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[emacs.git] / man / files.texi

@c This is part of the Emacs manual.
@c Copyright (C) 1985,86,87,93,94,95,97,1999 Free Software Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Files, Buffers, Fixit, Top
@chapter File Handling
@cindex files

  The operating system stores data permanently in named @dfn{files}.  So
most of the text you edit with Emacs comes from a file and is ultimately
stored in a file.

  To edit a file, you must tell Emacs to read the file and prepare a
buffer containing a copy of the file's text.  This is called
@dfn{visiting} the file.  Editing commands apply directly to text in the
buffer; that is, to the copy inside Emacs.  Your changes appear in the
file itself only when you @dfn{save} the buffer back into the file.

  In addition to visiting and saving files, Emacs can delete, copy,
rename, and append to files, keep multiple versions of them, and operate
on file directories.

@menu
* File Names::          How to type and edit file-name arguments.
* Visiting::            Visiting a file prepares Emacs to edit the file.
* Saving::              Saving makes your changes permanent.
* Reverting::           Reverting cancels all the changes not saved.
* Auto Save::           Auto Save periodically protects against loss of data.
* File Aliases::        Handling multiple names for one file.
* Version Control::     Version control systems (RCS, CVS and SCCS).
* Directories::         Creating, deleting, and listing file directories.
* Comparing Files::     Finding where two files differ.
* Misc File Ops::       Other things you can do on files.
* Compressed Files::    Accessing compressed files.
* Remote Files::        Accessing files on other sites.
* Quoted File Names::   Quoting special characters in file names.
@end menu

@node File Names
@section File Names
@cindex file names

  Most Emacs commands that operate on a file require you to specify the
file name.  (Saving and reverting are exceptions; the buffer knows which
file name to use for them.)  You enter the file name using the
minibuffer (@pxref{Minibuffer}).  @dfn{Completion} is available, to make
it easier to specify long file names.  @xref{Completion}.

  For most operations, there is a @dfn{default file name} which is used
if you type just @key{RET} to enter an empty argument.  Normally the
default file name is the name of the file visited in the current buffer;
this makes it easy to operate on that file with any of the Emacs file
commands.

@vindex default-directory
  Each buffer has a default directory, normally the same as the
directory of the file visited in that buffer.  When you enter a file
name without a directory, the default directory is used.  If you specify
a directory in a relative fashion, with a name that does not start with
a slash, it is interpreted with respect to the default directory.  The
default directory is kept in the variable @code{default-directory},
which has a separate value in every buffer.

  For example, if the default file name is @file{/u/rms/gnu/gnu.tasks} then
the default directory is @file{/u/rms/gnu/}.  If you type just @samp{foo},
which does not specify a directory, it is short for @file{/u/rms/gnu/foo}.
@samp{../.login} would stand for @file{/u/rms/.login}.  @samp{new/foo}
would stand for the file name @file{/u/rms/gnu/new/foo}.

@findex cd
@findex pwd
  The command @kbd{M-x pwd} prints the current buffer's default
directory, and the command @kbd{M-x cd} sets it (to a value read using
the minibuffer).  A buffer's default directory changes only when the
@code{cd} command is used.  A file-visiting buffer's default directory
is initialized to the directory of the file that is visited there.  If
you create a buffer with @kbd{C-x b}, its default directory is copied
from that of the buffer that was current at the time.

@vindex insert-default-directory
  The default directory actually appears in the minibuffer when the
minibuffer becomes active to read a file name.  This serves two
purposes: it @emph{shows} you what the default is, so that you can type
a relative file name and know with certainty what it will mean, and it
allows you to @emph{edit} the default to specify a different directory.
This insertion of the default directory is inhibited if the variable
@code{insert-default-directory} is set to @code{nil}.

  Note that it is legitimate to type an absolute file name after you
enter the minibuffer, ignoring the presence of the default directory
name as part of the text.  The final minibuffer contents may look
invalid, but that is not so.  For example, if the minibuffer starts out
with @samp{/usr/tmp/} and you add @samp{/x1/rms/foo}, you get
@samp{/usr/tmp//x1/rms/foo}; but Emacs ignores everything through the
first slash in the double slash; the result is @samp{/x1/rms/foo}.
@xref{Minibuffer File}.

  @samp{$} in a file name is used to substitute environment variables.
For example, if you have used the shell command @samp{export
FOO=rms/hacks} to set up an environment variable named @code{FOO}, then
you can use @file{/u/$FOO/test.c} or @file{/u/$@{FOO@}/test.c} as an
abbreviation for @file{/u/rms/hacks/test.c}.  The environment variable
name consists of all the alphanumeric characters after the @samp{$};
alternatively, it may be enclosed in braces after the @samp{$}.  Note
that shell commands to set environment variables affect Emacs only if
done before Emacs is started.

  To access a file with @samp{$} in its name, type @samp{$$}.  This pair
is converted to a single @samp{$} at the same time as variable
substitution is performed for single @samp{$}.  Alternatively, quote the
whole file name with @samp{/:} (@pxref{Quoted File Names}).

@findex substitute-in-file-name
  The Lisp function that performs the substitution is called
@code{substitute-in-file-name}.  The substitution is performed only on
file names read as such using the minibuffer.

  You can include non-ASCII characters in file names if you set the
variable @code{file-name-coding-system} to a non-@code{nil} value.
@xref{Specify Coding}.

@node Visiting
@section Visiting Files
@cindex visiting files

@c WideCommands
@table @kbd
@item C-x C-f
Visit a file (@code{find-file}).
@item C-x C-r
Visit a file for viewing, without allowing changes to it
(@code{find-file-read-only}).
@item C-x C-v
Visit a different file instead of the one visited last
(@code{find-alternate-file}).
@item C-x 4 f
Visit a file, in another window (@code{find-file-other-window}).  Don't
alter what is displayed in the selected window.
@item C-x 5 f
Visit a file, in a new frame (@code{find-file-other-frame}).  Don't
alter what is displayed in the selected frame.
@item M-x find-file-literally
Visit a file with no conversion of the contents.
@end table

@cindex files, visiting and saving
@cindex visiting files
@cindex saving files
  @dfn{Visiting} a file means copying its contents into an Emacs buffer
so you can edit them.  Emacs makes a new buffer for each file that you
visit.  We say that this buffer is visiting the file that it was created
to hold.  Emacs constructs the buffer name from the file name by
throwing away the directory, keeping just the name proper.  For example,
a file named @file{/usr/rms/emacs.tex} would get a buffer named
@samp{emacs.tex}.  If there is already a buffer with that name, a unique
name is constructed by appending @samp{<2>}, @samp{<3>}, or so on, using
the lowest number that makes a name that is not already in use.

  Each window's mode line shows the name of the buffer that is being displayed
in that window, so you can always tell what buffer you are editing.

  The changes you make with editing commands are made in the Emacs
buffer.  They do not take effect in the file that you visited, or any
place permanent, until you @dfn{save} the buffer.  Saving the buffer
means that Emacs writes the current contents of the buffer into its
visited file.  @xref{Saving}.

@cindex modified (buffer)
  If a buffer contains changes that have not been saved, we say the
buffer is @dfn{modified}.  This is important because it implies that
some changes will be lost if the buffer is not saved.  The mode line
displays two stars near the left margin to indicate that the buffer is
modified.

@kindex C-x C-f
@findex find-file
  To visit a file, use the command @kbd{C-x C-f} (@code{find-file}).  Follow
the command with the name of the file you wish to visit, terminated by a
@key{RET}.

  The file name is read using the minibuffer (@pxref{Minibuffer}), with
defaulting and completion in the standard manner (@pxref{File Names}).
While in the minibuffer, you can abort @kbd{C-x C-f} by typing @kbd{C-g}.

  Your confirmation that @kbd{C-x C-f} has completed successfully is the
appearance of new text on the screen and a new buffer name in the mode
line.  If the specified file does not exist and could not be created, or
cannot be read, then you get an error, with an error message displayed
in the echo area.

  If you visit a file that is already in Emacs, @kbd{C-x C-f} does not make
another copy.  It selects the existing buffer containing that file.
However, before doing so, it checks that the file itself has not changed
since you visited or saved it last.  If the file has changed, a warning
message is printed.  @xref{Interlocking,,Simultaneous Editing}.

@cindex creating files
  What if you want to create a new file?  Just visit it.  Emacs prints
@samp{(New File)} in the echo area, but in other respects behaves as if
you had visited an existing empty file.  If you make any changes and
save them, the file is created.

  Emacs recognizes from the contents of a file which convention it uses
to separate lines---newline (used on GNU/Linux and on Unix),
carriage-return linefeed (used on Microsoft systems), or just
carriage-return (used on the Macintosh)---and automatically converts the
contents to the normal Emacs convention, which is that the newline
character separates lines.  This is a part of the general feature of
coding system conversion (@pxref{Coding Systems}), and makes it possible
to edit files imported from various different operating systems with
equal convenience.  If you change the text and save the file, Emacs
performs the inverse conversion, changing newlines back into
carriage-return linefeed or just carriage-return if appropriate.

@vindex find-file-run-dired
  If the file you specify is actually a directory, @kbd{C-x C-f} invokes
Dired, the Emacs directory browser, so that you can ``edit'' the contents
of the directory (@pxref{Dired}).  Dired is a convenient way to delete,
look at, or operate on the files in the directory.  However, if the
variable @code{find-file-run-dired} is @code{nil}, then it is an error
to try to visit a directory.

  If the file name you specify contains wildcard characters, Emacs
visits all the files that match it.  @xref{Quoted File Names}, if you
want to visit a file whose name actually contains wildcard characters.

  If you visit a file that the operating system won't let you modify,
Emacs makes the buffer read-only, so that you won't go ahead and make
changes that you'll have trouble saving afterward.  You can make the
buffer writable with @kbd{C-x C-q} (@code{vc-toggle-read-only}).
@xref{Misc Buffer}.

@kindex C-x C-r
@findex find-file-read-only
  Occasionally you might want to visit a file as read-only in order to
protect yourself from entering changes accidentally; do so by visiting
the file with the command @kbd{C-x C-r} (@code{find-file-read-only}).

@kindex C-x C-v
@findex find-alternate-file
  If you visit a nonexistent file unintentionally (because you typed the
wrong file name), use the @kbd{C-x C-v} command
(@code{find-alternate-file}) to visit the file you really wanted.
@kbd{C-x C-v} is similar to @kbd{C-x C-f}, but it kills the current
buffer (after first offering to save it if it is modified).  When it
reads the file name to visit, it inserts the entire default file name in
the buffer, with point just after the directory part; this is convenient
if you made a slight error in typing the name.

  If you find a file which exists but cannot be read, @kbd{C-x C-f}
signals an error.

@kindex C-x 4 f
@findex find-file-other-window
  @kbd{C-x 4 f} (@code{find-file-other-window}) is like @kbd{C-x C-f}
except that the buffer containing the specified file is selected in another
window.  The window that was selected before @kbd{C-x 4 f} continues to
show the same buffer it was already showing.  If this command is used when
only one window is being displayed, that window is split in two, with one
window showing the same buffer as before, and the other one showing the
newly requested file.  @xref{Windows}.

@kindex C-x 5 f
@findex find-file-other-frame
  @kbd{C-x 5 f} (@code{find-file-other-frame}) is similar, but opens a
new frame, or makes visible any existing frame showing the file you
seek.  This feature is available only when you are using a window
system.  @xref{Frames}.

@findex find-file-literally
  If you wish to edit a file as a sequence of characters with no special
encoding or conversion, use the @kbd{M-x find-file-literally} command.
It visits a file, like @kbd{C-x C-f}, but does not do format conversion
(@pxref{Formatted Text}), character code conversion (@pxref{Coding
Systems}), or automatic uncompression (@pxref{Compressed Files}).
If you already have visited the same file in the usual (non-literal)
manner, this command asks you whether to visit it literally instead.

@vindex find-file-hooks
@vindex find-file-not-found-hooks
  Two special hook variables allow extensions to modify the operation of
visiting files.  Visiting a file that does not exist runs the functions
in the list @code{find-file-not-found-hooks}; this variable holds a list
of functions, and the functions are called one by one (with no
arguments) until one of them returns non-@code{nil}.  This is not a
normal hook, and the name ends in @samp{-hooks} rather than @samp{-hook}
to indicate that fact.

  Any visiting of a file, whether extant or not, expects
@code{find-file-hooks} to contain a list of functions, and calls them
all, one by one, with no arguments.  This variable is really a normal
hook, but it has an abnormal name for historical compatibility.  In the
case of a nonexistent file, the @code{find-file-not-found-hooks} are run
first.  @xref{Hooks}.

  There are several ways to specify automatically the major mode for
editing the file (@pxref{Choosing Modes}), and to specify local
variables defined for that file (@pxref{File Variables}).

@node Saving
@section Saving Files

  @dfn{Saving} a buffer in Emacs means writing its contents back into the file
that was visited in the buffer.

@table @kbd
@item C-x C-s
Save the current buffer in its visited file (@code{save-buffer}).
@item C-x s
Save any or all buffers in their visited files (@code{save-some-buffers}).
@item M-~
Forget that the current buffer has been changed (@code{not-modified}).
@item C-x C-w
Save the current buffer in a specified file (@code{write-file}).
@item M-x set-visited-file-name
Change file the name under which the current buffer will be saved.
@end table

@kindex C-x C-s
@findex save-buffer
  When you wish to save the file and make your changes permanent, type
@kbd{C-x C-s} (@code{save-buffer}).  After saving is finished, @kbd{C-x C-s}
displays a message like this:

@example
Wrote /u/rms/gnu/gnu.tasks
@end example

@noindent
If the selected buffer is not modified (no changes have been made in it
since the buffer was created or last saved), saving is not really done,
because it would have no effect.  Instead, @kbd{C-x C-s} displays a message
like this in the echo area:

@example
(No changes need to be saved)
@end example

@kindex C-x s
@findex save-some-buffers
  The command @kbd{C-x s} (@code{save-some-buffers}) offers to save any
or all modified buffers.  It asks you what to do with each buffer.  The
possible responses are analogous to those of @code{query-replace}:

@table @kbd
@item y
Save this buffer and ask about the rest of the buffers.
@item n
Don't save this buffer, but ask about the rest of the buffers.
@item !
Save this buffer and all the rest with no more questions.
@c following generates acceptable underfull hbox
@item @key{RET}
Terminate @code{save-some-buffers} without any more saving.
@item .
Save this buffer, then exit @code{save-some-buffers} without even asking
about other buffers.
@item C-r
View the buffer that you are currently being asked about.  When you exit
View mode, you get back to @code{save-some-buffers}, which asks the
question again.
@item C-h
Display a help message about these options.
@end table

  @kbd{C-x C-c}, the key sequence to exit Emacs, invokes
@code{save-some-buffers} and therefore asks the same questions.

@kindex M-~
@findex not-modified
  If you have changed a buffer but you do not want to save the changes,
you should take some action to prevent it.  Otherwise, each time you use
@kbd{C-x s} or @kbd{C-x C-c}, you are liable to save this buffer by
mistake.  One thing you can do is type @kbd{M-~} (@code{not-modified}),
which clears out the indication that the buffer is modified.  If you do
this, none of the save commands will believe that the buffer needs to be
saved.  (@samp{~} is often used as a mathematical symbol for `not'; thus
@kbd{M-~} is `not', metafied.)  You could also use
@code{set-visited-file-name} (see below) to mark the buffer as visiting
a different file name, one which is not in use for anything important.
Alternatively, you can cancel all the changes made since the file was
visited or saved, by reading the text from the file again.  This is
called @dfn{reverting}.  @xref{Reverting}.  You could also undo all the
changes by repeating the undo command @kbd{C-x u} until you have undone
all the changes; but reverting is easier.

@findex set-visited-file-name
  @kbd{M-x set-visited-file-name} alters the name of the file that the
current buffer is visiting.  It reads the new file name using the
minibuffer.  Then it specifies the visited file name and changes the
buffer name correspondingly (as long as the new name is not in use).
@code{set-visited-file-name} does not save the buffer in the newly
visited file; it just alters the records inside Emacs in case you do
save later.  It also marks the buffer as ``modified'' so that @kbd{C-x
C-s} in that buffer @emph{will} save.

@kindex C-x C-w
@findex write-file
  If you wish to mark the buffer as visiting a different file and save it
right away, use @kbd{C-x C-w} (@code{write-file}).  It is precisely
equivalent to @code{set-visited-file-name} followed by @kbd{C-x C-s}.
@kbd{C-x C-s} used on a buffer that is not visiting a file has the
same effect as @kbd{C-x C-w}; that is, it reads a file name, marks the
buffer as visiting that file, and saves it there.  The default file name in
a buffer that is not visiting a file is made by combining the buffer name
with the buffer's default directory.

  If the new file name implies a major mode, then @kbd{C-x C-w} switches
to that major mode, in most cases.  The command
@code{set-visited-file-name} also does this.  @xref{Choosing Modes}.

  If Emacs is about to save a file and sees that the date of the latest
version on disk does not match what Emacs last read or wrote, Emacs
notifies you of this fact, because it probably indicates a problem caused
by simultaneous editing and requires your immediate attention.
@xref{Interlocking,, Simultaneous Editing}.

@vindex require-final-newline
  If the variable @code{require-final-newline} is non-@code{nil}, Emacs
puts a newline at the end of any file that doesn't already end in one,
every time a file is saved or written.  The default is @code{nil}.

@menu
* Backup::              How Emacs saves the old version of your file.
* Interlocking::        How Emacs protects against simultaneous editing
                          of one file by two users.
@end menu

@node Backup
@subsection Backup Files
@cindex backup file
@vindex make-backup-files
@vindex vc-make-backup-files
@vindex backup-enable-predicate

  On most operating systems, rewriting a file automatically destroys all
record of what the file used to contain.  Thus, saving a file from Emacs
throws away the old contents of the file---or it would, except that
Emacs carefully copies the old contents to another file, called the
@dfn{backup} file, before actually saving.

  For most files, the variable @code{make-backup-files} determines
whether to make backup files.  On most operating systems, its default
value is @code{t}, so that Emacs does write backup files.

  For files managed by a version control system (@pxref{Version
Control}), the variable @code{vc-make-backup-files} determines whether
to make backup files.  By default, it is @code{nil}, since backup files
are redundant when you store all the previous versions in a version
control system.  @xref{VC Workfile Handling}.

  The default value of the @code{backup-enable-predicate} variable
prevents backup files being written for files in @file{/tmp}.

  At your option, Emacs can keep either a single backup file or a series of
numbered backup files for each file that you edit.

  Emacs makes a backup for a file only the first time the file is saved
from one buffer.  No matter how many times you save a file, its backup file
continues to contain the contents from before the file was visited.
Normally this means that the backup file contains the contents from before
the current editing session; however, if you kill the buffer and then visit
the file again, a new backup file will be made by the next save.

  You can also explicitly request making another backup file from a
buffer even though it has already been saved at least once.  If you save
the buffer with @kbd{C-u C-x C-s}, the version thus saved will be made
into a backup file if you save the buffer again.  @kbd{C-u C-u C-x C-s}
saves the buffer, but first makes the previous file contents into a new
backup file.  @kbd{C-u C-u C-u C-x C-s} does both things: it makes a
backup from the previous contents, and arranges to make another from the
newly saved contents, if you save again.

@menu
* Names: Backup Names.          How backup files are named;
                                  choosing single or numbered backup files.
* Deletion: Backup Deletion.    Emacs deletes excess numbered backups.
* Copying: Backup Copying.      Backups can be made by copying or renaming.
@end menu

@node Backup Names
@subsubsection Single or Numbered Backups

  If you choose to have a single backup file (this is the default),
the backup file's name is constructed by appending @samp{~} to the
file name being edited; thus, the backup file for @file{eval.c} would
be @file{eval.c~}.

  If you choose to have a series of numbered backup files, backup file
names are made by appending @samp{.~}, the number, and another @samp{~} to
the original file name.  Thus, the backup files of @file{eval.c} would be
called @file{eval.c.~1~}, @file{eval.c.~2~}, and so on, through names
like @file{eval.c.~259~} and beyond.

  If protection stops you from writing backup files under the usual names,
the backup file is written as @file{%backup%~} in your home directory.
Only one such file can exist, so only the most recently made such backup is
available.

@vindex version-control
  The choice of single backup or numbered backups is controlled by the
variable @code{version-control}.  Its possible values are

@table @code
@item t
Make numbered backups.
@item nil
Make numbered backups for files that have numbered backups already.
Otherwise, make single backups.
@item never
Do not in any case make numbered backups; always make single backups.
@end table

@noindent
You can set @code{version-control} locally in an individual buffer to
control the making of backups for that buffer's file.  For example,
Rmail mode locally sets @code{version-control} to @code{never} to make sure
that there is only one backup for an Rmail file.  @xref{Locals}.

@cindex @code{VERSION_CONTROL} environment variable
  If you set the environment variable @code{VERSION_CONTROL}, to tell
various GNU utilities what to do with backup files, Emacs also obeys the
environment variable by setting the Lisp variable @code{version-control}
accordingly at startup.  If the environment variable's value is @samp{t}
or @samp{numbered}, then @code{version-control} becomes @code{t}; if the
value is @samp{nil} or @samp{existing}, then @code{version-control}
becomes @code{nil}; if it is @samp{never} or @samp{simple}, then
@code{version-control} becomes @code{never}.

@node Backup Deletion
@subsubsection Automatic Deletion of Backups

  To prevent unlimited consumption of disk space, Emacs can delete numbered
backup versions automatically.  Generally Emacs keeps the first few backups
and the latest few backups, deleting any in between.  This happens every
time a new backup is made.

@vindex kept-old-versions
@vindex kept-new-versions
  The two variables @code{kept-old-versions} and
@code{kept-new-versions} control this deletion.  Their values are,
respectively the number of oldest (lowest-numbered) backups to keep and
the number of newest (highest-numbered) ones to keep, each time a new
backup is made.  Recall that these values are used just after a new
backup version is made; that newly made backup is included in the count
in @code{kept-new-versions}.  By default, both variables are 2.

@vindex delete-old-versions
  If @code{delete-old-versions} is non-@code{nil}, the excess
middle versions are deleted without a murmur.  If it is @code{nil}, the
default, then you are asked whether the excess middle versions should
really be deleted.

  Dired's @kbd{.} (Period) command can also be used to delete old versions.
@xref{Dired Deletion}.

@node Backup Copying
@subsubsection Copying vs.@: Renaming

  Backup files can be made by copying the old file or by renaming it.  This
makes a difference when the old file has multiple names.  If the old file
is renamed into the backup file, then the alternate names become names for
the backup file.  If the old file is copied instead, then the alternate
names remain names for the file that you are editing, and the contents
accessed by those names will be the new contents.

  The method of making a backup file may also affect the file's owner
and group.  If copying is used, these do not change.  If renaming is used,
you become the file's owner, and the file's group becomes the default
(different operating systems have different defaults for the group).

  Having the owner change is usually a good idea, because then the owner
always shows who last edited the file.  Also, the owners of the backups
show who produced those versions.  Occasionally there is a file whose
owner should not change; it is a good idea for such files to contain
local variable lists to set @code{backup-by-copying-when-mismatch}
locally (@pxref{File Variables}).

@vindex backup-by-copying
@vindex backup-by-copying-when-linked
@vindex backup-by-copying-when-mismatch
  The choice of renaming or copying is controlled by three variables.
Renaming is the default choice.  If the variable
@code{backup-by-copying} is non-@code{nil}, copying is used.  Otherwise,
if the variable @code{backup-by-copying-when-linked} is non-@code{nil},
then copying is used for files that have multiple names, but renaming
may still be used when the file being edited has only one name.  If the
variable @code{backup-by-copying-when-mismatch} is non-@code{nil}, then
copying is used if renaming would cause the file's owner or group to
change.  @code{backup-by-copying-when-mismatch} is @code{t} by default
if you start Emacs as the superuser.

  When a file is managed with a version control system (@pxref{Version
Control}), Emacs does not normally make backups in the usual way for
that file.  But check-in and check-out are similar in some ways to
making backups.  One unfortunate similarity is that these operations
typically break hard links, disconnecting the file name you visited from
any alternate names for the same file.  This has nothing to do with
Emacs---the version control system does it.

@node Interlocking
@subsection Protection against Simultaneous Editing

@cindex file dates
@cindex simultaneous editing
  Simultaneous editing occurs when two users visit the same file, both
make changes, and then both save them.  If nobody were informed that
this was happening, whichever user saved first would later find that his
changes were lost.

  On some systems, Emacs notices immediately when the second user starts
to change the file, and issues an immediate warning.  On all systems,
Emacs checks when you save the file, and warns if you are about to
overwrite another user's changes.  You can prevent loss of the other
user's work by taking the proper corrective action instead of saving the
file.

@findex ask-user-about-lock
@cindex locking files
  When you make the first modification in an Emacs buffer that is
visiting a file, Emacs records that the file is @dfn{locked} by you.
(It does this by creating a symbolic link in the same directory with a
different name.)  Emacs removes the lock when you save the changes.  The
idea is that the file is locked whenever an Emacs buffer visiting it has
unsaved changes.

@cindex collision
  If you begin to modify the buffer while the visited file is locked by
someone else, this constitutes a @dfn{collision}.  When Emacs detects a
collision, it asks you what to do, by calling the Lisp function
@code{ask-user-about-lock}.  You can redefine this function for the sake
of customization.  The standard definition of this function asks you a
question and accepts three possible answers:

@table @kbd
@item s
Steal the lock.  Whoever was already changing the file loses the lock,
and you gain the lock.
@item p
Proceed.  Go ahead and edit the file despite its being locked by someone else.
@item q
Quit.  This causes an error (@code{file-locked}) and the modification you
were trying to make in the buffer does not actually take place.
@end table

  Note that locking works on the basis of a file name; if a file has
multiple names, Emacs does not realize that the two names are the same file
and cannot prevent two users from editing it simultaneously under different
names.  However, basing locking on names means that Emacs can interlock the
editing of new files that will not really exist until they are saved.

  Some systems are not configured to allow Emacs to make locks, and
there are cases where lock files cannot be written.  In these cases,
Emacs cannot detect trouble in advance, but it still can detect the
collision when you try to save a file and overwrite someone else's
changes.

  If Emacs or the operating system crashes, this may leave behind lock
files which are stale.  So you may occasionally get warnings about
spurious collisions.  When you determine that the collision is spurious,
just use @kbd{p} to tell Emacs to go ahead anyway.

  Every time Emacs saves a buffer, it first checks the last-modification
date of the existing file on disk to verify that it has not changed since the
file was last visited or saved.  If the date does not match, it implies
that changes were made in the file in some other way, and these changes are
about to be lost if Emacs actually does save.  To prevent this, Emacs
prints a warning message and asks for confirmation before saving.
Occasionally you will know why the file was changed and know that it does
not matter; then you can answer @kbd{yes} and proceed.  Otherwise, you should
cancel the save with @kbd{C-g} and investigate the situation.

  The first thing you should do when notified that simultaneous editing
has already taken place is to list the directory with @kbd{C-u C-x C-d}
(@pxref{Directories}).  This shows the file's current author.  You
should attempt to contact him to warn him not to continue editing.
Often the next step is to save the contents of your Emacs buffer under a
different name, and use @code{diff} to compare the two files.@refill

@node Reverting
@section Reverting a Buffer
@findex revert-buffer
@cindex drastic changes

  If you have made extensive changes to a file and then change your mind
about them, you can get rid of them by reading in the previous version
of the file.  To do this, use @kbd{M-x revert-buffer}, which operates on
the current buffer.  Since reverting a buffer unintentionally could lose
a lot of work, you must confirm this command with @kbd{yes}.

  @code{revert-buffer} keeps point at the same distance (measured in
characters) from the beginning of the file.  If the file was edited only
slightly, you will be at approximately the same piece of text after
reverting as before.  If you have made drastic changes, the same value of
point in the old file may address a totally different piece of text.

  Reverting marks the buffer as ``not modified'' until another change is
made.

  Some kinds of buffers whose contents reflect data bases other than files,
such as Dired buffers, can also be reverted.  For them, reverting means
recalculating their contents from the appropriate data base.  Buffers
created explicitly with @kbd{C-x b} cannot be reverted; @code{revert-buffer}
reports an error when asked to do so.

@vindex revert-without-query
  When you edit a file that changes automatically and frequently---for
example, a log of output from a process that continues to run---it may be
useful for Emacs to revert the file without querying you, whenever you
visit the file again with @kbd{C-x C-f}.

  To request this behavior, set the variable @code{revert-without-query}
to a list of regular expressions.  When a file name matches one of these
regular expressions, @code{find-file} and @code{revert-buffer} will
revert it automatically if it has changed---provided the buffer itself
is not modified.  (If you have edited the text, it would be wrong to
discard your changes.)

@node Auto Save
@section Auto-Saving: Protection Against Disasters
@cindex Auto Save mode
@cindex mode, Auto Save
@cindex crashes

  Emacs saves all the visited files from time to time (based on counting
your keystrokes) without being asked.  This is called @dfn{auto-saving}.
It prevents you from losing more than a limited amount of work if the
system crashes.

  When Emacs determines that it is time for auto-saving, each buffer is
considered, and is auto-saved if auto-saving is turned on for it and it
has been changed since the last time it was auto-saved.  The message
@samp{Auto-saving...} is displayed in the echo area during auto-saving,
if any files are actually auto-saved.  Errors occurring during
auto-saving are caught so that they do not interfere with the execution
of commands you have been typing.

@menu
* Files: Auto Save Files.       The file where auto-saved changes are
                                  actually made until you save the file.
* Control: Auto Save Control.   Controlling when and how often to auto-save.
* Recover::                     Recovering text from auto-save files.
@end menu

@node Auto Save Files
@subsection Auto-Save Files

  Auto-saving does not normally save in the files that you visited, because
it can be very undesirable to save a program that is in an inconsistent
state when you have made half of a planned change.  Instead, auto-saving
is done in a different file called the @dfn{auto-save file}, and the
visited file is changed only when you request saving explicitly (such as
with @kbd{C-x C-s}).

  Normally, the auto-save file name is made by appending @samp{#} to the
front and rear of the visited file name.  Thus, a buffer visiting file
@file{foo.c} is auto-saved in a file @file{#foo.c#}.  Most buffers that
are not visiting files are auto-saved only if you request it explicitly;
when they are auto-saved, the auto-save file name is made by appending
@samp{#%} to the front and @samp{#} to the rear of buffer name.  For
example, the @samp{*mail*} buffer in which you compose messages to be
sent is auto-saved in a file named @file{#%*mail*#}.  Auto-save file
names are made this way unless you reprogram parts of Emacs to do
something different (the functions @code{make-auto-save-file-name} and
@code{auto-save-file-name-p}).  The file name to be used for auto-saving
in a buffer is calculated when auto-saving is turned on in that buffer.

  When you delete a substantial part of the text in a large buffer, auto
save turns off temporarily in that buffer.  This is because if you
deleted the text unintentionally, you might find the auto-save file more
useful if it contains the deleted text.  To reenable auto-saving after
this happens, save the buffer with @kbd{C-x C-s}, or use @kbd{C-u 1 M-x
auto-save}.

@vindex auto-save-visited-file-name
  If you want auto-saving to be done in the visited file, set the variable
@code{auto-save-visited-file-name} to be non-@code{nil}.  In this mode,
there is really no difference between auto-saving and explicit saving.

@vindex delete-auto-save-files
  A buffer's auto-save file is deleted when you save the buffer in its
visited file.  To inhibit this, set the variable @code{delete-auto-save-files}
to @code{nil}.  Changing the visited file name with @kbd{C-x C-w} or
@code{set-visited-file-name} renames any auto-save file to go with
the new visited name.

@node Auto Save Control
@subsection Controlling Auto-Saving

@vindex auto-save-default
@findex auto-save-mode
  Each time you visit a file, auto-saving is turned on for that file's
buffer if the variable @code{auto-save-default} is non-@code{nil} (but not
in batch mode; @pxref{Entering Emacs}).  The default for this variable is
@code{t}, so auto-saving is the usual practice for file-visiting buffers.
Auto-saving can be turned on or off for any existing buffer with the
command @kbd{M-x auto-save-mode}.  Like other minor mode commands, @kbd{M-x
auto-save-mode} turns auto-saving on with a positive argument, off with a
zero or negative argument; with no argument, it toggles.

@vindex auto-save-interval
  Emacs does auto-saving periodically based on counting how many characters
you have typed since the last time auto-saving was done.  The variable
@code{auto-save-interval} specifies how many characters there are between
auto-saves.  By default, it is 300.

@vindex auto-save-timeout
  Auto-saving also takes place when you stop typing for a while.  The
variable @code{auto-save-timeout} says how many seconds Emacs should
wait before it does an auto save (and perhaps also a garbage
collection).  (The actual time period is longer if the current buffer is
long; this is a heuristic which aims to keep out of your way when you
are editing long buffers, in which auto-save takes an appreciable amount
of time.)  Auto-saving during idle periods accomplishes two things:
first, it makes sure all your work is saved if you go away from the
terminal for a while; second, it may avoid some auto-saving while you
are actually typing.

  Emacs also does auto-saving whenever it gets a fatal error.  This
includes killing the Emacs job with a shell command such as @samp{kill
%emacs}, or disconnecting a phone line or network connection.

@findex do-auto-save
  You can request an auto-save explicitly with the command @kbd{M-x
do-auto-save}.

@node Recover
@subsection Recovering Data from Auto-Saves

@findex recover-file
  You can use the contents of an auto-save file to recover from a loss
of data with the command @kbd{M-x recover-file @key{RET} @var{file}
@key{RET}}.  This visits @var{file} and then (after your confirmation)
restores the contents from its auto-save file @file{#@var{file}#}.
You can then save with @kbd{C-x C-s} to put the recovered text into
@var{file} itself.  For example, to recover file @file{foo.c} from its
auto-save file @file{#foo.c#}, do:@refill

@example
M-x recover-file @key{RET} foo.c @key{RET}
yes @key{RET}
C-x C-s
@end example

  Before asking for confirmation, @kbd{M-x recover-file} displays a
directory listing describing the specified file and the auto-save file,
so you can compare their sizes and dates.  If the auto-save file
is older, @kbd{M-x recover-file} does not offer to read it.

@findex recover-session
  If Emacs or the computer crashes, you can recover all the files you
were editing from their auto save files with the command @kbd{M-x
recover-session}.  This first shows you a list of recorded interrupted
sessions.  Move point to the one you choose, and type @kbd{C-c C-c}.

  Then @code{recover-session} asks about each of the files that were
being edited during that session, asking whether to recover that file.
If you answer @kbd{y}, it calls @code{recover-file}, which works in its
normal fashion.  It shows the dates of the original file and its
auto-save file, and asks once again whether to recover that file.

  When @code{recover-session} is done, the files you've chosen to
recover are present in Emacs buffers.  You should then save them.  Only
this---saving them---updates the files themselves.

@vindex auto-save-list-file-prefix
  Interrupted sessions are recorded for later recovery in files named
@file{~/.saves-@var{pid}-@var{hostname}}.  The @samp{~/.saves} portion of
these names comes from the value of @code{auto-save-list-file-prefix}.
You can arrange to record sessions in a different place by setting that
variable in your @file{.emacs} file, but you'll have to redefine
@code{recover-session} as well to make it look in the new place.  If you
set @code{auto-save-list-file-prefix} to @code{nil} in your
@file{.emacs} file, sessions are not recorded for recovery.

@node File Aliases
@section File Name Aliases

  Symbolic links and hard links both make it possible for several file
names to refer to the same file.  Hard links are alternate names that
refer directly to the file; all the names are equally valid, and no one
of them is preferred.  By contrast, a symbolic link is a kind of defined
alias: when @file{foo} is a symbolic link to @file{bar}, you can use
either name to refer to the file, but @file{bar} is the real name, while
@file{foo} is just an alias.  More complex cases occur when symbolic
links point to directories.

  If you visit two names for the same file, normally Emacs makes
two different buffers, but it warns you about the situation.

@vindex find-file-existing-other-name
  If you wish to avoid visiting the same file in two buffers under
different names, set the variable @code{find-file-existing-other-name}
to a non-@code{nil} value.  Then @code{find-file} uses the existing
buffer visiting the file, no matter which of the file's names you
specify.

@vindex find-file-visit-truename
@cindex truenames of files
@cindex file truenames
  If the variable @code{find-file-visit-truename} is non-@code{nil},
then the file name recorded for a buffer is the file's @dfn{truename}
(made by replacing all symbolic links with their target names), rather
than the name you specify.  Setting @code{find-file-visit-truename} also
implies the effect of @code{find-file-existing-other-name}.

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

  @dfn{Version control systems} are packages that can record multiple
versions of a source file, usually storing the unchanged parts of the
file just once.  Version control systems also record history information
such as the creation time of each version, who created it, and a 
description of what was changed in that version.

  The Emacs version control interface is called VC.  Its commands work
with three version control systems---RCS, CVS and SCCS.  The GNU project
recommends RCS and CVS, which are free software and available from the
Free Software Foundation.

@menu
* Introduction to VC::  How version control works in general.
* VC Mode Line::     How the mode line shows version control status.
* Basic VC Editing::    How to edit a file under version control.
* Old Versions::        Examining and comparing old versions.
* Secondary VC Commands::    The commands used a little less frequently.
* Branches::            Multiple lines of development.
* Snapshots::           Sets of file versions treated as a unit.
* Miscellaneous VC::    Various other commands and features of VC.
* Customizing VC::      Variables that change VC's behavior.
@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.  VC
provides a uniform interface to version control, so that regardless of
which version control system is in use, you can use it the same way.

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

@menu
* Version Systems::  Supported version control back-end systems.
* VC Concepts::      Words and concepts related to version control.
@end menu

@node Version Systems
@subsubsection Supported Version Control Systems

@cindex RCS
@cindex back end (version control)
  VC currently works with three different version control systems or
``back ends'': RCS, CVS, and SCCS.

  RCS is a free version control system that is available from the Free
Software Foundation.  It is perhaps the most mature of the supported
back ends, and the VC commands are conceptually closest to RCS.  Almost
everything you can do with RCS can be done through VC.

@cindex CVS
  CVS is built on top of RCS, and extends the features of RCS, allowing
for more sophisticated release management, and concurrent multi-user
development.  VC supports basic editing operations under CVS, but for
some less common tasks you still need to call CVS from the command line.
Note also that before using CVS you must set up a repository, which is a
subject too complex to treat here.

@cindex SCCS
  SCCS is a proprietary but widely used version control system.  In
terms of capabilities, it is the weakest of the three that VC
supports.  VC compensates for certain features missing in SCCS
(snapshots, for example) by implementing them itself, but some other VC
features, such as multiple branches, are not available with SCCS.  You
should use SCCS only if for some reason you cannot use RCS.

@node VC Concepts
@subsubsection Concepts of Version Control

@cindex master file
@cindex registered file
   When a file is under version control, we also say that it is
@dfn{registered} in the version control system.  Each registered file
has a corresponding @dfn{master file} which represents the file's
present state plus its change history---enough to reconstruct the
current version or any earlier version.  Usually the master file also
records a @dfn{log entry} for each version, describing in words what was
changed in that version.

@cindex work file
@cindex checking out files
  The file that is maintained under version control is sometimes called
the @dfn{work file} corresponding to its master file.  You edit the work
file and make changes in it, as you would with an ordinary file.  (With
SCCS and RCS, you must @dfn{lock} the file before you start to edit it.)
After you are done with a set of changes, you @dfn{check the file in},
which records the changes in the master file, along with a log entry for
them.

  With CVS, there are usually multiple work files corresponding to a
single master file---often each user has his own copy.  It is also
possible to use RCS in this way, but this is not the usual way to use
RCS.

@cindex locking and version control
  A version control system typically has some mechanism to coordinate
between users who want to change the same file.  One method is
@dfn{locking} (analogous to the locking that Emacs uses to detect
simultaneous editing of a file, but distinct from it).  The other method
is to merge your changes with other people's changes when you check them
in.

  With version control locking, work files are normally read-only so
that you cannot change them.  You ask the version control system to make
a work file writable for you by locking it; only one user can do
this at any given time.  When you check in your changes, that unlocks
the file, making the work file read-only again.  This allows other users
to lock the file to make further changes.  SCCS always uses locking, and
RCS normally does.

  The other alternative for RCS is to let each user modify the work file
at any time.  In this mode, locking is not required, but it is
permitted; check-in is still the way to record a new version.

  CVS normally allows each user to modify his own copy of the work file
at any time, but requires merging with changes from other users at
check-in time.  However, CVS can also be set up to require locking.
(@pxref{Backend Options}).

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

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

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

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

  The principal VC command is an all-purpose command that performs
either locking or check-in, depending on the situation.

@table @kbd
@item C-x C-q
@itemx C-x v v
Perform the next logical version control operation on this file.
@end table

@findex vc-next-action
@findex vc-toggle-read-only
@kindex C-x v v
@kindex C-x C-q @r{(Version Control)}
  Strictly speaking, the command for this job is @code{vc-next-action},
bound to @kbd{C-x v v}.  However, the normal meaning of @kbd{C-x C-q} is
to make a read-only buffer writable, or vice versa; we have extended it
to do the same job properly for files managed by version control, by
performing the appropriate version control operations.  When you type
@kbd{C-x C-q} on a registered file, it acts like @kbd{C-x v v}.

  The precise action of this command depends on the state of the file,
and whether the version control system uses locking or not.  SCCS and
RCS normally use locking; CVS normally does not use locking.

@menu
* VC with Locking::     RCS in its default mode, SCCS, and optionally CVS.
* Without Locking::     Without locking: default mode for CVS.
* Log Buffer::          Features available in log entry buffers.
@end menu
               
@node VC with Locking                 
@subsubsection Basic Version Control with Locking

  If locking is used for the file (as with SCCS, and RCS in its default
mode), @kbd{C-x C-q} can either lock a file or check it in:

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

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

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

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

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

@node Without Locking
@subsubsection Basic Version Control without Locking

  When there is no locking---the default for CVS---work files are always
writable; you do not need to do anything before you begin to edit a
file.  The status indicator on the mode line is @samp{-} if the file is
unmodified; it flips to @samp{:} as soon as you save any changes in the
work file.

  Here is what @kbd{C-x C-q} does when using CVS:

@itemize @bullet
@item
If some other user has checked in changes into the master file,
Emacs asks you whether you want to merge those changes into your own
work file (@pxref{Merging}).  You must do this before you can check in
your own changes.

@item
If there are no new changes in the master file, but you have made
modifications in your work file, @kbd{C-x C-q} checks in your changes.
In order to do this, it first reads the log entry for the new version.
@xref{Log Buffer}.

@item
If the file is not modified, the @kbd{C-x C-q} does nothing.
@end itemize

  These rules also apply when you use RCS in the mode that does not
require locking, except that automatic merging of changes from the
master file is not implemented.  Unfortunately, this means that nothing
informs you if another user has checked in changes in the same file
since you began editing it, and when this happens, his changes will be
effectively removed when you check in your version (though they will
remain in the master file, so they will not be entirely lost).  You must
therefore verify the current version is unchanged, before you check in your
changes.  We hope to eliminate this risk and provide automatic merging
with RCS in a future Emacs version.

  In addition, locking is possible with RCS even in this mode, although
it is not required; @kbd{C-x C-q} with an unmodified file locks the
file, just as it does with RCS in its normal (locking) mode.

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

  When you check in changes, @kbd{C-x C-q} first reads a log entry.  It
pops up a buffer called @samp{*VC-Log*} for you to enter the log entry.
When you are finished, type @kbd{C-c C-c} in the @samp{*VC-Log*} buffer.
That is when check-in really happens.

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

  If you change several source files for the same reason, it is often
convenient to specify the same log entry for many of the files.  To do
this, use the history of previous log entries.  The commands @kbd{M-n},
@kbd{M-p}, @kbd{M-s} and @kbd{M-r} for doing this work just like the
minibuffer history commands (except that these versions are used outside
the minibuffer).

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

@node Old Versions
@subsection Examining And Comparing Old Versions

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

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

@item C-x v =
Compare the current buffer contents with the latest checked-in version
of the file.

@item C-u C-x v = @var{file} @key{RET} @var{oldvers} @key{RET} @var{newvers} @key{RET}
Compare the specified two versions of @var{file}.

@item C-x v g
Display the result of the CVS annotate command using colors.
@end table

@findex vc-version-other-window
@kindex C-x v ~
  To examine an old version in toto, visit the file and then type
@kbd{C-x v ~ @var{version} @key{RET}} (@code{vc-version-other-window}).
This puts the text of version @var{version} in a file named
@file{@var{filename}.~@var{version}~}, and visits it in its own buffer
in a separate window.  (In RCS, you can also select an old version
and create a branch from it.  @xref{Branches}.)

@findex vc-diff
@kindex C-x v =
  But usually it is more convenient to compare two versions of the file,
with the command @kbd{C-x v =} (@code{vc-diff}).  Plain @kbd{C-x v =}
compares the current buffer contents (saving them in the file if
necessary) with the last checked-in version of the file.  @kbd{C-u C-x v
=}, with a numeric argument, reads a file name and two version numbers,
then compares those versions of the specified file.

  If you supply a directory name instead of the name of a registered
file, this command compares the two specified versions of all registered
files in that directory and its subdirectories.

  You can specify a checked-in version by its number; an empty input
specifies the current contents of the work file (which may be different
from all the checked-in versions).  You can also specify a snapshot name
(@pxref{Snapshots}) instead of one or both version numbers.

  This command works by running the @code{diff} utility, getting the
options from the variable @code{diff-switches}.  It displays the output
in a special buffer in another window.  Unlike the @kbd{M-x diff}
command, @kbd{C-x v =} does not try to locate the changes in the old and
new versions.  This is because normally one or both versions do not
exist as files when you compare them; they exist only in the records of
the master file.  @xref{Comparing Files}, for more information about
@kbd{M-x diff}.

@findex vc-annotate
@kindex C-x v g
  For CVS-controlled files, you can display the result of the CVS
annotate command, using colors to enhance the visual appearance.  Use
the command @kbd{M-x vc-annotate} to do this.  Red means new, blue means
old, and intermediate colors indicate intermediate ages.  A prefix
argument @var{n} specifies a stretch factor for the time scale; it makes
each color cover a period @var{n} times as long.

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

  This section explains the secondary commands of VC; those that you might
use once a day.

@menu
* Registering::         Putting a file under version control.
* VC Status::           Viewing the VC status of files.
* VC Undo::             Cancelling changes before or after check-in.
* VC Dired Mode::       Listing files managed by version control. 
* VC Dired Commands::   Commands to use in a VC Dired buffer.
@end menu

@node Registering
@subsubsection Registering a File for Version Control

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

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

@vindex vc-default-back-end
  To register the file, Emacs must choose which version control system
to use for it.  You can specify your choice explicitly by setting
@code{vc-default-back-end} to @code{RCS}, @code{CVS} or @code{SCCS}.
Otherwise, if there is a subdirectory named @file{RCS}, @file{SCCS}, or
@file{CVS}, Emacs uses the corresponding version control system.  In the
absence of any specification, the default choice is RCS if RCS is
installed, otherwise SCCS.

  If locking is in use, @kbd{C-x v i} leaves the file unlocked and
read-only.  Type @kbd{C-x C-q} if you wish to start editing it.  After
registering a file with CVS, you must subsequently commit the initial
version by typing @kbd{C-x C-q}.

@vindex vc-default-init-version
  The initial version number for a newly registered file is 1.1, by
default.  You can specify a different default by setting the variable
@code{vc-default-init-version}, or you can give @kbd{C-x v i} a numeric
argument; then it reads the initial version number for this particular
file using the minibuffer.

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

@node VC Status
@subsubsection VC Status Commands

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

@kindex C-x v l
@findex vc-print-log
  To view the detailed version control status and history of a file,
type @kbd{C-x v l} (@code{vc-print-log}).  It displays the history of
changes to the current file, including the text of the log entries.  The
output appears in a separate window.

@node VC Undo
@subsubsection Undoing Version Control Actions

@table @kbd
@item C-x v u
Revert the buffer and the file to the last checked-in version.

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

@kindex C-x v u
@findex vc-revert-buffer
  If you want to discard your current set of changes and revert to the
last version checked in, use @kbd{C-x v u} (@code{vc-revert-buffer}).
This leaves the file unlocked; if locking is in use, you must first lock
the file again before you change it again.  @kbd{C-x v u} requires
confirmation, unless it sees that you haven't made any changes since the
last checked-in version.

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

@kindex C-x v c
@findex vc-cancel-version
  To cancel a change that you already checked in, use @kbd{C-x v c}
(@code{vc-cancel-version}).  This command discards all record of the
most recent checked-in version.  @kbd{C-x v c} also offers to revert
your work file and buffer to the previous version (the one that precedes
the version that is deleted).

  If you answer @kbd{no}, VC keeps your changes in the buffer, and locks
the file.  The no-revert option is useful when you have checked in a
change and then discover a trivial error in it; you can cancel the
erroneous check-in, fix the error, and check the file in again.

  When @kbd{C-x v c} does not revert the buffer, it unexpands all
version control headers in the buffer instead (@pxref{Version Headers}).
This is because the buffer no longer corresponds to any existing
version.  If you check it in again, the check-in process will expand the
headers properly for the new version number.

  However, it is impossible to unexpand the RCS @samp{@w{$}Log$} header
automatically.  If you use that header feature, you have to unexpand it
by hand---by deleting the entry for the version that you just canceled.

  Be careful when invoking @kbd{C-x v c}, as it is easy to lose a lot of
work with it.  To help you be careful, this command always requires
confirmation with @kbd{yes}.  Note also that this command is disabled
under CVS, because canceling versions is very dangerous and discouraged
with CVS.

@node VC Dired Mode
@subsubsection Dired under VC

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

@vindex vc-dired-terse-display
  @kbd{C-x v d} creates a buffer which uses VC Dired Mode.  This looks
much like an ordinary Dired buffer (@pxref{Dired}); however, normally it
shows only the noteworthy files (those locked or not up-to-date).  This
is called @dfn{terse display}.  If you set the variable
@code{vc-dired-terse-display} to @code{nil}, then VC Dired shows all
relevant files---those managed under version control, plus all
subdirectories (@dfn{full display}).  The command @kbd{v t} in a VC
Dired buffer toggles between terse display and full display (@pxref{VC
Dired Commands}).

@vindex vc-dired-recurse
  By default, VC Dired produces a recursive listing of noteworthy or
relevant files at or below the given directory.  You can change this by
setting the variable @code{vc-dired-recurse} to @code{nil}; then VC
Dired shows only the files in the given directory.

  The line for an individual file shows the version control state in the
place of the hard link count, owner, group, and size of the file.  If
the file is unmodified, in sync with the master file, the version
control state shown is blank.  Otherwise it consists of text in
parentheses.  Under RCS and SCCS, the name of the user locking the file
is shown; under CVS, an abbreviated version of the @samp{cvs status}
output is used.  Here is an example using RCS:

@smallexample
@group
  /home/jim/project:

  -rw-r--r-- (jim)      Apr  2 23:39 file1
  -r--r--r--            Apr  5 20:21 file2
@end group
@end smallexample

@noindent
The files @samp{file1} and @samp{file2} are under version control,
@samp{file1} is locked by user jim, and @samp{file2} is unlocked.

  Here is an example using CVS:

@smallexample
@group
  /home/joe/develop:

  -rw-r--r-- (modified) Aug  2  1997 file1.c
  -rw-r--r--            Apr  4 20:09 file2.c
  -rw-r--r-- (merge)    Sep 13  1996 file3.c
@end group
@end smallexample

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

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

  You can fine-tune VC Dired's format by typing @kbd{C-u C-x v d}---as in
ordinary Dired, that allows you to specify additional switches for the
@samp{ls} command.

@node VC Dired Commands
@subsubsection VC Dired Commands

  All the usual Dired commands work normally in VC Dired mode, except
for @kbd{v}, which is redefined as the version control prefix.  You can
invoke VC commands such as @code{vc-diff} and @code{vc-print-log} by
typing @kbd{v =}, or @kbd{v l}, and so on.  Most of these commands apply
to the file name on the current line.

  The command @kbd{v v} (@code{vc-next-action}) operates on all the
marked files, so that you can lock or check in several files at once.
If it operates on more than one file, it handles each file according to
its current state; thus, it might lock one file, but check in another
file.  This could be confusing; it is up to you to avoid confusing
behavior by marking a set of files that are in a similar state.

  If any files call for check-in, @kbd{v v} reads a single log entry,
then uses it for all the files being checked in.  This is convenient for
registering or checking in several files at once, as part of the same
change.

@findex vc-dired-toggle-terse-mode
@findex vc-dired-mark-locked
  You can toggle between terse display (only locked files, or files not
up-to-date) and full display at any time by typing @kbd{v t}
@code{vc-dired-toggle-terse-mode}.  There is also a special command
@kbd{* l} (@code{vc-dired-mark-locked}), which marks all files currently
locked (or, with CVS, all files not up-to-date).  Thus, typing @kbd{* l
t k} is another way to delete from the buffer all files except those
currently locked.

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

  One use of version control is to maintain multiple ``current''
versions of a file.  For example, you might have different versions of a
program in which you are gradually adding various unfinished new
features.  Each such independent line of development is called a
@dfn{branch}.  VC allows you to create branches, switch between
different branches, and merge changes from one branch to another.
Please note, however, that branches are only supported for RCS at the
moment.

  A file's main line of development is usually called the @dfn{trunk}.
The versions on the trunk are normally numbered 1.1, 1.2, 1.3, etc.  At
any such version, you can start an independent branch.  A branch
starting at version 1.2 would have version number 1.2.1.1, and consecutive
versions on this branch would have numbers 1.2.1.2, 1.2.1.3, 1.2.1.4,
and so on.  If there is a second branch also starting at version 1.2, it
would consist of versions 1.2.2.1, 1.2.2.2, 1.2.2.3, etc.

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

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

@node Switching Branches
@subsubsection Switching between Branches

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

  You can omit the minor version number, thus giving only the branch
number; this takes you to the head version on the chosen branch.  If you
only type @key{RET}, Emacs goes to the highest version on the trunk.

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

@node Creating Branches
@subsubsection Creating New Branches

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

  To create a new branch at an older version (one that is no longer the
head of a branch), first select that version (@pxref{Switching
Branches}), then lock it with @kbd{C-x C-q}.  You'll be asked to
confirm, when you lock the old version, that you really mean to create a
new branch---if you say no, you'll be offered a chance to lock the
latest version instead.

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

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

@node Merging
@subsubsection Merging Branches

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

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

@kindex C-x v m
@findex vc-merge
  @kbd{C-x v m} (@code{vc-merge}) takes a set of changes and merges it
into the current version of the work file.  It first asks you for a
branch number or a pair of version numbers in the minibuffer.  Then it
finds the changes from that branch, or between the two versions you
specified, and merges them into the current version of the current file.

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

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

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

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

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

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

@cindex vc-resolve-conflicts
  Then you can resolve the conflicts by editing the file manually.  Or
you can type @code{M-x vc-resolve-conflicts} after visiting the file.
This starts an Ediff session, as described above.

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

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

  This technique works reliably and automatically, provided that the
source files contain RCS version headers (@pxref{Version Headers}).  The
headers enable Emacs to be sure, at all times, which version number is
present in the work file.

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

@node Snapshots
@subsection Snapshots
@cindex snapshots and version control

  A @dfn{snapshot} is a named set of file versions (one for each
registered file) that you can treat as a unit.  One important kind of
snapshot is a @dfn{release}, a (theoretically) stable version of the
system that is ready for distribution to users.

@menu
* Making Snapshots::            The snapshot facilities.
* Snapshot Caveats::            Things to be careful of when using snapshots.
@end menu

@node Making Snapshots
@subsubsection Making and Using Snapshots

  There are two basic commands for snapshots; one makes a
snapshot with a given name, the other retrieves a named snapshot.

@table @code
@kindex C-x v s
@findex vc-create-snapshot
@item C-x v s @var{name} @key{RET}
Define the last saved versions of every registered file in or under the
current directory as a snapshot named @var{name}
(@code{vc-create-snapshot}).

@kindex C-x v r
@findex vc-retrieve-snapshot
@item C-x v r @var{name} @key{RET}
For all registered files at or below the current directory level, select
whatever versions correspond to the snapshot @var{name}
(@code{vc-retrieve-snapshot}).

This command reports an error if any files are locked at or below the
current directory, without changing anything; this is to avoid
overwriting work in progress.
@end table

  A snapshot uses a very small amount of resources---just enough to record
the list of file names and which version belongs to the snapshot.  Thus,
you need not hesitate to create snapshots whenever they are useful.

  You can give a snapshot name as an argument to @kbd{C-x v =} or
@kbd{C-x v ~} (@pxref{Old Versions}).  Thus, you can use it to compare a
snapshot against the current files, or two snapshots against each other,
or a snapshot against a named version.

@node Snapshot Caveats
@subsubsection Snapshot Caveats

@cindex named configurations (RCS)
  VC's snapshot facilities are modeled on RCS's named-configuration
support.  They use RCS's native facilities for this, so under VC
snapshots made using RCS are visible even when you bypass VC.

@c worded verbosely to avoid overfull hbox.
  For SCCS, VC implements snapshots itself.  The files it uses contain
name/file/version-number triples.  These snapshots are visible only
through VC.

  A snapshot is a set of checked-in versions.  So make sure that all the
files are checked in and not locked when you make a snapshot.

  File renaming and deletion can create some difficulties with snapshots.
This is not a VC-specific problem, but a general design issue in version
control systems that no one has solved very well yet.

  If you rename a registered file, you need to rename its master along
with it (the command @code{vc-rename-file} does this automatically).  If
you are using SCCS, you must also update the records of the snapshot, to
mention the file by its new name (@code{vc-rename-file} does this,
too).  An old snapshot that refers to a master file that no longer
exists under the recorded name is invalid; VC can no longer retrieve
it.  It would be beyond the scope of this manual to explain enough about
RCS and SCCS to explain how to update the snapshots by hand.

  Using @code{vc-rename-file} makes the snapshot remain valid for
retrieval, but it does not solve all problems.  For example, some of the
files in the program probably refer to others by name.  At the very
least, the makefile probably mentions the file that you renamed.  If you
retrieve an old snapshot, the renamed file is retrieved under its new
name, which is not the name that the makefile expects.  So the program
won't really work as retrieved.

@node Miscellaneous VC
@subsection Miscellaneous Commands and Features of VC

  This section explains the less-frequently-used features of VC.

@menu
* Change Logs and VC::  Generating a change log file from log entries.
* Renaming and VC::     A command to rename both the source and master 
                          file correctly.
* Version Headers::     Inserting version control headers into working files.
@end menu

@node Change Logs and VC
@subsubsection Change Logs and VC

  If you use RCS or CVS for a program and also maintain a change log
file for it (@pxref{Change Log}), you can generate change log entries
automatically from the version control log entries:

@table @kbd
@item C-x v a
@kindex C-x v a
@findex vc-update-change-log
Visit the current directory's change log file and, for registered files
in that directory, create new entries for versions checked in since the
most recent entry in the change log file.
(@code{vc-update-change-log}).

This command works with RCS or CVS only, not with SCCS.

@item C-u C-x v a
As above, but only find entries for the current buffer's file.

@item M-1 C-x v a
As above, but find entries for all the currently visited files that are
maintained with version control.  This works only with RCS, and it puts
all entries in the log for the default directory, which may not be
appropriate.
@end table

  For example, suppose the first line of @file{ChangeLog} is dated
1999-04-10, and that the only check-in since then was by Nathaniel
Bowditch to @file{rcs2log} on 1999-05-22 with log text @samp{Ignore log
messages that start with `#'.}.  Then @kbd{C-x v a} visits
@file{ChangeLog} and inserts text like this:

@iftex
@medbreak
@end iftex
@smallexample
@group
1999-05-22  Nathaniel Bowditch  <nat@@apn.org>

        * rcs2log: Ignore log messages that start with `#'.
@end group
@end smallexample
@iftex
@medbreak
@end iftex

@noindent
You can then edit the new change log entry further as you wish.

  Unfortunately, timestamps in ChangeLog files are only dates, so some
of the new change log entry may duplicate what's already in ChangeLog.
You will have to remove these duplicates by hand.

  Normally, the log entry for file @file{foo} is displayed as @samp{*
foo: @var{text of log entry}}.  The @samp{:} after @file{foo} is omitted
if the text of the log entry starts with @w{@samp{(@var{functionname}):
}}.  For example, if the log entry for @file{vc.el} is
@samp{(vc-do-command): Check call-process status.}, then the text in
@file{ChangeLog} looks like this:

@iftex
@medbreak
@end iftex
@smallexample
@group
1999-05-06  Nathaniel Bowditch  <nat@@apn.org>

        * vc.el (vc-do-command): Check call-process status.
@end group
@end smallexample
@iftex
@medbreak
@end iftex

  When @kbd{C-x v a} adds several change log entries at once, it groups
related log entries together if they all are checked in by the same
author at nearly the same time.  If the log entries for several such
files all have the same text, it coalesces them into a single entry.
For example, suppose the most recent check-ins have the following log
entries:

@flushleft
@bullet{} For @file{vc.texinfo}: @samp{Fix expansion typos.}
@bullet{} For @file{vc.el}: @samp{Don't call expand-file-name.}
@bullet{} For @file{vc-hooks.el}: @samp{Don't call expand-file-name.}
@end flushleft

@noindent
They appear like this in @file{ChangeLog}:

@iftex
@medbreak
@end iftex
@smallexample
@group
1999-04-01  Nathaniel Bowditch  <nat@@apn.org>

        * vc.texinfo: Fix expansion typos.

        * vc.el, vc-hooks.el: Don't call expand-file-name.
@end group
@end smallexample
@iftex
@medbreak
@end iftex

  Normally, @kbd{C-x v a} separates log entries by a blank line, but you
can mark several related log entries to be clumped together (without an
intervening blank line) by starting the text of each related log entry
with a label of the form @w{@samp{@{@var{clumpname}@} }}.  The label
itself is not copied to @file{ChangeLog}.  For example, suppose the log
entries are:

@flushleft
@bullet{} For @file{vc.texinfo}: @samp{@{expand@} Fix expansion typos.}
@bullet{} For @file{vc.el}: @samp{@{expand@} Don't call expand-file-name.}
@bullet{} For @file{vc-hooks.el}: @samp{@{expand@} Don't call expand-file-name.}
@end flushleft

@noindent
Then the text in @file{ChangeLog} looks like this:

@iftex
@medbreak
@end iftex
@smallexample
@group
1999-04-01  Nathaniel Bowditch  <nat@@apn.org>

        * vc.texinfo: Fix expansion typos.
        * vc.el, vc-hooks.el: Don't call expand-file-name.
@end group
@end smallexample
@iftex
@medbreak
@end iftex

  A log entry whose text begins with @samp{#} is not copied to
@file{ChangeLog}.  For example, if you merely fix some misspellings in
comments, you can log the change with an entry beginning with @samp{#}
to avoid putting such trivia into @file{ChangeLog}.

@node Renaming and VC
@subsubsection Renaming VC Work Files and Master Files

@findex vc-rename-file
  When you rename a registered file, you must also rename its master
file correspondingly to get proper results.  Use @code{vc-rename-file}
to rename the source file as you specify, and rename its master file
accordingly.  It also updates any snapshots (@pxref{Snapshots}) that
mention the file, so that they use the new name; despite this, the
snapshot thus modified may not completely work (@pxref{Snapshot
Caveats}).

  You cannot use @code{vc-rename-file} on a file that is locked by
someone else.

@node Version Headers
@subsubsection Inserting Version Control Headers

   Sometimes it is convenient to put version identification strings
directly into working files.  Certain special strings called
@dfn{version headers} are replaced in each successive version by the
number of that version.

  If you are using RCS, and version headers are present in your working
files, Emacs can use them to determine the current version and the
locking state of the files.  This is more reliable than referring to the
master files, which is done when there are no version headers.  Note
that in a multi-branch environment, version headers are necessary to
make VC behave correctly (@pxref{Multi-User Branching}).

  Searching for version headers is controlled by the variable
@code{vc-consult-headers}.  If it is non-@code{nil}, Emacs searches for
headers to determine the version number you are editing.  Setting it to
@code{nil} disables this feature.

@kindex C-x v h
@findex vc-insert-headers
  You can use the @kbd{C-x v h} command (@code{vc-insert-headers}) to
insert a suitable header string.

@table @kbd
@item C-x v h
Insert headers in a file for use with your version-control system.
@end table

@vindex vc-header-alist
  The default header string is @samp{@w{$}Id$} for RCS and
@samp{@w{%}W%} for SCCS.  You can specify other headers to insert by
setting the variable @code{vc-header-alist}.  Its value is a list of
elements of the form @code{(@var{program} . @var{string})} where
@var{program} is @code{RCS} or @code{SCCS} and @var{string} is the
string to use.

  Instead of a single string, you can specify a list of strings; then
each string in the list is inserted as a separate header on a line of
its own.

  It is often necessary to use ``superfluous'' backslashes when writing
the strings that you put in this variable.  This is to prevent the
string in the constant from being interpreted as a header itself if the
Emacs Lisp file containing it is maintained with version control.

@vindex vc-comment-alist
  Each header is inserted surrounded by tabs, inside comment delimiters,
on a new line at point.  Normally the ordinary comment
start and comment end strings of the current mode are used, but for
certain modes, there are special comment delimiters for this purpose;
the variable @code{vc-comment-alist} specifies them.  Each element of
this list has the form @code{(@var{mode} @var{starter} @var{ender})}.

@vindex vc-static-header-alist
  The variable @code{vc-static-header-alist} specifies further strings
to add based on the name of the buffer.  Its value should be a list of
elements of the form @code{(@var{regexp} . @var{format})}.  Whenever
@var{regexp} matches the buffer name, @var{format} is inserted as part
of the header.  A header line is inserted for each element that matches
the buffer name, and for each string specified by
@code{vc-header-alist}.  The header line is made by processing the
string from @code{vc-header-alist} with the format taken from the
element.  The default value for @code{vc-static-header-alist} is as follows:

@example
@group
(("\\.c$" .
  "\n#ifndef lint\nstatic char vcid[] = \"\%s\";\n\
#endif /* lint */\n"))
@end group
@end example

@noindent
It specifies insertion of text of this form:

@example
@group

#ifndef lint
static char vcid[] = "@var{string}";
#endif /* lint */
@end group
@end example

@noindent
Note that the text above starts with a blank line.

  If you use more than one version header in a file, put them close
together in the file.  The mechanism in @code{revert-buffer} that
preserves markers may not handle markers positioned between two version
headers.

@node Customizing VC
@subsection Customizing VC

  There are many ways of customizing VC.  The options you can set fall
into four categories, described in the following sections.

@vindex vc-ignore-vc-files
@cindex Version control, deactivating
  In addition, it is possible to turn VC on and off generally by setting
the variable @code{vc-ignore-vc-files}.  Normally VC will notice the
presence of version control on a file you visit and automatically invoke
the relevant program to check the file's state.  Change
@code{vc-ignore-vc-files} if this isn't the right thing, for instance,
if you edit files under version control but don't have the relevant
version control programs available.

@menu
* Backend Options::       Customizing the back-end to your needs.
* VC Workfile Handling::  Various options concerning working files.
* VC Status Retrieval::   How VC finds the version control status of a file,
                            and how to customize this.
* VC Command Execution::  Which commands VC should run, and how.
@end menu

@node Backend Options
@subsubsection Options for VC Backends

@cindex backend options (VC)
@cindex locking under version control
  You can tell RCS and CVS whether to use locking for a file or not
(@pxref{VC Concepts}, for a description of locking).  VC automatically
recognizes what you have chosen, and behaves accordingly.

@cindex non-strict locking (RCS)
@cindex locking, non-strict (RCS)
  For RCS, the default is to use locking, but there is a mode called
@dfn{non-strict locking} in which you can check-in changes without
locking the file first.  Use @samp{rcs -U} to switch to non-strict
locking for a particular file, see the @samp{rcs} manpage for details.

@cindex locking (CVS)
  Under CVS, the default is not to use locking; anyone can change a work
file at any time.  However, there are ways to restrict this, resulting
in behavior that resembles locking.

@cindex CVSREAD environment variable (CVS)
  For one thing, you can set the @code{CVSREAD} environment variable to
an arbitrary value.  If this variable is defined, CVS makes your work
files read-only by default.  In Emacs, you must type @kbd{C-x C-q} to
make the file writeable, so that editing works in fact similar as if
locking was used.  Note however, that no actual locking is performed, so
several users can make their files writeable at the same time.  When
setting @code{CVSREAD} for the first time, make sure to check out all
your modules anew, so that the file protections are set correctly.

@cindex cvs watch feature
@cindex watching files (CVS)
  Another way to achieve something similar to locking is to use the
@dfn{watch} feature of CVS.  If a file is being watched, CVS makes it
read-only by default, and you must also use @kbd{C-x C-q} in Emacs to
make it writable.  VC calls @code{cvs edit} to make the file writeable,
and CVS takes care to notify other developers of the fact that you
intend to change the file.  See the CVS documentation for details on
using the watch feature.

@vindex vc-handle-cvs
  You can turn off use of VC for CVS-managed files by setting the
variable @code{vc-handle-cvs} to @code{nil}.  If you do this, Emacs
treats these files as if they were not registered, and the VC commands
are not available for them.  You must do all CVS operations manually.

@node VC Workfile Handling
@subsubsection VC Workfile Handling

@vindex vc-make-backup-files
  Emacs normally does not save backup files for source files that are
maintained with version control.  If you want to make backup files even
for files that use version control, set the variable
@code{vc-make-backup-files} to a non-@code{nil} value.

@vindex vc-keep-workfiles
  Normally the work file exists all the time, whether it is locked or
not.  If you set @code{vc-keep-workfiles} to @code{nil}, then checking
in a new version with @kbd{C-x C-q} deletes the work file; but any
attempt to visit the file with Emacs creates it again.  (With CVS, work
files are always kept.)

@vindex vc-follow-symlinks
  Editing a version-controlled file through a symbolic link can be
dangerous.  It bypasses the version control system---you can edit the
file without locking it, and fail to check your changes in.  Also,
your changes might overwrite those of another user.  To protect against
this, VC checks each symbolic link that you visit, to see if it points
to a file under version control.

  The variable @code{vc-follow-symlinks} controls what to do when a
symbolic link points to a version-controlled file.  If it is @code{nil},
VC only displays a warning message.  If it is @code{t}, VC automatically
follows the link, and visits the real file instead, telling you about
this in the echo area.  If the value is @code{ask} (the default), VC
asks you each time whether to follow the link.

@node VC Status Retrieval
@subsubsection VC Status Retrieval
@c There is no need to tell users about vc-master-templates.

  When deducing the locked/unlocked state of a file, VC first looks for
an RCS version header string in the file (@pxref{Version Headers}).  If
there is no header string, or if you are using SCCS, VC normally looks
at the file permissions of the work file; this is fast.  But there might
be situations when the file permissions cannot be trusted.  In this case
the master file has to be consulted, which is rather expensive.  Also
the master file can only tell you @emph{if} there's any lock on the
file, but not whether your work file really contains that locked
version.

@vindex vc-consult-headers
  You can tell VC not to use version headers to determine lock status by
setting @code{vc-consult-headers} to @code{nil}.  VC then always uses
the file permissions (if it can trust them), or else checks the master
file.

@vindex vc-mistrust-permissions
  You can specify the criterion for whether to trust the file
permissions by setting the variable @code{vc-mistrust-permissions}.  Its
value can be @code{t} (always mistrust the file permissions and check
the master file), @code{nil} (always trust the file permissions), or a
function of one argument which makes the decision.  The argument is the
directory name of the @file{RCS}, @file{CVS} or @file{SCCS}
subdirectory.  A non-@code{nil} value from the function says to mistrust
the file permissions.  If you find that the file permissions of work
files are changed erroneously, set @code{vc-mistrust-permissions} to
@code{t}.  Then VC always checks the master file to determine the file's
status.

@node VC Command Execution
@subsubsection VC Command Execution

@vindex vc-suppress-confirm
  If @code{vc-suppress-confirm} is non-@code{nil}, then @kbd{C-x C-q}
and @kbd{C-x v i} can save the current buffer without asking, and
@kbd{C-x v u} also operates without asking for confirmation.  (This
variable does not affect @kbd{C-x v c}; that operation is so drastic
that it should always ask for confirmation.)

@vindex vc-command-messages
  VC mode does much of its work by running the shell commands for RCS,
CVS and SCCS.  If @code{vc-command-messages} is non-@code{nil}, VC
displays messages to indicate which shell commands it runs, and
additional messages when the commands finish.

@vindex vc-path
  You can specify additional directories to search for version control
programs by setting the variable @code{vc-path}.  These directories are
searched before the usual search path.  But the proper files are usually
found automatically.

@node Directories
@section File Directories

@cindex file directory
@cindex directory listing
  The file system groups files into @dfn{directories}.  A @dfn{directory
listing} is a list of all the files in a directory.  Emacs provides
commands to create and delete directories, and to make directory
listings in brief format (file names only) and verbose format (sizes,
dates, and authors included).  There is also a directory browser called
Dired; see @ref{Dired}.

@table @kbd
@item C-x C-d @var{dir-or-pattern} @key{RET}
Display a brief directory listing (@code{list-directory}).
@item C-u C-x C-d @var{dir-or-pattern} @key{RET}
Display a verbose directory listing.
@item M-x make-directory @key{RET} @var{dirname} @key{RET}
Create a new directory named @var{dirname}.
@item M-x delete-directory @key{RET} @var{dirname} @key{RET}
Delete the directory named @var{dirname}.  It must be empty,
or you get an error.
@end table

@findex list-directory
@kindex C-x C-d
  The command to display a directory listing is @kbd{C-x C-d}
(@code{list-directory}).  It reads using the minibuffer a file name
which is either a directory to be listed or a wildcard-containing
pattern for the files to be listed.  For example,

@example
C-x C-d /u2/emacs/etc @key{RET}
@end example

@noindent
lists all the files in directory @file{/u2/emacs/etc}.  Here is an
example of specifying a file name pattern:

@example
C-x C-d /u2/emacs/src/*.c @key{RET}
@end example

  Normally, @kbd{C-x C-d} prints a brief directory listing containing
just file names.  A numeric argument (regardless of value) tells it to
make a verbose listing including sizes, dates, and authors (like
@samp{ls -l}).

@vindex list-directory-brief-switches
@vindex list-directory-verbose-switches
  The text of a directory listing is obtained by running @code{ls} in an
inferior process.  Two Emacs variables control the switches passed to
@code{ls}: @code{list-directory-brief-switches} is a string giving the
switches to use in brief listings (@code{"-CF"} by default), and
@code{list-directory-verbose-switches} is a string giving the switches to
use in a verbose listing (@code{"-l"} by default).

@node Comparing Files
@section Comparing Files
@cindex comparing files

@findex diff
@vindex diff-switches
  The command @kbd{M-x diff} compares two files, displaying the
differences in an Emacs buffer named @samp{*Diff*}.  It works by running
the @code{diff} program, using options taken from the variable
@code{diff-switches}, whose value should be a string.

  The buffer @samp{*Diff*} has Compilation mode as its major mode, so
you can use @kbd{C-x `} to visit successive changed locations in the two
source files.  You can also move to a particular hunk of changes and
type @key{RET} or @kbd{C-c C-c}, or click @kbd{Mouse-2} on it, to move
to the corresponding source location.  You can also use the other
special commands of Compilation mode: @key{SPC} and @key{DEL} for
scrolling, and @kbd{M-p} and @kbd{M-n} for cursor motion.
@xref{Compilation}.

@findex diff-backup
  The command @kbd{M-x diff-backup} compares a specified file with its most
recent backup.  If you specify the name of a backup file,
@code{diff-backup} compares it with the source file that it is a backup
of.

@findex compare-windows
  The command @kbd{M-x compare-windows} compares the text in the current
window with that in the next window.  Comparison starts at point in each
window, and each starting position is pushed on the mark ring in its
respective buffer.  Then point moves forward in each window, a character
at a time, until a mismatch between the two windows is reached.  Then
the command is finished.  For more information about windows in Emacs,
@ref{Windows}.

@vindex compare-ignore-case
  With a numeric argument, @code{compare-windows} ignores changes in
whitespace.  If the variable @code{compare-ignore-case} is
non-@code{nil}, it ignores differences in case as well.

  See also @ref{Emerge}, for convenient facilities for merging two
similar files.

@node Misc File Ops
@section Miscellaneous File Operations

  Emacs has commands for performing many other operations on files.
All operate on one file; they do not accept wildcard file names.

@findex view-file
@cindex viewing
@cindex View mode
@cindex mode, View
  @kbd{M-x view-file} allows you to scan or read a file by sequential
screenfuls.  It reads a file name argument using the minibuffer.  After
reading the file into an Emacs buffer, @code{view-file} displays the
beginning.  You can then type @key{SPC} to scroll forward one windowful,
or @key{DEL} to scroll backward.  Various other commands are provided
for moving around in the file, but none for changing it; type @kbd{?}
while viewing for a list of them.  They are mostly the same as normal
Emacs cursor motion commands.  To exit from viewing, type @kbd{q}.
The commands for viewing are defined by a special major mode called View
mode.

  A related command, @kbd{M-x view-buffer}, views a buffer already present
in Emacs.  @xref{Misc Buffer}.

@findex insert-file
  @kbd{M-x insert-file} inserts a copy of the contents of the specified
file into the current buffer at point, leaving point unchanged before the
contents and the mark after them.

@findex write-region
  @kbd{M-x write-region} is the inverse of @kbd{M-x insert-file}; it
copies the contents of the region into the specified file.  @kbd{M-x
append-to-file} adds the text of the region to the end of the specified
file.  @xref{Accumulating Text}.

@findex delete-file
@cindex deletion (of files)
  @kbd{M-x delete-file} deletes the specified file, like the @code{rm}
command in the shell.  If you are deleting many files in one directory, it
may be more convenient to use Dired (@pxref{Dired}).

@findex rename-file
  @kbd{M-x rename-file} reads two file names @var{old} and @var{new} using
the minibuffer, then renames file @var{old} as @var{new}.  If a file named
@var{new} already exists, you must confirm with @kbd{yes} or renaming is not
done; this is because renaming causes the old meaning of the name @var{new}
to be lost.  If @var{old} and @var{new} are on different file systems, the
file @var{old} is copied and deleted.

@findex add-name-to-file
  The similar command @kbd{M-x add-name-to-file} is used to add an
additional name to an existing file without removing its old name.
The new name must belong on the same file system that the file is on.

@findex copy-file
@cindex copying files
  @kbd{M-x copy-file} reads the file @var{old} and writes a new file named
@var{new} with the same contents.  Confirmation is required if a file named
@var{new} already exists, because copying has the consequence of overwriting
the old contents of the file @var{new}.

@findex make-symbolic-link
  @kbd{M-x make-symbolic-link} reads two file names @var{target} and
@var{linkname}, then creates a symbolic link named @var{linkname} and
pointing at @var{target}.  The effect is that future attempts to open file
@var{linkname} will refer to whatever file is named @var{target} at the
time the opening is done, or will get an error if the name @var{target} is
not in use at that time.  This command does not expand the argument
@var{target}, so that it allows you to specify a relative name
as the target of the link.

  Confirmation is required when creating the link if @var{linkname} is
in use.  Note that not all systems support symbolic links.

@node Compressed Files
@section Accessing Compressed Files
@cindex compression
@cindex uncompression
@cindex Auto Compression mode
@cindex mode, Auto Compression
@pindex gzip

@findex auto-compression-mode
  Emacs comes with a library that can automatically uncompress
compressed files when you visit them, and automatically recompress them
if you alter them and save them.  To enable this feature, type the
command @kbd{M-x auto-compression-mode}.

  When automatic compression (which implies automatic uncompression as
well) is enabled, Emacs recognizes compressed files by their file names.
File names ending in @samp{.gz} indicate a file compressed with
@code{gzip}.  Other endings indicate other compression programs.

  Automatic uncompression and compression apply to all the operations in
which Emacs uses the contents of a file.  This includes visiting it,
saving it, inserting its contents into a buffer, loading it, and byte
compiling it.

@node Remote Files
@section Remote Files

@cindex FTP
@cindex remote file access
  You can refer to files on other machines using a special file name syntax:

@example
@group
/@var{host}:@var{filename}
/@var{user}@@@var{host}:@var{filename}
/@var{user}@@@var{host}#@var{port}:@var{filename}
@end group
@end example

@noindent
When you do this, Emacs uses the FTP program to read and write files on
the specified host.  It logs in through FTP using your user name or the
name @var{user}.  It may ask you for a password from time to time; this
is used for logging in on @var{host}.  The form using @var{port} allows
you to access servers running on a non-default TCP port.

@cindex ange-ftp
@vindex ange-ftp-default-user
  Normally, if you do not specify a user name in a remote file name,
that means to use your own user name.  But if you set the variable
@code{ange-ftp-default-user} to a string, that string is used instead.
(The Emacs package that implements FTP file access is called
@code{ange-ftp}.)

@vindex file-name-handler-alist
  You can entirely turn off the FTP file name feature by removing the
entries @var{ange-ftp-completion-hook-function} and
@var{ange-ftp-hook-function} from the variable
@code{file-name-handler-alist}.

@node Quoted File Names
@section Quoted File Names

@cindex quoting file names
  You can @dfn{quote} an absolute file name to prevent special
characters and syntax in it from having their special effects.
The way to do this is to add @samp{/:} at the beginning.

  For example, you can quote a local file name which appears remote, to
prevent it from being treated as a remote file name.  Thus, if you have
a directory named @file{/foo:} and a file named @file{bar} in it, you
can refer to that file in Emacs as @samp{/:/foo:/bar}.

  @samp{/:} can also prevent @samp{~} from being treated as a special
character for a user's home directory.  For example, @file{/:/tmp/~hack}
refers to a file whose name is @file{~hack} in directory @file{/tmp}.

  Likewise, quoting with @samp{/:} is one way to enter in the minibuffer
a file name that contains @samp{$}.  However, the @samp{/:} must be at
the beginning of the buffer in order to quote @samp{$}.

  You can also quote wildcard characters with @samp{/:}, for visiting.
For example, @file{/:/tmp/foo*bar} visits the file @file{/tmp/foo*bar}.
However, in most cases you can simply type the wildcard characters for
themselves.  For example, if the only file name in @file{/tmp} that
starts with @samp{foo} and ends with @samp{bar} is @file{foo*bar}, then
specifying @file{/tmp/foo*bar} will visit just @file{/tmp/foo*bar}.