reads the log
entry for the new revision.  @xref{Log Buffer}.

@item
If the file is locked by you, but you have not changed it since you
locked it, @kbd{C-x v v} releases the lock and makes the file
read-only again.

@item
If the file is locked by some other user, @kbd{C-x v v} asks you whether
you want to ``steal the lock'' from that user.  If you say yes, the file
becomes locked by you, but a message is sent to the person who had
formerly locked the file, to inform him of what has happened.
@end itemize

  These rules also apply when you use CVS in locking mode, except
that there is no such thing as stealing a lock.

@node Advanced C-x v v
@subsubsection Advanced Control in @kbd{C-x v v}

@cindex revision ID to check in/out
  When you give a prefix argument to @code{vc-next-action} (@kbd{C-u
C-x v v}), it still performs the next logical version control
operation, but accepts additional arguments to specify precisely how
to do the operation.

@itemize @bullet
@item
If the file is modified (or locked), you can specify the revision ID
to use for the new version that you check in.  This is one way
to create a new branch (@pxref{Branches}).

@item
If the file is not modified (and unlocked), you can specify the
revision to select; this lets you start working from an older
revision, or on another branch.  If you do not enter any revision,
that takes you to the highest (``head'') revision on the current
branch; therefore @kbd{C-u C-x v v @key{RET}} is a convenient way to
get the latest version of a file from the repository.

@item
@cindex specific version control system
Instead of the revision ID, you can also specify the name of a
version control system.  This is useful when one file is being managed
with two version control systems at the same time
@iftex
(@pxref{Local Version Control,,,emacs-xtra, Specialized Emacs
Features}).
@end iftex
@ifnottex
(@pxref{Local Version Control}).
@end ifnottex

@end itemize

@node Log Buffer
@subsubsection Features of the Log Entry Buffer

  When you check in changes, Emacs pops up a buffer called
@samp{*VC-Log*} for you to enter a log entry.

  After you have finished editing the log message, type @kbd{C-c C-c}
to exit the buffer and commit the change.

@findex log-edit-show-files
@findex log-edit-show-diff
  In the @samp{*VC-Log*} buffer, typing @kbd{C-c C-f}
(@code{log-edit-show-files}) displays a list of files in the VC
fileset you are committing.  If you called @kbd{C-x v v} directly from
a work file, the VC fileset consists of that single file, so this
command is not very useful.  If you called @kbd{C-x v v} from a VC
directory buffer, the VC fileset may consist of multiple files
(@pxref{VC Directory Mode}).

@findex log-edit-insert-changelog
  Type @kbd{C-c C-d} (@code{log-edit-show-diff}) to show a ``diff'' of
the changes you have made (i.e., the differences between the work file
and the repository revision from which you started editing the file).
The diff is displayed in a special buffer in another window.
@xref{Comparing Files}.

  If you have written an entry in the @file{ChangeLog} (@pxref{Change
Log}), type @kbd{C-c C-a} (@code{log-edit-insert-changelog}) to pull
it into the @samp{*VC-Log*} buffer.  If the topmost item in the
@file{ChangeLog} was made under your user name on the current date,
this command searches that item for entries that match the file(s) to
be committed; if found, these entries are inserted.
@iftex
@xref{Change Logs and VC,,,emacs-xtra, Specialized Emacs Features},
@end iftex
@ifnottex
@xref{Change Logs and VC},
@end ifnottex
for the opposite way of working---generating ChangeLog entries from
the revision control log.

  To abort a check-in, just @strong{don't} type @kbd{C-c C-c} in that
buffer.  You can switch buffers and do other editing.  As long as you
don't try to check in another file, the entry you were editing remains
in the @samp{*VC-Log*} buffer, and you can go back to that buffer at
any time to complete the check-in.

  If you change several source files for the same reason, it is often
convenient to specify the same log entry for many of the files.  (This
is the normal way to do things on a changeset-oriented system, where
comments are attached to changesets rather than the history of
individual files.)  The most convenient way to do this is to mark all
the files in VC Directory Mode and check in from there; the log buffer
will carry the fileset information with it and do a group commit when
you type @kbd{C-c C-c}.

  You can also browse the history of previous log entries to duplicate
a checkin comment. This can be useful when you want several files to
have checkin comments that vary only slightly from each other. The
commands @kbd{M-n}, @kbd{M-p}, @kbd{M-s} and @kbd{M-r} for doing this
work just like the minibuffer history commands (except that these
versions are used outside the minibuffer).

@vindex vc-log-mode-hook
  Each time you check in a change, the log entry buffer is put into VC
Log Edit mode, which involves running two hooks: @code{text-mode-hook}
and @code{vc-log-mode-hook}.  @xref{Hooks}.

@node Old Revisions
@subsection Examining And Comparing Old Revisions

  One of the convenient features of version control is the ability
to examine any revision of a file, or compare two revisions.

@table @kbd
@item C-x v ~ @var{revision} @key{RET}
Examine revision @var{revision} of the visited file, in a buffer of its
own.

@item C-x v =
Compare the buffer contents associated with the current
fileset with the working revision(s) from which you started editing.

@item C-u C-x v = @key{RET} @var{oldvers} @key{RET} @var{newvers} @key{RET}
Compare the specified two repository revisions of the current fileset.

@item C-x v g
Display an annotated version of the file: for each line, show the
latest revision in which it was modified.
@end table

@findex vc-revision-other-window
@kindex C-x v ~
  To examine an old revision, visit the work file and type @kbd{C-x v
~ @var{revision} @key{RET}} (@code{vc-revision-other-window}).  Here,
@var{revision} is either the desired revision ID (@pxref{VCS
Concepts}), or the name of a tag or branch
@iftex
(@pxref{Tags,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
@ifnottex
(@pxref{Tags}).
@end ifnottex
This command puts the text of the old revision in a file named
@file{@var{filename}.~@var{revision}~}, and visits it in its own
buffer in a separate window.

@findex vc-diff
@kindex C-x v =
  @kbd{C-x v =} (@code{vc-diff}) compares the current buffer contents
of each file in the current VC fileset (saving them if necessary) with
the repository revision from which you started editing.  Note that the
latter may or may not be the latest revision of the file(s).  The diff
is displayed in a special buffer in another window.  @xref{Comparing
Files}.

@findex vc-diff
@kindex C-u C-x v =
  To compare two arbitrary revisions of the current VC fileset, call
@code{vc-diff} with a prefix argument: @kbd{C-u C-x v =}.  This
prompts for two revision IDs, using the minibuffer, and displays the
diff in a special buffer in another window.  Instead of providing a
revision ID, you can give an empty input, which specifies the current
contents of the work file; or a tag or branch name
@iftex
(@pxref{Tags,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
@ifnottex
(@pxref{Tags}).
@end ifnottex
If your version control system is file-based (e.g. CVS) rather than
changeset-based (Subversion, GNU Arch, git, Mercurial), supplying a
revision ID for a multi-file fileset (as opposed to a symbolic tag
name) is unlikely to return diffs that are connected in any meaningful
way.

  If you invoke @kbd{C-x v =} or @kbd{C-u C-x v =} from a buffer that
is neither visiting a version-controlled file nor a VC directory
buffer, these commands generate a diff of all registered files in the
current directory and its subdirectories.

@vindex vc-diff-switches
@vindex vc-rcs-diff-switches
  @kbd{C-x v =} works by running a variant of the @code{diff} utility
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designed to work with the version control system in use.  The options
to pass to the @code{diff} command are taken from the first non-@code{nil}
value of @code{vc-@var{backend}-diff-switches}, @code{vc-diff-switches},
and @code{diff-switches} (@pxref{Comparing Files}), in that order.
Since @code{nil} means to check the next variable in the sequence,
either of the first two may use the value @code{t} to mean no switches at all.
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Most of the @samp{vc@dots{}diff-switches} variables default to
@code{nil}, but some default to @code{t}.  These are for those version
control systems (e.g. SVN) whose @code{diff} implementations do not
accept common options (e.g. @samp{-c}) likely to be in
@code{diff-switches}.
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  The buffer produced by @kbd{C-x v =} supports the commands of
Compilation mode (@pxref{Compilation Mode}), such as @kbd{C-x `} and
@kbd{C-c C-c}, in both the ``old'' and ``new'' text, and they always
find the corresponding locations in the current work file.  (Older
revisions are not, in general, present as files on your disk.)

@findex vc-annotate
@kindex C-x v g
  For some back ends, you can display the file @dfn{annotated} with
per-line revision information, by typing @kbd{C-x v g}
(@code{vc-annotate}).  This creates a new buffer (the ``annotate
buffer'') displaying the file's text, with each part colored to show
how old it is.  Text colored red is new, blue means old, and
intermediate colors indicate intermediate ages.  By default, the color
is scaled over the full range of ages, such that the oldest changes
are blue, and the newest changes are red.

  When you give a prefix argument to this command, Emacs reads two
arguments using the minibuffer: the ID of which revision to display and
annotate (instead of the current file contents), and the time span in
days the color range should cover.

  From the annotate buffer, these and other color scaling options are
available from the @samp{VC-Annotate} menu.  In this buffer, you can
also use the following keys to browse the annotations of past revisions,
view diffs, or view log entries:

@table @kbd
@item p
Annotate the previous revision, that is to say, the revision before
the one currently annotated.  A numeric prefix argument is a repeat
count, so @kbd{C-u 10 p} would take you back 10 revisions.

@item n
Annotate the next revision---the one after the revision currently
annotated.  A numeric prefix argument is a repeat count.

@item j
Annotate the revision indicated by the current line.

@item a
Annotate the revision before the one indicated by the current line.
This is useful to see the state the file was in before the change on
the current line was made.

@item f
Show in a buffer the file revision indicated by the current line.

@item d
Display the diff between the current line's revision and the previous
revision.  This is useful to see what the current line's revision
actually changed in the file.

@item D
Display the diff between the current line's revision and the previous
revision for all files in the changeset (for VC systems that support
changesets).  This is useful to see what the current line's revision
actually changed in the tree.

@item l
Show the log of the current line's revision.  This is useful to see
the author's description of the changes in the revision on the current
line.

@item w
Annotate the working revision--the one you are editing.  If you used
@kbd{p} and @kbd{n} to browse to other revisions, use this key to
return to your working revision.

@item v
Toggle the annotation visibility.  This is useful for looking just at
the file contents without distraction from the annotations.
@end table

@node Secondary VC Commands
@subsection The Secondary Commands of VC

  This section explains the secondary commands of VC.

@menu
* Registering::         Putting a file under version control.
* VC Status::           Viewing the VC status of files.
* VC Undo::             Canceling changes before or after check-in.
@end menu

@node Registering
@subsubsection Registering a File for Version Control

@kindex C-x v i
@findex vc-register
  You can put any file under version control by simply visiting it, and
then typing @w{@kbd{C-x v i}} (@code{vc-register}).

@table @kbd
@item C-x v i
Register the visited file for version control.
@end table

  To register the file, Emacs must choose which version control system
to use for it.  If the file's directory already contains files
registered in a version control system, Emacs uses that system.  If
there is more than one system in use for a directory, Emacs uses the
one that appears first in @code{vc-handled-backends}
@iftex
(@pxref{Customizing VC,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
@ifnottex
(@pxref{Customizing VC}).
@end ifnottex
On the other hand, if there are no files already registered, Emacs uses
the first system from @code{vc-handled-backends} that could register
the file (for example, you cannot register a file under CVS if its
directory is not already part of a CVS tree); with the default value
of @code{vc-handled-backends}, this means that Emacs uses RCS in this
situation.

  If locking is in use, @kbd{C-x v i} leaves the file unlocked and
read-only.  Type @kbd{C-x v v} if you wish to start editing it.  After
registering a file with CVS, you must subsequently commit the initial
revision by typing @kbd{C-x v v}.  Until you do that, the revision ID
appears as @samp{@@@@} in the mode line.

@vindex vc-default-init-revision
@cindex initial revision ID to register
  The default initial revision ID for a newly registered file
varies by what VCS you are using; normally it will be 1.1 on VCSes
that use dot-pair revision IDs and 1 on VCSes that use monotonic IDs.
You can specify a different default by setting the variable
@code{vc-default-init-revision}, or you can give @kbd{C-x v i} a
numeric argument; then it reads the initial revision ID for this
particular file using the minibuffer.

@vindex vc-initial-comment
  If @code{vc-initial-comment} is non-@code{nil}, @kbd{C-x v i} reads an
initial comment to describe the purpose of this source file.  Reading
the initial comment works like reading a log entry (@pxref{Log Buffer}).

@node VC Status
@subsubsection VC Status Commands

@table @kbd
@item C-x v l
Display revision control state and change history.
@end table

@kindex C-x v l
@findex vc-print-log
  To view the detailed revision control status and history of a file,
type @kbd{C-x v l} (@code{vc-print-log}).  This pops up a special
buffer named @samp{*vc-change-log*}, in a new window, that displays
the history of changes to the current file, including the text of the
log entries.  The point is centered at the revision of the file that
is currently being visited.

  In the @samp{*vc-change-log*} buffer, you can use the following keys
to move between the logs of revisions and of files, to view past
revisions, to modify change comments, to view annotations and to view
diffs:

@table @kbd
@item p
Move to the previous revision-item in the buffer.  (Revision entries in the log
buffer are usually in reverse-chronological order, so the previous
revision-item usually corresponds to a newer revision.)  A numeric
prefix argument is a repeat count.

@item n
Move to the next revision-item (which most often corresponds to the
previous revision of the file).  A numeric prefix argument is a repeat
count.

@item P
Move to the log of the previous file, when the logs of multiple files
are in the log buffer (@pxref{VC Directory Mode}).  Otherwise, just
move to the beginning of the log.  A numeric prefix argument is a
repeat count, so @kbd{C-u 10 P} would move backward 10 files.

@item N
Move to the log of the next file, when the logs of multiple files are
in the log buffer (@pxref{VC Directory Mode}).  It also takes a
numeric prefix argument as a repeat count.

@item a
Annotate the revision indicated by the current line.

@item e
Modify the change comment displayed at point.  Note that not all VC
systems support modifying change comments.

@item f
Visit the revision indicated at the current line, like typing @kbd{C-x
v ~} and specifying this revision's ID (@pxref{Old Revisions}).

@item d
Display the diff (@pxref{Comparing Files}) between the revision
indicated at the current line and the next earlier revision.  This is
useful to see what actually changed in the file when the revision
indicated on the current line was committed.

@item D
Display the changeset diff (@pxref{Comparing Files}) between the
revision indicated at the current line and the next earlier revision.
This is useful to see all the changes to all files that the revision
indicated on the current line did when it was committed.
@end table

@node VC Undo
@subsubsection Undoing Version Control Actions

@table @kbd
@item C-x v u
Revert the buffer and the file to the working revision from which you started
editing the file.

@item C-x v c
Remove the last-entered change from the master for the visited file.
This undoes your last check-in.
@end table

@kindex C-x v u
@findex vc-revert-buffer
  If you want to discard your current set of changes and revert to the
working revision from which you started editing the file, use @kbd{C-x
v u} (@code{vc-revert-buffer}).  If the version control system is
locking-based, this leaves the file unlocked, and you must lock it
again before making new changes.  @kbd{C-x v u} requires confirmation,
unless it sees that you haven't made any changes with respect to the
master copy of the working revision.

  @kbd{C-x v u} is also the command to unlock a file if you lock it and
then decide not to change it.

@kindex C-x v c
@findex vc-rollback
  To cancel a change that you already checked in, use @kbd{C-x v c}
(@code{vc-rollback}).  This command discards all record of the most
recent checked-in revision, but only if your work file corresponds to
that revision---you cannot use @kbd{C-x v c} to cancel a revision that
is not the latest on its branch.  Note that many version control
systems do not support rollback at all; this command is something of a
historical relic.

@node VC Directory Mode
@subsection VC Directory Mode

@kindex C-x v d
@findex vc-dir
  When you are working on a large program, it is often useful to find
out which files have changed within an entire directory tree, or to
view the status of all files under version control at once, and to
perform version control operations on collections of files.  You can
use the command @kbd{C-x v d} (@code{vc-dir}) to make a directory
listing that includes only files relevant for version control.  This
creates a @dfn{VC Directory buffer} and displays it in a separate
window.

@cindex PCL-CVS
@pindex cvs
@cindex CVS directory mode
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  The VC Directory buffer works with all the version control systems
that VC supports.  For CVS, Emacs also offers a more powerful facility
called PCL-CVS.  @xref{Top, , About PCL-CVS, pcl-cvs, PCL-CVS --- The
Emacs Front-End to CVS}.

@menu
* Buffer: VC Directory Buffer.      What the buffer looks like and means.
* Commands: VC Directory Commands.  Commands to use in a VC directory buffer.
@end menu

@node VC Directory Buffer
@subsubsection The VC Directory Buffer
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  The VC Directory buffer contains a list of version-controlled files
in the current directory and its subdirectories.  Files which are
up-to-date (have no local differences from the repository copy) are
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usually hidden; if all files in a subdirectory are up-to-date, the
subdirectory is hidden as well.  There is an exception to this rule:
if VC mode detects that a file has changed to an up-to-date state
since you last looked at it, that file and its state are shown.
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  If a directory uses more that one version control system, you can
select which system to use for the @code{vc-dir} command by invoking
@code{vc-dir} with a prefix argument: @kbd{C-u C-x v d}.

  The line for an individual file shows the version control state of
the file.  Under RCS and SCCS, the name of the user locking the file
is shown; under CVS, an abbreviated version of the @samp{cvs status}
output is used.  Here is an example using CVS:

@smallexample
@group
                       ./
    modified           file1.c
    needs-update       file2.c
    needs-merge        file3.c
@end group
@end smallexample

@noindent
In this example, @samp{file1.c} is modified with respect to the
repository, and @samp{file2.c} is not.  @samp{file3.c} is modified,
but other changes have also been checked in to the repository---you
need to merge them with the work file before you can check it in.

@vindex vc-stay-local
@vindex vc-cvs-stay-local
  In the above, if the repository were on a remote machine, VC only
contacts it when the variable @code{vc-stay-local} (or
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@code{vc-cvs-stay-local}) is @code{nil} (@pxref{CVS Options}).  This is
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because access to the repository may be slow, or you may be working
offline and not have access to the repository at all.  As a
consequence, VC would not be able to tell you that @samp{file3.c} is
in the ``merge'' state; you would learn that only when you try to
check-in your modified copy of the file, or use a command such as
@kbd{C-x v m}.

  In practice, this is not a problem because CVS handles this case
consistently whenever it arises.  In VC, you'll simply get prompted to
merge the remote changes into your work file first.  The benefits of
less network communication usually outweigh the disadvantage of not
seeing remote changes immediately.

@vindex vc-directory-exclusion-list
  When a VC directory displays subdirectories it omits some that
should never contain any files under version control.  By default,
this includes Version Control subdirectories such as @samp{RCS} and
@samp{CVS}; you can customize this by setting the variable
@code{vc-directory-exclusion-list}.

@node VC Directory Commands
@subsubsection VC Directory Commands

  VC Directory mode has a full set of navigation and marking commands
for picking out filesets.  Some of these are also available in a
context menu invoked by the @kbd{mouse-2} button.

  Up- and down-arrow keys move in the buffer; @kbd{n} and @kbd{p}  also
move vertically as in other list-browsing modes.  @key{SPC} and
@key{TAB} behave like down-arrow, and @key{BackTab} behaves like
up-arrow.

  Both @kbd{C-m} and @kbd{f} visit the file on the current
line.  @kbd{o} visits that file in another window.  @kbd{q} dismisses
the directory buffer.

  @kbd{x} toggles hiding of up-to-date files.

  @kbd{m} marks the file or directory on the current line.  If the
region is active, @kbd{m} marks all the files in the region.  There
are some restrictions when marking: a file cannot be marked if any of
its parent directories are marked, and a directory cannot be marked if
any files in it or in its child directories are marked.

  @kbd{M} marks all the files with the same VC state as the current
file if the cursor is on a file.  If the cursor is on a directory, it
marks all child files.  With a prefix argument: marks all files and
directories.

  @kbd{u} unmarks the file or directory on the current line.  If the
region is active, it unmarks all the files in the region.

  @kbd{U} marks all the files with the same VC state as the current file
if the cursor is on a file.  If the cursor is on a directory, it
unmarks all child files.  With a prefix argument: unmarks all marked
files and directories.

  It is possible to do search, search and replace, incremental search,
and incremental regexp search on multiple files.  These commands will
work on all the marked files or the current file if nothing is marked.
If a directory is marked, the files in that directory shown in the VC
directory buffer will be used.

  @kbd{S} searches the marked files.

  @kbd{Q} does a query replace on the marked files.

  @kbd{M-s a C-s} does an incremental search on the marked files.

  @kbd{M-s a C-M-s} does an incremental search on the marked files.

  Commands are also accessible from the VC-dir menu.  Note that some VC
backends use the VC-dir menu to make available extra backend specific
commands.

  Normal VC commands with the @kbd{C-x v} prefix work in VC directory
buffers.  Some single-key shortcuts are available as well; @kbd{=},
@kbd{+}, @kbd{l}, @kbd{i}, and @kbd{v} behave as through prefixed with
@kbd{C-x v}.

  The command @kbd{C-x v v} (@code{vc-next-action}) operates on all the
marked files, so that you can check in several files at once.
If the underlying VC supports atomic commits of multiple-file
changesets, @kbd{C-x v v} with a selected set of modified but not
committed files will commit all of them at once as a single changeset.

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  When @kbd{C-x v v} (@code{vc-next-action}) operates on multiple
files, all of those files must be either in the same state or in
compatible states (added, modified and removed states are considered
compatible).  Otherwise it signals an error.  This differs from the
behavior of older versions of VC, which did not have fileset
operations and simply did @code{vc-next-action} on each file
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individually.

  If any files are in a state that calls for commit, @kbd{C-x v v} reads a
single log entry and uses it for the changeset as a whole.  If the
underling VCS is file- rather than changeset-oriented, the log entry
will be replicated into the history of each file.

@node Branches
@subsection Multiple Branches of a File
@cindex branch (version control)
@cindex trunk (version control)

  One use of version control is to maintain multiple ``current''
revisions of a file.  For example, you might have different revisions of a
program in which you are gradually adding various unfinished new
features.  Each such independent line of development is called a
@dfn{branch}.  VC allows you to create branches, switch between
different branches, and merge changes from one branch to another.
Please note, however, that branches are not supported for SCCS.

  A file's main line of development is usually called the @dfn{trunk}.
You can create multiple branches from the trunk.  How the difference
between trunk and branch is made visible is dependent on whether the
VCS uses dot-pair or monotonic version IDs.

  In VCSes with dot-pair revision IDs, the revisions on the trunk are
normally IDed 1.1, 1.2, 1.3, etc.  At any such revision, you can
start an independent branch.  A branch starting at revision 1.2 would
have revision ID 1.2.1.1, and consecutive revisions on this branch
would have IDs 1.2.1.2, 1.2.1.3, 1.2.1.4, and so on.  If there is
a second branch also starting at revision 1.2, it would consist of
revisions 1.2.2.1, 1.2.2.2, 1.2.2.3, etc.

   In VCSes with monotonic revision IDs, trunk revisions are IDed as
1, 2, 3, etc.  A branch from (say) revision 2 might start with 2.1 and
continue through 2.2, 2.3, etc.  But naming conventions for branches
and subbranches vary widely on these systems, and some (like
Mercurial) never depart from the monotonic integer sequence at all.
Consult the documentation of the VCS you are using.

@cindex head revision
  If you omit the final component of a dot-pair revision ID, that is called a
@dfn{branch ID}.  It refers to the highest existing revision on that
branch---the @dfn{head revision} of that branch.  The branches in the
dot-pair example above have branch IDs 1.2.1 and 1.2.2.

@menu
* Switching Branches::    How to get to another existing branch.
* Creating Branches::     How to start a new branch.
* Merging::               Transferring changes between branches.
* Multi-User Branching::  Multiple users working at multiple branches
                            in parallel.
@end menu

@node Switching Branches
@subsubsection Switching between Branches

  To switch between branches, type @kbd{C-u C-x v v} and specify the
revision ID you want to select.  On a locking-based system, this
version is then visited @emph{unlocked} (write-protected), so you can
examine it before locking it.  Switching branches in this way is allowed
only when the file is not locked.

  On a VCS with dot-pair IDs, you can omit the minor part, thus giving
only the branch ID; this takes you to the head version on the
chosen branch.  If you only type @key{RET}, Emacs goes to the highest
version on the trunk.

  After you have switched to any branch (including the main branch), you
stay on it for subsequent VC commands, until you explicitly select some
other branch.

@node Creating Branches
@subsubsection Creating New Branches

  To create a new branch from a head revision (one that is the latest in
the branch that contains it), first select that revision if necessary,
lock it with @kbd{C-x v v}, and make whatever changes you want.  Then,
when you check in the changes, use @kbd{C-u C-x v v}.  This lets you
specify the revision ID for the new revision.  You should specify a
suitable branch ID for a branch starting at the current revision.
For example, if the current revision is 2.5, the branch ID should be
2.5.1, 2.5.2, and so on, depending on the number of existing branches at
that point.

  To create a new branch at an older revision (one that is no longer the
head of a branch), first select that revision (@pxref{Switching
Branches}).  Your procedure will then differ depending on whether you
are using a locking or merging-based VCS.

  On a locking VCS, you will need to lock the old revision branch with
@kbd{C-x v v}.  You'll be asked to confirm, when you lock the old
revision, that you really mean to create a new branch---if you say no,
you'll be offered a chance to lock the latest revision instead.  On
a merging-based VCS you will skip this step.

  Then make your changes and type @kbd{C-x v v} again to check in a new
revision.  This automatically creates a new branch starting from the
selected revision.  You need not specially request a new branch, because
that's the only way to add a new revision at a point that is not the head
of a branch.

  After the branch is created, you ``stay'' on it.  That means that
subsequent check-ins create new revisions on that branch.  To leave the
branch, you must explicitly select a different revision with @kbd{C-u C-x
v v}.  To transfer changes from one branch to another, use the merge
command, described in the next section.

@node Merging
@subsubsection Merging Branches

@cindex merging changes
  When you have finished the changes on a certain branch, you will
often want to incorporate them into the file's main line of development
(the trunk).  This is not a trivial operation, because development might
also have proceeded on the trunk, so that you must @dfn{merge} the
changes into a file that has already been changed otherwise.  VC allows
you to do this (and other things) with the @code{vc-merge} command.

@table @kbd
@item C-x v m (vc-merge)
Merge changes into the work file.
@end table

@kindex C-x v m
@findex vc-merge
  @kbd{C-x v m} (@code{vc-merge}) takes a set of changes and merges it
into the current version of the work file.  It firsts asks you in the
minibuffer where the changes should come from.  If you just type
@key{RET}, Emacs merges any changes that were made on the same branch
since you checked the file out (we call this @dfn{merging the news}).
This is the common way to pick up recent changes from the repository,
regardless of whether you have already changed the file yourself.

  You can also enter a branch ID or a pair of revision IDs in
the minibuffer.  Then @kbd{C-x v m} finds the changes from that
branch, or the differences between the two revisions you specified, and
merges them into the current revision of the current file.

  As an example, suppose that you have finished a certain feature on
branch 1.3.1.  In the meantime, development on the trunk has proceeded
to revision 1.5.  To merge the changes from the branch to the trunk,
first go to the head revision of the trunk, by typing @kbd{C-u C-x v v
@key{RET}}.  Revision 1.5 is now current.  If locking is used for the file,
type @kbd{C-x v v} to lock revision 1.5 so that you can change it.  Next,
type @kbd{C-x v m 1.3.1 @key{RET}}.  This takes the entire set of changes on
branch 1.3.1 (relative to revision 1.3, where the branch started, up to
the last revision on the branch) and merges it into the current revision
of the work file.  You can now check in the changed file, thus creating
revision 1.6 containing the changes from the branch.

  It is possible to do further editing after merging the branch, before
the next check-in.  But it is usually wiser to check in the merged
revision, then lock it and make the further changes.  This will keep
a better record of the history of changes.

@cindex conflicts
@cindex resolving conflicts
  When you merge changes into a file that has itself been modified, the
changes might overlap.  We call this situation a @dfn{conflict}, and
reconciling the conflicting changes is called @dfn{resolving a
conflict}.

  Whenever conflicts occur during merging, VC detects them, tells you
about them in the echo area, and asks whether you want help in merging.
If you say yes, it starts an Ediff session (@pxref{Top,
Ediff, Ediff, ediff, The Ediff Manual}).

  If you say no, the conflicting changes are both inserted into the
file, surrounded by @dfn{conflict markers}.  The example below shows how
a conflict region looks; the file is called @samp{name} and the current
master file revision with user B's changes in it is 1.11.

@c @w here is so CVS won't think this is a conflict.
@smallexample
@group
@w{<}<<<<<< name
  @var{User A's version}
=======
  @var{User B's version}
@w{>}>>>>>> 1.11
@end group
@end smallexample

@cindex vc-resolve-conflicts
  Then you can resolve the conflicts by editing the file manually.  Or
you can type @code{M-x vc-resolve-conflicts} after visiting the file.
This starts an Ediff session, as described above.  Don't forget to
check in the merged version afterwards.

@node Multi-User Branching
@subsubsection Multi-User Branching

  It is often useful for multiple developers to work simultaneously on
different branches of a file.  CVS and later systems allow this by
default; for RCS, it is possible if you create multiple source
directories.  Each source directory should have a link named
@file{RCS} which points to a common directory of RCS master files.
Then each source directory can have its own choice of selected
revisions, but all share the same common RCS records.

  This technique works reliably and automatically, provided that the
source files contain RCS version headers
@iftex
(@pxref{Version Headers,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
@ifnottex
(@pxref{Version Headers}).
@end ifnottex
The headers enable Emacs to be sure, at all times, which revision
ID is present in the work file.

  If the files do not have version headers, you must instead tell Emacs
explicitly in each session which branch you are working on.  To do this,
first find the file, then type @kbd{C-u C-x v v} and specify the correct
branch ID.  This ensures that Emacs knows which branch it is using
during this particular editing session.

@ifnottex
@include vc1-xtra.texi
@end ifnottex

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@node Change Log
@section Change Logs

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@cindex change log
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  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.

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@menu
* Change Log Commands:: Commands for editing change log files.
* Format of ChangeLog:: What the change log file looks like.
@end menu

@node Change Log Commands
@subsection Change Log Commands

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@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.

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

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

@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.

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You can use the @code{next-error} command (by default bound to
@kbd{C-x `}) to move between entries in the Change Log, when Change
Log mode is on.  You will jump to the actual site in the file that was
changed, not just to the next Change Log entry.  You can also use
@code{previous-error} to move back in the same list.

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@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.

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  Version control systems are another way to keep track of changes in
your program and keep a change log.  In the VC log buffer, typing
@kbd{C-c C-a} (@code{log-edit-insert-changelog}) inserts the relevant
Change Log entry, if one exists (@pxref{Log Buffer}).  You can also
insert a VC log entry into a Change Log buffer by typing @kbd{C-x v a}
(@code{vc-update-change-log}) in the Change Log buffer
@iftex
(@pxref{Change Logs and VC,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
@ifnottex
(@pxref{Change Logs and VC}).
@end ifnottex
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@node Format of ChangeLog
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@subsection Format of ChangeLog
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  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{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 @dfn{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.

@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, 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:

@smallexample
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 smallexample

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

@node Tags
@section Tags Tables
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@cindex tags and tag tables
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  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, @code{#undef} 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}, and so are struct members, unless you specify
@samp{--no-members}.  Use of @samp{--no-globals}, @samp{--no-defines}
and @samp{--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 @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; member variables are also recognized,
unless you use the @samp{--no-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},
@code{\index}, @code{\def}, @code{\newcommand}, @code{\renewcommand},
@code{\newenvironment} or @code{\renewenvironment} 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="mycommand:myothercommand"
export TEXTAGS
@end example

@noindent
specifies (using Bourne shell syntax) that the commands
@samp{\mycommand} and @samp{\myothercommand} 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 HTML input files, the tags are the @code{title} and the @code{h1},
@code{h2}, @code{h3} headers.  Also, tags are @code{name=} in anchors
and all occurrences of @code{id=}.

@item
In Lua input files, all functions are tags.

@item
In makefiles, targets are tags; additionally, variables are tags
unless you specify @samp{--no-globals}.

@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.  Vars are tags
too, unless you use the @samp{--no-members} option.

@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
@ifnottex
@ref{Tag Syntax}.
@end ifnottex
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
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table named @file{TAGS} in the current working directory.  You can
optionally specify a different file name for the tags table by using the
@samp{--output=@var{file}} option; specifying @file{-} as a file name
prints the tags table to standard output.
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  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 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 @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
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files.  If the tags file is @file{-} or is in the @file{/dev} directory,
however, the file names are 
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