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[gnu-emacs] / lispref / files.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998 Free Software Foundation, Inc. 
@c See the file elisp.texi for copying conditions.
@setfilename ../info/files
@node Files, Backups and Auto-Saving, Documentation, Top
@comment  node-name,  next,  previous,  up
@chapter Files

  In Emacs, you can find, create, view, save, and otherwise work with
files and file directories.  This chapter describes most of the
file-related functions of Emacs Lisp, but a few others are described in
@ref{Buffers}, and those related to backups and auto-saving are
described in @ref{Backups and Auto-Saving}.

  Many of the file functions take one or more arguments that are file
names.  A file name is actually a string.  Most of these functions
expand file name arguments by calling @code{expand-file-name}, so that
@file{~} is handled correctly, as are relative file names (including
@samp{../}).  These functions don't recognize environment variable
substitutions such as @samp{$HOME}.  @xref{File Name Expansion}.

  When file I/O functions signal Lisp errors, they usually use the
condition @code{file-error} (@pxref{Handling Errors}).  The error
message is in most cases obtained from the operating system, according
to locale @code{system-message-locale}, and decoded using coding system
@code{locale-coding-system} (@pxref{Locales}).

@menu
* Visiting Files::           Reading files into Emacs buffers for editing.
* Saving Buffers::           Writing changed buffers back into files.
* Reading from Files::       Reading files into buffers without visiting.
* Writing to Files::         Writing new files from parts of buffers.
* File Locks::               Locking and unlocking files, to prevent
                               simultaneous editing by two people.
* Information about Files::  Testing existence, accessibility, size of files.
* Changing Files::           Renaming files, changing protection, etc.
* File Names::               Decomposing and expanding file names.
* Contents of Directories::  Getting a list of the files in a directory.
* Create/Delete Dirs::       Creating and Deleting Directories.
* Magic File Names::         Defining "magic" special handling
                               for certain file names.
* Format Conversion::        Conversion to and from various file formats.
@end menu

@node Visiting Files
@section Visiting Files
@cindex finding files
@cindex visiting files

  Visiting a file means reading a file into a buffer.  Once this is
done, we say that the buffer is @dfn{visiting} that file, and call the
file ``the visited file'' of the buffer.

  A file and a buffer are two different things.  A file is information
recorded permanently in the computer (unless you delete it).  A buffer,
on the other hand, is information inside of Emacs that will vanish at
the end of the editing session (or when you kill the buffer).  Usually,
a buffer contains information that you have copied from a file; then we
say the buffer is visiting that file.  The copy in the buffer is what
you modify with editing commands.  Such changes to the buffer do not
change the file; therefore, to make the changes permanent, you must
@dfn{save} the buffer, which means copying the altered buffer contents
back into the file.

  In spite of the distinction between files and buffers, people often
refer to a file when they mean a buffer and vice-versa.  Indeed, we say,
``I am editing a file,'' rather than, ``I am editing a buffer that I
will soon save as a file of the same name.''  Humans do not usually need
to make the distinction explicit.  When dealing with a computer program,
however, it is good to keep the distinction in mind.

@menu
* Visiting Functions::         The usual interface functions for visiting.
* Subroutines of Visiting::    Lower-level subroutines that they use.
@end menu

@node Visiting Functions
@subsection Functions for Visiting Files

  This section describes the functions normally used to visit files.
For historical reasons, these functions have names starting with
@samp{find-} rather than @samp{visit-}.  @xref{Buffer File Name}, for
functions and variables that access the visited file name of a buffer or
that find an existing buffer by its visited file name.

  In a Lisp program, if you want to look at the contents of a file but
not alter it, the fastest way is to use @code{insert-file-contents} in a
temporary buffer.  Visiting the file is not necessary and takes longer.
@xref{Reading from Files}.

@deffn Command find-file filename &optional wildcards
This command selects a buffer visiting the file @var{filename},
using an existing buffer if there is one, and otherwise creating a 
new buffer and reading the file into it.  It also returns that buffer.

The body of the @code{find-file} function is very simple and looks
like this:

@example
(switch-to-buffer (find-file-noselect filename))
@end example

@noindent
(See @code{switch-to-buffer} in @ref{Displaying Buffers}.)

If @var{wildcards} is non-@code{nil}, which is always true in an
interactive call, then @code{find-file} expands wildcard characters in
@var{filename} and visits all the matching files.

When @code{find-file} is called interactively, it prompts for
@var{filename} in the minibuffer.
@end deffn

@defun find-file-noselect filename &optional nowarn rawfile wildcards
This function is the guts of all the file-visiting functions.  It finds
or creates a buffer visiting the file @var{filename}, and returns it.
It uses an existing buffer if there is one, and otherwise creates a new
buffer and reads the file into it.  You may make the buffer current or
display it in a window if you wish, but this function does not do so.

If @var{wildcards} is non-@code{nil},
then @code{find-file-noselect} expands wildcard
characters in @var{filename} and visits all the matching files.

When @code{find-file-noselect} uses an existing buffer, it first
verifies that the file has not changed since it was last visited or
saved in that buffer.  If the file has changed, then this function asks
the user whether to reread the changed file.  If the user says
@samp{yes}, any changes previously made in the buffer are lost.

This function displays warning or advisory messages in various peculiar
cases, unless the optional argument @var{nowarn} is non-@code{nil}.  For
example, if it needs to create a buffer, and there is no file named
@var{filename}, it displays the message @samp{(New file)} in the echo
area, and leaves the buffer empty.

The @code{find-file-noselect} function normally calls
@code{after-find-file} after reading the file (@pxref{Subroutines of
Visiting}).  That function sets the buffer major mode, parses local
variables, warns the user if there exists an auto-save file more recent
than the file just visited, and finishes by running the functions in
@code{find-file-hooks}.

If the optional argument @var{rawfile} is non-@code{nil}, then
@code{after-find-file} is not called, and the
@code{find-file-not-found-hooks} are not run in case of failure.  What's
more, a non-@code{nil} @var{rawfile} value suppresses coding system
conversion (@pxref{Coding Systems}) and format conversion (@pxref{Format
Conversion}).

The @code{find-file-noselect} function returns the buffer that is
visiting the file @var{filename}.

@example
@group
(find-file-noselect "/etc/fstab")
     @result{} #<buffer fstab>
@end group
@end example
@end defun

@deffn Command find-file-other-window filename &optional wildcards
This command selects a buffer visiting the file @var{filename}, but
does so in a window other than the selected window.  It may use another
existing window or split a window; see @ref{Displaying Buffers}.

When this command is called interactively, it prompts for
@var{filename}.
@end deffn

@deffn Command find-file-read-only filename &optional wildcards
This command selects a buffer visiting the file @var{filename}, like
@code{find-file}, but it marks the buffer as read-only.  @xref{Read Only
Buffers}, for related functions and variables.

When this command is called interactively, it prompts for
@var{filename}.
@end deffn

@deffn Command view-file filename
This command visits @var{filename} using View mode, returning to the
previous buffer when you exit View mode.  View mode is a minor mode that
provides commands to skim rapidly through the file, but does not let you
modify the text.  Entering View mode runs the normal hook
@code{view-mode-hook}.  @xref{Hooks}.

When @code{view-file} is called interactively, it prompts for
@var{filename}.
@end deffn

@tindex find-file-wildcards
@defvar find-file-wildcards
If this variable is non-@code{nil}, then the various @code{find-file}
commands check for wildcard characters and visit all the files that
match them.  If this is @code{nil}, then wildcard characters are
not treated specially.
@end defvar

@defvar find-file-hooks
The value of this variable is a list of functions to be called after a
file is visited.  The file's local-variables specification (if any) will
have been processed before the hooks are run.  The buffer visiting the
file is current when the hook functions are run.

This variable works just like a normal hook, but we think that renaming
it would not be advisable.  @xref{Hooks}.
@end defvar

@defvar find-file-not-found-hooks
The value of this variable is a list of functions to be called when
@code{find-file} or @code{find-file-noselect} is passed a nonexistent
file name.  @code{find-file-noselect} calls these functions as soon as
it detects a nonexistent file.  It calls them in the order of the list,
until one of them returns non-@code{nil}.  @code{buffer-file-name} is
already set up.

This is not a normal hook because the values of the functions are
used, and in many cases only some of the functions are called.
@end defvar

@node Subroutines of Visiting
@comment  node-name,  next,  previous,  up
@subsection Subroutines of Visiting

  The @code{find-file-noselect} function uses two important subroutines
which are sometimes useful in user Lisp code: @code{create-file-buffer}
and @code{after-find-file}.  This section explains how to use them.

@defun create-file-buffer filename
This function creates a suitably named buffer for visiting
@var{filename}, and returns it.  It uses @var{filename} (sans directory)
as the name if that name is free; otherwise, it appends a string such as
@samp{<2>} to get an unused name.  See also @ref{Creating Buffers}.

@strong{Please note:} @code{create-file-buffer} does @emph{not}
associate the new buffer with a file and does not select the buffer.
It also does not use the default major mode.

@example
@group
(create-file-buffer "foo")
     @result{} #<buffer foo>
@end group
@group
(create-file-buffer "foo")
     @result{} #<buffer foo<2>>
@end group
@group
(create-file-buffer "foo")
     @result{} #<buffer foo<3>>
@end group
@end example

This function is used by @code{find-file-noselect}.
It uses @code{generate-new-buffer} (@pxref{Creating Buffers}).
@end defun

@defun after-find-file &optional error warn noauto after-find-file-from-revert-buffer nomodes
This function sets the buffer major mode, and parses local variables
(@pxref{Auto Major Mode}).  It is called by @code{find-file-noselect}
and by the default revert function (@pxref{Reverting}).

@cindex new file message
@cindex file open error
If reading the file got an error because the file does not exist, but
its directory does exist, the caller should pass a non-@code{nil} value
for @var{error}.  In that case, @code{after-find-file} issues a warning:
@samp{(New file)}.  For more serious errors, the caller should usually not
call @code{after-find-file}.

If @var{warn} is non-@code{nil}, then this function issues a warning
if an auto-save file exists and is more recent than the visited file.

If @var{noauto} is non-@code{nil}, that says not to enable or disable
Auto-Save mode.  The mode remains enabled if it was enabled before.

If @var{after-find-file-from-revert-buffer} is non-@code{nil}, that
means this call was from @code{revert-buffer}.  This has no direct
effect, but some mode functions and hook functions check the value
of this variable.

If @var{nomodes} is non-@code{nil}, that means don't alter the buffer's
major mode, don't process local variables specifications in the file,
and don't run @code{find-file-hooks}.  This feature is used by
@code{revert-buffer} in some cases.

The last thing @code{after-find-file} does is call all the functions
in the list @code{find-file-hooks}.
@end defun

@node Saving Buffers
@section Saving Buffers

  When you edit a file in Emacs, you are actually working on a buffer
that is visiting that file---that is, the contents of the file are
copied into the buffer and the copy is what you edit.  Changes to the
buffer do not change the file until you @dfn{save} the buffer, which
means copying the contents of the buffer into the file.

@deffn Command save-buffer &optional backup-option
This function saves the contents of the current buffer in its visited
file if the buffer has been modified since it was last visited or saved.
Otherwise it does nothing.

@code{save-buffer} is responsible for making backup files.  Normally,
@var{backup-option} is @code{nil}, and @code{save-buffer} makes a backup
file only if this is the first save since visiting the file.  Other
values for @var{backup-option} request the making of backup files in
other circumstances:

@itemize @bullet
@item
With an argument of 4 or 64, reflecting 1 or 3 @kbd{C-u}'s, the
@code{save-buffer} function marks this version of the file to be
backed up when the buffer is next saved.

@item
With an argument of 16 or 64, reflecting 2 or 3 @kbd{C-u}'s, the
@code{save-buffer} function unconditionally backs up the previous
version of the file before saving it.
@end itemize
@end deffn

@deffn Command save-some-buffers &optional save-silently-p pred
This command saves some modified file-visiting buffers.  Normally it
asks the user about each buffer.  But if @var{save-silently-p} is
non-@code{nil}, it saves all the file-visiting buffers without querying
the user.

The optional @var{pred} argument controls which buffers to ask about.
If it is @code{nil}, that means to ask only about file-visiting buffers.
If it is @code{t}, that means also offer to save certain other non-file
buffers---those that have a non-@code{nil} buffer-local value of
@code{buffer-offer-save}.  (A user who says @samp{yes} to saving a
non-file buffer is asked to specify the file name to use.)  The
@code{save-buffers-kill-emacs} function passes the value @code{t} for
@var{pred}.

If @var{pred} is neither @code{t} nor @code{nil}, then it should be
a function of no arguments.  It will be called in each buffer to decide
whether to offer to save that buffer.  If it returns a non-@code{nil}
value in a certain buffer, that means do offer to save that buffer.
@end deffn

@deffn Command write-file filename &optional confirm
This function writes the current buffer into file @var{filename}, makes
the buffer visit that file, and marks it not modified.  Then it renames
the buffer based on @var{filename}, appending a string like @samp{<2>}
if necessary to make a unique buffer name.  It does most of this work by
calling @code{set-visited-file-name} (@pxref{Buffer File Name}) and
@code{save-buffer}.

If @var{confirm} is non-@code{nil}, that means to ask for confirmation
before overwriting an existing file.
@end deffn

  Saving a buffer runs several hooks.  It also performs format
conversion (@pxref{Format Conversion}), and may save text properties in
``annotations'' (@pxref{Saving Properties}).

@defvar write-file-hooks
The value of this variable is a list of functions to be called before
writing out a buffer to its visited file.  If one of them returns
non-@code{nil}, the file is considered already written and the rest of
the functions are not called, nor is the usual code for writing the file
executed.

If a function in @code{write-file-hooks} returns non-@code{nil}, it
is responsible for making a backup file (if that is appropriate).
To do so, execute the following code:

@example
(or buffer-backed-up (backup-buffer))
@end example

You might wish to save the file modes value returned by
@code{backup-buffer} and use that to set the mode bits of the file that
you write.  This is what @code{save-buffer} normally does.

The hook functions in @code{write-file-hooks} are also responsible for
encoding the data (if desired): they must choose a suitable coding
system (@pxref{Lisp and Coding Systems}), perform the encoding
(@pxref{Explicit Encoding}), and set @code{last-coding-system-used} to
the coding system that was used (@pxref{Encoding and I/O}).

Do not make this variable buffer-local.  To set up buffer-specific hook
functions, use @code{write-contents-hooks} instead.

Even though this is not a normal hook, you can use @code{add-hook} and
@code{remove-hook} to manipulate the list.  @xref{Hooks}.
@end defvar

@c Emacs 19 feature
@defvar local-write-file-hooks
This works just like @code{write-file-hooks}, but it is intended to be
made buffer-local in particular buffers, and used for hooks that pertain
to the file name or the way the buffer contents were obtained.

The variable is marked as a permanent local, so that changing the major
mode does not alter a buffer-local value.  This is convenient for
packages that read ``file'' contents in special ways, and set up hooks
to save the data in a corresponding way.
@end defvar

@c Emacs 19 feature
@defvar write-contents-hooks
This works just like @code{write-file-hooks}, but it is intended for
hooks that pertain to the contents of the file, as opposed to hooks that
pertain to where the file came from.  Such hooks are usually set up by
major modes, as buffer-local bindings for this variable.

This variable automatically becomes buffer-local whenever it is set;
switching to a new major mode always resets this variable.  When you use
@code{add-hooks} to add an element to this hook, you should @emph{not}
specify a non-@code{nil} @var{local} argument, since this variable is
used @emph{only} buffer-locally.
@end defvar

@c Emacs 19 feature
@defvar after-save-hook
This normal hook runs after a buffer has been saved in its visited file.
One use of this hook is in Fast Lock mode; it uses this hook to save the
highlighting information in a cache file.
@end defvar

@defvar file-precious-flag
If this variable is non-@code{nil}, then @code{save-buffer} protects
against I/O errors while saving by writing the new file to a temporary
name instead of the name it is supposed to have, and then renaming it to
the intended name after it is clear there are no errors.  This procedure
prevents problems such as a lack of disk space from resulting in an
invalid file.

As a side effect, backups are necessarily made by copying.  @xref{Rename
or Copy}.  Yet, at the same time, saving a precious file always breaks
all hard links between the file you save and other file names.

Some modes give this variable a non-@code{nil} buffer-local value
in particular buffers.
@end defvar

@defopt require-final-newline
This variable determines whether files may be written out that do
@emph{not} end with a newline.  If the value of the variable is
@code{t}, then @code{save-buffer} silently adds a newline at the end of
the file whenever the buffer being saved does not already end in one.
If the value of the variable is non-@code{nil}, but not @code{t}, then
@code{save-buffer} asks the user whether to add a newline each time the
case arises.

If the value of the variable is @code{nil}, then @code{save-buffer}
doesn't add newlines at all.  @code{nil} is the default value, but a few
major modes set it to @code{t} in particular buffers.
@end defopt

  See also the function @code{set-visited-file-name} (@pxref{Buffer File
Name}).

@node Reading from Files
@comment  node-name,  next,  previous,  up
@section Reading from Files

  You can copy a file from the disk and insert it into a buffer
using the @code{insert-file-contents} function.  Don't use the user-level
command @code{insert-file} in a Lisp program, as that sets the mark.

@defun insert-file-contents filename &optional visit beg end replace
This function inserts the contents of file @var{filename} into the
current buffer after point.  It returns a list of the absolute file name
and the length of the data inserted.  An error is signaled if
@var{filename} is not the name of a file that can be read.

The function @code{insert-file-contents} checks the file contents
against the defined file formats, and converts the file contents if
appropriate.  @xref{Format Conversion}.  It also calls the functions in
the list @code{after-insert-file-functions}; see @ref{Saving
Properties}.  Normally, one of the functions in the
@code{after-insert-file-functions} list determines the coding system
(@pxref{Coding Systems}) used for decoding the file's contents.

If @var{visit} is non-@code{nil}, this function additionally marks the
buffer as unmodified and sets up various fields in the buffer so that it
is visiting the file @var{filename}: these include the buffer's visited
file name and its last save file modtime.  This feature is used by
@code{find-file-noselect} and you probably should not use it yourself.

If @var{beg} and @var{end} are non-@code{nil}, they should be integers
specifying the portion of the file to insert.  In this case, @var{visit}
must be @code{nil}.  For example,

@example
(insert-file-contents filename nil 0 500)
@end example

@noindent
inserts the first 500 characters of a file.

If the argument @var{replace} is non-@code{nil}, it means to replace the
contents of the buffer (actually, just the accessible portion) with the
contents of the file.  This is better than simply deleting the buffer
contents and inserting the whole file, because (1) it preserves some
marker positions and (2) it puts less data in the undo list.

It is possible to read a special file (such as a FIFO or an I/O device)
with @code{insert-file-contents}, as long as @var{replace} and
@var{visit} are @code{nil}.
@end defun

@defun insert-file-contents-literally filename &optional visit beg end replace
@tindex insert-file-contents-literally
This function works like @code{insert-file-contents} except that it does
not do format decoding (@pxref{Format Conversion}), does not do
character code conversion (@pxref{Coding Systems}), does not run
@code{find-file-hooks}, does not perform automatic uncompression, and so
on.
@end defun

If you want to pass a file name to another process so that another
program can read the file, use the function @code{file-local-copy}; see
@ref{Magic File Names}.

@node Writing to Files
@comment  node-name,  next,  previous,  up
@section Writing to Files

  You can write the contents of a buffer, or part of a buffer, directly
to a file on disk using the @code{append-to-file} and
@code{write-region} functions.  Don't use these functions to write to
files that are being visited; that could cause confusion in the
mechanisms for visiting.

@deffn Command append-to-file start end filename
This function appends the contents of the region delimited by
@var{start} and @var{end} in the current buffer to the end of file
@var{filename}.  If that file does not exist, it is created.  This
function returns @code{nil}.

An error is signaled if @var{filename} specifies a nonwritable file,
or a nonexistent file in a directory where files cannot be created.
@end deffn

@deffn Command write-region start end filename &optional append visit lockname mustbenew
This function writes the region delimited by @var{start} and @var{end}
in the current buffer into the file specified by @var{filename}.

@c Emacs 19 feature
If @var{start} is a string, then @code{write-region} writes or appends
that string, rather than text from the buffer.  @var{end} is ignored in
this case.

If @var{append} is non-@code{nil}, then the specified text is appended
to the existing file contents (if any).

If @var{mustbenew} is non-@code{nil}, then @code{write-region} asks
for confirmation if @var{filename} names an existing file.
Starting in Emacs 21, if @var{mustbenew} is the symbol @code{excl}, 
then @code{write-region} does not ask for confirmation, but instead
it signals an error @code{file-already-exists} if the file already
exists.

The test for an existing file, when @var{mustbenew} is @code{excl}, uses
a special system feature.  At least for files on a local disk, there is
no chance that some other program could create a file of the same name
before Emacs does, without Emacs's noticing.

If @var{visit} is @code{t}, then Emacs establishes an association
between the buffer and the file: the buffer is then visiting that file.
It also sets the last file modification time for the current buffer to
@var{filename}'s modtime, and marks the buffer as not modified.  This
feature is used by @code{save-buffer}, but you probably should not use
it yourself.

@c Emacs 19 feature
If @var{visit} is a string, it specifies the file name to visit.  This
way, you can write the data to one file (@var{filename}) while recording
the buffer as visiting another file (@var{visit}).  The argument
@var{visit} is used in the echo area message and also for file locking;
@var{visit} is stored in @code{buffer-file-name}.  This feature is used
to implement @code{file-precious-flag}; don't use it yourself unless you
really know what you're doing.

The optional argument @var{lockname}, if non-@code{nil}, specifies the
file name to use for purposes of locking and unlocking, overriding
@var{filename} and @var{visit} for that purpose.

The function @code{write-region} converts the data which it writes to
the appropriate file formats specified by @code{buffer-file-format}.
@xref{Format Conversion}.  It also calls the functions in the list
@code{write-region-annotate-functions}; see @ref{Saving Properties}.

Normally, @code{write-region} displays the message @samp{Wrote
@var{filename}} in the echo area.  If @var{visit} is neither @code{t}
nor @code{nil} nor a string, then this message is inhibited.  This
feature is useful for programs that use files for internal purposes,
files that the user does not need to know about.
@end deffn

@defmac with-temp-file file body...
@tindex with-temp-file
The @code{with-temp-file} macro evaluates the @var{body} forms with a
temporary buffer as the current buffer; then, at the end, it writes the
buffer contents into file @var{file}.  It kills the temporary buffer
when finished, restoring the buffer that was current before the
@code{with-temp-file} form.  Then it returns the value of the last form
in @var{body}.

The current buffer is restored even in case of an abnormal exit via
@code{throw} or error (@pxref{Nonlocal Exits}).

See also @code{with-temp-buffer} in @ref{Current Buffer}.
@end defmac

@node File Locks
@section File Locks
@cindex file locks

  When two users edit the same file at the same time, they are likely to
interfere with each other.  Emacs tries to prevent this situation from
arising by recording a @dfn{file lock} when a file is being modified.
Emacs can then detect the first attempt to modify a buffer visiting a
file that is locked by another Emacs job, and ask the user what to do.
The file lock is really a file, a symbolic link with a special name,
stored in the same directory as the file you are editing.

  When you access files using NFS, there may be a small probability that
you and another user will both lock the same file ``simultaneously''.
If this happens, it is possible for the two users to make changes
simultaneously, but Emacs will still warn the user who saves second.
Also, the detection of modification of a buffer visiting a file changed
on disk catches some cases of simultaneous editing; see
@ref{Modification Time}.

@defun file-locked-p filename
This function returns @code{nil} if the file @var{filename} is not
locked.  It returns @code{t} if it is locked by this Emacs process, and
it returns the name of the user who has locked it if it is locked by
some other job.

@example
@group
(file-locked-p "foo")
     @result{} nil
@end group
@end example
@end defun

@defun lock-buffer &optional filename
This function locks the file @var{filename}, if the current buffer is
modified.  The argument @var{filename} defaults to the current buffer's
visited file.  Nothing is done if the current buffer is not visiting a
file, or is not modified.
@end defun

@defun unlock-buffer
This function unlocks the file being visited in the current buffer,
if the buffer is modified.  If the buffer is not modified, then
the file should not be locked, so this function does nothing.  It also
does nothing if the current buffer is not visiting a file.
@end defun

  File locking is not supported on some systems.  On systems that do not
support it, the functions @code{lock-buffer}, @code{unlock-buffer} and
@code{file-locked-p} do nothing and return @code{nil}.

@defun ask-user-about-lock file other-user
This function is called when the user tries to modify @var{file}, but it
is locked by another user named @var{other-user}.  The default
definition of this function asks the user to say what to do.  The value
this function returns determines what Emacs does next:

@itemize @bullet
@item
A value of @code{t} says to grab the lock on the file.  Then
this user may edit the file and @var{other-user} loses the lock.

@item
A value of @code{nil} says to ignore the lock and let this
user edit the file anyway.

@item
@kindex file-locked
This function may instead signal a @code{file-locked} error, in which
case the change that the user was about to make does not take place.

The error message for this error looks like this:

@example
@error{} File is locked: @var{file} @var{other-user}
@end example

@noindent
where @code{file} is the name of the file and @var{other-user} is the
name of the user who has locked the file.
@end itemize

If you wish, you can replace the @code{ask-user-about-lock} function
with your own version that makes the decision in another way.  The code
for its usual definition is in @file{userlock.el}.
@end defun

@node Information about Files
@section Information about Files

  The functions described in this section all operate on strings that
designate file names.  All the functions have names that begin with the
word @samp{file}.  These functions all return information about actual
files or directories, so their arguments must all exist as actual files
or directories unless otherwise noted.

@menu
* Testing Accessibility::   Is a given file readable?  Writable?
* Kinds of Files::          Is it a directory?  A symbolic link?
* Truenames::               Eliminating symbolic links from a file name.
* File Attributes::         How large is it?  Any other names?  Etc.
@end menu

@node Testing Accessibility
@comment  node-name,  next,  previous,  up
@subsection Testing Accessibility
@cindex accessibility of a file
@cindex file accessibility

  These functions test for permission to access a file in specific ways.

@defun file-exists-p filename
This function returns @code{t} if a file named @var{filename} appears to
exist.  This does not mean you can necessarily read the file, only that
you can find out its attributes.  (On Unix and GNU/Linux, this is true
if the file exists and you have execute permission on the containing
directories, regardless of the protection of the file itself.)

If the file does not exist, or if fascist access control policies
prevent you from finding the attributes of the file, this function
returns @code{nil}.
@end defun

@defun file-readable-p filename
This function returns @code{t} if a file named @var{filename} exists
and you can read it.  It returns @code{nil} otherwise.

@example
@group
(file-readable-p "files.texi")
     @result{} t
@end group
@group
(file-exists-p "/usr/spool/mqueue")
     @result{} t
@end group
@group
(file-readable-p "/usr/spool/mqueue")
     @result{} nil
@end group
@end example
@end defun

@c Emacs 19 feature
@defun file-executable-p filename
This function returns @code{t} if a file named @var{filename} exists and
you can execute it.  It returns @code{nil} otherwise.  On Unix and
GNU/Linux, if the file is a directory, execute permission means you can
check the existence and attributes of files inside the directory, and
open those files if their modes permit.
@end defun

@defun file-writable-p filename
This function returns @code{t} if the file @var{filename} can be written
or created by you, and @code{nil} otherwise.  A file is writable if the
file exists and you can write it.  It is creatable if it does not exist,
but the specified directory does exist and you can write in that
directory.

In the third example below, @file{foo} is not writable because the
parent directory does not exist, even though the user could create such
a directory.

@example
@group
(file-writable-p "~/foo")
     @result{} t
@end group
@group
(file-writable-p "/foo")
     @result{} nil
@end group
@group
(file-writable-p "~/no-such-dir/foo")
     @result{} nil
@end group
@end example
@end defun

@c Emacs 19 feature
@defun file-accessible-directory-p dirname
This function returns @code{t} if you have permission to open existing
files in the directory whose name as a file is @var{dirname}; otherwise
(or if there is no such directory), it returns @code{nil}.  The value
of @var{dirname} may be either a directory name or the file name of a
file which is a directory.

Example: after the following,

@example
(file-accessible-directory-p "/foo")
     @result{} nil
@end example

@noindent
we can deduce that any attempt to read a file in @file{/foo/} will
give an error.
@end defun

@defun access-file filename string
@tindex access-file
This function opens file @var{filename} for reading, then closes it and
returns @code{nil}.  However, if the open fails, it signals an error
using @var{string} as the error message text.
@end defun

@defun file-ownership-preserved-p filename
This function returns @code{t} if deleting the file @var{filename} and
then creating it anew would keep the file's owner unchanged.
@end defun

@defun file-newer-than-file-p filename1 filename2
@cindex file age
@cindex file modification time
This function returns @code{t} if the file @var{filename1} is
newer than file @var{filename2}.  If @var{filename1} does not
exist, it returns @code{nil}.  If @var{filename2} does not exist,
it returns @code{t}.

In the following example, assume that the file @file{aug-19} was written
on the 19th, @file{aug-20} was written on the 20th, and the file
@file{no-file} doesn't exist at all.

@example
@group
(file-newer-than-file-p "aug-19" "aug-20")
     @result{} nil
@end group
@group
(file-newer-than-file-p "aug-20" "aug-19")
     @result{} t
@end group
@group
(file-newer-than-file-p "aug-19" "no-file")
     @result{} t
@end group
@group
(file-newer-than-file-p "no-file" "aug-19")
     @result{} nil
@end group
@end example

You can use @code{file-attributes} to get a file's last modification
time as a list of two numbers.  @xref{File Attributes}.
@end defun

@node Kinds of Files
@comment  node-name,  next,  previous,  up
@subsection Distinguishing Kinds of Files

  This section describes how to distinguish various kinds of files, such
as directories, symbolic links, and ordinary files.

@defun file-symlink-p filename
@cindex file symbolic links
If the file @var{filename} is a symbolic link, the @code{file-symlink-p}
function returns the file name to which it is linked.  This may be the
name of a text file, a directory, or even another symbolic link, or it
may be a nonexistent file name.

If the file @var{filename} is not a symbolic link (or there is no such file),
@code{file-symlink-p} returns @code{nil}.  

@example
@group
(file-symlink-p "foo")
     @result{} nil
@end group
@group
(file-symlink-p "sym-link")
     @result{} "foo"
@end group
@group
(file-symlink-p "sym-link2")
     @result{} "sym-link"
@end group
@group
(file-symlink-p "/bin")
     @result{} "/pub/bin"
@end group
@end example

@c !!! file-symlink-p: should show output of ls -l for comparison
@end defun

@defun file-directory-p filename
This function returns @code{t} if @var{filename} is the name of an
existing directory, @code{nil} otherwise.

@example
@group
(file-directory-p "~rms")
     @result{} t
@end group
@group
(file-directory-p "~rms/lewis/files.texi")
     @result{} nil
@end group
@group
(file-directory-p "~rms/lewis/no-such-file")
     @result{} nil
@end group
@group
(file-directory-p "$HOME")
     @result{} nil
@end group
@group
(file-directory-p
 (substitute-in-file-name "$HOME"))
     @result{} t
@end group
@end example
@end defun

@defun file-regular-p filename
This function returns @code{t} if the file @var{filename} exists and is
a regular file (not a directory, named pipe, terminal, or
other I/O device).
@end defun

@node Truenames
@subsection Truenames
@cindex truename (of file)

@c Emacs 19 features
  The @dfn{truename} of a file is the name that you get by following
symbolic links at all levels until none remain, then simplifying away
@samp{.}@: and @samp{..}@: appearing as name components.  This results
in a sort of canonical name for the file.  A file does not always have a
unique truename; the number of distinct truenames a file has is equal to
the number of hard links to the file.  However, truenames are useful
because they eliminate symbolic links as a cause of name variation.

@defun file-truename filename
The function @code{file-truename} returns the truename of the file
@var{filename}.  The argument must be an absolute file name.
@end defun

@defun file-chase-links filename
This function follows symbolic links, starting with @var{filename},
until it finds a file name which is not the name of a symbolic link.
Then it returns that file name.
@end defun

  To illustrate the difference between @code{file-chase-links} and
@code{file-truename}, suppose that @file{/usr/foo} is a symbolic link to
the directory @file{/home/foo}, and @file{/home/foo/hello} is an
ordinary file (or at least, not a symbolic link) or nonexistent.  Then
we would have:

@example
(file-chase-links "/usr/foo/hello")
     ;; @r{This does not follow the links in the parent directories.}
     @result{} "/usr/foo/hello"
(file-truename "/usr/foo/hello")
     ;; @r{Assuming that @file{/home} is not a symbolic link.}
     @result{} "/home/foo/hello"
@end example

  @xref{Buffer File Name}, for related information.

@node File Attributes
@comment  node-name,  next,  previous,  up
@subsection Other Information about Files

  This section describes the functions for getting detailed information
about a file, other than its contents.  This information includes the
mode bits that control access permission, the owner and group numbers,
the number of names, the inode number, the size, and the times of access
and modification.

@defun file-modes filename
@cindex permission
@cindex file attributes
This function returns the mode bits of @var{filename}, as an integer.
The mode bits are also called the file permissions, and they specify
access control in the usual Unix fashion.  If the low-order bit is 1,
then the file is executable by all users, if the second-lowest-order bit
is 1, then the file is writable by all users, etc.

The highest value returnable is 4095 (7777 octal), meaning that
everyone has read, write, and execute permission, that the @sc{suid} bit
is set for both others and group, and that the sticky bit is set.

@example
@group
(file-modes "~/junk/diffs")
     @result{} 492               ; @r{Decimal integer.}
@end group
@group
(format "%o" 492)
     @result{} "754"             ; @r{Convert to octal.}
@end group

@group
(set-file-modes "~/junk/diffs" 438)
     @result{} nil
@end group

@group
(format "%o" 438)
     @result{} "666"             ; @r{Convert to octal.}
@end group

@group
% ls -l diffs
  -rw-rw-rw-  1 lewis 0 3063 Oct 30 16:00 diffs
@end group
@end example
@end defun

@defun file-nlinks filename
This functions returns the number of names (i.e., hard links) that
file @var{filename} has.  If the file does not exist, then this function
returns @code{nil}.  Note that symbolic links have no effect on this
function, because they are not considered to be names of the files they
link to.

@example
@group
% ls -l foo*
-rw-rw-rw-  2 rms       4 Aug 19 01:27 foo
-rw-rw-rw-  2 rms       4 Aug 19 01:27 foo1
@end group

@group
(file-nlinks "foo")
     @result{} 2
@end group
@group
(file-nlinks "doesnt-exist")
     @result{} nil
@end group
@end example
@end defun

@defun file-attributes filename
This function returns a list of attributes of file @var{filename}.  If
the specified file cannot be opened, it returns @code{nil}.

The elements of the list, in order, are:

@enumerate 0
@item
@code{t} for a directory, a string for a symbolic link (the name
linked to), or @code{nil} for a text file.

@c Wordy so as to prevent an overfull hbox.  --rjc 15mar92
@item
The number of names the file has.  Alternate names, also known as hard
links, can be created by using the @code{add-name-to-file} function
(@pxref{Changing Files}).

@item
The file's @sc{uid}.

@item
The file's @sc{gid}.

@item
The time of last access, as a list of two integers.
The first integer has the high-order 16 bits of time,
the second has the low 16 bits.  (This is similar to the
value of @code{current-time}; see @ref{Time of Day}.)

@item
The time of last modification as a list of two integers (as above).

@item
The time of last status change as a list of two integers (as above).

@item
The size of the file in bytes.  If the size is too large to fit in a
Lisp integer, this is a floating point number.

@item
The file's modes, as a string of ten letters or dashes,
as in @samp{ls -l}.

@item
@code{t} if the file's @sc{gid} would change if file were
deleted and recreated; @code{nil} otherwise.

@item
The file's inode number.  If possible, this is an integer.  If the inode
number is too large to be represented as an integer in Emacs Lisp, then
the value has the form @code{(@var{high} . @var{low})}, where @var{low}
holds the low 16 bits.

@item
The file system number of the file system that the file is in.  This
element and the file's inode number together give enough information to
distinguish any two files on the system---no two files can have the same
values for both of these numbers.
@end enumerate

For example, here are the file attributes for @file{files.texi}:

@example
@group
(file-attributes "files.texi")
     @result{}  (nil 1 2235 75 
          (8489 20284) 
          (8489 20284) 
          (8489 20285)
          14906 "-rw-rw-rw-" 
          nil 129500 -32252)
@end group
@end example

@noindent
and here is how the result is interpreted:

@table @code
@item nil
is neither a directory nor a symbolic link.

@item 1
has only one name (the name @file{files.texi} in the current default
directory).

@item 2235
is owned by the user with @sc{uid} 2235.

@item 75
is in the group with @sc{gid} 75.

@item (8489 20284)
was last accessed on Aug 19 00:09.

@item (8489 20284)
was last modified on Aug 19 00:09.

@item (8489 20285)
last had its inode changed on Aug 19 00:09.

@item 14906
is 14906 bytes long.  (It may not contain 14906 characters, though,
if some of the bytes belong to multibyte sequences.)

@item "-rw-rw-rw-"
has a mode of read and write access for the owner, group, and world.

@item nil
would retain the same @sc{gid} if it were recreated.

@item 129500
has an inode number of 129500.
@item -32252
is on file system number -32252.
@end table
@end defun

@node Changing Files
@section Changing File Names and Attributes
@cindex renaming files
@cindex copying files
@cindex deleting files
@cindex linking files
@cindex setting modes of files

  The functions in this section rename, copy, delete, link, and set the
modes of files.

  In the functions that have an argument @var{newname}, if a file by the
name of @var{newname} already exists, the actions taken depend on the
value of the argument @var{ok-if-already-exists}:

@itemize @bullet
@item
Signal a @code{file-already-exists} error if
@var{ok-if-already-exists} is @code{nil}.

@item
Request confirmation if @var{ok-if-already-exists} is a number.

@item
Replace the old file without confirmation if @var{ok-if-already-exists}
is any other value.
@end itemize

@defun add-name-to-file oldname newname &optional ok-if-already-exists
@cindex file with multiple names
@cindex file hard link
This function gives the file named @var{oldname} the additional name
@var{newname}.  This means that @var{newname} becomes a new ``hard
link'' to @var{oldname}.

In the first part of the following example, we list two files,
@file{foo} and @file{foo3}.

@example
@group
% ls -li fo*
81908 -rw-rw-rw-  1 rms       29 Aug 18 20:32 foo
84302 -rw-rw-rw-  1 rms       24 Aug 18 20:31 foo3
@end group
@end example

Now we create a hard link, by calling @code{add-name-to-file}, then list
the files again.  This shows two names for one file, @file{foo} and
@file{foo2}.

@example
@group
(add-name-to-file "foo" "foo2")
     @result{} nil
@end group

@group
% ls -li fo*
81908 -rw-rw-rw-  2 rms       29 Aug 18 20:32 foo
81908 -rw-rw-rw-  2 rms       29 Aug 18 20:32 foo2
84302 -rw-rw-rw-  1 rms       24 Aug 18 20:31 foo3
@end group
@end example

Finally, we evaluate the following:

@example
(add-name-to-file "foo" "foo3" t)
@end example

@noindent
and list the files again.  Now there are three names
for one file: @file{foo}, @file{foo2}, and @file{foo3}.  The old
contents of @file{foo3} are lost.

@example
@group
(add-name-to-file "foo1" "foo3")
     @result{} nil
@end group

@group
% ls -li fo*
81908 -rw-rw-rw-  3 rms       29 Aug 18 20:32 foo
81908 -rw-rw-rw-  3 rms       29 Aug 18 20:32 foo2
81908 -rw-rw-rw-  3 rms       29 Aug 18 20:32 foo3
@end group
@end example

This function is meaningless on operating systems where multiple names
for one file are not allowed.  Some systems implement multiple names
by copying the file instead.

See also @code{file-nlinks} in @ref{File Attributes}.
@end defun

@deffn Command rename-file filename newname &optional ok-if-already-exists
This command renames the file @var{filename} as @var{newname}.

If @var{filename} has additional names aside from @var{filename}, it
continues to have those names.  In fact, adding the name @var{newname}
with @code{add-name-to-file} and then deleting @var{filename} has the
same effect as renaming, aside from momentary intermediate states.

In an interactive call, this function prompts for @var{filename} and
@var{newname} in the minibuffer; also, it requests confirmation if
@var{newname} already exists.
@end deffn

@deffn Command copy-file oldname newname &optional ok-if-exists time
This command copies the file @var{oldname} to @var{newname}.  An
error is signaled if @var{oldname} does not exist.

If @var{time} is non-@code{nil}, then this function gives the new file
the same last-modified time that the old one has.  (This works on only
some operating systems.)  If setting the time gets an error,
@code{copy-file} signals a @code{file-date-error} error.

In an interactive call, this function prompts for @var{filename} and
@var{newname} in the minibuffer; also, it requests confirmation if
@var{newname} already exists.
@end deffn

@deffn Command delete-file filename
@pindex rm
This command deletes the file @var{filename}, like the shell command
@samp{rm @var{filename}}.  If the file has multiple names, it continues
to exist under the other names.

A suitable kind of @code{file-error} error is signaled if the file does
not exist, or is not deletable.  (On Unix and GNU/Linux, a file is
deletable if its directory is writable.)

See also @code{delete-directory} in @ref{Create/Delete Dirs}.
@end deffn

@deffn Command make-symbolic-link filename newname  &optional ok-if-exists
@pindex ln
@kindex file-already-exists
This command makes a symbolic link to @var{filename}, named
@var{newname}.  This is like the shell command @samp{ln -s
@var{filename} @var{newname}}.

In an interactive call, this function prompts for @var{filename} and
@var{newname} in the minibuffer; also, it requests confirmation if
@var{newname} already exists.

This function is not available on systems that don't support symbolic
links.
@end deffn

@defun define-logical-name varname string
This function defines the logical name @var{name} to have the value
@var{string}.  It is available only on VMS.
@end defun

@defun set-file-modes filename mode
This function sets mode bits of @var{filename} to @var{mode} (which must
be an integer).  Only the low 12 bits of @var{mode} are used.
@end defun

@c Emacs 19 feature
@defun set-default-file-modes mode
This function sets the default file protection for new files created by
Emacs and its subprocesses.  Every file created with Emacs initially has
this protection.  On Unix and GNU/Linux, the default protection is the
bitwise complement of the ``umask'' value.

The argument @var{mode} must be an integer.  On most systems, only the
low 9 bits of @var{mode} are meaningful.

Saving a modified version of an existing file does not count as creating
the file; it does not change the file's mode, and does not use the
default file protection.
@end defun

@defun default-file-modes
This function returns the current default protection value.
@end defun

@cindex MS-DOS and file modes
@cindex file modes and MS-DOS
  On MS-DOS, there is no such thing as an ``executable'' file mode bit.
So Emacs considers a file executable if its name ends in one of the
standard executable extensions, such as @file{.com}, @file{.bat},
@file{.exe}, and some others.  Files that begin with the Unix-standard
@samp{#!} signature, such as shell and Perl scripts, are also considered
as executable files.  This is reflected in the values returned by
@code{file-modes} and @code{file-attributes}.  Directories are also
reported with executable bit set, for compatibility with Unix.

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

  Files are generally referred to by their names, in Emacs as elsewhere.
File names in Emacs are represented as strings.  The functions that
operate on a file all expect a file name argument.

  In addition to operating on files themselves, Emacs Lisp programs
often need to operate on file names; i.e., to take them apart and to use
part of a name to construct related file names.  This section describes
how to manipulate file names.

  The functions in this section do not actually access files, so they
can operate on file names that do not refer to an existing file or
directory.

  On VMS, all these functions understand both VMS file-name syntax and
Unix syntax.  This is so that all the standard Lisp libraries can
specify file names in Unix syntax and work properly on VMS without
change.  On MS-DOS and MS-Windows, these functions understand MS-DOS or
MS-Windows file-name syntax as well as Unix syntax.

@menu
* File Name Components::  The directory part of a file name, and the rest.
* Directory Names::       A directory's name as a directory
                            is different from its name as a file.
* Relative File Names::   Some file names are relative to a current directory.
* File Name Expansion::   Converting relative file names to absolute ones.
* Unique File Names::     Generating names for temporary files.
* File Name Completion::  Finding the completions for a given file name.
* Standard File Names::   If your package uses a fixed file name,
                            how to handle various operating systems simply.
@end menu

@node File Name Components
@subsection File Name Components
@cindex directory part (of file name)
@cindex nondirectory part (of file name)
@cindex version number (in file name)

  The operating system groups files into directories.  To specify a
file, you must specify the directory and the file's name within that
directory.  Therefore, Emacs considers a file name as having two main
parts: the @dfn{directory name} part, and the @dfn{nondirectory} part
(or @dfn{file name within the directory}).  Either part may be empty.
Concatenating these two parts reproduces the original file name.

  On most systems, the directory part is everything up to and including
the last slash; the nondirectory part is the rest.  The rules in VMS
syntax are complicated.

  For some purposes, the nondirectory part is further subdivided into
the name proper and the @dfn{version number}.  On most systems, only
backup files have version numbers in their names.  On VMS, every file
has a version number, but most of the time the file name actually used
in Emacs omits the version number, so that version numbers in Emacs are
found mostly in directory lists.

@defun file-name-directory filename
This function returns the directory part of @var{filename} (or
@code{nil} if @var{filename} does not include a directory part).  On
most systems, the function returns a string ending in a slash.  On VMS,
it returns a string ending in one of the three characters @samp{:},
@samp{]}, or @samp{>}.

@example
@group
(file-name-directory "lewis/foo")  ; @r{Unix example}
     @result{} "lewis/"
@end group
@group
(file-name-directory "foo")        ; @r{Unix example}
     @result{} nil
@end group
@group
(file-name-directory "[X]FOO.TMP") ; @r{VMS example}
     @result{} "[X]"
@end group
@end example
@end defun

@defun file-name-nondirectory filename
This function returns the nondirectory part of @var{filename}.

@example
@group
(file-name-nondirectory "lewis/foo")
     @result{} "foo"
@end group
@group
(file-name-nondirectory "foo")
     @result{} "foo"
@end group
@group
;; @r{The following example is accurate only on VMS.}
(file-name-nondirectory "[X]FOO.TMP")
     @result{} "FOO.TMP"
@end group
@end example
@end defun

@defun file-name-sans-versions filename &optional keep-backup-version
This function returns @var{filename} with any file version numbers,
backup version numbers, or trailing tildes discarded.

If @var{keep-backup-version} is non-@code{nil}, then true file version
numbers understood as such by the file system are discarded from the
return value, but backup version numbers are kept.

@example
@group
(file-name-sans-versions "~rms/foo.~1~")
     @result{} "~rms/foo"
@end group
@group
(file-name-sans-versions "~rms/foo~")
     @result{} "~rms/foo"
@end group
@group
(file-name-sans-versions "~rms/foo")
     @result{} "~rms/foo"
@end group
@group
;; @r{The following example applies to VMS only.}
(file-name-sans-versions "foo;23")
     @result{} "foo"
@end group
@end example
@end defun

@defun file-name-sans-extension filename
This function returns @var{filename} minus its ``extension,'' if any.
The extension, in a file name, is the part that starts with the last
@samp{.} in the last name component.  For example,

@example
(file-name-sans-extension "foo.lose.c")
     @result{} "foo.lose"
(file-name-sans-extension "big.hack/foo")
     @result{} "big.hack/foo"
@end example
@end defun

@node Directory Names
@comment  node-name,  next,  previous,  up
@subsection Directory Names
@cindex directory name
@cindex file name of directory

  A @dfn{directory name} is the name of a directory.  A directory is a
kind of file, and it has a file name, which is related to the directory
name but not identical to it.  (This is not quite the same as the usual
Unix terminology.)  These two different names for the same entity are
related by a syntactic transformation.  On most systems, this is simple: a
directory name ends in a slash, whereas the directory's name as a file
lacks that slash.  On VMS, the relationship is more complicated.

  The difference between a directory name and its name as a file is
subtle but crucial.  When an Emacs variable or function argument is
described as being a directory name, a file name of a directory is not
acceptable.

  The following two functions convert between directory names and file
names.  They do nothing special with environment variable substitutions
such as @samp{$HOME}, and the constructs @samp{~}, and @samp{..}.

@defun file-name-as-directory filename
This function returns a string representing @var{filename} in a form
that the operating system will interpret as the name of a directory.  On
most systems, this means appending a slash to the string (if it does not
already end in one).  On VMS, the function converts a string of the form
@file{[X]Y.DIR.1} to the form @file{[X.Y]}.

@example
@group
(file-name-as-directory "~rms/lewis")
     @result{} "~rms/lewis/"
@end group
@end example
@end defun

@defun directory-file-name dirname
This function returns a string representing @var{dirname} in a form that
the operating system will interpret as the name of a file.  On most
systems, this means removing the final slash from the string.  On VMS,
the function converts a string of the form @file{[X.Y]} to
@file{[X]Y.DIR.1}.

@example
@group
(directory-file-name "~lewis/")
     @result{} "~lewis"
@end group
@end example
@end defun

@cindex directory name abbreviation
  Directory name abbreviations are useful for directories that are
normally accessed through symbolic links.  Sometimes the users recognize
primarily the link's name as ``the name'' of the directory, and find it
annoying to see the directory's ``real'' name.  If you define the link
name as an abbreviation for the ``real'' name, Emacs shows users the
abbreviation instead.

@defvar directory-abbrev-alist
The variable @code{directory-abbrev-alist} contains an alist of
abbreviations to use for file directories.  Each element has the form
@code{(@var{from} . @var{to})}, and says to replace @var{from} with
@var{to} when it appears in a directory name.  The @var{from} string is
actually a regular expression; it should always start with @samp{^}.
The function @code{abbreviate-file-name} performs these substitutions.

You can set this variable in @file{site-init.el} to describe the
abbreviations appropriate for your site.

Here's an example, from a system on which file system @file{/home/fsf}
and so on are normally accessed through symbolic links named @file{/fsf}
and so on.

@example
(("^/home/fsf" . "/fsf")
 ("^/home/gp" . "/gp")
 ("^/home/gd" . "/gd"))
@end example
@end defvar

  To convert a directory name to its abbreviation, use this
function:

@defun abbreviate-file-name dirname
This function applies abbreviations from @code{directory-abbrev-alist}
to its argument, and substitutes @samp{~} for the user's home
directory.
@end defun

@node Relative File Names
@subsection Absolute and Relative File Names
@cindex absolute file name
@cindex relative file name

  All the directories in the file system form a tree starting at the
root directory.  A file name can specify all the directory names
starting from the root of the tree; then it is called an @dfn{absolute}
file name.  Or it can specify the position of the file in the tree
relative to a default directory; then it is called a @dfn{relative} file
name.  On Unix and GNU/Linux, an absolute file name starts with a slash
or a tilde (@samp{~}), and a relative one does not.  On MS-DOS and
MS-Windows, an absolute file name starts with a slash or a backslash, or
with a drive specification @samp{@var{x}:/}, where @var{x} is the
@dfn{drive letter}.  The rules on VMS are complicated.

@defun file-name-absolute-p filename
This function returns @code{t} if file @var{filename} is an absolute
file name, @code{nil} otherwise.  On VMS, this function understands both
Unix syntax and VMS syntax.

@example
@group
(file-name-absolute-p "~rms/foo")
     @result{} t
@end group
@group
(file-name-absolute-p "rms/foo")
     @result{} nil
@end group
@group
(file-name-absolute-p "/user/rms/foo")
     @result{} t
@end group
@end example
@end defun

@node File Name Expansion
@subsection Functions that Expand Filenames
@cindex expansion of file names

  @dfn{Expansion} of a file name means converting a relative file name
to an absolute one.  Since this is done relative to a default directory,
you must specify the default directory name as well as the file name to
be expanded.  Expansion also simplifies file names by eliminating
redundancies such as @file{./} and @file{@var{name}/../}.

@defun expand-file-name filename &optional directory
This function converts @var{filename} to an absolute file name.  If
@var{directory} is supplied, it is the default directory to start with
if @var{filename} is relative.  (The value of @var{directory} should
itself be an absolute directory name; it may start with @samp{~}.)
Otherwise, the current buffer's value of @code{default-directory} is
used.  For example:

@example
@group
(expand-file-name "foo")
     @result{} "/xcssun/users/rms/lewis/foo"
@end group
@group
(expand-file-name "../foo")
     @result{} "/xcssun/users/rms/foo"
@end group
@group
(expand-file-name "foo" "/usr/spool/")
     @result{} "/usr/spool/foo"
@end group
@group
(expand-file-name "$HOME/foo")
     @result{} "/xcssun/users/rms/lewis/$HOME/foo"
@end group
@end example

Filenames containing @samp{.} or @samp{..} are simplified to their
canonical form:

@example
@group
(expand-file-name "bar/../foo")
     @result{} "/xcssun/users/rms/lewis/foo"
@end group
@end example

Note that @code{expand-file-name} does @emph{not} expand environment
variables; only @code{substitute-in-file-name} does that.
@end defun

@c Emacs 19 feature
@defun file-relative-name filename &optional directory
This function does the inverse of expansion---it tries to return a
relative name that is equivalent to @var{filename} when interpreted
relative to @var{directory}.  If @var{directory} is omitted or
@code{nil}, it defaults to the current buffer's default directory.

On some operating systems, an absolute file name begins with a device
name.  On such systems, @var{filename} has no relative equivalent based
on @var{directory} if they start with two different device names.  In
this case, @code{file-relative-name} returns @var{filename} in absolute
form.

@example
(file-relative-name "/foo/bar" "/foo/")
     @result{} "bar"
(file-relative-name "/foo/bar" "/hack/")
     @result{} "/foo/bar"
@end example
@end defun

@defvar default-directory
The value of this buffer-local variable is the default directory for the
current buffer.  It should be an absolute directory name; it may start
with @samp{~}.  This variable is buffer-local in every buffer.

@code{expand-file-name} uses the default directory when its second
argument is @code{nil}.

Aside from VMS, the value is always a string ending with a slash.

@example
@group
default-directory
     @result{} "/user/lewis/manual/"
@end group
@end example
@end defvar

@defun substitute-in-file-name filename
This function replaces environment variables references in
@var{filename} with the environment variable values.  Following standard
Unix shell syntax, @samp{$} is the prefix to substitute an environment
variable value.

The environment variable name is the series of alphanumeric characters
(including underscores) that follow the @samp{$}.  If the character following
the @samp{$} is a @samp{@{}, then the variable name is everything up to the
matching @samp{@}}.

@c Wordy to avoid overfull hbox.  --rjc 15mar92
Here we assume that the environment variable @code{HOME}, which holds
the user's home directory name, has value @samp{/xcssun/users/rms}.

@example
@group
(substitute-in-file-name "$HOME/foo")
     @result{} "/xcssun/users/rms/foo"
@end group
@end example

After substitution, if a @samp{~} or a @samp{/} appears following a
@samp{/}, everything before the following @samp{/} is discarded:

@example
@group
(substitute-in-file-name "bar/~/foo")
     @result{} "~/foo"
@end group
@group
(substitute-in-file-name "/usr/local/$HOME/foo")
     @result{} "/xcssun/users/rms/foo"
     ;; @r{@file{/usr/local/} has been discarded.}
@end group
@end example

On VMS, @samp{$} substitution is not done, so this function does nothing
on VMS except discard superfluous initial components as shown above.
@end defun

@node Unique File Names
@subsection Generating Unique File Names

  Some programs need to write temporary files.  Here is the usual way to
construct a name for such a file, starting in Emacs 21:

@example
(make-temp-file @var{name-of-application})
@end example

@noindent
The job of @code{make-temp-file} is to prevent two different users or
two different jobs from trying to use the exact same file name.

@defun make-temp-file prefix &optional dir-flag
@tindex make-temp-file
This function creates a temporary file and returns its name.
The name starts with @var{prefix}; it also contains a number that is
different in each Emacs job.  If @var{prefix} is a relative file name,
it is expanded against @code{temporary-file-directory}.

@example
@group
(make-temp-file "foo")
     @result{} "/tmp/foo232J6v"
@end group
@end example

When @code{make-temp-file} returns, the file has been created and is
empty.  At that point, you should write the intended contents into the
file.

If @var{dir-flag} is non-@code{nil}, @code{make-temp-file} creates
an empty directory instead of an empty file.

To prevent conflicts among different libraries running in the same
Emacs, each Lisp program that uses @code{make-temp-file} should have its
own @var{prefix}.  The number added to the end of @var{prefix}
distinguishes between the same application running in different Emacs
jobs.  Additional added characters permit a large number of distinct
names even in one Emacs job.
@end defun

  The default directory for temporary files is controlled by the
variable @code{temporary-file-directory}.  This variable gives the user
a uniform way to specify the directory for all temporary files.  Some
programs use @code{small-temporary-file-directory} instead, if that is
non-@code{nil}.  To use it, you should expand the prefix against
the proper directory before calling @code{make-temp-file}.

  In older Emacs versions where @code{make-temp-file} does not exist,
you should use @code{make-temp-name} instead:

@example
(make-temp-name
 (expand-file-name @var{name-of-application}
                   temporary-file-directory))
@end example

@defun make-temp-name string
This function generates a string that can be used as a unique file name.
The name starts with @var{string}, and contains a number that is
different in each Emacs job.  It is like @code{make-temp-file} except
that it just constructs a name, and does not create a file.  On MS-DOS,
the @var{string} prefix can be truncated to fit into the 8+3 file-name
limits.
@end defun

@defvar temporary-file-directory
@cindex @code{TMPDIR} environment variable
@cindex @code{TMP} environment variable
@cindex @code{TEMP} environment variable
This variable specifies the directory name for creating temporary files.
Its value should be a directory name (@pxref{Directory Names}), but it
is good for Lisp programs to cope if the value is a directory's file
name instead.  Using the value as the second argument to
@code{expand-file-name} is a good way to achieve that.

The default value is determined in a reasonable way for your operating
system; it is based on the @code{TMPDIR}, @code{TMP} and @code{TEMP}
environment variables, with a fall-back to a system-dependent name if
none of these variables is defined.

Even if you do not use @code{make-temp-name} to choose the temporary
file's name, you should still use this variable to decide which
directory to put the file in.  However, if you expect the file to be
small, you should use @code{small-temporary-file-directory} first if
that is non-@code{nil}.
@end defvar

@tindex small-temporary-file-directory
@defvar small-temporary-file-directory
This variable (new in Emacs 21) specifies the directory name for
creating certain temporary files, which are likely to be small.

If you want to write a temporary file which is likely to be small, you
should compute the directory like this:

@example
(make-temp-file
  (expand-file-name @var{prefix}
                    (or small-temporary-file-directory
                        temporary-file-directory)))
@end example
@end defvar

@node File Name Completion
@subsection File Name Completion
@cindex file name completion subroutines
@cindex completion, file name

  This section describes low-level subroutines for completing a file
name.  For other completion functions, see @ref{Completion}.

@defun file-name-all-completions partial-filename directory
This function returns a list of all possible completions for a file
whose name starts with @var{partial-filename} in directory
@var{directory}.  The order of the completions is the order of the files
in the directory, which is unpredictable and conveys no useful
information.

The argument @var{partial-filename} must be a file name containing no
directory part and no slash.  The current buffer's default directory is
prepended to @var{directory}, if @var{directory} is not absolute.

In the following example, suppose that @file{~rms/lewis} is the current
default directory, and has five files whose names begin with @samp{f}:
@file{foo}, @file{file~}, @file{file.c}, @file{file.c.~1~}, and
@file{file.c.~2~}.@refill

@example
@group
(file-name-all-completions "f" "")
     @result{} ("foo" "file~" "file.c.~2~" 
                "file.c.~1~" "file.c")
@end group

@group
(file-name-all-completions "fo" "")  
     @result{} ("foo")
@end group
@end example
@end defun

@defun file-name-completion filename directory
This function completes the file name @var{filename} in directory
@var{directory}.  It returns the longest prefix common to all file names
in directory @var{directory} that start with @var{filename}.

If only one match exists and @var{filename} matches it exactly, the
function returns @code{t}.  The function returns @code{nil} if directory
@var{directory} contains no name starting with @var{filename}.

In the following example, suppose that the current default directory
has five files whose names begin with @samp{f}: @file{foo},
@file{file~}, @file{file.c}, @file{file.c.~1~}, and
@file{file.c.~2~}.@refill

@example
@group
(file-name-completion "fi" "")
     @result{} "file"
@end group

@group
(file-name-completion "file.c.~1" "")
     @result{} "file.c.~1~"
@end group

@group
(file-name-completion "file.c.~1~" "")
     @result{} t
@end group

@group
(file-name-completion "file.c.~3" "")
     @result{} nil
@end group
@end example
@end defun

@defopt completion-ignored-extensions
@code{file-name-completion} usually ignores file names that end in any
string in this list.  It does not ignore them when all the possible
completions end in one of these suffixes or when a buffer showing all
possible completions is displayed.@refill

A typical value might look like this:

@example
@group
completion-ignored-extensions
     @result{} (".o" ".elc" "~" ".dvi")
@end group
@end example
@end defopt

@node Standard File Names
@subsection Standard File Names

  Most of the file names used in Lisp programs are entered by the user.
But occasionally a Lisp program needs to specify a standard file name
for a particular use---typically, to hold customization information
about each user.  For example, abbrev definitions are stored (by
default) in the file @file{~/.abbrev_defs}; the @code{completion}
package stores completions in the file @file{~/.completions}.  These are
two of the many standard file names used by parts of Emacs for certain
purposes.

  Various operating systems have their own conventions for valid file
names and for which file names to use for user profile data.  A Lisp
program which reads a file using a standard file name ought to use, on
each type of system, a file name suitable for that system.  The function
@code{convert-standard-filename} makes this easy to do.

@defun convert-standard-filename filename
This function alters the file name @var{filename} to fit the conventions
of the operating system in use, and returns the result as a new string.
@end defun

  The recommended way to specify a standard file name in a Lisp program
is to choose a name which fits the conventions of GNU and Unix systems,
usually with a nondirectory part that starts with a period, and pass it
to @code{convert-standard-filename} instead of using it directly.  Here
is an example from the @code{completion} package:

@example
(defvar save-completions-file-name
        (convert-standard-filename "~/.completions")
  "*The file name to save completions to.")
@end example

  On GNU and Unix systems, and on some other systems as well,
@code{convert-standard-filename} returns its argument unchanged.  On
some other systems, it alters the name to fit the system's conventions.

  For example, on MS-DOS the alterations made by this function include
converting a leading @samp{.}  to @samp{_}, converting a @samp{_} in the
middle of the name to @samp{.} if there is no other @samp{.}, inserting
a @samp{.} after eight characters if there is none, and truncating to
three characters after the @samp{.}.  (It makes other changes as well.)
Thus, @file{.abbrev_defs} becomes @file{_abbrev.def}, and
@file{.completions} becomes @file{_complet.ion}.

@node Contents of Directories
@section Contents of Directories
@cindex directory-oriented functions
@cindex file names in directory

  A directory is a kind of file that contains other files entered under
various names.  Directories are a feature of the file system.

  Emacs can list the names of the files in a directory as a Lisp list,
or display the names in a buffer using the @code{ls} shell command.  In
the latter case, it can optionally display information about each file,
depending on the options passed to the @code{ls} command.

@defun directory-files directory &optional full-name match-regexp nosort
This function returns a list of the names of the files in the directory
@var{directory}.  By default, the list is in alphabetical order.

If @var{full-name} is non-@code{nil}, the function returns the files'
absolute file names.  Otherwise, it returns the names relative to
the specified directory.

If @var{match-regexp} is non-@code{nil}, this function returns only
those file names that contain a match for that regular expression---the
other file names are excluded from the list.

@c Emacs 19 feature
If @var{nosort} is non-@code{nil}, @code{directory-files} does not sort
the list, so you get the file names in no particular order.  Use this if
you want the utmost possible speed and don't care what order the files
are processed in.  If the order of processing is visible to the user,
then the user will probably be happier if you do sort the names.

@example
@group
(directory-files "~lewis")
     @result{} ("#foo#" "#foo.el#" "." ".."
         "dired-mods.el" "files.texi" 
         "files.texi.~1~")
@end group
@end example

An error is signaled if @var{directory} is not the name of a directory
that can be read.
@end defun

@defun file-name-all-versions file dirname
This function returns a list of all versions of the file named
@var{file} in directory @var{dirname}.
@end defun

@tindex file-expand-wildcards
@defun file-expand-wildcards pattern &optional full
This function expands the wildcard pattern @var{pattern}, returning
a list of file names that match it.

If @var{pattern} is written as an absolute file name,
the values are absolute also.

If @var{pattern} is written as a relative file name, it is interpreted
relative to the current default directory.  The file names returned are
normally also relative to the current default directory.  However, if
@var{full} is non-@code{nil}, they are absolute.
@end defun

@defun insert-directory file switches &optional wildcard full-directory-p
This function inserts (in the current buffer) a directory listing for
directory @var{file}, formatted with @code{ls} according to
@var{switches}.  It leaves point after the inserted text.

The argument @var{file} may be either a directory name or a file
specification including wildcard characters.  If @var{wildcard} is
non-@code{nil}, that means treat @var{file} as a file specification with
wildcards.

If @var{full-directory-p} is non-@code{nil}, that means the directory
listing is expected to show the full contents of a directory.  You
should specify @code{t} when @var{file} is a directory and switches do
not contain @samp{-d}.  (The @samp{-d} option to @code{ls} says to
describe a directory itself as a file, rather than showing its
contents.)

On most systems, this function works by running a directory listing
program whose name is in the variable @code{insert-directory-program}.
If @var{wildcard} is non-@code{nil}, it also runs the shell specified by
@code{shell-file-name}, to expand the wildcards.

MS-DOS and MS-Windows systems usually lack the standard Unix program
@code{ls}, so this function emulates the standard Unix program @code{ls}
with Lisp code.
@end defun

@defvar insert-directory-program
This variable's value is the program to run to generate a directory listing
for the function @code{insert-directory}.  It is ignored on systems
which generate the listing with Lisp code.
@end defvar

@node Create/Delete Dirs
@section Creating and Deleting Directories
@c Emacs 19 features

  Most Emacs Lisp file-manipulation functions get errors when used on
files that are directories.  For example, you cannot delete a directory
with @code{delete-file}.  These special functions exist to create and
delete directories.

@defun make-directory dirname &optional parents
This function creates a directory named @var{dirname}.
If @var{parents} is non-@code{nil}, that means to create
the parent directories first, if they don't already exist.
@end defun

@defun delete-directory dirname
This function deletes the directory named @var{dirname}.  The function
@code{delete-file} does not work for files that are directories; you
must use @code{delete-directory} for them.  If the directory contains
any files, @code{delete-directory} signals an error.
@end defun

@node Magic File Names
@section Making Certain File Names ``Magic''
@cindex magic file names

@c Emacs 19 feature
  You can implement special handling for certain file names.  This is
called making those names @dfn{magic}.  The principal use for this
feature is in implementing remote file names (@pxref{Remote Files,,
Remote Files, emacs, The GNU Emacs Manual}).

  To define a kind of magic file name, you must supply a regular
expression to define the class of names (all those that match the
regular expression), plus a handler that implements all the primitive
Emacs file operations for file names that do match.

  The variable @code{file-name-handler-alist} holds a list of handlers,
together with regular expressions that determine when to apply each
handler.  Each element has this form:

@example
(@var{regexp} . @var{handler})
@end example

@noindent
All the Emacs primitives for file access and file name transformation
check the given file name against @code{file-name-handler-alist}.  If
the file name matches @var{regexp}, the primitives handle that file by
calling @var{handler}.

The first argument given to @var{handler} is the name of the primitive;
the remaining arguments are the arguments that were passed to that
operation.  (The first of these arguments is typically the file name
itself.)  For example, if you do this:

@example
(file-exists-p @var{filename})
@end example

@noindent
and @var{filename} has handler @var{handler}, then @var{handler} is
called like this:

@example
(funcall @var{handler} 'file-exists-p @var{filename})
@end example

Here are the operations that a magic file name handler gets to handle:

@ifinfo
@noindent
@code{add-name-to-file}, @code{copy-file}, @code{delete-directory},
@code{delete-file},
@code{diff-latest-backup-file},
@code{directory-file-name},
@code{directory-files},
@code{dired-call-process},
@code{dired-compress-file}, @code{dired-uncache},
@code{expand-file-name},
@code{file-accessible-directory-p},@*
@code{file-attributes},
@code{file-directory-p},
@code{file-executable-p}, @code{file-exists-p},@*
@code{file-local-copy},
@code{file-modes}, @code{file-name-all-completions},@*
@code{file-name-as-directory},
@code{file-name-completion},
@code{file-name-directory},
@code{file-name-nondirectory},
@code{file-name-sans-versions}, @code{file-newer-than-file-p},
@code{file-ownership-preserved-p},
@code{file-readable-p}, @code{file-regular-p}, @code{file-symlink-p},
@code{file-truename}, @code{file-writable-p},
@code{find-backup-file-name},
@code{get-file-buffer},@*
@code{insert-directory},
@code{insert-file-contents},
@code{load}, @code{make-directory},
@code{make-symbolic-link}, @code{rename-file}, @code{set-file-modes},
@code{set-visited-file-modtime}, @code{shell-command},@*
@code{unhandled-file-name-directory},
@code{vc-registered},
@code{verify-visited-file-modtime},@*
@code{write-region}.
@end ifinfo
@iftex
@noindent
@flushleft
@code{add-name-to-file}, @code{copy-file}, @code{delete-directory},
@code{delete-file},
@code{diff-latest-backup-file},
@code{directory-file-name},
@code{directory-files},
@code{dired-call-process},
@code{dired-compress-file}, @code{dired-uncache},
@code{expand-file-name},
@code{file-accessible-direc@discretionary{}{}{}tory-p},
@code{file-attributes},
@code{file-direct@discretionary{}{}{}ory-p},
@code{file-executable-p}, @code{file-exists-p},
@code{file-local-copy},
@code{file-modes}, @code{file-name-all-completions},
@code{file-name-as-directory},
@code{file-name-completion},
@code{file-name-directory},
@code{file-name-nondirec@discretionary{}{}{}tory},
@code{file-name-sans-versions}, @code{file-newer-than-file-p},
@code{file-ownership-pre@discretionary{}{}{}served-p},
@code{file-readable-p}, @code{file-regular-p}, @code{file-symlink-p},
@code{file-truename}, @code{file-writable-p},
@code{find-backup-file-name},
@code{get-file-buffer},
@code{insert-directory},
@code{insert-file-contents},
@code{load}, @code{make-direc@discretionary{}{}{}tory},
@code{make-symbolic-link}, @code{rename-file}, @code{set-file-modes},
@code{set-visited-file-modtime}, @code{shell-command},
@code{unhandled-file-name-directory},
@code{vc-regis@discretionary{}{}{}tered},
@code{verify-visited-file-modtime},
@code{write-region}.
@end flushleft
@end iftex

Handlers for @code{insert-file-contents} typically need to clear the
buffer's modified flag, with @code{(set-buffer-modified-p nil)}, if the
@var{visit} argument is non-@code{nil}.  This also has the effect of
unlocking the buffer if it is locked.

The handler function must handle all of the above operations, and
possibly others to be added in the future.  It need not implement all
these operations itself---when it has nothing special to do for a
certain operation, it can reinvoke the primitive, to handle the
operation ``in the usual way''.  It should always reinvoke the primitive
for an operation it does not recognize.  Here's one way to do this:

@smallexample
(defun my-file-handler (operation &rest args)
  ;; @r{First check for the specific operations}
  ;; @r{that we have special handling for.}
  (cond ((eq operation 'insert-file-contents) @dots{})
        ((eq operation 'write-region) @dots{})
        @dots{}
        ;; @r{Handle any operation we don't know about.}
        (t (let ((inhibit-file-name-handlers
                  (cons 'my-file-handler 
                        (and (eq inhibit-file-name-operation operation)
                             inhibit-file-name-handlers)))
                 (inhibit-file-name-operation operation))
             (apply operation args)))))
@end smallexample

When a handler function decides to call the ordinary Emacs primitive for
the operation at hand, it needs to prevent the primitive from calling
the same handler once again, thus leading to an infinite recursion.  The
example above shows how to do this, with the variables
@code{inhibit-file-name-handlers} and
@code{inhibit-file-name-operation}.  Be careful to use them exactly as
shown above; the details are crucial for proper behavior in the case of
multiple handlers, and for operations that have two file names that may
each have handlers.

@defvar inhibit-file-name-handlers
This variable holds a list of handlers whose use is presently inhibited
for a certain operation.
@end defvar

@defvar inhibit-file-name-operation
The operation for which certain handlers are presently inhibited.
@end defvar

@defun find-file-name-handler file operation
This function returns the handler function for file name @var{file}, or
@code{nil} if there is none.  The argument @var{operation} should be the
operation to be performed on the file---the value you will pass to the
handler as its first argument when you call it.  The operation is needed
for comparison with @code{inhibit-file-name-operation}.
@end defun

@defun file-local-copy filename
This function copies file @var{filename} to an ordinary non-magic file,
if it isn't one already.

If @var{filename} specifies a magic file name, which programs
outside Emacs cannot directly read or write, this copies the contents to
an ordinary file and returns that file's name.

If @var{filename} is an ordinary file name, not magic, then this function
does nothing and returns @code{nil}.
@end defun

@defun unhandled-file-name-directory filename
This function returns the name of a directory that is not magic.  It
uses the directory part of @var{filename} if that is not magic.  For a
magic file name, it invokes the file name handler, which therefore
decides what value to return.

This is useful for running a subprocess; every subprocess must have a
non-magic directory to serve as its current directory, and this function
is a good way to come up with one.
@end defun

@node Format Conversion
@section File Format Conversion

@cindex file format conversion
@cindex encoding file formats
@cindex decoding file formats
  The variable @code{format-alist} defines a list of @dfn{file formats},
which describe textual representations used in files for the data (text,
text-properties, and possibly other information) in an Emacs buffer.
Emacs performs format conversion if appropriate when reading and writing
files.

@defvar format-alist
This list contains one format definition for each defined file format.
@end defvar

@cindex format definition
Each format definition is a list of this form:

@example
(@var{name} @var{doc-string} @var{regexp} @var{from-fn} @var{to-fn} @var{modify} @var{mode-fn})
@end example

Here is what the elements in a format definition mean:

@table @var
@item name
The name of this format.

@item doc-string
A documentation string for the format.

@item regexp
A regular expression which is used to recognize files represented in
this format.

@item from-fn
A shell command or function to decode data in this format (to convert
file data into the usual Emacs data representation).

A shell command is represented as a string; Emacs runs the command as a
filter to perform the conversion.

If @var{from-fn} is a function, it is called with two arguments, @var{begin}
and @var{end}, which specify the part of the buffer it should convert.
It should convert the text by editing it in place.  Since this can
change the length of the text, @var{from-fn} should return the modified
end position.

One responsibility of @var{from-fn} is to make sure that the beginning
of the file no longer matches @var{regexp}.  Otherwise it is likely to
get called again.

@item to-fn
A shell command or function to encode data in this format---that is, to
convert the usual Emacs data representation into this format.

If @var{to-fn} is a string, it is a shell command; Emacs runs the
command as a filter to perform the conversion.

If @var{to-fn} is a function, it is called with two arguments, @var{begin}
and @var{end}, which specify the part of the buffer it should convert.
There are two ways it can do the conversion:

@itemize @bullet
@item
By editing the buffer in place.  In this case, @var{to-fn} should
return the end-position of the range of text, as modified.

@item
By returning a list of annotations.  This is a list of elements of the
form @code{(@var{position} . @var{string})}, where @var{position} is an
integer specifying the relative position in the text to be written, and
@var{string} is the annotation to add there.  The list must be sorted in
order of position when @var{to-fn} returns it.

When @code{write-region} actually writes the text from the buffer to the
file, it intermixes the specified annotations at the corresponding
positions.  All this takes place without modifying the buffer.
@end itemize

@item modify
A flag, @code{t} if the encoding function modifies the buffer, and
@code{nil} if it works by returning a list of annotations.

@item mode
A mode function to call after visiting a file converted from this
format.
@end table

The function @code{insert-file-contents} automatically recognizes file
formats when it reads the specified file.  It checks the text of the
beginning of the file against the regular expressions of the format
definitions, and if it finds a match, it calls the decoding function for
that format.  Then it checks all the known formats over again.
It keeps checking them until none of them is applicable.

Visiting a file, with @code{find-file-noselect} or the commands that use
it, performs conversion likewise (because it calls
@code{insert-file-contents}); it also calls the mode function for each
format that it decodes.  It stores a list of the format names in the
buffer-local variable @code{buffer-file-format}.

@defvar buffer-file-format
This variable states the format of the visited file.  More precisely,
this is a list of the file format names that were decoded in the course
of visiting the current buffer's file.  It is always buffer-local in all
buffers.
@end defvar

When @code{write-region} writes data into a file, it first calls the
encoding functions for the formats listed in @code{buffer-file-format},
in the order of appearance in the list.

@deffn Command format-write-file file format
This command writes the current buffer contents into the file @var{file}
in format @var{format}, and makes that format the default for future
saves of the buffer.  The argument @var{format} is a list of format
names.
@end deffn

@deffn Command format-find-file file format
This command finds the file @var{file}, converting it according to
format @var{format}.  It also makes @var{format} the default if the
buffer is saved later.

The argument @var{format} is a list of format names.  If @var{format} is
@code{nil}, no conversion takes place.  Interactively, typing just
@key{RET} for @var{format} specifies @code{nil}.
@end deffn

@deffn Command format-insert-file file format &optional beg end
This command inserts the contents of file @var{file}, converting it
according to format @var{format}.  If @var{beg} and @var{end} are
non-@code{nil}, they specify which part of the file to read, as in
@code{insert-file-contents} (@pxref{Reading from Files}).

The return value is like what @code{insert-file-contents} returns: a
list of the absolute file name and the length of the data inserted
(after conversion).

The argument @var{format} is a list of format names.  If @var{format} is
@code{nil}, no conversion takes place.  Interactively, typing just
@key{RET} for @var{format} specifies @code{nil}.
@end deffn

@defvar auto-save-file-format
This variable specifies the format to use for auto-saving.  Its value is
a list of format names, just like the value of
@code{buffer-file-format}; however, it is used instead of
@code{buffer-file-format} for writing auto-save files.  This variable is
always buffer-local in all buffers.
@end defvar