root/trunk/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, 1999, 2001,
@c   2002, 2003, 2004, 2005, 2006, 2007, 2008  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.

Aside from some technical details, the body of the @code{find-file}
function is basically equivalent to:

@smallexample
(switch-to-buffer (find-file-noselect filename nil nil wildcards))
@end smallexample

@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
returns a buffer visiting the file @var{filename}.  You may make the
buffer current or display it in a window if you wish, but this
function does not do so.

The function returns an existing buffer if there is one; otherwise it
creates a new buffer and reads the file into it.  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, this function asks the user
whether to reread the changed file.  If the user says @samp{yes}, any
edits previously made in the buffer are lost.

Reading the file involves decoding the file's contents (@pxref{Coding
Systems}), including end-of-line conversion, and format conversion
(@pxref{Format Conversion}).  If @var{wildcards} is non-@code{nil},
then @code{find-file-noselect} expands wildcard characters in
@var{filename} and visits all the matching files.

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-hook}.

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-functions} are not run in case of failure.
What's more, a non-@code{nil} @var{rawfile} value suppresses coding
system conversion and format conversion.

The @code{find-file-noselect} function usually returns the buffer that
is visiting the file @var{filename}.  But, if wildcards are actually
used and expanded, it returns a list of buffers that are visiting the
various files.

@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

@defopt 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 (when invoked interactively or when their @var{wildcards}
argument is non-@code{nil}).  If this option is @code{nil}, then
the @code{find-file} commands ignore their @var{wildcards} argument
and never treat wildcard characters specially.
@end defopt

@defvar find-file-hook
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 is a normal hook.  @xref{Hooks}.
@end defvar

@defvar find-file-not-found-functions
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-hook}.  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-hook}.
@end defun

@node Saving Buffers
@section Saving Buffers
@cindex 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.

@item
With an argument of 0, unconditionally do @emph{not} make any backup file.
@end itemize
@end deffn

@deffn Command save-some-buffers &optional save-silently-p pred
@anchor{Definition of save-some-buffers}
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
(or to save silently if @var{save-silently-p} is non-@code{nil}).
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} (@pxref{Killing Buffers}).  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
@anchor{Definition of write-file}
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.  Interactively, confirmation is
required, unless the user supplies a prefix argument.

If @var{filename} is an existing directory, or a symbolic link to one,
@code{write-file} uses the name of the visited file, in directory
@var{filename}.  If the buffer is not visiting a file, it uses the
buffer name instead.
@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-functions
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-functions} 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 (if non-@code{nil}) to set the mode
bits of the file that you write.  This is what @code{save-buffer}
normally does. @xref{Making Backups,, Making Backup Files}.

The hook functions in @code{write-file-functions} are also responsible
for encoding the data (if desired): they must choose a suitable coding
system and end-of-line conversion (@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}).

If you set this hook locally in a buffer, it is assumed to be
associated with the file or the way the contents of the buffer were
obtained.  Thus the variable is marked as a permanent local, so that
changing the major mode does not alter a buffer-local value.  On the
other hand, calling @code{set-visited-file-name} will reset it.
If this is not what you want, you might like to use
@code{write-contents-functions} 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 write-contents-functions
This works just like @code{write-file-functions}, but it is intended
for hooks that pertain to the buffer's contents, not to the particular
visited file or its location.  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, but calling
@code{set-visited-file-name} does not.

If any of the functions in this hook returns non-@code{nil}, the file
is considered already written and the rest are not called and neither
are the functions in @code{write-file-functions}.
@end defvar

@defopt before-save-hook
This normal hook runs before a buffer is saved in its visited file,
regardless of whether that is done normally or by one of the hooks
described above.  For instance, the @file{copyright.el} program uses
this hook to make sure the file you are saving has the current year in
its copyright notice.
@end defopt

@c Emacs 19 feature
@defopt 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 defopt

@defopt 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 defopt

@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
@cindex 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,
including end-of-line conversion.

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
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-hook}, 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
@cindex 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.

When called from Lisp, this function is completely equivalent to:

@example
(write-region start end filename t)
@end example
@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}.

If @var{start} is @code{nil}, then the command writes the entire buffer
contents (@emph{not} just the accessible portion) to the file and
ignores @var{end}.

@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{append} is an
integer, @code{write-region} seeks to that byte offset from the start
of the file and writes the data from there.

If @var{mustbenew} is non-@code{nil}, then @code{write-region} asks
for confirmation if @var{filename} names an existing file.  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@dots{}
@anchor{Definition of 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{Definition of
with-temp-buffer,, The Current Buffer}.
@end defmac

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

  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.  (File locks are not implemented on Microsoft systems.)
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, or if the system does not support locking.
@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, or if the
system does not support locking.
@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
@cindex file, information about

  The functions described in this section all operate on strings that
designate file names.  With a few exceptions, 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.
* Locating Files::          How to find a file in standard places.
@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.  Unless explicitly stated otherwise, they recursively follow
symbolic links for their file name arguments, at all levels (at the
level of the file itself and at all levels of parent directories).

@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}.

Directories are files, so @code{file-exists-p} returns @code{t} when
given a directory name.  However, symbolic links are treated
specially; @code{file-exists-p} returns @code{t} for a symbolic link
name only if the target file exists.
@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 (such as
@file{/foo/}) or the file name of a file which is a directory
(such as @file{/foo}, without the final slash).

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
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.  It also
returns @code{t} for nonexistent files.

If @var{filename} is a symbolic link, then, unlike the other functions
discussed here, @code{file-ownership-preserved-p} does @emph{not}
replace @var{filename} with its target.  However, it does recursively
follow symbolic links at all levels of parent directories.
@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{filename1} does exist, but
@var{filename2} does not, 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 (non-recursive) link target
as a string.  (Determining the file name that the link points to from
the target is nontrivial.)  First, this function recursively follows
symbolic links at all levels of parent directories.

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

The next two functions recursively follow symbolic links at
all levels for @var{filename}.

@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.

This function does not expand environment variables.  Only
@code{substitute-in-file-name} does that.  @xref{Definition of
substitute-in-file-name}.

If you may need to follow symbolic links preceding @samp{..}@:
appearing as a name component, you should make sure to call
@code{file-truename} without prior direct or indirect calls to
@code{expand-file-name}, as otherwise the file name component
immediately preceding @samp{..} will be ``simplified away'' before
@code{file-truename} is called.  To eliminate the need for a call to
@code{expand-file-name}, @code{file-truename} handles @samp{~} in the
same way that @code{expand-file-name} does.  @xref{File Name
Expansion,, Functions that Expand Filenames}.
@end defun

@defun file-chase-links filename &optional limit
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.  This function does @emph{not} follow
symbolic links at the level of parent directories.

If you specify a number for @var{limit}, then after chasing through
that many links, the function just returns what it has even if that is
still a symbolic link.
@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 @acronym{SUID} bit
is set for both others and group, and that the sticky bit is set.

If @var{filename} does not exist, @code{file-modes} returns @code{nil}.

This function recursively follows symbolic links at all levels.

@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

If the @var{filename} argument to the next two functions is a symbolic
link, then these function do @emph{not} replace it with its target.
However, they both recursively follow symbolic links at all levels of
parent directories.

@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 &optional id-format
@anchor{Definition of file-attributes}
This function returns a list of attributes of file @var{filename}.  If
the specified file cannot be opened, it returns @code{nil}.
The optional parameter @var{id-format} specifies the preferred format
of attributes @acronym{UID} and @acronym{GID} (see below)---the
valid values are @code{'string} and @code{'integer}.  The latter is
the default, but we plan to change that, so you should specify a
non-@code{nil} value for @var{id-format} if you use the returned
@acronym{UID} or @acronym{GID}.

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 @acronym{UID}, normally as a string.  However, if it does
not correspond to a named user, the value is an integer or a floating
point number.

@item
The file's @acronym{GID}, likewise.

@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).
@cindex modification time of file

@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 @acronym{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.
Depending on the magnitude of the value, this can be either an integer
or a cons cell, in the same manner as the inode number.  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