@c This is part of the Emacs manual., Abbrevs, This is part of the Emacs manual., Top
@c Copyright (C) 1985--1987, 1993--1995, 1997, 1999--2024 Free Software
@c Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Maintaining
@chapter Maintaining Large Programs
@cindex maintaining large programs
@cindex large programming projects, maintaining

  This chapter describes Emacs features for maintaining medium- to
large-size programs and packages.  These features include:

@itemize @minus
@item
Unified interface to Support for Version Control Systems
(@acronym{VCS}) that record the history of changes to source files.

@item
Commands for handling programming projects.

@item
A specialized mode for maintaining @file{ChangeLog} files that provide
a chronological log of program changes.

@item
@acronym{Xref}, a set of commands for displaying definitions of
symbols (a.k.a.@: ``identifiers'') and their references.

@item
@acronym{EDE}, the Emacs's own IDE.

@ifnottex
@item
A mode for merging changes to program sources made on separate
branches of development.
@end ifnottex

@item
A minor-mode for highlighting bug references and visiting the
referenced bug reports in their issue tracker.
@end itemize

If you are maintaining a large Lisp program, then in addition to the
features described here, you may find the Emacs Lisp Regression
Testing (@acronym{ERT}) library useful (@pxref{Top,,ERT,ert, Emacs
Lisp Regression Testing}).

@menu
* Version Control::     Using version control systems.
* Projects::            Commands for handling source files in a project.
* Change Log::          Maintaining a change history for your program.
* Xref::                Find definitions and references of any function,
                          method, struct, macro, @dots{} in your program.
* EDE::                 An integrated development environment for Emacs.
@ifnottex
* Emerge::              A convenient way of merging two versions of a program.
@end ifnottex
* Bug Reference::       Highlighting references to bug reports and browsing
                          them in their issue trackers.
@end menu

@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
time of each version, who made it, and a description of what was
changed.

@cindex VC
  The Emacs version control interface is called @dfn{VC}@.  VC
commands work with several different version control systems;
currently, it supports Bazaar, CVS, Git, Mercurial, Monotone, RCS,
SRC, SCCS/CSSC, and Subversion.  Of these, the GNU project distributes
CVS, 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}
@iftex
(@pxref{Customizing VC,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
@ifnottex
(@pxref{Customizing VC}).
@end ifnottex

@findex vc-refresh-state
@findex vc-state-refresh
  To update the VC state information for the file visited in the
current buffer, use the command @code{vc-refresh-state}.  This command
is useful when you perform version control commands outside Emacs
(e.g., from the shell prompt), or if you put the buffer's file under a
different version control system, or remove it from version control
entirely.

@cindex VC commands, in Dired buffers
@cindex filesets, VC, in Dired buffers
  VC is also enabled automatically in Dired buffers (@pxref{Dired})
showing directories whose files are controlled by a VCS@.  All VC
commands described in this section can be invoked from any Dired
buffer showing a directory with VC-controlled files; any files that
are marked in a Dired buffer (@pxref{Marks vs Flags}) are considered
to belong to the current fileset, and VC commands operate on the files
in this fileset.  This allows you to construct VC filesets including
any files you want, regardless of their VC state.  (If no files are
marked when a VC command is invoked from a Dired buffer, the file
shown on the current line in the buffer is considered the only file in
the fileset.)

@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.
* Log Buffer::          Features available in log entry buffers.
* Registering::         Putting a file under version control.
* Old Revisions::       Examining and comparing old versions.
* VC Change Log::       Viewing the VC Change Log.
* VC Undo::             Canceling changes before or after committing.
* VC Ignore::           Ignore files under version control system.
* VC Directory Mode::   Listing files managed by version control.
* Branches::            Multiple lines of development.
@ifnottex
* 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.  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 VC, e.g., via the command line.

  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.
* VCS Merging::             How file conflicts are handled.
* VCS Changesets::          How changes are grouped.
* VCS Repositories::        Where version control repositories are stored.
* 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.
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,
which it refers to as @dfn{back ends}:

@itemize @bullet

@cindex git
@item
Git is a decentralized version control system originally invented by
Linus Torvalds to support development of Linux (his kernel).  VC
supports many common Git operations, but others, such as repository
syncing, must be done from the command line.

@cindex CVS
@item
CVS is the free version control system that was, until circa 2008,
used by the majority of free software projects.  Since then, it has
been superseded by newer systems.  CVS allows concurrent 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.

@cindex SVN
@cindex Subversion
@item
Subversion (svn) is a free version control system designed to be
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).

@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.  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.

@cindex hg
@cindex Mercurial
@item
Mercurial (hg) is a decentralized version control system broadly
resembling Git.  VC supports most Mercurial commands, with the
exception of repository sync operations.

@cindex bzr
@cindex Bazaar
@item
Bazaar (bzr) is a decentralized version control system that supports
both repository-based and decentralized versioning.  VC supports most
basic editing operations under Bazaar.

@cindex SRC
@cindex src
@item
SRC (src) is RCS, reloaded---a specialized version-control system
designed for single-file projects worked on by only one person.  It
allows multiple files with independent version-control histories to
exist in one directory, and is thus particularly well suited for
maintaining small documents, scripts, and dotfiles.  While it uses RCS
for revision storage, it presents a modern user interface featuring
lockless operation and integer sequential version numbers.  VC
supports almost all SRC operations.
@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
  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
may @dfn{commit} (or @dfn{check in}) the changes; this records the
changes in the repository, along with a descriptive log entry.

@cindex working tree
  A directory tree of work files is called a @dfn{working tree}.

@cindex revision
@cindex revision ID
  Each commit creates a new @dfn{revision} in the repository.  The
version control system keeps track of all past revisions and the
changes that were made in each revision.  Each revision is named by a
@dfn{revision ID}, whose format 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
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.

@node VCS Merging
@subsubsection Merge-based vs Lock-based Version Control

  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.

@cindex merging-based version
  In a version control system that uses merging, each user may 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.

@cindex locking-based version
  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
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 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
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.

  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.
Decentralized version control systems, such as Git and Mercurial, are
exclusively merging-based.

  VC mode supports both locking and merging version control.  The
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
possible.

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

@cindex file-based version control
  On SCCS, RCS, CVS, and other early version control systems (and also
in SRC), version control operations are @dfn{file-based}: each file
has its own comment and revision history separate from that of all
other files.  Newer systems, beginning with Subversion, are
@dfn{changeset-based}: a commit 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.

@cindex changeset-based version control
  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.

@node VCS Repositories
@subsubsection Decentralized vs Centralized Repositories

@cindex centralized version control
@cindex decentralized version control
@cindex distributed 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, Subversion, and SRC share this
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{distributed} or
@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.  In
effect, there is one repository for each developer, and repository
merges take the place of commit operations.

  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.

@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 commit 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.
@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.  Another
advantage is that commit logs can't be fixed in many version control
systems.

  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 committing the change (@pxref{Log Buffer}).  Or you
can write the entry in the log buffer while committing the change
(with the help of @kbd{C-c C-w}), 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
@cindex VC mode line indicator

  When you visit a file that is under version control, Emacs indicates
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.

@cindex version control status
  The character between the back-end name and the revision ID
indicates the @dfn{version control status} of the work file.  In a
merge-based version control system, a @samp{-} character indicates
that the work file is 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.

  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, identical to
@samp{Tools / Version Control} on the menu bar.

@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
@cindex VC filesets
   Most VC commands operate on @dfn{VC filesets}.  A VC fileset is a
collection of one or more files that a VC operation acts upon.  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
consists of the marked files (@pxref{VC Directory Mode}).  Likewise,
when you invoke a VC command from a Dired buffer, the VC fileset
consists of the marked files (@pxref{Marks vs Flags}), defaulting to
the file shown on the current line if no files are marked.

  With modern changeset-based version control systems (@pxref{VCS
Changesets}), such as Git, Mercurial, and Bazaar, VC commands handle
multi-file VC filesets as a group.  For example, committing a
multi-file VC fileset generates a single revision, containing the
changes to all those files.  On older file-based version control
systems like CVS, each file in a multi-file VC fileset is handled
individually; thus, committing a fileset generates one revision for
each changed file in the fileset.

@table @kbd
@item C-x v v
Perform the next appropriate version control operation on the current
VC fileset.
@end table

@findex vc-next-action
@kindex C-x v v
  The principal VC command is a multi-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 for each situation are described in detail in the
following subsections.  You can use @kbd{C-x v v} either in a
file-visiting buffer, in a Dired buffer, or in a VC Directory buffer;
in the latter two cases the command operates on the fileset consisting
of the marked files.  You can also use @kbd{C-x v v}, in a buffer with
patches under Diff Mode (@pxref{Diff Mode}), in which case the command
operates on the files whose diffs are shown in the 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.

@menu
* VC With A Merging VCS::  Without locking: default mode for Git, Hg, SVN, 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

  With a modern merging-based version control system (such as Git and Hg;
@pxref{VCS Merging}), @kbd{C-x v v} does the following when invoked
from a buffer that visits a version-controlled file or a VC Directory
or Dired buffer:

@itemize @bullet
@item
If there is more than one file in the VC fileset and the files have
inconsistent version control statuses, signal an error.  (Note,
however, that a fileset is allowed to include both newly-added files
and modified files; @pxref{Registering}.)  Also signal an error if the
files in the fileset are missing (removed from the filesystem, but
still tracked by version control), or are ignored by version control.

@item
If every file in the VC fileset is registered and unchanged with
respect to the last revision, do nothing.

@item
If none of the files in the VC fileset are registered with a version
control system, register the newly-added files in the VC fileset,
i.e., place them under version control.  @xref{Registering}.  If Emacs
cannot find a system to register under, it prompts for a repository
type, creates a new repository, and registers the VC fileset with it.
You can also specify the system explicitly, see @ref{Advanced C-x v
v}.  Note that registering the files doesn't commit them; you must
invoke @w{@kbd{C-x v v}} again to commit, see below.

@item
If every file in the VC fileset has been either newly-added or
modified, commit the changed files.  To do this, Emacs pops up a
@file{*vc-log*} buffer; type the desired log entry for the changes,
followed by @kbd{C-c C-c} to commit.  @xref{Log Buffer}.

With modern decentralized version control systems (Git, Mercurial,
etc.), the changes are committed locally and not automatically
propagated to the upstream repository (which is usually on a remote
host).  In these cases, if the repository has been changed since your
last update, the commit may fail.  In that case, you must update from
upstream and then try again.  Use @kbd{C-x v +} (@pxref{Pulling /
Pushing}) or @kbd{C-x v m} (@pxref{Merging}) for that.

With a centralized version control system, if the commit fails due to
upstream changes, type @kbd{C-x v v} again to merge in the upstream
repository changes.

@item
Finally, if you are using a centralized version control system, if any
file in the VC fileset is outdated with respect to the upstream
repository, offer to update the fileset from the repository.
@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.
Nothing informs you if another user has committed changes in the same
file since you began editing it; when you commit your revision, that
other user's 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 committing 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}).

  If @kbd{C-x v v} is invoked from a buffer under Diff Mode, the
command assumes the buffer holds a set of patches for one or more
files.  It then applies the changes to the respective files and
commits the changes after popping up the @file{*vc-log*} buffer to
allow you to type a suitable commit log message.

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

  With 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 there is more than one file in the VC fileset and the files have
inconsistent version control statuses, signal an error.  Also signal
an error if the files in the fileset are missing (removed from the
filesystem, but still tracked by version control).

@item
If each file in the VC fileset is not registered with a version
control system, register the newly-added files in the fileset.
@xref{Registering}.  If Emacs cannot find a system to register under,
it prompts for a repository type, creates a new repository, and
registers the VC fileset with it.  You can also specify the system
explicitly, see @ref{Advanced C-x v v}.

@item
If each file is registered and unlocked, check the files out: lock
each one and make it writable, so that you can begin to edit it.

@item
If each file is locked by you and contains changes, commit (a.k.a.@:
``check-in'') the changes.  To do this, Emacs pops up a
@file{*vc-log*} buffer; type the desired log entry for the new
revision, followed by @kbd{C-c C-c} to commit (@pxref{Log Buffer}).

@item
If each file is locked by you, but you have not changed it, release
the lock and make the file read-only again.  This undoes previous
check-out operation for files that were not changed since the
checkout.

@item
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
file.

@item
If files in the fileset are unlocked, but have changes with respect to
their last revision, offer to claim the lock for each such file or to
revert the file to the last checked-in revision.  (This situation is
exceptional and should not normally happen.)
@end itemize

  These rules also apply when you use CVS in locking mode, except
that CVS does not support stealing locks.

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

@cindex revision ID in version control
  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
@cindex specific version control system
You can specify the name of a version control system.  This is useful
if the fileset can be managed by more than one version control system,
and Emacs fails to detect the correct one.

@item
Otherwise, if using CVS, RCS or SRC, you can specify a revision ID.

If the fileset is modified (or locked), this makes Emacs commit the
files with that revision ID@.  You can create a new branch by
supplying an appropriate revision ID (@pxref{Branches}).

If the fileset is unmodified (and unlocked), this checks out the
specified revision into the working tree.  You can also specify a
revision on another branch by giving its revision or branch ID
(@pxref{Switching Branches}).  An empty argument (i.e., @kbd{C-u C-x v
v @key{RET}}) checks out the latest (a.k.a.@: ``head'') revision on
the current branch.

Specifying revision ID in this manner is silently ignored by a
decentralized version control system.  Those systems do not let you
specify your own revision IDs, nor do they use the concept of checking
out individual files.
@end itemize

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

@kindex C-c C-c @r{(Log Edit mode)}
@findex log-edit-done
  When you tell VC to commit a change, it pops up a buffer named
@file{*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} (@code{log-edit-done}) to exit
the buffer and commit the change, together with your log entry.

@cindex Log Edit mode
@cindex mode, Log Edit
@vindex vc-log-mode-hook
@c FIXME: Mention log-edit-mode-hook here?  --xfq
  The major mode for the @file{*vc-log*} buffer is Log Edit mode, a
variant of Text mode (@pxref{Text Mode}).  On entering Log Edit mode,
Emacs runs the hooks @code{text-mode-hook} and @code{vc-log-mode-hook}
(@pxref{Hooks}).

  In the @file{*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:

@smallexample
Author: J. R. Hacker <jrh@@example.com>
@end smallexample

@noindent
Apart from the @samp{Author} header, Emacs recognizes the headers
@samp{Summary} (a one-line summary of the changeset), @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.  If you specify a header for a system that does not
support it, the header is treated as part of the log entry.

@kindex C-c C-f @r{(Log Edit mode)}
@findex log-edit-show-files
@kindex C-c C-d @r{(Log Edit mode)}
@findex log-edit-show-diff
  While in the @file{*vc-log*} buffer, the current VC fileset is
considered to be the fileset that will be committed if you type
@w{@kbd{C-c C-c}}.  To view a list of the files in the VC fileset,
type @w{@kbd{C-c C-f}} (@code{log-edit-show-files}).  To view a diff
of changes between the VC fileset and the version from which you
started editing (@pxref{Old Revisions}), type @kbd{C-c C-d}
(@code{log-edit-show-diff}).

@kindex C-c C-w @r{(Log Edit mode)}
@findex log-edit-generate-changelog-from-diff
@vindex diff-add-log-use-relative-names
  To help generate ChangeLog entries, type @kbd{C-c C-w}
(@code{log-edit-generate-changelog-from-diff}), to generate skeleton
ChangeLog entries, listing all changed file and function names based
on the diff of the VC fileset.  Consecutive entries left empty will be
combined by @kbd{M-q} (@code{fill-paragraph}).  By default the
skeleton will just include the file name, without any leading
directories.  If you wish to prepend the leading directories up to the
VC root, customize @code{diff-add-log-use-relative-names}.

@kindex C-c C-a @r{(Log Edit mode)}
@findex log-edit-insert-changelog
  If the 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 @file{*vc-log*} buffer.  If the topmost item in each
@file{ChangeLog} was made under your user name on the current date,
this command searches that item for entries matching the file(s) to be
committed, and inserts them.
@ifnottex
If you are using CVS or RCS, see @ref{Change Logs and VC}, for the
opposite way of working---generating ChangeLog entries from the Log
Edit buffer.
@end ifnottex

  To abort a commit, just @emph{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 make another commit, the entry you were editing remains
in the @file{*vc-log*} buffer, and you can go back to that buffer at
any time to complete the commit.

@kindex M-n @r{(Log Edit mode)}
@kindex M-p @r{(Log Edit mode)}
@kindex M-s @r{(Log Edit mode)}
@kindex M-r @r{(Log Edit mode)}
  You can also browse the history of previous log entries to duplicate
a commit comment.  This can be useful when you want to make several
commits with similar comments.  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 (@pxref{Minibuffer History}), except that they are
used outside the minibuffer.

@node Registering
@subsection Registering a File for Version Control

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

@kindex C-x v i
@findex vc-register
  The command @kbd{C-x v i} (@code{vc-register}) @dfn{registers} each
file in the current VC fileset, placing it under version control.
This is essentially equivalent to the action of @kbd{C-x v v} on an
unregistered VC fileset (@pxref{Basic VC Editing}), except that if the
VC fileset is already registered, @kbd{C-x v i} signals an error
whereas @kbd{C-x v v} performs some other action.

  To register a file, Emacs must choose a version control system.  For
a multi-file VC fileset, the VC Directory buffer specifies the system
to use (@pxref{VC Directory Mode}).  For a single-file VC fileset, if
the file's directory already contains files registered in a version
control system, or if the directory is part of a directory tree
controlled by a version control system, Emacs chooses that system.  In
the event that more than one version control system is applicable,
Emacs uses the one that appears first in the variable
@iftex
@code{vc-handled-backends}.
@end iftex
@ifnottex
@code{vc-handled-backends} (@pxref{Customizing VC}).
@end ifnottex
If Emacs cannot find a version control system to register the file
under, it prompts for a repository type, creates a new repository, and
registers the file into that repository.

@cindex added files, VC
@cindex files added to VCS
  With most version control systems, registering a file with
@w{@kbd{C-x v i}} or @w{@kbd{C-x v v}} adds it to the working tree,
but does not commit it, i.e., doesn't add it to the repository.  Such
files are labeled as @dfn{added} in the VC Directory buffer, and the
mode line of the buffers visiting such files shows a revision ID of
@samp{@@@@}.  To make the registration take effect in the repository,
you must commit the newly-added files (@pxref{Basic VC Editing}).
Note that a single commit can include both file additions and edits to
files already known to the VCS.

  With a locking-based version control system (@pxref{VCS Merging}),
registering a file leaves it unlocked and read-only.  Type @kbd{C-x v
v} to check-out the file and start editing it.

@node Old Revisions
@subsection Examining And Comparing Old Revisions

@table @kbd
@item C-x v =
Compare the work files in the current VC fileset with the versions you
started from (@code{vc-diff}).  With a prefix argument, prompt for two
revisions of the current VC fileset and compare them.  You can also
call this command from a Dired buffer (@pxref{Dired}).

@ifnottex
@item M-x vc-ediff
Like @kbd{C-x v =}, but using Ediff.  @xref{Top,, Ediff, ediff, The
Ediff Manual}.
@end ifnottex

@item C-x v D
Compare the entire working tree to the revision you started from
(@code{vc-root-diff}).  With a prefix argument, prompt for two
revisions and compare their trees.

@item C-x v ~
Prompt for a revision of the current file, and visit it in a separate
buffer (@code{vc-revision-other-window}).

@item C-x v g
Display an annotated version of the current file: for each line, show
the latest revision in which it was modified (@code{vc-annotate}).
@end table

@findex vc-diff
@kindex C-x v =
  @kbd{C-x v =} (@code{vc-diff}) displays a @dfn{diff} which compares
each work file in the current VC fileset to the version(s) from which
you started editing.  The diff is displayed in another window, in a
Diff mode buffer (@pxref{Diff Mode}) named @file{*vc-diff*}.  The
usual Diff mode commands are available in this buffer.  In particular,
the @kbd{g} (@code{revert-buffer}) command performs the file
comparison again, generating a new 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 (@pxref{VCS Concepts}), and displays a
diff between those versions of the fileset.  This will not work
reliably for multi-file VC filesets, if the version control system is
file-based rather than changeset-based (e.g., CVS), since then
revision IDs for different files would not be related in any
meaningful way.

  Instead of the revision ID, some version control systems let you
specify revisions in other formats.  For instance, under Bazaar you
can enter @samp{date:yesterday} for the argument to @kbd{C-u C-x v =}
(and related commands) to specify the first revision committed after
yesterday.  See the documentation of the version control system for
details.

  If you invoke @kbd{C-x v =} or @kbd{C-u C-x v =} from a Dired buffer
(@pxref{Dired}), the file listed on the current line is treated as the
current VC fileset.  The VC fileset can also include several marked files.

@ifnottex
@findex vc-ediff
  @kbd{M-x vc-ediff} works like @kbd{C-x v =}, except that it uses an
Ediff session.  @xref{Top,, Ediff, ediff, The Ediff Manual}.
@end ifnottex

@findex vc-root-diff
@kindex C-x v D
  @kbd{C-x v D} (@code{vc-root-diff}) is similar to @kbd{C-x v =}, but
it displays the changes in the entire current working tree (i.e., the
working tree containing the current VC fileset).  If you invoke this
command from a Dired buffer, it applies to the working tree containing
the directory.

@findex vc-root-version-diff
@kindex C-u C-x v D
  To compare two arbitrary revisions of the whole trees, call
@code{vc-root-diff} with a prefix argument: @kbd{C-u C-x v D}.  This
prompts for two revision IDs (@pxref{VCS Concepts}), and displays a
diff between those versions of the entire version-controlled directory
trees (RCS, SCCS, CVS, and SRC do not support this feature).

@vindex vc-diff-switches
  You can customize the @command{diff} options that @kbd{C-x v =} and
@kbd{C-x v D} use for generating diffs.  The options used are taken
from the first non-@code{nil} value amongst the variables
@code{vc-@var{backend}-diff-switches}, @code{vc-diff-switches}, and
@code{diff-switches} (@pxref{Comparing Files}), in that order.  Here,
@var{backend} stands for the relevant version control system,
e.g., @code{bzr} for Bazaar.  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.  Most of the
@code{vc-@var{backend}-diff-switches} variables default to @code{nil},
but some default to @code{t}; these are for version control systems
whose @code{diff} implementations do not accept common diff options,
such as Subversion.

@findex vc-revision-other-window
@kindex C-x v ~
  To directly examine an older version of a file, visit the work file
and type @kbd{C-x v ~ @var{revision} @key{RET}}
(@code{vc-revision-other-window}).  This retrieves the file version
corresponding to @var{revision}, saves it to
@file{@var{filename}.~@var{revision}~}, and visits it in a separate
window.

@findex vc-annotate
@vindex vc-annotate-background-mode
@kindex C-x v g
  Many version control systems allow you to view files @dfn{annotated}
with per-line revision information, by typing @kbd{C-x v g}
(@code{vc-annotate}).  This creates a new ``annotate'' buffer
displaying the file's text, with each line colored to show how old it
is.  Red text is new, blue is 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.  If the variable @code{vc-annotate-background-mode}
is non-@code{nil}, the colors expressing the age of each line are
applied to the background color, leaving the foreground at its default
color.

@vindex vc-annotate-switches
  You can customize the @code{annotate} options that @kbd{C-x v g}
uses by customizing @code{vc-@var{backend}-annotate-switches} and
@code{vc-annotate-switches}.  They function similarly to
@code{vc-@var{backend}-diff-switches} and @code{vc-diff-switches},
described above.

  When you give a prefix argument to @kbd{C-x v g}, Emacs reads two
arguments using the minibuffer: the 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, i.e., 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, i.e., the revision after the one
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 VC Change Log
@subsection VC Change Log
@cindex VC change log

@table @kbd
@item C-x v l
Display the change history for the current fileset
(@code{vc-print-log}).

@item C-x v L
Display the change history for the current repository
(@code{vc-print-root-log}).

@item C-x v b l
Display the change history for another branch
(@code{vc-print-branch-log}).

@item C-x v I
Display the changes that a ``pull'' operation will retrieve
(@code{vc-log-incoming}).

@item C-x v O
Display the changes that will be sent by the next ``push'' operation
(@code{vc-log-outgoing}).

@item C-x v h
Display the history of changes made in the region of file visited by
the current buffer (@code{vc-region-history}).

@item M-x vc-log-search @key{RET}
Search the change history for a specified pattern.
@end table

@kindex C-x v l
@findex vc-print-log
  @kbd{C-x v l} (@code{vc-print-log}) displays a buffer named
@file{*vc-change-log*}, showing the history of changes made to the
current fileset in the long form, including who made the changes, the
dates, and the log entry for each change (these are the same log
entries you would enter via the @file{*vc-log*} buffer; @pxref{Log
Buffer}).  When invoked from a buffer visiting a file, the current
fileset consists of that single file, and point in the displayed
@file{*vc-change-log*} buffer is centered at the revision of that
file.  When invoked from a VC Directory buffer (@pxref{VC Directory
Mode}) or from a Dired buffer (@pxref{Dired}), the fileset consists of
all the marked files, defaulting to the file shown on the current line
in the directory buffer if no file is marked.

  If the fileset includes one or more directories, the resulting
@file{*vc-change-log*} buffer shows a short log of changes (one line
for each change), if the VC backend supports that; otherwise it shows
the log in the long form.

  With a prefix argument, the command prompts for the revision to
center on in the @file{*vc-change-log*} buffer and for the maximum
number of revisions to display.

@kindex C-x v L
@findex vc-print-root-log
@findex log-view-toggle-entry-display
  @kbd{C-x v L} (@code{vc-print-root-log}) displays a
@file{*vc-change-log*} buffer showing the history of the entire
version-controlled directory tree (RCS, SCCS, CVS, and SRC do not
support this feature).  With a prefix argument, the command prompts
for the maximum number of revisions to display.  A numeric prefix
argument specifies the maximum number of revisions without prompting.
When the numeric prefix argument is 1, as in @w{@kbd{C-1 C-x v L}} or
@w{@kbd{C-u 1 C-x v L}}, the command prompts for the revision ID, and
displays the log entry of that revision together with the changes
(diffs) it introduced.  (Some less capable version control systems,
such as RCS and CVS, don't have commands to show a revision log with
its diffs; for them the command displays only the log entry, and you
can request to show the diffs by typing @kbd{d} or @kbd{D}, see
below.)

  The @kbd{C-x v L} history is shown in a compact form, usually
showing only the first line of each log entry.  However, you can type
@key{RET} (@code{log-view-toggle-entry-display}) in the
@file{*vc-change-log*} buffer to reveal the entire log entry for the
revision at point.  A second @key{RET} hides it again.

@kindex C-x v b l
@findex vc-print-branch-log
  @kbd{C-x v b l @var{branch-name} @key{RET}} (@code{vc-print-branch-log})
displays a @file{*vc-change-log*} buffer showing the history of the
version-controlled directory tree, like @code{vc-print-root-log} does,
but it shows the history of a branch other than the current one; it
prompts for the branch whose history to display.

@kindex C-x v I
@kindex C-x v O
@findex vc-log-incoming
@findex vc-log-outgoing
  On a decentralized 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
remote location (@pxref{Pulling / Pushing}).  This other remote location 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 remote location.  Similarly, @kbd{C-x v O}
(@code{vc-log-outgoing}) shows the changes that will be sent to
another remote location, the next time you run the push command; with a
prefix argument, it prompts for a specific destination that
in case of some version control system can be a branch name.

@cindex VC log buffer, commands in
@cindex vc-log buffer
  In the @file{*vc-change-log*} buffer, you can use the following keys
to move between the logs of revisions and of files, and to examine and
compare past revisions (@pxref{Old Revisions}):

@table @kbd
@item p
Move to the previous revision entry.  (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 entry.  A numeric prefix argument is a
repeat count.

@item a
Annotate the revision on the current line (@pxref{Old Revisions}).

@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.

@item d
Display a diff between the revision at point and the next earlier
revision, for the specific file.

@item D
Display the changeset diff between the revision at point and the next
earlier revision.  This shows the changes to all files made in that
revision.

@item @key{RET}
In a compact-style log buffer (e.g., the one created by @kbd{C-x v
L}), toggle between showing and hiding the full log entry for the
revision at point.
@end table

@vindex vc-log-show-limit
Because fetching many log entries can be slow, the
@file{*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
@file{*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, CVS, and SRC do not support this feature.

@kindex C-x v h
@findex vc-region-history
A useful variant of examining history of changes is provided by the command
@kbd{vc-region-history} (by default bound to @kbd{C-x v h}), which shows
a @file{*VC-history*} buffer with the history of changes made in the region
of the current buffer's file between point and the mark (@pxref{Mark}).  The
history of changes includes the commit log messages and also the
changes themselves in the Diff format.

Invoke this command after marking in the current buffer the region in
whose changes you are interested.  In the @file{*VC-history*} buffer
it pops up, you can use all of the commands available in the
@file{*vc-change-log*} buffer described above, and also the commands
defined by Diff mode (@pxref{Diff Mode}).

This command is currently available only with Git and Mercurial (hg).

@findex vc-log-search
The command @code{vc-log-search} allows searching for a pattern in the
log of changes.  It prompts for a pattern (a regular expression), and
displays all entries in the change history whose log messages match
the pattern.  When invoked with a prefix argument, the command will
also prompt for a specific VCS shell command to run for this purpose.

@node VC Undo
@subsection Undoing Version Control Actions

@table @kbd
@item C-x v u
Revert the work file(s) in the current VC fileset to the last revision
(@code{vc-revert}).
@end table

@kindex C-x v u
@findex vc-revert
@vindex vc-revert-show-diff
  If you want to discard all the changes you have made to the current
VC fileset, type @kbd{C-x v u} (@code{vc-revert}).  This will ask you
for confirmation before discarding the changes.  If you agree, the
fileset is reverted.

  If @code{vc-revert-show-diff} is non-@code{nil}, this command will
show you a diff between the work file(s) and the revision from which
you started editing.  Afterwards, the diff buffer will either be
killed (if this variable is @code{kill}), or the buffer will be buried
(any other non-@code{nil} value).  If you don't want @kbd{C-x v u} to
show a diff, set this variable to @code{nil} (you can still view the
diff directly with @kbd{C-x v =}; @pxref{Old Revisions}).

  On locking-based version control systems, @kbd{C-x v u} leaves files
unlocked; you must lock again to resume editing.  You can also use
@kbd{C-x v u} to unlock a file if you lock it and then decide not to
change it.

@node VC Ignore
@subsection Ignore Version Control Files

@table @kbd
@item C-x v G
Ignore a file under current version control system.  (@code{vc-ignore}).
@end table

@kindex C-x v G
@findex vc-ignore
  Many source trees contain some files that do not need to be
versioned, such as editor backups, object or bytecode files, and built
programs.  You can simply not add them, but then they'll always crop
up as unknown files.  You can also tell the version control system to
ignore these files by adding them to the ignore file at the top of the
tree.  @kbd{C-x v G} (@code{vc-ignore}) can help you do this.  When
called with a prefix argument, you can remove a file from the ignored
file list.

@node VC Directory Mode
@subsection VC Directory Mode

@cindex VC Directory buffer
  The @dfn{VC Directory buffer} is a specialized buffer for viewing
the version control statuses of the files in a directory tree, and
performing version control operations on those files.  In particular,
it is used to specify multi-file VC filesets for commands like
@w{@kbd{C-x v v}} to act on (@pxref{VC Directory Commands}).

@kindex C-x v d
@findex vc-dir
  To use the VC Directory buffer, type @kbd{C-x v d} (@code{vc-dir}).
This reads a directory's name using the minibuffer, and switches to a VC
Directory buffer for that directory.  By default, the buffer is named
@file{*vc-dir*}.  Its contents are described
@iftex
below.
@end iftex
@ifnottex
in @ref{VC Directory Buffer}.
@end ifnottex

  The @code{vc-dir} command automatically detects the version control
system to be used in the specified directory.  In the event that more
than one system is being used in the directory, you should invoke the
command with a prefix argument, @kbd{C-u C-x v d}; this prompts for
the version control system which the VC Directory buffer should use.

@ifnottex
@cindex PCL-CVS
@pindex cvs
@cindex CVS directory mode
  In addition to the VC Directory buffer, Emacs has a similar facility
called PCL-CVS which is specialized for CVS@.  @xref{Top, , About
PCL-CVS, pcl-cvs, PCL-CVS---The Emacs Front-End to CVS}.
@end ifnottex

  You can also invoke VC commands from Dired buffers (@pxref{Dired}).
In that case, any VC command you invoke considers the marked files as
the current fileset (@pxref{Basic VC Editing}), defaulting to the file
on the current line if no files are marked.

@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

  The VC Directory buffer contains a list of version-controlled files
and their version control statuses.  It lists files in the current
directory (the one specified when you called @kbd{C-x v d}) and its
subdirectories, but only those with a noteworthy status.  Files
that are up-to-date (i.e., the same as in the repository) are
omitted.  If all the files in a subdirectory are up-to-date, the
subdirectory is not listed either.  As an exception, if a file has
become up-to-date as a direct result of a VC command, it is listed.

  Here is an example of a VC Directory buffer listing:

@smallexample
@group
                     ./
    edited           configure.ac
*   added            README
    unregistered     temp.txt
                     src/
*   edited           src/main.c
@end group
@end smallexample

@noindent
Two work files have been modified but not committed:
@file{configure.ac} in the current directory, and @file{main.c} in the
@file{src/} subdirectory.  The file named @file{README} has been added
but is not yet committed, while @file{temp.txt} is not under version
control (@pxref{Registering}).

The @samp{*} characters next to the entries for @file{README} and
@file{src/main.c} indicate that the user has marked these files as
the current VC fileset
@iftex
(see below).
@end iftex
@ifnottex
(@pxref{VC Directory Commands}).
@end ifnottex

  The above example is typical for a decentralized version control
system like Bazaar, Git, or Mercurial.  Other systems can show other
statuses.  For instance, CVS shows the @samp{needs-update} status if
the repository has changes that have not been applied to the work
file.  RCS and SCCS show the name of the user locking a file as its
status.

@ifnottex
  On CVS, the @code{vc-dir} command normally contacts the repository,
which may be on a remote machine, to check for updates.  If you change
the variable @code{vc-cvs-stay-local} to @code{nil} (@pxref{CVS
Options}), then Emacs avoids contacting a remote repository when
generating the VC Directory buffer (it will still contact it when
necessary, e.g., when doing a commit).  This may be desirable if you
are working offline or the network is slow.
@end ifnottex

@vindex vc-directory-exclusion-list
  The VC Directory buffer omits subdirectories listed in the variable
@code{vc-directory-exclusion-list}.  Its default value contains
directories that are used internally by version control systems.

@node VC Directory Commands
@subsubsection VC Directory Commands

  Emacs provides several commands for navigating the VC Directory
buffer, and for marking files as belonging to the current VC
fileset.

@table @kbd
@item n
@itemx @key{SPC}
Move point to the next entry (@code{vc-dir-next-line}).

@item p
Move point to the previous entry (@code{vc-dir-previous-line}).

@item @key{TAB}
Move to the next directory entry (@code{vc-dir-next-directory}).

@item S-@key{TAB}
Move to the previous directory entry
(@code{vc-dir-previous-directory}).

@item @key{RET}
@itemx f
Visit the file or directory listed on the current line
(@code{vc-dir-find-file}).

@item o
Visit the file or directory on the current line, in a separate window
(@code{vc-dir-find-file-other-window}).

@item m
Mark the file or directory on the current line (@code{vc-dir-mark}),
putting it in the current VC fileset.  If the region is active, mark
all files in the region.

A file cannot be marked with this command if it is already in a marked
directory, or one of its subdirectories.  Similarly, a directory
cannot be marked with this command if any file in its tree is marked.

@item M
If point is on a file entry, mark all files with the same status; if
point is on a directory entry, mark all files in that directory tree
(@code{vc-dir-mark-all-files}).  With a prefix argument, mark all
listed files and directories.

@findex vc-dir-mark-by-regexp
@item % m
@itemx * %
You can use this command to mark files by regexp
(@code{vc-dir-mark-by-regexp}).  If given a prefix, unmark files
instead.

@findex vc-dir-mark-registered-files
@item * r
You can use this command to mark files that are in one of registered
states, including edited, added or removed.
(@code{vc-dir-mark-registered-files}).

@item G
Add the file under point to the list of files that the VC should
ignore (@code{vc-dir-ignore}).  For instance, if the VC is Git, it
will append this file to the @file{.gitignore} file.  If given a
prefix, do this with all the marked files.

@item q
Quit the VC Directory buffer, and bury it (@code{quit-window}).

@item u
Unmark the file or directory on the current line.  If the region is
active, unmark all the files in the region (@code{vc-dir-unmark}).

@item U
If point is on a file entry, unmark all files with the same status; if
point is on a directory entry, unmark all files in that directory tree
(@code{vc-dir-unmark-all-files}).  With a prefix argument, unmark all
files and directories.

@item x
Hide files with @samp{up-to-date} or @samp{ignored} status
(@code{vc-dir-hide-up-to-date}).  With a prefix argument, hide items
whose state is that of the item at point.
@end table

@findex vc-dir-mark
@findex vc-dir-mark-all-files
  While in the VC Directory buffer, all the files that you mark with
@kbd{m} (@code{vc-dir-mark}) or @kbd{M} (@code{vc-dir-mark-all-files})
are in the current VC fileset.  If you mark a directory entry with
@kbd{m}, all the listed files in that directory tree are in the
current VC fileset.  The files and directories that belong to the
current VC fileset are indicated with a @samp{*} character in the VC
Directory buffer, next to their VC status.  In this way, you can set
up a multi-file VC fileset to be acted on by VC commands like
@w{@kbd{C-x v v}} (@pxref{Basic VC Editing}), @w{@kbd{C-x v =}}
(@pxref{Old Revisions}), and @w{@kbd{C-x v u}} (@pxref{VC Undo}).

  The VC Directory buffer also defines some single-key shortcuts for
VC commands with the @kbd{C-x v} prefix: @kbd{=}, @kbd{+}, @kbd{l},
@kbd{i}, @kbd{D}, @kbd{L}, @kbd{G}, @kbd{I}, @kbd{O}, and @kbd{v}.

  For example, you can commit a set of edited files by opening a VC
Directory buffer, where the files are listed with the @samp{edited}
status; marking the files; and typing @kbd{v} or @kbd{C-x v v}
(@code{vc-next-action}).  If the version control system is
changeset-based, Emacs will commit the files in a single revision.

  While in the VC Directory buffer, you can also perform search and
replace on the current VC fileset, with the following commands:

@table @kbd
@item S
Search the fileset (@code{vc-dir-search}).

@item Q
Do a regular expression query replace on the fileset
(@code{vc-dir-query-replace-regexp}).

@item M-s a C-s
Do an incremental search on the fileset (@code{vc-dir-isearch}).

@item M-s a C-M-s
Do an incremental regular expression search on the fileset
(@code{vc-dir-isearch-regexp}).
@end table

@noindent
Apart from acting on multiple files, these commands behave much like
their single-buffer counterparts (@pxref{Search}).

  The VC Directory buffer additionally defines some branch-related
commands starting with the prefix @kbd{b}:

@table @kbd
@item b c
Create a new branch (@code{vc-create-branch}).  @xref{Creating
Branches}.

@item b l
Prompt for the name of a branch and display the change history of that
branch (@code{vc-print-branch-log}).

@item b s
Switch to a branch (@code{vc-switch-branch}).  @xref{Switching
Branches}.

@item d
Delete the marked files, or the current file if no marks
(@code{vc-dir-clean-delete)}.  The files will not be marked as
deleted in the version control system, so this function is mostly
useful for unregistered files.
@end table

@cindex stashes in version control
@cindex shelves in version control
  The above commands are also available via the menu bar, and via a
context menu invoked by @kbd{mouse-2}.  Furthermore, some VC backends
use the menu to provide extra backend-specific commands.  For example,
Git and Bazaar allow you to manipulate @dfn{stashes} and @dfn{shelves}
(which are a way to temporarily put aside uncommitted changes, and
bring them back at a later time).

@node Branches
@subsection Version Control Branches
@cindex branch (version control)

  One use of version control is to support multiple independent lines
of development, which are called @dfn{branches}.  Amongst other
things, branches can be used for maintaining separate stable and
development versions of a program, and for developing unrelated
features in isolation from one another.

  VC's support for branch operations is currently fairly limited.  For
decentralized version control systems, it provides commands for
@dfn{updating} one branch with the contents of another, and for
@dfn{merging} the changes made to two different branches
(@pxref{Merging}).  For centralized version control systems, it
supports checking out different branches and committing into new or
different branches.

@menu
* Switching Branches::    How to get to another existing branch.
* Pulling / Pushing::     Receiving/sending changes from/to elsewhere.
* Merging::               Transferring changes between branches.
* Creating Branches::     How to start a new branch.
@end menu

@node Switching Branches
@subsubsection Switching between Branches

  The various version control systems differ in how branches are
implemented, and these differences cannot be entirely concealed by VC.

  On some decentralized version control systems, including Bazaar and
Mercurial in its normal mode of operation, each branch has its own
working directory tree, so switching between branches just involves
switching directories.  On Git, branches are normally @dfn{co-located}
in the same directory, and switching between branches is done using
the @kbd{git checkout} command, which changes the contents of the
working tree to match the branch you switch to.  Bazaar also supports
co-located branches, in which case the @kbd{bzr switch} command
will switch branches in the current directory.  With Subversion, you
switch to another branch using the @kbd{svn switch} command.  With
Mercurial, command @kbd{hg update} is used to switch to another
branch.

@kindex C-x v b s
@findex vc-switch-branch
  The VC command to switch to another branch in the current directory
is @kbd{C-x v b s @var{branch-name} @key{RET}} (@code{vc-switch-branch}).

  On centralized version control systems, you can also switch between
branches by typing @kbd{C-u C-x v v} in an up-to-date work file
(@pxref{Advanced C-x v v}), and entering the revision ID for a
revision on another branch.  On CVS, for instance, revisions on the
@dfn{trunk} (the main line of development) normally have IDs of the
form 1.1, 1.2, 1.3, @dots{}, while the first branch created from (say)
revision 1.2 has revision IDs 1.2.1.1, 1.2.1.2, @dots{}, the second
branch created from revision 1.2 has revision IDs 1.2.2.1, 1.2.2.2,
@dots{}, and so forth.  You can also specify the @dfn{branch ID},
which is a branch revision ID omitting its final component
(e.g., 1.2.1), to switch to the latest revision on that branch.

  On a locking-based system, switching to a different branch also
unlocks (write-protects) the working tree.

  Once you have switched to a branch, VC commands will apply to that
branch until you switch away; for instance, any VC filesets that you
commit will be committed to that specific branch.

@node Pulling / Pushing
@subsubsection Pulling/Pushing Changes into/from a Branch

@table @kbd
@cindex push changes to upstream (VC)
@item C-x v P
With a decentralized version control system, update another repository
with locally-committed changes from the current branch (a.k.a.@:
@dfn{push} changes).  This concept does not exist for centralized
version control systems

@cindex pull changes from upstream (VC)
@item C-x v +
With a decentralized version control system, update the current branch
of the local repository by @dfn{pulling in} changes from another
repository.

With a centralized version control system, update the current VC
fileset from the repository.
@end table

@kindex C-x v P
@findex vc-push
@cindex upstream repository
On a decentralized version control system, the command @kbd{C-x v P}
(@code{vc-push}) updates another location, commonly known as the
@dfn{upstream repository}, with locally-committed changes from the
current branch.  With a prefix argument, it prompts for the exact
version control command to run, which lets you specify where to push
changes; the default is @kbd{bzr push} with Bazaar, @kbd{git push}
with Git, and @kbd{hg push} with Mercurial.  The default commands
always push to the repository in the default location determined by
the version control system from your branch configuration.

Prior to pushing, you can use @kbd{C-x v O} (@code{vc-log-outgoing})
to view a log buffer of the changes to be sent upstream.  @xref{VC
Change Log}.

@cindex bound branch (Bazaar VCS)
This command is currently supported only by Bazaar, Git, and Mercurial.
The concept of ``pushing'' does not exist for centralized version
control systems, where this operation is a part of committing a
changeset, so invoking this command on a centralized VCS signals an
error.  This command also signals an error when attempted in a Bazaar
@dfn{bound branch}, where committing a changeset automatically pushes
the changes to the remote repository to which the local branch is
bound.

@kindex C-x v +
@findex vc-pull
  With a decentralized version control system, the command @kbd{C-x v
+} (@code{vc-pull}) updates the current branch of the local repository
and it working tree with changes made in the upstream repository.  It
is typically used to update a copy (a.k.a.@: @dfn{clone}) of a remote
branch.  If you supply a prefix argument, the command prompts for the
exact version control command to use, which lets you specify where to
pull changes from.  Otherwise, it pulls from the repository in the
default location determined by the version control system from your
branch configuration.

  Amongst decentralized version control systems, @kbd{C-x v +} is
currently supported only by Bazaar, Git, and Mercurial.  With Bazaar,
it calls @kbd{bzr pull} for ordinary branches (to pull from a
master branch into a mirroring branch), and @kbd{bzr update} for a
bound branch (to pull from a central repository).  With Git, it calls
@kbd{git pull} to fetch changes from a remote repository and merge
it into the current branch.  With Mercurial, it calls @kbd{hg pull
-u} to fetch changesets from the default remote repository and update
the working directory.

  Prior to pulling, you can use @kbd{C-x v I} (@code{vc-log-incoming})
to view a log buffer of the changes to be applied.  @xref{VC Change
Log}.

  With a centralized version control system like CVS, @kbd{C-x v +}
updates the current VC fileset from the repository.

@node Merging
@subsubsection Merging Branches
@cindex merging changes

@table @kbd
@item C-x v m
With a decentralized version control system, merge changes from another
branch into the current one.

With a centralized version control system, merge changes from another
branch into the current VC fileset.
@end table

  While developing a branch, you may sometimes need to @dfn{merge} in
changes that have already been made in another branch.  This is not a
trivial operation, as overlapping and conflicting changes may have
been made to the two branches.

  With a decentralized version control system, you merge changes with
the command @kbd{C-x v m} (@code{vc-merge}).  With Bazaar, this
prompts for the exact arguments to pass to the @command{bzr merge}
command, offering a sensible default if possible.  With Git, this
prompts for the name of a branch to merge from, with completion (based
on the branch names known to the current repository).  With Mercurial,
this prompts for argument to pass to @command{hg merge}.  The output
from running the merge command is shown in a separate buffer.

  With a centralized version control system like CVS, @kbd{C-x v m}
prompts for a branch ID, or a pair of revision IDs (@pxref{Switching
Branches}); then it finds the changes from that branch, or the changes
between the two revisions you specified, and merges those changes into
the current VC fileset.  If you just type @kbd{@key{RET}} at the
prompt, Emacs simply merges any changes that were made on the same
branch since you checked the file out.

@cindex conflicts, VC
@cindex resolving conflicts
  Immediately after performing a merge, only the working tree is
modified, and you can review the changes produced by the merge with
@kbd{C-x v D} and related commands (@pxref{Old Revisions}).  If the
two branches contained overlapping changes, merging produces a
@dfn{conflict}; a warning appears in the output of the merge command,
and @dfn{conflict markers} are inserted into each affected work file,
surrounding the two sets of conflicting changes.  You must then
resolve the conflict by editing the conflicted files; by default,
Emacs will place buffers with VC conflicts in the special Smerge mode,
which provides special commands for resolving the merge conflicts.
Once you are done with resolving the conflicts and have saved the
files with resolved conflicts, those files must be committed in the
usual way for the merge to take effect (@pxref{Basic VC Editing}).

@node Creating Branches
@subsubsection Creating New Branches

  On centralized version control systems like CVS, Emacs supports
creating new branches as part of a commit operation.  When committing
a modified VC fileset, type @kbd{C-u C-x v v} (@code{vc-next-action}
with a prefix argument; @pxref{Advanced C-x v v}).  Then Emacs prompts
for a 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.

@kindex C-x v b c
@findex vc-create-branch
  This procedure will not work for distributed version control systems
like git or Mercurial.  For those systems you should use the command
@code{vc-create-branch} (@w{@kbd{C-x v b c @var{branch-name} @key{RET}}})
instead.

  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 creates a new branch starting from the selected
revision.

  After the branch is created, subsequent commits 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}.

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

@node Projects
@section Working with Projects
@cindex projects

@cindex project root
  A @dfn{project} is a collection of files used for producing one or
more programs.  Files that belong to a project are typically stored in
a hierarchy of directories; the top-level directory of the hierarchy
is known as the @dfn{project root}.

@cindex project back-end
  Whether a given directory is a root of some project is determined by
the project-specific infrastructure, known as @dfn{project back-end}.
Emacs currently supports two such back-ends: VC-aware (@pxref{Version
Control}), whereby a VCS repository is considered a project; and EDE
(@pxref{EDE}).  This is expected to be extended in the future to
support additional types of projects.

  Which files do or don't belong to a project is also determined by
the project back-end.  For example, the VC-aware back-end doesn't
consider ``ignored'' files (@pxref{VC Ignore}) to be part of the
project.  Also, the VC-aware Project back-end considers ``untracked''
files by default.  That behavior is controllable with the variable
@code{project-vc-include-untracked}.

@menu
* Project File Commands::   Commands for handling project files.
* Project Buffer Commands:: Commands for handling project buffers.
* Switching Projects::      Switching between projects.
* Managing Projects::       Managing the project list file.
@end menu

@node Project File Commands
@subsection Project Commands That Operate on Files

@table @kbd
@item C-x p f
Visit a file that belongs to the current project
(@code{project-find-file}).
@item C-x p g
Find matches for a regexp in all files that belong to the current
project (@code{project-find-regexp}).
@item M-x project-search
Interactively search for regexp matches in all files that belong to
the current project.
@item C-x p r
Perform query-replace for a regexp in all files that belong to the
current project (@code{project-query-replace-regexp}).
@item C-x p d
Run Dired in the current project's root directory
(@code{project-dired}).
@item C-x p v
Run @code{vc-dir} in the current project's root directory
(@code{project-vc-dir}).
@item C-x p s
Start an inferior shell in the current project's root directory
(@code{project-shell}).
@item C-x p e
Start Eshell in the current project's root directory
(@code{project-eshell}).
@item C-x p c
Run compilation in the current project's root directory
(@code{project-compile}).
@item C-x p !
Run shell command in the current project's root directory
(@code{project-shell-command}).
@item C-x p &
Run shell command asynchronously in the current project's root
directory (@code{project-async-shell-command}).
@end table

  Emacs provides commands for handling project files conveniently.
This subsection describes these commands.

@cindex current project
  All of the commands described here share the notion of the
@dfn{current project}.  The current project is determined by the
@code{default-directory} (@pxref{File Names}) of the buffer that is
the current buffer when the command is invoked.  If that directory
doesn't seem to belong to a recognizable project, these commands
prompt you for the project directory.

@findex project-find-file
@vindex vc-directory-exclusion-list
  The command @kbd{C-x p f} (@code{project-find-file}) is a convenient
way of visiting files (@pxref{Visiting}) that belong to the current
project.  Unlike @kbd{C-x C-f}, this command doesn't require to type
the full file name of the file to visit, you can type only the file's
base name (i.e., omit the leading directories).  In addition, the
completion candidates considered by the command include only the files
belonging to the current project, and nothing else.  If there's a file
name at point, this command offers that file as the first element of
the ``future history''.  If given a prefix, include all files under
the project root, except for @acronym{VCS} directories listed in
@code{vc-directory-exclusion-list}.

@findex project-find-regexp
  The command @kbd{C-x p g} (@code{project-find-regexp}) is similar to
@code{rgrep} (@pxref{Grep Searching}), but it searches only the files
that belong to the current project.  The command prompts for the
regular expression to search, and pops up an Xref mode buffer with the
search results, where you can select a match using the Xref mode
commands (@pxref{Xref Commands}).  When invoked with a prefix
argument, this command additionally prompts for the base directory
from which to start the search; this allows, for example, to limit the
search only to project files under a certain subdirectory of the
project root.  The way this command displays the matches is affected
by the value of @code{xref-auto-jump-to-first-xref} (@pxref{Identifier
Search}).

@findex project-search
  @kbd{M-x project-search} is a sequential variant of
@code{project-find-regexp}.  It prompts for a regular expression to
search in the current project's files, but instead of finding all the
matches and displaying them, it stops when it finds a match and visits
the matched file at the locus of the match, allowing you to edit the
matched file.  To find the rest of the matches, type @w{@kbd{M-x
fileloop-continue @key{RET}}}.

@findex project-query-replace-regexp
  @kbd{C-x p r} (@code{project-query-replace-regexp}) is similar to
@code{project-search}, but it prompts you for whether to replace each
match it finds, like @code{query-replace} does (@pxref{Query
Replace}), and continues to the next match after you respond.  If your
response causes Emacs to exit the query-replace loop, you can later
continue with @w{@kbd{M-x fileloop-continue @key{RET}}}.

@findex project-find-dir
  The command @kbd{C-x p d} (@code{project-find-dir}) prompts you to
choose a directory inside the current project, with completion.
And opens a Dired buffer (@pxref{Dired}) listing the files in it.

@findex project-dired
  The command @kbd{C-x p D} (@code{project-dired}) opens a Dired
buffer (@pxref{Dired}) listing the files in the current project's root
directory.

@findex project-vc-dir
  The command @kbd{C-x p v} (@code{project-vc-dir}) opens a VC
Directory buffer (@pxref{VC Directory Mode}) listing the version
control statuses of the files in a directory tree under the current
project's root directory.

@findex project-shell
  The command @kbd{C-x p s} (@code{project-shell}) starts a shell
session (@pxref{Shell}) in a new buffer with the current project's
root as the working directory.

@findex project-eshell
  The command @kbd{C-x p e} (@code{project-eshell}) starts an Eshell
session in a new buffer with the current project's root as the working
directory.  @xref{Top,Eshell,Eshell, eshell, Eshell: The Emacs Shell}.

@findex project-compile
  The command @kbd{C-x p c} (@code{project-compile}) runs compilation
(@pxref{Compilation}) in the current project's root directory.

@findex project-shell-command
  The command @kbd{C-x p !} (@code{project-shell-command}) runs
@code{shell-command} in the current project's root directory.

@findex project-async-shell-command
  The command @kbd{C-x p &} (@code{project-async-shell-command}) runs
@code{async-shell-command} in the current project's root directory.

@node Project Buffer Commands
@subsection Project Commands That Operate on Buffers

@table @kbd
@item C-x p b
Switch to another buffer belonging to the current project
(@code{project-switch-to-buffer}).
@item C-x p C-b
List the project buffers (@code{project-list-buffers}).
@item C-x p k
Kill all live buffers that belong to the current project
(@code{project-kill-buffers}).
@end table

@findex project-switch-to-buffer
  Working on a project could potentially involve having many buffers
visiting files that belong to the project, and also buffers that
belong to the project, but don't visit any files (like the
@file{*compilation*} buffer created by @code{project-compile}).  The
command @kbd{C-x p b} (@code{project-switch-to-buffer}) helps you
switch between buffers that belong to the current project by prompting
for a buffer to switch and considering only the current project's
buffers as candidates for completion.

@findex project-list-buffers
  Like the command @code{list-buffers} (@pxref{List Buffers}), the
command @kbd{C-x p C-b} (@code{project-list-buffers}) displays a list
of existing buffers, but only belonging to the current project.

@findex project-kill-buffers
@vindex project-kill-buffer-conditions
@vindex project-kill-buffers-display-buffer-list
  When you finish working on the project, you may wish to kill all the
buffers that belong to the project, to keep your Emacs session
smaller.  The command @kbd{C-x p k} (@code{project-kill-buffers})
accomplishes that: it kills all the buffers that belong to the current
project that satisfy any of @code{project-kill-buffer-conditions}.  If
@code{project-kill-buffers-display-buffer-list} is non-@code{nil}, the
buffers to be killed will be displayed first.

@node Switching Projects
@subsection Switching Projects

@table @kbd
@item C-x p p
Run an Emacs command for another project (@code{project-switch-project}).
@end table

@findex project-switch-project
@vindex project-switch-commands
  Commands that operate on project files (@pxref{Project File
Commands}) will conveniently prompt you for a project directory when
no project is current.  When you are inside some project, but you want
to operate on a different project, use the @kbd{C-x p p} command
(@code{project-switch-project}).  This command prompts you to choose a
directory among known project roots, and then displays the menu of
available commands to operate on the project you choose.  The variable
@code{project-switch-commands} controls which commands are available
in the menu, and which key invokes each command.

@vindex project-list-file
  The variable @code{project-list-file} names the file in which Emacs
records the list of known projects.  It defaults to the file
@file{projects} in @code{user-emacs-directory} (@pxref{Find Init}).

@node Managing Projects
@subsection Managing the Project List File

@table @kbd
@item M-x project-forget-project
Remove a known project from the @code{project-list-file}.
@end table

@findex project-forget-project
  Normally Emacs automatically adds and removes projects to and from the
@code{project-list-file}, but sometimes you may want to manually edit
the available projects.  @kbd{M-x project-forget-project}
prompts you to choose one of the available projects, and then removes
it from the file.

@node Change Log
@section Change Logs

@cindex change log
  Many software projects keep a @dfn{change log}.  This is a file,
normally named @file{ChangeLog}, containing a chronological record of
when and how the program was changed.  Sometimes, these files are
automatically generated from the change log entries stored in version
control systems, or are used to generate these change log entries.
Sometimes, there are several change log files, each recording the
changes in one directory or directory tree.

@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

@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.

@c Not worth it.
@c @vindex change-log-directory-files
To find the change log file, Emacs searches up the directory tree from
the file you are editing.  By default, it stops if it finds a
directory that seems to be the root of a version-control repository.
To change this, customize @code{change-log-directory-files}.

@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.

You can combine multiple changes of the same nature.  If you don't
enter any text after the initial @kbd{C-x 4 a}, any subsequent
@kbd{C-x 4 a} adds another symbol to the change log entry.

@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.

@findex change-log-goto-source
  You can use the command @code{change-log-goto-source} (by default
bound to @kbd{C-c C-c}) to go to the source location of the change log
entry near point, when Change Log mode is on.  Then subsequent
invocations of the @code{next-error} command (by default bound to
@kbd{M-g M-n} and @kbd{C-x `}) will move between entries in the change
log.  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 through the change log entries.

@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.

@vindex add-log-dont-create-changelog-file
  Version control systems are another way to keep track of changes in
your program and keep a change log.  Many projects that use a VCS don't
keep a separate versioned change log file nowadays, so you may wish to
avoid having such a file in the repository.  If the value of
@code{add-log-dont-create-changelog-file} is non-@code{nil}, commands
like @kbd{C-x 4 a} (@code{add-change-log-entry-other-window}) will
record changes in a suitably named temporary buffer instead of a file,
if such a file does not already exist.

Whether you have a change log file or use a temporary buffer for
change logs, you can type @kbd{C-c C-a}
(@code{log-edit-insert-changelog}) in the VC Log buffer to insert the
relevant change log entries, if they exist.  @xref{Log Buffer}.

@node Format of ChangeLog
@subsection Format of ChangeLog

  A change log entry starts with a header line that contains the
current date, your name (taken from the variable
@code{add-log-full-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 Xref
@section Find Identifier References
@cindex xref

@cindex tag
  An @dfn{identifier} is a name of a syntactical subunit of the
program: a function, a subroutine, a method, a class, a data type, a
macro, etc.  In a programming language, each identifier is a symbol in
the language's syntax.  Identifiers are also known as @dfn{tags}.

Program development and maintenance requires capabilities to quickly
find where each identifier was defined and referenced, to rename
identifiers across the entire project, etc.  These capabilities are
also useful for finding references in major modes other than those
defined to support programming languages.  For example, chapters,
sections, appendices, etc.@: of a text or a @TeX{} document can be
treated as subunits as well, and their names can be used as
identifiers.  In this chapter, we use the term ``identifiers'' to
collectively refer to the names of any kind of subunits, in program
source and in other kinds of text alike.

Emacs provides a unified interface to these capabilities, called
@samp{xref}.

@cindex xref backend
To do its job, @code{xref} needs to make use of information and to
employ methods specific to the major mode.  What files to search for
identifiers, how to find references to identifiers, how to complete on
identifiers---all this and more is mode-specific knowledge.
@code{xref} delegates the mode-specific parts of its job to a
@dfn{backend} provided by the mode; it also includes defaults for some
of its commands, for those modes that don't provide their own.

A backend can implement its capabilities in a variety of ways.  Here
are a few examples:

@enumerate a
@item
Some major modes provide built-in means for looking up the language
symbols.  For example, Emacs Lisp symbols can be identified by
searching the package load history, maintained by the Emacs Lisp
interpreter, and by consulting the built-in documentation strings; the
Emacs Lisp mode uses these facilities in its backend to allow finding
definitions of symbols.  (One disadvantage of this kind of backend is
that it only knows about subunits that were loaded into the
interpreter.)

@item
If Eglot is activated for the current buffer's project
(@pxref{Projects}) and the current buffer's major mode, Eglot consults
an external language server program and provides the data supplied by
the server regarding the definitions of the identifiers in the
project.  @xref{Eglot Features,,, eglot, Eglot: The Emacs LSP Client}.

@item
An external program can extract references by scanning the relevant
files, and build a database of these references.  A backend can then
access this database whenever it needs to list or look up references.
The Emacs distribution includes @command{etags}, a command for tagging
identifier definitions in programs, which supports many programming
languages and other major modes, such as HTML, by extracting
references into @dfn{tags tables}.  @xref{Create Tags Table}.  Major
modes for languages supported by @command{etags} can use tags tables
as basis for their backend.  (One disadvantage of this kind of backend
is that tags tables need to be kept reasonably up to date, by
rebuilding them from time to time.)
@end enumerate

@menu
* Find Identifiers::    Commands to find where an identifier is defined
                          or referenced, to list identifiers, etc.
* Tags Tables::         Tags table records which file defines a symbol.
* Select Tags Table::   How to visit a specific tags table.
@end menu

@node Find Identifiers
@subsection Find Identifiers

  This subsection describes the commands that find references to
identifiers and perform various queries about identifiers.  Each such
reference could @emph{define} an identifier, e.g., provide the
implementation of a program subunit or the text of a document section;
or it could @emph{use} the identifier, e.g., call a function or a
method, assign a value to a variable, mention a chapter in a
cross-reference, etc.

@menu
* Looking Up Identifiers:: Commands to find the definition of an identifier.
* Xref Commands::          Commands in the @file{*xref*} buffer.
* Identifier Search::      Searching and replacing identifiers.
* List Identifiers::       Listing identifiers and completing on them.
@end menu

@node Looking Up Identifiers
@subsubsection Looking Up Identifiers
@cindex find definition of symbols
@cindex identifier, finding definition of
@cindex find references to symbols

  The most important thing that @code{xref} enables you to do is to find
the definition of a specific identifier.

@table @kbd
@item M-.@:
Find definitions of an identifier (@code{xref-find-definitions}).
@item C-M-. @var{pattern} @key{RET}
Find all identifiers whose name matches @var{pattern}
(@code{xref-find-apropos}).
@item C-x 4 .@: @key{RET}
Find definitions of identifier, but display it in another window
(@code{xref-find-definitions-other-window}).
@item C-x 5 .@: @key{RET}
Find definition of identifier, and display it in a new frame
(@code{xref-find-definitions-other-frame}).
@item M-x xref-find-definitions-at-mouse
Find definition of identifier at mouse click.
@item M-,
Go back to where you previously invoked @kbd{M-.} and friends
(@code{xref-go-back}).
@item C-M-,
Go forward to where you previously invoked @kbd{M-,}
(@code{xref-go-forward}).
@item M-x xref-etags-mode
Switch @code{xref} to use the @code{etags} backend.
@end table

@kindex M-.
@findex xref-find-definitions
@vindex xref-prompt-for-identifier
  @kbd{M-.}@: (@code{xref-find-definitions}) shows the definition of
the identifier at point.  With a prefix argument, or if there's no
identifier at point, it prompts for the identifier.  (If you want it
to always prompt, customize @code{xref-prompt-for-identifier} to
@code{t}.)

  When entering the identifier argument to @kbd{M-.}, you can use the
usual minibuffer completion commands (@pxref{Completion}), with the
known identifier names being the completion candidates.

@kindex C-x 4 .
@findex xref-find-definitions-other-window
@kindex C-x 5 .
@findex xref-find-definitions-other-frame
  Like most commands that can switch buffers,
@code{xref-find-definitions} has a variant that displays the new
buffer in another window, and one that makes a new frame for it.  The
former is @w{@kbd{C-x 4 .}}
(@code{xref-find-definitions-other-window}), and the latter is
@w{@kbd{C-x 5 .}}  (@code{xref-find-definitions-other-frame}).

  The command @code{xref-find-definitions-at-mouse} works like
@code{xref-find-definitions}, but it looks for the identifier name at
or around the place of a mouse event.  This command is intended to be
bound to a mouse event, such as @kbd{C-M-mouse-1}, for example.

@kindex C-M-.
@findex xref-find-apropos
@vindex tags-apropos-additional-actions
  The command @kbd{C-M-.}@: (@code{xref-find-apropos}) is like
@code{apropos} for tags (@pxref{Apropos}).  It displays a list of
identifiers in the selected tags table whose names match the specified
@var{regexp}.  This is just like @kbd{M-.}, except that it does regexp
matching of identifiers instead of matching symbol names as fixed
strings.  By default, the command pops up the @file{*xref*} buffer,
like @kbd{M-.}, but you can display additional output by customizing
the variable @code{tags-apropos-additional-actions}; see its
documentation for details.

@vindex xref-auto-jump-to-first-definition
  If any of the above commands finds more than one matching
definition, it by default pops up the @file{*xref*} buffer showing the
matching candidates and selects that buffer's window.  (@kbd{C-M-.}@:
@emph{always} pops up the @file{*xref*} buffer if it finds at least
one match.)  Each candidate is normally shown in that buffer as the
name of a file and the matching identifier(s) in that file.  In that
buffer, you can select any of the candidates for display, and you have
several additional commands, described in @ref{Xref Commands}.
However, if the value of the variable
@code{xref-auto-jump-to-first-definition} is @code{move}, Emacs
automatically moves point to the first of these candidates in the
@file{*xref*} buffer, so just typing @key{RET} will display the
definition of that candidate.  If the value of the variable is
@code{t} or @code{show}, the first candidate is automatically shown in
its own window; @code{t} also selects the window showing the first
candidate's definition, while @code{show} leaves the window of the
@file{*xfer*} buffer selected.  The default value is @code{nil}, which
just shows the candidates in the @file{*xref*} buffer, but neither
selects any of them nor shows their definition, until you select a
candidate in the @file{*xref*} buffer.

@findex next-error, in @file{*xref*} buffer
@findex previous-error, in @file{*xref*} buffer
@kindex M-g M-n, for navigation in @file{*xref*} buffer
@kindex M-g M-p, for navigation in @file{*xref*} buffer
  If you switch away of the window showing the @file{*xref*} buffer
which displays several candidates, you can move from one candidate to
another using the commands @w{@kbd{M-g M-n}} (@code{next-error}) and
@w{@kbd{M-g M-p}} (@code{previous-error}).  @xref{Compilation Mode}.

@kindex M-,
@findex xref-go-back
  To go back to places @emph{from where} you've displayed the
definition, use @kbd{M-,} (@code{xref-go-back}).  It jumps back to the
point of the last invocation of @kbd{M-.}.  Thus you can find and
examine the definition of something with @kbd{M-.} and then return to
where you were with @kbd{M-,}.  @kbd{M-,} allows you to retrace the
steps you made forward in the history of places, all the way to the
first place in history, where you first invoked @kbd{M-.}, or to any
place in-between.

@kindex C-M-,
@findex xref-go-forward
  If you previously went back too far with @kbd{M-,}, or want to
re-examine a place from which you went back, you can use @kbd{C-M-,}
(@code{xref-go-forward}) to go forward again.  This is similar to
using @kbd{M-.}, except that you don't need on each step to move point
to the identifier whose definition you want to look up.  @kbd{C-M-,}
allows you to retrace all the steps you made back in the history of
places, all the way to the last place in history, where you invoked
@kbd{M-,}, or to any place in-between.

@findex xref-etags-mode
  Some major modes install @code{xref} support facilities that might
sometimes fail to find certain identifiers.  For example, in Emacs
Lisp mode (@pxref{Lisp Eval}) @kbd{M-.} will by default find only
functions and variables from Lisp packages which are loaded into the
current Emacs session or are auto-loaded (@pxref{Autoload,,, elisp,
The Emacs Lisp Reference Manual}).  If @kbd{M-.} fails to find some
identifiers, you can try forcing @code{xref} to use the @code{etags}
backend (@pxref{Xref}).  To this end, turn on the Xref Etags minor
mode with @w{@kbd{M-x xref-etags-mode}}, then invoke @kbd{M-.} again.
(For this to work, be sure to run @command{etags} to create the tags
table in the directory tree of the source files, see @ref{Create Tags
Table}.)

@node Xref Commands
@subsubsection Commands Available in the @file{*xref*} Buffer
@cindex commands in @file{*xref*} buffers
@cindex XREF mode

  The following commands are provided in the @file{*xref*} buffer by
the special XREF mode:

@table @kbd
@item @key{RET}
@itemx mouse-1
Display the reference on the current line (@code{xref-goto-xref}).
With prefix argument, also bury the @file{*xref*} buffer.

@item mouse-2
@findex xref-select-and-show-xref
The same as @code{mouse-1}, but make the window displaying the
@file{*xref*} buffer the selected window
(@code{xref-select-and-show-xref}).

@item n
@itemx .
@findex xref-next-line
Move to the next reference and display it in the other window
(@code{xref-next-line}).

@item N
@findex xref-next-group
Move to the first reference of the next reference group and display it
in the other window (@code{xref-next-group}).

@item p
@itemx ,
@findex xref-prev-line
Move to the previous reference and display it in the other window
(@code{xref-prev-line}).

@item P
@findex xref-prev-group
Move to the first reference of the previous reference group and
display it in the other window (@code{xref-prev-group}).

@item C-o
@findex xref-show-location-at-point
Display the reference on the current line in the other window
(@code{xref-show-location-at-point}).

@item r @var{pattern} @key{RET} @var{replacement} @key{RET}
Perform interactive query-replace on references that match
@var{pattern} (@code{xref-query-replace-in-results}), replacing
the match with @var{replacement}.  This command can only be used in
@file{*xref*} buffers that show all the matches for an identifier in
all the relevant files.  @xref{Identifier Search}.

@item g
@findex xref-revert-buffer
Refresh the contents of the @file{*xref*} buffer
(@code{xref-revert-buffer}).

@item M-,
@findex xref-quit-and-pop-marker-stack
Quit the window showing the @file{*xref*} buffer, and then jump to the
previous Xref stack location (@code{xref-quit-and-pop-marker-stack}).

@item q
@findex xref-quit
Quit the window showing the @file{*xref*} buffer (@code{xref-quit}).
@end table

In addition, the usual navigation commands, such as the arrow keys,
@kbd{C-n}, and @kbd{C-p} are available for moving around the buffer
without displaying the references.

@node Identifier Search
@subsubsection Searching and Replacing with Identifiers
@cindex search and replace in multiple source files
@cindex multiple source file search and replace

  The commands in this section perform various search and replace
operations either on identifiers themselves or on files that reference
them.

@table @kbd
@item M-?
Find all the references for the identifier at point.

@item r
@itemx M-x xref-query-replace-in-results @key{RET} @var{replacement} @key{RET}
@itemx C-u M-x xref-query-replace-in-results @key{RET} @var{regexp} @key{RET} @var{replacement} @key{RET}
Interactively replace @var{regexp} with @var{replacement} in the names
of all the identifiers shown in the @file{*xref*} buffer.

@item M-x xref-find-references-and-replace @key{RET} @var{from} @key{RET} @var{to} @key{RET}
Interactively rename all instances of the identifier @var{from} to the
new name @var{to}.

@item M-x tags-search @key{RET} @var{regexp} @key{RET}
Search for @var{regexp} through the files in the selected tags
table.

@item M-x tags-query-replace @key{RET} @var{regexp} @key{RET} @var{replacement} @key{RET}
Perform a @code{query-replace-regexp} on each file in the selected tags table.

@item M-x fileloop-continue
Restart one of the last 2 commands above, from the current location of point.
@end table

@kindex M-?
@findex xref-find-references
  @kbd{M-?} finds all the references for the identifier at point,
prompting for the identifier as needed, with completion.  Depending on
the current backend (@pxref{Xref}), the command may prompt even if it
finds a valid identifier at point.  When invoked with a prefix
argument, it always prompts for the identifier.  (If you want it to
prompt always, customize the value of the variable
@code{xref-prompt-for-identifier} to @code{t}; or set it to @code{nil}
to prompt only if there's no usable identifier at point.)  The command
then presents the @file{*xref*} buffer with all the references to the
identifier, showing the file name and the line where the identifier is
referenced.  The XREF mode commands are available in this buffer, see
@ref{Xref Commands}.

@vindex xref-auto-jump-to-first-xref
  If the value of the variable @code{xref-auto-jump-to-first-xref} is
@code{t}, @code{xref-find-references} automatically jumps to the first
result in the @file{*xref*} buffer and selects the window where that
reference is displayed; you can select the other results with
@w{@kbd{M-g M-n}} (@code{next-error}) and @w{@kbd{M-g M-p}}
(@code{previous-error}) (@pxref{Compilation Mode}).  If the value is
@code{show}, the first result is displayed, but the window showing the
@file{*xref*} buffer is left selected.  If the value is @code{move},
the first result is selected in the @file{*xref*} buffer, but is not
displayed; you can then use @key{RET} to actually display the
reference.  The default value is @code{nil}, which just shows the
results in the @file{*xref*} buffer, but doesn't select any of them,
and doesn't display the reference itself.

@findex xref-query-replace-in-results
  @kbd{r} (@code{xref-query-replace-in-results}) reads a @var{replacement}
string, just like ordinary @kbd{M-x query-replace-regexp}.  It then
renames the identifiers shown in the @file{*xref*} buffer in all the
places in all the files where these identifiers are referenced, such
that their new name is @var{replacement}.  This is useful when you
rename your identifiers as part of refactoring.  This command should
be invoked in the @file{*xref*} buffer generated by @kbd{M-?}.  By
default, the command replaces the entire name of each identifier with
@var{replacement}, but if invoked with a prefix argument, the command
prompts for a regexp to match identifier names, and replaces only the
matches of that regexp in the names of the identifiers with
@var{replacement}.

@findex xref-find-references-and-replace
  @kbd{M-x xref-find-references-and-replace} works similarly to
@code{xref-query-replace-in-results}, but is more convenient when you
want to rename a single identifier specified by its name @var{from}.

@findex tags-search
  @kbd{M-x tags-search} reads a regexp using the minibuffer, then
searches for matches in all the files in the selected tags table, one
file at a time.  It displays the name of the file being searched so
you can follow its progress.  As soon as it finds an occurrence,
@code{tags-search} returns.  This command requires tags tables to be
available (@pxref{Tags Tables}).

@findex fileloop-continue
  Having found one match with @code{tags-search}, you probably want to
find all the rest.  @kbd{M-x fileloop-continue} resumes the
@code{tags-search}, finding one more match.  This searches the rest of
the current buffer, followed by the remaining files of the tags table.

@findex tags-query-replace
  @kbd{M-x tags-query-replace} performs a single
@code{query-replace-regexp} through all the files in the tags table.  It
reads a regexp to search for and a string to replace with, just like
ordinary @kbd{M-x query-replace-regexp}.  It searches much like @kbd{M-x
tags-search}, but repeatedly, processing matches according to your
input.  @xref{Query Replace}, for more information on query replace.

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

  It is possible to get through all the files in the tags table with a
single invocation of @kbd{M-x tags-query-replace}.  But often it is
useful to exit temporarily, which you can do with any input event that
has no special query replace meaning.  You can resume the query
replace subsequently by typing @kbd{M-x fileloop-continue}; this
command resumes the last tags search or replace command that you did.
For instance, to skip the rest of the current file, you can type
@w{@kbd{M-> M-x fileloop-continue}}.

  Note that the commands described above carry out much broader
searches than the @code{xref-find-definitions} family.  The
@code{xref-find-definitions} commands search only for definitions of
identifiers that match your string or regexp.  The commands
@code{xref-find-references}, @code{tags-search}, and
@code{tags-query-replace} find every occurrence of the identifier or
regexp, as ordinary search commands and replace commands do in the
current buffer.

  As an alternative to @code{xref-find-references} and
@code{tags-search}, you can run @command{grep} as a subprocess and
have Emacs show you the matching lines one by one.  @xref{Grep
Searching}.

@node List Identifiers
@subsubsection Identifier Inquiries

@table @kbd
@item C-M-i
@itemx M-@key{TAB}
Perform completion on the text around point, possibly using the
selected tags table if one is loaded (@code{completion-at-point}).

@item M-x list-tags @key{RET} @var{file} @key{RET}
Display a list of the identifiers defined in the program file
@var{file}.

@item C-M-. @var{regexp} @key{RET}
Display a list of all identifiers matching @var{regexp}
(@code{xref-find-apropos}).  @xref{Looking Up Identifiers}.

@item M-x tags-next-file
Visit files recorded in the selected tags table.
@end table

  In most programming language modes, you can type @kbd{C-M-i} or
@kbd{M-@key{TAB}} (@code{completion-at-point}) to complete the symbol
at point.  Some modes provide specialized completion for this command
tailored to the mode; for those that don't, if there is a tags table
loaded, this command can use it to generate completion candidates.
@xref{Symbol Completion}.

@findex list-tags
  @kbd{M-x list-tags} reads the name of one of the files covered by
the selected tags table, and displays a list of tags defined in that
file.  Do not include a directory as part of the file name unless the
file name recorded in the tags table includes a directory.  This
command works only with the etags backend, and requires a tags table
for the project to be available.  @xref{Tags Tables}.  If used
interactively, the default tag is file name of the current buffer if
used interactively.

@findex tags-next-file
  @kbd{M-x tags-next-file} visits files covered by the selected tags table.
The first time it is called, it visits the first file covered by the
table.  Each subsequent call visits the next covered file, unless a
prefix argument is supplied, in which case it returns to the first
file.  This command requires a tags table to be selected.

@node Tags Tables
@subsection Tags Tables
@cindex tags and tag tables

  A @dfn{tags table} records the tags@footnote{
A @dfn{tag} is a synonym for identifier reference.  Commands and
features based on the @code{etags} package traditionally use ``tag''
with this meaning, and this subsection follows that tradition.
} extracted by scanning the source code of a certain program or a
certain document.  Tags extracted from generated files reference the
original files, rather than the generated files that were scanned
during tag extraction.  Examples of generated files include C files
generated from Cweb source files, from a Yacc parser, or from Lex
scanner definitions; @file{.i} preprocessed C files; and Fortran files
produced by preprocessing @file{.fpp} source files.

@cindex etags
  To produce a tags table, you run the @command{etags} shell command
on a document or the source code file.  The @samp{etags} program
writes the tags to a @dfn{tags table file}, or @dfn{tags file} in
short.  The conventional name for a tags file is @file{TAGS}@.
@xref{Create Tags Table}.  (It is also possible to create a tags table
by using one of the commands from other packages that can produce such
tables in the same format.)

  Emacs uses the tags tables via the @code{etags} package as one of
the supported backends for @code{xref}.  Because tags tables are
produced by the @command{etags} command that is part of an Emacs
distribution, we describe tags tables in more detail here.

@cindex C++ class browser, tags
@cindex tags, C++
@cindex class browser, C++
@cindex Ebrowse
  The Ebrowse facility is similar to @command{etags} but specifically
tailored for C++.  @xref{Top,, Ebrowse, ebrowse, Ebrowse User's
Manual}.  The Semantic package provides another way to generate and
use tags, separate from the @command{etags} facility.
@xref{Semantic}.

@menu
* Tag Syntax::          Tag syntax for various types of code and text files.
* Create Tags Table::   Creating a tags table with @command{etags}.
* Etags Regexps::       Create arbitrary tags using regular expressions.
@end menu

@node Tag Syntax
@subsubsection 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
@command{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.  @code{operator}
definitions have tag names like @samp{operator+}.  If you specify the
@samp{--class-qualify} option, tags for variables and functions in
classes are named @samp{@var{class}::@var{variable}} and
@samp{@var{class}::@var{function}}.  By default, class methods and
members are not class-qualified, which allows to identify their names in
the sources more accurately.

@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 @LaTeX{} documents, the arguments for @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} and @code{\renewenvironment} are tags.

Other commands can make tags as well, if you specify them in the
environment variable @env{TEXTAGS} before invoking @command{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.  As an exception, expressions of the form
@code{(defvar @var{foo})} are treated as declarations, and are only
tagged if the @samp{--declarations} option is given.

@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 names 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 start 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 Go code, packages, functions, and types 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{use constant},
@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.

@item
In Ruby code, @code{def} or @code{class} or @code{module} at the
beginning of a line generate a tag.  Constants also generate tags.

@item
In Rust code, tags anything defined with @code{fn}, @code{enum},
@code{struct} or @code{macro_rules!}.
@end itemize

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

@node Create Tags Table
@subsubsection Creating Tags Tables
@cindex @command{etags} program

  The @command{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 @command{etags}:

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

@noindent
The @command{etags} program reads the specified files, and writes a tags
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.  You can also append the
newly created tags table to an existing file by using the @samp{--append}
option.

  If the specified files don't exist, @command{etags} looks for
compressed versions of them and uncompresses them to read them.  Under
MS-DOS, @command{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.

  If the tags table becomes outdated due to changes in the files
described in it, you can update it by running the @command{etags}
program again.  If the tags table does not record a tag, or records it
for the wrong file, then Emacs will not be able to find that
definition until you update the tags table.  But if the position
recorded in the tags table becomes a little bit wrong (due to other
editing), Emacs will still be able to find the right position, with a
slight delay.

   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.

  You can make a tags table @dfn{include} another tags table, by
passing the @samp{--include=@var{file}} option to @command{etags}.  It
then covers all the files covered by the included tags file, as well
as its own.

  If you specify the source files with relative file names when you run
@command{etags}, the tags file will contain file names relative to the
directory where the tags file was initially written.  This way, you can
move an entire directory tree containing both the tags file and the
source files, and the tags file will still refer correctly to the source
files.  If the tags file is @file{-} or is in the @file{/dev} directory,
however, the file names are
made relative to the current working directory.  This is useful, for
example, when writing the tags to the standard output.

  When using a relative file name, it should not be a symbolic link
pointing to a tags file in a different directory, because this would
generally render the file names invalid.

  If you specify absolute file names as arguments to @command{etags}, then
the tags file will contain absolute file names.  This way, the tags file
will still refer to the same files even if you move it, as long as the
source files remain in the same place.  Absolute file names start with
@samp{/}, or with @samp{@var{device}:/} on MS-DOS and MS-Windows.

   When you want to make a tags table from a great number of files,
you may have problems listing them on the command line, because some
systems have a limit on its length.  You can circumvent this limit by
telling @command{etags} to read the file names from its standard
input, by typing a dash in place of the file names, like this:

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

  @command{etags} recognizes the language used in an input file based on
its file name and contents.  It first tries to match the file's name and
extension to the ones commonly used with certain languages.  Some
languages have interpreters with known names (e.g., @command{perl} for
Perl or @command{pl} for Prolog), so @command{etags} next looks for an
interpreter specification of the form @samp{#!@var{interp}} on the first
line of an input file, and matches that against known interpreters.  If
none of that works, or if you want to override the automatic detection of
the language, you can specify the language explicitly with the
@samp{--language=@var{name}} option.  You can intermix these options with
file names; each one applies to the file names that follow it.  Specify
@samp{--language=auto} to tell @command{etags} to resume guessing the
language from the file names and file contents.  Specify
@samp{--language=none} to turn off language-specific processing entirely;
then @command{etags} recognizes tags by regexp matching alone
(@pxref{Etags Regexps}).  This comes in handy when an input file uses a
language not yet supported by @command{etags}, and you want to avoid
having @command{etags} fall back on Fortran and C as the default
languages.

  The option @samp{--parse-stdin=@var{file}} is mostly useful when
calling @command{etags} from programs.  It can be used (only once) in
place of a file name on the command line.  @command{etags} will read from
standard input and mark the produced tags as belonging to the file
@var{file}.

  For C and C++, if the source files don't observe the GNU Coding
Standards' convention if having braces (@samp{@{} and @samp{@}}) in
column zero only for top-level definitions, like functions and
@code{struct} definitions, we advise that you use the
@samp{--ignore-indentation} option, to prevent @command{etags} from
incorrectly interpreting closing braces in column zero.

  @samp{etags --help} outputs the list of the languages @command{etags}
knows, and the file name rules for guessing the language.  It also prints
a list of all the available @command{etags} options, together with a short
explanation.  If followed by one or more @samp{--language=@var{lang}}
options, it outputs detailed information about how tags are generated for
@var{lang}.

@node Etags Regexps
@subsubsection Etags Regexps

  The @samp{--regex} option to @command{etags} allows tags to be
recognized by regular expression matching.  You can intermix this
option with file names; each one applies to the source files that
follow it.  If you specify multiple @samp{--regex} options, all of
them are used in parallel.  The syntax is:

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

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

  In these regular expressions, @samp{\} quotes the next character,
and all the C character escape sequences are supported: @samp{\a} for
bell, @samp{\b} for back space, @samp{\e} for escape, @samp{\f} for
formfeed, @samp{\n} for newline, @samp{\r} for carriage return,
@samp{\t} for tab, and @samp{\v} for vertical tab.  In addition,
@samp{\d} stands for the @code{DEL} character.

  Ideally, @var{tagregexp} should not match more characters than are
needed to recognize what you want to tag.  If the syntax requires you
to write @var{tagregexp} so it matches more characters beyond the tag
itself, you should add a @var{nameregexp}, to pick out just the tag.
This will enable Emacs to find tags more accurately and to do
completion on tag names more reliably.  In @var{nameregexp}, it is
frequently convenient to use ``back references'' (@pxref{Regexp
Backslash}) to parenthesized groupings @w{@samp{\( @dots{} \)}} in
@var{tagregexp}.  For example, @samp{\1} refers to the first such
parenthesized grouping.  You can find some examples of this below.

  The @var{modifiers} are a sequence of zero or more characters that
modify the way @command{etags} does the matching.  A regexp with no
modifiers is applied sequentially to each line of the input file, in a
case-sensitive way.  The modifiers and their meanings are:

@table @samp
@item i
Ignore case when matching this regexp.
@item m
Match this regular expression against the whole file, so that
multi-line matches are possible.
@item s
Match this regular expression against the whole file, and allow
@samp{.} in @var{tagregexp} to match newlines.
@end table

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

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

@noindent
Here @command{etags} chooses the parsing language for @file{voo.doo} and
@file{bar.ber} according to their contents.  @command{etags} also uses
@var{reg1} to recognize additional tags in @file{voo.doo}, and both
@var{reg1} and @var{reg2} to recognize additional tags in
@file{bar.ber}.  @var{reg1} is checked against each line of
@file{voo.doo} and @file{bar.ber}, in a case-insensitive way, while
@var{reg2} is checked against the whole @file{bar.ber} file,
permitting multi-line matches, in a case-sensitive way.  @command{etags}
uses only the Lisp tags rules, with no user-specified regexp matching,
to recognize tags in @file{los.er}.

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

@smallexample
--regex='@{c@}/[ \t]*DEFVAR_[A-Z_ \t(]+"\([^"]+\)"/\1/'
@end smallexample

@noindent
When you have complex regular expressions, you can store the list of
them in a file.  The following option syntax instructs @command{etags} to
read two files of regular expressions.  The regular expressions
contained in the second file are matched without regard to case.

@smallexample
--regex=@@@var{case-sensitive-file} --ignore-case-regex=@@@var{ignore-case-file}
@end smallexample

@noindent
A regex file for @command{etags} contains one regular expression per
line.  Empty lines, and lines beginning with space or tab are ignored.
When the first character in a line is @samp{@@}, @command{etags} assumes
that the rest of the line is the name of another file of regular
expressions; thus, one such file can include another file.  All the
other lines are taken to be regular expressions.  If the first
non-whitespace text on the line is @samp{--}, that line is a comment.

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

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

@noindent
and then use it like this:

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

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

@itemize @bullet

@item
Tag Octave files:

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

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

@item
Tag Tcl files:

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

@item
Tag VHDL files:

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

@node Select Tags Table
@subsection Selecting a Tags Table

@findex visit-tags-table
  Emacs has at any time at most one @dfn{selected} tags table.  All
the commands for working with tags tables use the selected one first.
To select a tags table, type @kbd{M-x visit-tags-table}, which reads
the tags table file name as an argument, with @file{TAGS} defaulting
to the first directory that contains a file named @file{TAGS}
encountered when recursively searching upward from the default
directory.

@vindex tags-file-name
  Emacs does not actually read in the tags table contents until you
try to use them; all @code{visit-tags-table} does is store the file
name in the variable @code{tags-file-name}, and not much more.  The
variable's initial value is @code{nil}; that value tells all the
commands for working with tags tables that they must ask for a tags
table file name to use.

  In addition to the selected tags table, Emacs maintains the list of
several tags tables that you use together.  For example, if you are
working on a program that uses a library, you may wish to have the
tags tables of both the program and the library available, so that
Emacs could easily find identifiers from both.  If the selected tags
table doesn't have the identifier or doesn't mention the source file a
tags command needs, the command will try using all the other tags
tables in the current list of tags tables.

  Using @code{visit-tags-table} to load a new tags table when another
tags table is already loaded gives you a choice: you can add the new
tags table to the current list of tags tables, or discard the current
list and start a new list.  If you start a new list, the new tags
table is used @emph{instead} of others.  If you add the new table to
the current list, it is used @emph{as well as} the others.

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

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

@noindent
This tells the tags commands to look at the @file{TAGS} files in your
@file{~/.emacs.d} directory and in the @file{/usr/local/lib/emacs/src}
directory.  The order depends on which file you are in and which tags
table mentions that file.

  Do not set both @code{tags-file-name} and @code{tags-table-list}.

@node EDE
@section Emacs Development Environment
@cindex EDE (Emacs Development Environment)
@cindex Emacs Development Environment
@cindex Integrated development environment

EDE (@dfn{Emacs Development Environment}) is a package that simplifies
the task of creating, building, and debugging large programs with
Emacs.  It provides some of the features of an IDE, or @dfn{Integrated
Development Environment}, in Emacs.

This section provides a brief description of EDE usage.
@ifnottex
For full details, see @ref{Top, EDE,, ede, Emacs Development Environment}.
@end ifnottex
@iftex
For full details on Ede, type @kbd{C-h i} and then select the EDE
manual.
@end iftex

  EDE is implemented as a global minor mode (@pxref{Minor Modes}).  To
enable it, type @kbd{M-x global-ede-mode} or click on the
@samp{Project Support (EDE)} item in the @samp{Tools} menu.  You can
also enable EDE each time you start Emacs, by adding the following
line to your initialization file:

@smallexample
(global-ede-mode t)
@end smallexample

@noindent
Activating EDE adds a menu named @samp{Development} to the menu bar.
Many EDE commands, including the ones described below, can be invoked
from this menu.

  EDE organizes files into @dfn{projects}, which correspond to
directory trees.  The @dfn{project root} is the topmost directory of a
project.  To define a new project, visit a file in the desired project
root and type @kbd{M-x ede-new}.  This command prompts for a
@dfn{project type}, which refers to the underlying method that EDE
will use to manage the project (@pxref{Creating a project, EDE,, ede,
Emacs Development Environment}).  The most common project types are
@samp{Make}, which uses Makefiles, and @samp{Automake}, which uses GNU
Automake (@pxref{Top, Automake,, automake, Automake}).  In both cases,
EDE also creates a file named @file{Project.ede}, which stores
information about the project.

  A project may contain one or more @dfn{targets}.  A target can be an
object file, executable program, or some other type of file, which is
built from one or more of the files in the project.

  To add a new @dfn{target} to a project, type @kbd{C-c . t}
(@code{M-x ede-new-target}).  This command also asks if you wish to
add the current file to that target, which means that the target
is to be built from that file.  After you have defined a target, you
can add more files to it by typing @kbd{C-c . a}
(@code{ede-add-file}).

  To build a target, type @kbd{C-c . c} (@code{ede-compile-target}).
To build all the targets in the project, type @kbd{C-c . C}
(@code{ede-compile-project}).  EDE uses the file types to guess how
the target should be built.

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


@node Bug Reference
@section Bug Reference
@cindex bug reference

Most projects with a certain amount of users track bug reports in some
issue tracking software which assigns each report a unique and short
number or identifier.  Those are used to reference a given bug, e.g.,
in a source code comment above the code fixing some bug, in
documentation files, or in discussions on some mailing list or IRC
channel.

@findex bug-reference-mode
@findex bug-reference-prog-mode
  The minor modes @code{bug-reference-mode} and
@code{bug-reference-prog-mode} highlight such bug references and make
it possible to follow them to the corresponding bug report on the
project's issue tracker.  @code{bug-reference-prog-mode} is a variant
of @code{bug-reference-mode} which highlights bug references only
inside source code comments and strings.

@vindex bug-reference-bug-regexp
@vindex bug-reference-url-format
  For its working, bug reference mode needs to know the syntax of bug
references (@code{bug-reference-bug-regexp}), and the URL of the
tracker where bug reports can be looked up
(@code{bug-reference-url-format}).  Since those are typically
different from project to project, it makes sense to specify them in
@pxref{Directory Variables} or @pxref{File Variables}.

For example, let's assume in our project, we usually write references
to bug reports as bug#1234, or Bug-1234 and that this bug's page on
the issue tracker is @url{https://project.org/issues/1234}, then
these local variables section would do.

@smallexample
;; Local Variables:
;; bug-reference-bug-regexp: "\\([Bb]ug[#-]\\([0-9]+\\)\\)"
;; bug-reference-url-format: "https://project.org/issues/%s"
;; End:
@end smallexample

The string captured by the first regexp group defines the bounds of
the overlay bug-reference creates, i.e., the part which is highlighted
and made clickable.

The string captured by the second regexp group in
@code{bug-reference-bug-regexp} is used to replace the @code{%s}
template in the @code{bug-reference-url-format}.

Note that @code{bug-reference-url-format} may also be a function in
order to cater for more complex scenarios, e.g., when different parts
of the bug reference have to be used to distinguish between issues and
merge requests resulting in different URLs.


@heading Automatic Setup

@vindex bug-reference-auto-setup-functions
If @code{bug-reference-mode} is activated,
@code{bug-reference-mode-hook} has been run, and either
@code{bug-reference-bug-regexp} or @code{bug-reference-url-format} is
still @code{nil}, the mode will try to automatically find a suitable
value for these two variables by calling the functions in
@code{bug-reference-auto-setup-functions} one by one until one
succeeds.

@vindex bug-reference-setup-from-vc-alist
@vindex bug-reference-forge-alist
@vindex bug-reference-setup-from-mail-alist
@vindex bug-reference-setup-from-irc-alist
Right now, there are three types of setup functions.

@enumerate
@item
Setup for version-controlled files configurable by the variables
@code{bug-reference-forge-alist}, and
@code{bug-reference-setup-from-vc-alist}.  The defaults are able to
set up GNU projects where @url{https://debbugs.gnu.org} is used as
issue tracker and issues are usually referenced as @code{bug#13} (but
many different notations are considered, too), as well as several
other kinds of software forges such as GitLab, Gitea, SourceHut, and
GitHub.  If you deploy a self-hosted instance of such a forge, the
easiest way to tell bug-reference about it is through
@code{bug-reference-forge-alist}.

@item
Setup for email guessing from mail folder/mbox names, and mail header
values configurable by the variable
@code{bug-reference-setup-from-mail-alist}.  The built-in news- and
mailreader @ref{Gnus} and @ref{Rmail} are supported.

@item
Setup for IRC channels configurable by the variable
@code{bug-reference-setup-from-irc-alist}.  The built-in IRC clients
Rcirc, @xref{Top, Rcirc,, rcirc, The Rcirc Manual}, and ERC,
@xref{Top, ERC,, erc, The ERC Manual}, are supported.
@end enumerate

For almost all of those modes, it's enough to simply enable
@code{bug-reference-mode}; only Rmail requires a slightly different
setup.

@smallexample
;; Use VC-based setup if file is under version control.
(add-hook 'prog-mode-hook #'bug-reference-prog-mode)

;; Gnus (summary & article buffers)
(add-hook 'gnus-mode-hook #'bug-reference-mode)

;; Rmail
(add-hook 'rmail-show-message-hook #'bug-reference-mode-force-auto-setup)

;; Rcirc
(add-hook 'rcirc-mode-hook #'bug-reference-mode)

;; ERC
(add-hook 'erc-mode-hook #'bug-reference-mode)
@end smallexample

In the Rmail case, instead of the mode hook, the
@code{rmail-show-message-hook} has to be used in combination with the
function @code{bug-reference-mode-force-auto-setup} which activates
@code{bug-reference-mode} and forces auto-setup.  The reason is that
with Rmail all messages reside in the same buffer but the setup needs
to be performed whenever another messages is displayed.


@heading Adding support for third-party packages

@vindex bug-reference-auto-setup-functions
Adding support for bug-reference auto-setup is usually quite
straightforward: write a setup function of zero arguments which
gathers the required information (e.g., List-Id/To/From/Cc mail header
values in the case of a MUA), and then calls one of the following
helper functions:

@itemize @bullet
@item
@code{bug-reference-maybe-setup-from-vc}, which does the setup
according to @code{bug-reference-setup-from-vc-alist};

@item
@code{bug-reference-maybe-setup-from-mail}, which does the setup
according to @code{bug-reference-setup-from-mail-alist}; and

@item
@code{bug-reference-maybe-setup-from-irc}, which does the setup
according to @code{bug-reference-setup-from-irc-alist}.
@end itemize

A setup function should return non-@code{nil} if it could set up
bug-reference mode, which is the case if the last thing the function
does is call one of the helper functions above.

Finally, the setup function has to be added to
@code{bug-reference-auto-setup-functions}.

Note that these auto-setup functions should check as a first step if
they are applicable, e.g., by checking the value of @code{major-mode}.


@heading Integration with the debbugs package

@findex debbugs-browse-mode
If your project's issues are tracked on the server
@url{https://debbugs.gnu.org}, you can browse and reply to reports
directly in Emacs using the @code{debbugs} package, which can be
downloaded via the Package Menu (@pxref{Packages}).  This package adds
the minor mode @code{debbugs-browse-mode}, which can be activated on
top of @code{bug-reference-mode} and @code{bug-reference-prog-mode} as
follows:

@smallexample
(add-hook 'bug-reference-mode-hook 'debbugs-browse-mode)
(add-hook 'bug-reference-prog-mode-hook 'debbugs-browse-mode)
@end smallexample
