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@c This is part of the Emacs manual.
@c Copyright (C) 1985-1987, 1993-1995, 1997, 1999-2011
@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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* Version Control::     Using version control systems.
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* Change Log::          Maintaining a change history for your program.
* Tags::                Go directly to any function in your program in one
                          command.  Tags remembers which file it is in.
* EDE::                 An integrated development environment for Emacs.
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* 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 program that can record multiple
versions of a source file, storing information such as the creation
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time of each version, who made it, and a description of what was

  The Emacs version control interface is called @dfn{VC}.  VC 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,
Arch, RCS, and Bazaar.

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

* 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.
* Log Buffer::          Features available in log entry buffers.
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* 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.
* Remote Repositories:: Efficient access to remote CVS servers.
* Revision Tags::       Symbolic names for revisions.
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* 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,
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integrating the version control operations smoothly with editing.  It
provides a uniform interface for common operations in many version
control operations.

  Some uncommon or intricate version control operations, such as
altering repository settings, are not supported in VC.  You should
perform such tasks outside Emacs, e.g.@: via the command line.
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  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.

* Why Version Control?::    Understanding the problems it addresses.
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* Version Control Systems:: Supported version control back-end systems.
* VCS Concepts::            Words and concepts related to version control.
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* VCS Merging::             How file conflicts are handled.
* VCS Changesets::          How changes are grouped.
* VCS Repositories::        Where version control repositories are stored.
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* 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

@itemize @bullet
@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.

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

@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)
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  VC currently works with many different version control systems,
which it refers to as @dfn{back ends}:
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@itemize @bullet

@cindex SCCS
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
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itself.  Other VC features, such as multiple branches, are simply
unavailable.  Since SCCS is non-free, we recommend avoiding it.

@cindex CSSC
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
RCS is the free version control system around which VC was initially
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built.  It is relatively primitive: it cannot be used over the
network, and works at the level of individual files.  Almost
everything you can do with RCS can be done through VC.
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@cindex CVS
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
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multi-user development either locally or over the network.  Unlike
newer systems, it lacks support for atomic commits and file
moving/renaming.  VC supports all basic editing operations under CVS.
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@cindex SVN
@cindex Subversion
Subversion (svn) is a free version control system designed to be
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similar to CVS but without its problems (e.g., it supports atomic
commits of filesets, and versioning of directories, symbolic links,
meta-data, renames, copies, and deletes).
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@cindex GNU Arch
@cindex Arch
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GNU Arch is one of the earliest @dfn{distributed} version control
systems (the other being Monotone).  @xref{VCS Concepts}, for a
description of distributed version control systems.  It is no longer
under active development, and has been deprecated in favor of Bazaar.
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@cindex git
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Git is a distributed version control system originally invented by
Linus Torvalds to support development of Linux (his kernel).  VC
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supports many common git operations, but others, such as repository
syncing, must be done from the command line.
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@cindex hg
@cindex Mercurial
Mercurial (hg) is a distributed version control system broadly
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resembling git.  VC supports most Mercurial commands, with the
exception of repository sync operations.
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@cindex bzr
@cindex Bazaar
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Bazaar (bzr) is a distributed version control system that supports
both repository-based and distributed versioning.  VC supports most
basic editing operations under Bazaar.
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@end itemize

@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
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  The copy of a version-controlled file that you actually edit is
called the @dfn{work file}.  You can change each work file as you
would an ordinary file.  After you are done with a set of changes, you
@dfn{commit} (or @dfn{check in}) the changes; this records the changes
in the repository, along with a descriptive log entry.
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@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 revision is
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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

  To go beyond these basic concepts, you will need to understand three
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aspects in which version control systems differ.  As explained in the
next three sections, they can be lock-based or merge-based; file-based
or changeset-based; and centralized or decentralized.  VC handles all
these modes of operation, but it cannot hide the differences.

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@node VCS Merging
@subsubsection Merge-based vs lock-based Version Control
@cindex locking versus merging

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  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 committed, with the latest changes that others have committed.
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  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 commit your changes, that unlocks
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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
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conflicts} happen when you commit a change to a file that conflicts
with a change committed 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
Mercurial, are exclusively merging-based.

  VC mode supports both locking and merging version control.  The
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terms ``commit'' and ``update'' are used in newer version control
systems; older lock-based systems use the terms ``check in'' and
``check out''.  VC hides the differences between them as much as

@node VCS Changesets
@subsubsection Changeset-based vs File-based Version Control

@cindex changesets
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  On SCCS, RCS, CVS, and other early version control systems, version
control operations are @dfn{file-based}: each file has its own comment
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and revision history separate from that of all other files.  Newer
systems, beginning with Subversion, are @dfn{changeset-based}: a
checkin may include changes to several files, and the entire set of
changes is handled as a unit.  Any comment associated with the change
does not belong to a single file, but to the changeset itself.
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  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
all of it.

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@node VCS Repositories
@subsubsection Decentralized vs Centralized Repositories

@cindex centralized version control
@cindex decentralized version control
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  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.

  GNU Arch pioneered the concept of @dfn{decentralized} version
control, later implemented in Git, Mercurial, and Bazaar.  A project
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may have several different repositories, and these systems support a
sort of super-merge between repositories that tries to reconcile their
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change histories.  In effect, there is one repository for each
developer, and repository merges take the place of commit operations.

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  VC helps you manage the traffic between your personal workfiles and
a repository.  Whether the repository is a single master, or one of a
network of peer repositories, is not something VC has to care about.
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@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:
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each time you commit a change, you fill out a @dfn{log entry} for the
change (@pxref{Log Buffer}).  This is called the @dfn{version control
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  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
(@pxref{Change Logs and VC,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
(@pxref{Change Logs and VC}).
@end ifnottex

@node VC Mode Line
@subsection Version Control and the Mode Line
@cindex VC, mode line indicator
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  When you visit a file that is under version control, Emacs indicates
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this on the mode line.  For example, @samp{Bzr-1223} says that Bazaar
is used for that file, and the current revision ID is 1223.
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  The character between the back-end name and the revision ID
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indicates the status of the work file.  In a merge-based version
control system, a @samp{-} character indicates that the work file is
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unmodified, and @samp{:} indicates that it has been modified.
@samp{!} indicates that the file contains conflicts as result of a
recent merge operation (@pxref{Merging}), or that the file was removed
from the version control.  Finally, @samp{?}  means that the file is
under version control, but is missing from the working tree.

  In a lock-based system, @samp{-} indicates an unlocked file, and
@samp{:} a locked file; if the file is locked by another user (for
instance, @samp{jim}), that is displayed as @samp{RCS:jim:1.3}.
@samp{@@} means that the file was locally added, but not yet committed
to the master repository.
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  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}
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over the indicator pops up a menu of VC commands, identical to
@samp{Tools / Version Control} on the menu bar.
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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

@cindex filesets, VC
   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}).

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  On Subversion and on decentralized version control systems,
multi-file VC filesets are handled as a single group by the relevant
version control commands.  For example, committing a multi-file VC
fileset generates a single revision, consisting of all the changes to
those files.  But on older version control systems which lack support
for group operations, such as CVS, the files in a multi-file VC
fileset are passed individually to version control commands (e.g.@: a
commit generates one revision for each changed file).
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@table @kbd
@itemx C-x v v
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Perform the next appropriate version control operation on the current
VC fileset.
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@end table

@findex vc-next-action
@kindex C-x v v
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  The principal VC command is an all-purpose command, @kbd{C-x v v}
(@code{vc-next-action}), which performs the ``most appropriate''
action on the current VC fileset: either registering it with a version
control system, or committing it, or unlocking it, or merging changes
into it.  The precise actions are described in detail in the following
subsections.  You can use @kbd{C-x v v} either in a file-visiting
buffer or in a VC Directory buffer.

  Note that 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.
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* 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.
@end menu

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

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  On a merging-based version control system (i.e.@: most modern ones;
@pxref{VCS Merging}), @kbd{C-x v v} does the following:
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@itemize @bullet
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If there is more than one file in the VC fileset and the files have
inconsistent version control states, signal an error.
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If each file in the VC fileset is not registered with a version
control system, register the VC fileset.  If the fileset is in a
directory controlled by a version control system, register it with
that system; otherwise, prompt for a repository type, create a new
repository, and register the VC fileset with it.
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If each work file in the VC fileset is unchanged, do nothing.
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If each work file in the VC fileset has been modified, commit the
changes.  To do this, Emacs pops up a @samp{*vc-log*} buffer; type the
desired log entry for the new revision, followed by @kbd{C-c C-c} to
commit (@pxref{Log Buffer}).

If committing to a shared repository, the commit may fail if the
repository that has been changed since your last update.  In that
case, you must perform an update before trying again.  If using a
decentralized version control system, use @kbd{C-x v +} or @kbd{C-x v
m} (@pxref{Merging}).  If using a centralized version control system,
type @kbd{C-x v v} again to merge in the repository changes.
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Finally, if you are using a centralized version control system, check
if each work file in the VC fileset is up-to-date.  If any file has
been changed in the repository, offer to update it.
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@end itemize

  These rules also apply when you use RCS in its ``non-locking'' mode,
except that changes are not automatically merged from the repository.
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Nothing informs you if another user has committed changes in the same
file since you began editing it; when you commit your revision, his
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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}).
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@node VC With A Locking VCS
@subsubsection Basic Version Control with Locking

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  On 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
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If there is more than one file in the VC fileset and the files have
inconsistent version control states, signal an error.

If each file in the VC fileset is not registered with a version
control system, register the VC fileset.  If the fileset is in a
directory controlled by a version control system, register it with
that system; otherwise, prompt for a repository type, create a new
repository, and register the VC fileset with it.

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If each file is registered and unlocked, lock it and make it writable,
so that you can begin to edit it.
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If each file is locked by you and contains changes, commit the
changes.  To do this, Emacs pops up a @samp{*vc-log*} buffer; type the
desired log entry for the new revision, followed by @kbd{C-c C-c} to
commit (@pxref{Log Buffer}).
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If each file is locked by you, but you have not changed it, release
the lock and make the file read-only again.
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If each file is locked by another user, ask whether you want to
``steal the lock''.  If you say yes, the file becomes locked by you,
and a warning message is sent to the user who had formerly locked the
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@end itemize

  These rules also apply when you use CVS in locking mode, except
that CVS does not support stealing locks.
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@node Advanced C-x v v
@subsubsection Advanced Control in @kbd{C-x v v}

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

@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
(@pxref{Local Version Control,,,emacs-xtra, Specialized Emacs
@end iftex
(@pxref{Local Version Control}).
@end ifnottex

@end itemize

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

  When you tell VC to commit a change, it pops up a buffer called
@samp{*VC-Log*}.  In this buffer, you should write a @dfn{log entry}
describing the changes you have made (@pxref{Why Version Control?}).
After you are done, type @kbd{C-c C-c}; this exits the buffer and
commits the change, together with your log entry.

  While in the @samp{*VC-Log*} buffer, you can write one or more
@dfn{header lines}, specifying additional information to be supplied
to the version control system.  Each header line must occupy a single
line at the top of the buffer; the first line that is not a header
line is treated as the start of the log entry.  For example, the
following header line states that the present change was not written
by you, but by another developer:

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Author: J. R. Hacker <>
@end smallexample

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Apart from the @samp{Author} header, Emacs recognizes the headers
@samp{Date} (a manually-specified commit time) and @samp{Fixes} (a
reference to a bug fixed by the change).  Not all version control
systems recognize all headers: Bazaar recognizes all three headers,
while git, Mercurial, and Monotone recognizes only @samp{Author} and
@samp{Summary}.  If you specify a header for a version control that
does not support it, the header is treated as part of the log entry.
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@findex log-edit-show-files
@findex log-edit-show-diff
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  Type @kbd{C-c C-f} (@code{log-edit-show-files}) to display a list of
files in the current VC fileset.  If you called @kbd{C-x v v} directly
from a work file, the fileset consists of that single file; if you
called @kbd{C-x v v} from a VC directory buffer (@pxref{VC Directory
Mode}), the fileset may consist of multiple files.
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@findex log-edit-insert-changelog
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  Type @kbd{C-c C-d} (@code{log-edit-show-diff}) to show a @dfn{diff}
of the changes you have made (i.e., the differences between the work
file and the repository revision from which you started editing).
@xref{Old Revisions}.

  If the current VC fileset includes one or more @file{ChangeLog}
files (@pxref{Change Log}), type @kbd{C-c C-a}
(@code{log-edit-insert-changelog}) to pull the relevant entries into
the @samp{*VC-Log*} buffer.  If the topmost item in each
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@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.
@xref{Change Logs and VC,,,emacs-xtra, Specialized Emacs Features},
@end iftex
@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 commit another file, the entry you were editing remains
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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 commit from there; the log buffer
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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 commit a change, the log entry buffer is put into VC
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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 ~
Prompt for a revision of the current file, and visit it in a buffer of
its own (@code{vc-revision-other-window}).
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@item C-x v =
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Compare the files in the current fileset with the working revision(s)
you started from (@code{vc-diff}).  With a prefix argument, prompt for
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two revisions of the current fileset and compare them.  You can call
this command from a Dired buffer (@pxref{Dired}).
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@item C-x v D
Compare the entire tree corresponding to the current fileset with the
tree you started from (@code{vc-root-diff}).  With a prefix argument,
prompt for two revisions and compare their trees.
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@item C-x v g
Display an annotated version of the file: for each line, show the
latest revision in which it was modified (@code{vc-annotate}).
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@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
(@pxref{Tags,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
@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 =
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  @kbd{C-x v =} (@code{vc-diff}) compares each file in the current VC
fileset (saving them if necessary) with the repository revision(s)
from which you started editing.  Note that the latter may or may not
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be the latest revision of the file(s).

  The diff is displayed in another window, in a Diff mode buffer
(@pxref{Diff Mode}) named @file{*vc-diff*}.  In this buffer, the
@kbd{g} (@code{revert-buffer}) command performs the file comparison
again, generating a new diff.
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@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
(@pxref{Tags,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
@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

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  The command @kbd{C-x v D} (@code{vc-root-diff}) is similar to
@kbd{C-x v =}, but it compares the entire tree associated with the
current VC fileset with the tree you started with.  This means all the
files controlled by the current version control repository, even those
that are not part of the current VC fileset.

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  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.

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@findex vc-ediff
The function @code{vc-ediff} works like @code{vc-diff} and provides a way to
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visually compare two revisions of a file in an Ediff session, @pxref{Top,
Ediff, ediff, The Ediff Manual}.  It compares the file associated with the
current buffer with the last repository revision.  To compare two arbitrary
revisions of the current file, call @code{vc-ediff} with a prefix argument.

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@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
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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

@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.

* Registering::         Putting a file under version control.
* VC Change Log::       Viewing the VC Change Log.
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* 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}
(@pxref{Customizing VC,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
(@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

  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.

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

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@node VC Change Log
@subsubsection VC Change Log
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@table @kbd
@item C-x v l
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Display revision control state and change history

@item C-x v L
Display the change history for the current repository
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@item C-x v I
Display the changes that will be received with a pull operation

@item C-x v O
Display the changes that will be sent by the next push operation
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@end table

@kindex C-x v l
@findex vc-print-log
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  The command @kbd{C-x v l} (@code{vc-print-log}) displays a buffer
named @samp{*vc-change-log*} in a new window.  This buffer lists the
changes to the current file, including the associated log entries.
(These are the log entries associated with the version control system,
i.e. the ones you enter via the @samp{*VC-Log*} buffer.  @xref{Log
Buffer}.)  Point is centered at the revision of the file currently
being visited.  With a prefix argument, the command prompts for the
revision to center on, and the maximum number of revisions to display.
You can call this command from a Dired buffer (@pxref{Dired}).
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@findex vc-print-root-log
  Type @kbd{C-x v L} (@code{vc-print-root-log}) to display a
@samp{*vc-change-log*} buffer showing the history of the
version-controlled directory tree as a whole.  With a prefix argument,
the command prompts for the maximum number of revisions to display.
RCS, SCCS, and CVS do not support this feature.

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  On a distributed version control system, the @kbd{C-x v I}
(@code{vc-log-incoming}) command displays a log buffer showing the
changes that will be applied, the next time you run the version
control system's ``pull'' command to get new revisions from another
repository.  This other repository is the default one from which
changes are pulled, as defined by the version control system; with a
prefix argument, @code{vc-log-incoming} prompts for a specific
repository from which changes would be pulled, and lists the changes
accordingly.  Similarly, @kbd{C-x v O} (@code{vc-log-outgoing}) shows
the changes that will be sent to another repository, the next time you
run the ``push'' command; with a prefix argument, it prompts for a
specific repository to which changes would be pushed.

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  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

@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

@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

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@vindex vc-log-show-limit
Because fetching many log entries can be slow, the
@samp{*vc-change-log*} buffer displays no more than 2000 revisions by
default.  The variable @code{vc-log-show-limit} specifies this limit;
if you set the value to zero, that removes the limit.  You can also
increase the number of revisions shown in an existing
@samp{*vc-change-log*} buffer by clicking on the @samp{Show 2X
entries} or @samp{Show unlimited entries} buttons at the end of the
buffer.  However, RCS, SCCS, and CVS do not support this feature.

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@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 committed, use @kbd{C-x v c}
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(@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

@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}.

* 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:

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

In this example, @samp{file1.c} is modified with respect to the
repository, and @samp{file2.c} is not.  @samp{file3.c} is modified,
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but other changes have also been committed---you need to merge them
with the work file before you can check it in.
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@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,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
(@pxref{CVS Options}).
@end ifnottex
This is 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
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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

@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 @kbd{mouse-2}.
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  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

  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} hides up-to-date files.
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  @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

  @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.

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Bob Rogers committed
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  @kbd{M-s a C-M-s} does an incremental regular expression search
on the marked files.

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@cindex stashes in version control
@cindex shelves in version control
  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.  For example, Git and Bazaar allow you to
manipulate @dfn{stashes} and @dfn{shelves}.  (These provide a
mechanism to temporarily store uncommitted changes somewhere out of
the way, and bring them back at a later time.)
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  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}.

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  The command @kbd{C-x v v} (@code{vc-next-action}) operates on all
the marked files, so that you can commit 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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  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, and consecutive revisions on this branch
would have IDs,,, and so on.  If there is
a second branch also starting at revision 1.2, it would consist of
revisions,,, 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.

* 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

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  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 commit 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.
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  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 commit a new
revision.  This automatically creates a new branch starting from the
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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.
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