Mon Dec 18 13:40:37 1995 Roland McGrath <roland@churchy.gnu.ai.mit.edu>

[glibc.git] / manual / filesys.texi

@node File System Interface, Pipes and FIFOs, Low-Level I/O, Top
@chapter File System Interface

This chapter describes the GNU C library's functions for manipulating
files.  Unlike the input and output functions described in
@ref{I/O on Streams} and @ref{Low-Level I/O}, these
functions are concerned with operating on the files themselves, rather
than on their contents.

Among the facilities described in this chapter are functions for
examining or modifying directories, functions for renaming and deleting
files, and functions for examining and setting file attributes such as
access permissions and modification times.

@menu
* Working Directory::           This is used to resolve relative
                                 file names.
* Accessing Directories::       Finding out what files a directory
                                 contains.
* Hard Links::                  Adding alternate names to a file.
* Symbolic Links::              A file that ``points to'' a file name.
* Deleting Files::              How to delete a file, and what that means.
* Renaming Files::              Changing a file's name.
* Creating Directories::        A system call just for creating a directory.
* File Attributes::             Attributes of individual files.
* Making Special Files::        How to create special files.
* Temporary Files::             Naming and creating temporary files.
@end menu

@node Working Directory
@section Working Directory

@cindex current working directory
@cindex working directory
@cindex change working directory
Each process has associated with it a directory, called its @dfn{current
working directory} or simply @dfn{working directory}, that is used in
the resolution of relative file names (@pxref{File Name Resolution}).

When you log in and begin a new session, your working directory is
initially set to the home directory associated with your login account
in the system user database.  You can find any user's home directory
using the @code{getpwuid} or @code{getpwnam} functions; see @ref{User
Database}.

Users can change the working directory using shell commands like
@code{cd}.  The functions described in this section are the primitives
used by those commands and by other programs for examining and changing
the working directory.
@pindex cd

Prototypes for these functions are declared in the header file
@file{unistd.h}.
@pindex unistd.h

@comment unistd.h
@comment POSIX.1
@deftypefun {char *} getcwd (char *@var{buffer}, size_t @var{size})
The @code{getcwd} function returns an absolute file name representing
the current working directory, storing it in the character array
@var{buffer} that you provide.  The @var{size} argument is how you tell
the system the allocation size of @var{buffer}.

The GNU library version of this function also permits you to specify a
null pointer for the @var{buffer} argument.  Then @code{getcwd}
allocates a buffer automatically, as with @code{malloc}
(@pxref{Unconstrained Allocation}).  If the @var{size} is greater than
zero, then the buffer is that large; otherwise, the buffer is as large
as necessary to hold the result.

The return value is @var{buffer} on success and a null pointer on failure.
The following @code{errno} error conditions are defined for this function:

@table @code
@item EINVAL
The @var{size} argument is zero and @var{buffer} is not a null pointer.

@item ERANGE
The @var{size} argument is less than the length of the working directory
name.  You need to allocate a bigger array and try again.

@item EACCES
Permission to read or search a component of the file name was denied.
@end table
@end deftypefun

Here is an example showing how you could implement the behavior of GNU's
@w{@code{getcwd (NULL, 0)}} using only the standard behavior of
@code{getcwd}:

@smallexample
char *
gnu_getcwd ()
@{
  int size = 100;
  char *buffer = (char *) xmalloc (size);

  while (1)
    @{
      char *value = getcwd (buffer, size);
      if (value != 0)
        return buffer;
      size *= 2;
      free (buffer);
      buffer = (char *) xmalloc (size);
    @}
@}
@end smallexample

@noindent
@xref{Malloc Examples}, for information about @code{xmalloc}, which is
not a library function but is a customary name used in most GNU
software.

@comment unistd.h
@comment BSD
@deftypefun {char *} getwd (char *@var{buffer})
This is similar to @code{getcwd}, but has no way to specify the size of
the buffer.  The GNU library provides @code{getwd} only
for backwards compatibility with BSD.

The @var{buffer} argument should be a pointer to an array at least
@code{PATH_MAX} bytes long (@pxref{Limits for Files}).  In the GNU
system there is no limit to the size of a file name, so this is not
necessarily enough space to contain the directory name.  That is why
this function is deprecated.
@end deftypefun

@comment unistd.h
@comment POSIX.1
@deftypefun int chdir (const char *@var{filename})
This function is used to set the process's working directory to
@var{filename}.

The normal, successful return value from @code{chdir} is @code{0}.  A
value of @code{-1} is returned to indicate an error.  The @code{errno}
error conditions defined for this function are the usual file name
syntax errors (@pxref{File Name Errors}), plus @code{ENOTDIR} if the
file @var{filename} is not a directory.
@end deftypefun


@node Accessing Directories
@section Accessing Directories
@cindex accessing directories
@cindex reading from a directory
@cindex directories, accessing

The facilities described in this section let you read the contents of a
directory file.  This is useful if you want your program to list all the
files in a directory, perhaps as part of a menu.

@cindex directory stream
The @code{opendir} function opens a @dfn{directory stream} whose
elements are directory entries.  You use the @code{readdir} function on
the directory stream to retrieve these entries, represented as
@w{@code{struct dirent}} objects.  The name of the file for each entry is
stored in the @code{d_name} member of this structure.  There are obvious
parallels here to the stream facilities for ordinary files, described in
@ref{I/O on Streams}.

@menu
* Directory Entries::           Format of one directory entry.
* Opening a Directory::         How to open a directory stream.
* Reading/Closing Directory::   How to read directory entries from the stream.
* Simple Directory Lister::     A very simple directory listing program.
* Random Access Directory::     Rereading part of the directory
                                 already read with the same stream.
@end menu

@node Directory Entries
@subsection Format of a Directory Entry

@pindex dirent.h
This section describes what you find in a single directory entry, as you
might obtain it from a directory stream.  All the symbols are declared
in the header file @file{dirent.h}.

@comment dirent.h
@comment POSIX.1
@deftp {Data Type} {struct dirent}
This is a structure type used to return information about directory
entries.  It contains the following fields:

@table @code
@item char d_name[]
This is the null-terminated file name component.  This is the only
field you can count on in all POSIX systems.

@item ino_t d_fileno
This is the file serial number.  For BSD compatibility, you can also
refer to this member as @code{d_ino}.  In the GNU system and most POSIX
systems, for most files this the same as the @code{st_ino} member that
@code{stat} will return for the file.  @xref{File Attributes}.

@item unsigned char d_namlen
This is the length of the file name, not including the terminating null
character.  Its type is @code{unsigned char} because that is the integer
type of the appropriate size

@item unsigned char d_type
This is the type of the file, possibly unknown.  The following constants
are defined for its value:

@table @code
@item DT_UNKNOWN
The type is unknown.  On some systems this is the only value returned.

@item DT_REG
A regular file.

@item DT_DIR
A directory.

@item DT_FIFO
A named pipe, or FIFO.  @xref{FIFO Special Files}.

@item DT_SOCK
A local-domain socket.  @c !!! @xref{Local Domain}.

@item DT_CHR
A character device.

@item DT_BLK
A block device.
@end table

This member is a BSD extension.  Each value except DT_UNKNOWN
corresponds to the file type bits in the @code{st_mode} member of
@code{struct statbuf}.  These two macros convert between @code{d_type}
values and @code{st_mode} values:

@deftypefun int IFTODT (mode_t @var{mode})
This returns the @code{d_type} value corresponding to @var{mode}.
@end deftypefun

@deftypefun mode_t DTTOIF (int @var{dirtype})
This returns the @code{st_mode} value corresponding to @var{dirtype}.
@end deftypefun
@end table

This structure may contain additional members in the future.

When a file has multiple names, each name has its own directory entry.
The only way you can tell that the directory entries belong to a
single file is that they have the same value for the @code{d_fileno}
field.

File attributes such as size, modification times, and the like are part
of the file itself, not any particular directory entry.  @xref{File
Attributes}.
@end deftp

@node Opening a Directory
@subsection Opening a Directory Stream

@pindex dirent.h
This section describes how to open a directory stream.  All the symbols
are declared in the header file @file{dirent.h}.

@comment dirent.h
@comment POSIX.1
@deftp {Data Type} DIR
The @code{DIR} data type represents a directory stream.  
@end deftp

You shouldn't ever allocate objects of the @code{struct dirent} or
@code{DIR} data types, since the directory access functions do that for
you.  Instead, you refer to these objects using the pointers returned by
the following functions.

@comment dirent.h
@comment POSIX.1
@deftypefun {DIR *} opendir (const char *@var{dirname})
The @code{opendir} function opens and returns a directory stream for
reading the directory whose file name is @var{dirname}.  The stream has
type @code{DIR *}.

If unsuccessful, @code{opendir} returns a null pointer.  In addition to
the usual file name errors (@pxref{File Name Errors}), the
following @code{errno} error conditions are defined for this function:

@table @code
@item EACCES
Read permission is denied for the directory named by @code{dirname}.

@item EMFILE
The process has too many files open.

@item ENFILE
The entire system, or perhaps the file system which contains the
directory, cannot support any additional open files at the moment.
(This problem cannot happen on the GNU system.)
@end table

The @code{DIR} type is typically implemented using a file descriptor,
and the @code{opendir} function in terms of the @code{open} function.
@xref{Low-Level I/O}.  Directory streams and the underlying
file descriptors are closed on @code{exec} (@pxref{Executing a File}).
@end deftypefun

@node Reading/Closing Directory
@subsection Reading and Closing a Directory Stream

@pindex dirent.h
This section describes how to read directory entries from a directory
stream, and how to close the stream when you are done with it.  All the
symbols are declared in the header file @file{dirent.h}.

@comment dirent.h
@comment POSIX.1
@deftypefun {struct dirent *} readdir (DIR *@var{dirstream})
This function reads the next entry from the directory.  It normally
returns a pointer to a structure containing information about the file.
This structure is statically allocated and can be rewritten by a
subsequent call.

@strong{Portability Note:} On some systems, @code{readdir} may not
return entries for @file{.} and @file{..}, even though these are always
valid file names in any directory.  @xref{File Name Resolution}.

If there are no more entries in the directory or an error is detected,
@code{readdir} returns a null pointer.  The following @code{errno} error
conditions are defined for this function:

@table @code
@item EBADF
The @var{dirstream} argument is not valid.
@end table
@end deftypefun

@comment dirent.h
@comment POSIX.1
@deftypefun int closedir (DIR *@var{dirstream})
This function closes the directory stream @var{dirstream}.  It returns
@code{0} on success and @code{-1} on failure.  

The following @code{errno} error conditions are defined for this
function:

@table @code
@item EBADF
The @var{dirstream} argument is not valid.
@end table
@end deftypefun

@node Simple Directory Lister
@subsection Simple Program to List a Directory

Here's a simple program that prints the names of the files in
the current working directory:

@smallexample
@include dir.c.texi
@end smallexample

The order in which files appear in a directory tends to be fairly
random.  A more useful program would sort the entries (perhaps by
alphabetizing them) before printing them; see @ref{Array Sort Function}.

@c ??? not documented: scandir, alphasort

@node Random Access Directory
@subsection Random Access in a Directory Stream

@pindex dirent.h
This section describes how to reread parts of a directory that you have
already read from an open directory stream.  All the symbols are
declared in the header file @file{dirent.h}.

@comment dirent.h
@comment POSIX.1
@deftypefun void rewinddir (DIR *@var{dirstream})
The @code{rewinddir} function is used to reinitialize the directory
stream @var{dirstream}, so that if you call @code{readdir} it
returns information about the first entry in the directory again.  This
function also notices if files have been added or removed to the
directory since it was opened with @code{opendir}.  (Entries for these
files might or might not be returned by @code{readdir} if they were
added or removed since you last called @code{opendir} or
@code{rewinddir}.)
@end deftypefun

@comment dirent.h
@comment BSD
@deftypefun off_t telldir (DIR *@var{dirstream})
The @code{telldir} function returns the file position of the directory
stream @var{dirstream}.  You can use this value with @code{seekdir} to
restore the directory stream to that position.
@end deftypefun

@comment dirent.h
@comment BSD
@deftypefun void seekdir (DIR *@var{dirstream}, off_t @var{pos})
The @code{seekdir} function sets the file position of the directory
stream @var{dirstream} to @var{pos}.  The value @var{pos} must be the
result of a previous call to @code{telldir} on this particular stream;
closing and reopening the directory can invalidate values returned by
@code{telldir}.
@end deftypefun

@node Hard Links
@section Hard Links
@cindex hard link
@cindex link, hard
@cindex multiple names for one file
@cindex file names, multiple

In POSIX systems, one file can have many names at the same time.  All of
the names are equally real, and no one of them is preferred to the
others.

To add a name to a file, use the @code{link} function.  (The new name is
also called a @dfn{hard link} to the file.)  Creating a new link to a
file does not copy the contents of the file; it simply makes a new name
by which the file can be known, in addition to the file's existing name
or names.

One file can have names in several directories, so the the organization
of the file system is not a strict hierarchy or tree.

In most implementations, it is not possible to have hard links to the
same file in multiple file systems.  @code{link} reports an error if you
try to make a hard link to the file from another file system when this
cannot be done.

The prototype for the @code{link} function is declared in the header
file @file{unistd.h}.
@pindex unistd.h

@comment unistd.h
@comment POSIX.1
@deftypefun int link (const char *@var{oldname}, const char *@var{newname})
The @code{link} function makes a new link to the existing file named by
@var{oldname}, under the new name @var{newname}.

This function returns a value of @code{0} if it is successful and
@code{-1} on failure.  In addition to the usual file name errors
(@pxref{File Name Errors}) for both @var{oldname} and @var{newname}, the
following @code{errno} error conditions are defined for this function:

@table @code
@item EACCES
You are not allowed to write the directory in which the new link is to
be written.
@ignore 
Some implementations also require that the existing file be accessible
by the caller, and use this error to report failure for that reason.
@end ignore

@item EEXIST
There is already a file named @var{newname}.  If you want to replace
this link with a new link, you must remove the old link explicitly first.

@item EMLINK
There are already too many links to the file named by @var{oldname}.
(The maximum number of links to a file is @w{@code{LINK_MAX}}; see
@ref{Limits for Files}.)

@item ENOENT
The file named by @var{oldname} doesn't exist.  You can't make a link to
a file that doesn't exist.

@item ENOSPC
The directory or file system that would contain the new link is full
and cannot be extended.

@item EPERM
In the GNU system and some others, you cannot make links to directories.
Many systems allow only privileged users to do so.  This error
is used to report the problem.

@item EROFS
The directory containing the new link can't be modified because it's on
a read-only file system.

@item EXDEV
The directory specified in @var{newname} is on a different file system
than the existing file.

@item EIO
A hardware error occurred while trying to read or write the to filesystem.
@end table
@end deftypefun

@node Symbolic Links
@section Symbolic Links
@cindex soft link
@cindex link, soft
@cindex symbolic link
@cindex link, symbolic

The GNU system supports @dfn{soft links} or @dfn{symbolic links}.  This
is a kind of ``file'' that is essentially a pointer to another file
name.  Unlike hard links, symbolic links can be made to directories or
across file systems with no restrictions.  You can also make a symbolic
link to a name which is not the name of any file.  (Opening this link
will fail until a file by that name is created.)  Likewise, if the
symbolic link points to an existing file which is later deleted, the
symbolic link continues to point to the same file name even though the
name no longer names any file.

The reason symbolic links work the way they do is that special things
happen when you try to open the link.  The @code{open} function realizes
you have specified the name of a link, reads the file name contained in
the link, and opens that file name instead.  The @code{stat} function
likewise operates on the file that the symbolic link points to, instead
of on the link itself.

By contrast, other operations such as deleting or renaming the file
operate on the link itself.  The functions @code{readlink} and
@code{lstat} also refrain from following symbolic links, because their
purpose is to obtain information about the link.  So does @code{link},
the function that makes a hard link---it makes a hard link to the
symbolic link, which one rarely wants.

Prototypes for the functions listed in this section are in
@file{unistd.h}.
@pindex unistd.h

@comment unistd.h
@comment BSD
@deftypefun int symlink (const char *@var{oldname}, const char *@var{newname})
The @code{symlink} function makes a symbolic link to @var{oldname} named
@var{newname}.

The normal return value from @code{symlink} is @code{0}.  A return value
of @code{-1} indicates an error.  In addition to the usual file name
syntax errors (@pxref{File Name Errors}), the following @code{errno}
error conditions are defined for this function:

@table @code
@item EEXIST
There is already an existing file named @var{newname}.

@item EROFS
The file @var{newname} would exist on a read-only file system.

@item ENOSPC
The directory or file system cannot be extended to make the new link.

@item EIO
A hardware error occurred while reading or writing data on the disk.

@ignore
@comment not sure about these
@item ELOOP
There are too many levels of indirection.  This can be the result of
circular symbolic links to directories.

@item EDQUOT
The new link can't be created because the user's disk quota has been
exceeded.
@end ignore
@end table
@end deftypefun

@comment unistd.h
@comment BSD
@deftypefun int readlink (const char *@var{filename}, char *@var{buffer}, size_t @var{size})
The @code{readlink} function gets the value of the symbolic link
@var{filename}.  The file name that the link points to is copied into
@var{buffer}.  This file name string is @emph{not} null-terminated;
@code{readlink} normally returns the number of characters copied.  The
@var{size} argument specifies the maximum number of characters to copy,
usually the allocation size of @var{buffer}.

If the return value equals @var{size}, you cannot tell whether or not
there was room to return the entire name.  So make a bigger buffer and
call @code{readlink} again.  Here is an example:

@smallexample
char *
readlink_malloc (char *filename)
@{
  int size = 100;

  while (1)
    @{
      char *buffer = (char *) xmalloc (size);
      int nchars = readlink (filename, buffer, size);
      if (nchars < size)
        return buffer;
      free (buffer);
      size *= 2;
    @}
@}
@end smallexample

@c @group  Invalid outside example.
A value of @code{-1} is returned in case of error.  In addition to the
usual file name errors (@pxref{File Name Errors}), the following
@code{errno} error conditions are defined for this function:

@table @code
@item EINVAL
The named file is not a symbolic link.

@item EIO
A hardware error occurred while reading or writing data on the disk.
@end table
@c @end group
@end deftypefun

@node Deleting Files
@section Deleting Files
@cindex deleting a file
@cindex removing a file
@cindex unlinking a file

You can delete a file with the functions @code{unlink} or @code{remove}.

Deletion actually deletes a file name.  If this is the file's only name,
then the file is deleted as well.  If the file has other names as well
(@pxref{Hard Links}), it remains accessible under its other names.

@comment unistd.h
@comment POSIX.1
@deftypefun int unlink (const char *@var{filename})
The @code{unlink} function deletes the file name @var{filename}.  If
this is a file's sole name, the file itself is also deleted.  (Actually,
if any process has the file open when this happens, deletion is
postponed until all processes have closed the file.)

@pindex unistd.h
The function @code{unlink} is declared in the header file @file{unistd.h}.

This function returns @code{0} on successful completion, and @code{-1}
on error.  In addition to the usual file name errors
(@pxref{File Name Errors}), the following @code{errno} error conditions are 
defined for this function:

@table @code
@item EACCES
Write permission is denied for the directory from which the file is to be
removed, or the directory has the sticky bit set and you do not own the file.

@item EBUSY
This error indicates that the file is being used by the system in such a
way that it can't be unlinked.  For example, you might see this error if
the file name specifies the root directory or a mount point for a file
system.

@item ENOENT
The file name to be deleted doesn't exist.

@item EPERM
On some systems, @code{unlink} cannot be used to delete the name of a
directory, or can only be used this way by a privileged user.
To avoid such problems, use @code{rmdir} to delete directories.
(In the GNU system @code{unlink} can never delete the name of a directory.)

@item EROFS
The directory in which the file name is to be deleted is on a read-only
file system, and can't be modified.
@end table
@end deftypefun

@comment unistd.h
@comment POSIX.1
@deftypefun int rmdir (const char *@var{filename})
@cindex directories, deleting
@cindex deleting a directory
The @code{rmdir} function deletes a directory.  The directory must be
empty before it can be removed; in other words, it can only contain
entries for @file{.} and @file{..}.

In most other respects, @code{rmdir} behaves like @code{unlink}.  There
are two additional @code{errno} error conditions defined for
@code{rmdir}:

@table @code
@item ENOTEMPTY
@itemx EEXIST
The directory to be deleted is not empty.  
@end table

These two error codes are synonymous; some systems use one, and some use
the other.  The GNU system always uses @code{ENOTEMPTY}.

The prototype for this function is declared in the header file
@file{unistd.h}.
@pindex unistd.h
@end deftypefun

@comment stdio.h
@comment ANSI
@deftypefun int remove (const char *@var{filename})
This is the ANSI C function to remove a file.  It works like
@code{unlink} for files and like @code{rmdir} for directories.
@code{remove} is declared in @file{stdio.h}.
@pindex stdio.h
@end deftypefun

@node Renaming Files
@section Renaming Files

The @code{rename} function is used to change a file's name.

@cindex renaming a file
@comment stdio.h
@comment ANSI
@deftypefun int rename (const char *@var{oldname}, const char *@var{newname})
The @code{rename} function renames the file name @var{oldname} with
@var{newname}.  The file formerly accessible under the name
@var{oldname} is afterward accessible as @var{newname} instead.  (If the
file had any other names aside from @var{oldname}, it continues to have
those names.)

The directory containing the name @var{newname} must be on the same
file system as the file (as indicated by the name @var{oldname}).

One special case for @code{rename} is when @var{oldname} and
@var{newname} are two names for the same file.  The consistent way to
handle this case is to delete @var{oldname}.  However, POSIX requires
that in this case @code{rename} do nothing and report success---which is
inconsistent.  We don't know what your operating system will do.

If the @var{oldname} is not a directory, then any existing file named
@var{newname} is removed during the renaming operation.  However, if
@var{newname} is the name of a directory, @code{rename} fails in this
case.

If the @var{oldname} is a directory, then either @var{newname} must not
exist or it must name a directory that is empty.  In the latter case,
the existing directory named @var{newname} is deleted first.  The name
@var{newname} must not specify a subdirectory of the directory
@code{oldname} which is being renamed.

One useful feature of @code{rename} is that the meaning of the name
@var{newname} changes ``atomically'' from any previously existing file
by that name to its new meaning (the file that was called
@var{oldname}).  There is no instant at which @var{newname} is
nonexistent ``in between'' the old meaning and the new meaning.  If
there is a system crash during the operation, it is possible for both
names to still exist; but @var{newname} will always be intact if it
exists at all.

If @code{rename} fails, it returns @code{-1}.  In addition to the usual
file name errors (@pxref{File Name Errors}), the following
@code{errno} error conditions are defined for this function:

@table @code
@item EACCES
One of the directories containing @var{newname} or @var{oldname}
refuses write permission; or @var{newname} and @var{oldname} are
directories and write permission is refused for one of them.

@item EBUSY
A directory named by @var{oldname} or @var{newname} is being used by
the system in a way that prevents the renaming from working.  This includes
directories that are mount points for filesystems, and directories
that are the current working directories of processes.

@item ENOTEMPTY
@itemx EEXIST
The directory @var{newname} isn't empty.  The GNU system always returns
@code{ENOTEMPTY} for this, but some other systems return @code{EEXIST}.

@item EINVAL
The @var{oldname} is a directory that contains @var{newname}.

@item EISDIR
The @var{newname} names a directory, but the @var{oldname} doesn't.

@item EMLINK
The parent directory of @var{newname} would have too many links.

@item ENOENT
The file named by @var{oldname} doesn't exist.

@item ENOSPC
The directory that would contain @var{newname} has no room for another
entry, and there is no space left in the file system to expand it.

@item EROFS
The operation would involve writing to a directory on a read-only file
system.

@item EXDEV
The two file names @var{newname} and @var{oldnames} are on different
file systems.
@end table
@end deftypefun

@node Creating Directories
@section Creating Directories
@cindex creating a directory
@cindex directories, creating

@pindex mkdir
Directories are created with the @code{mkdir} function.  (There is also
a shell command @code{mkdir} which does the same thing.)
@c !!! umask

@comment sys/stat.h
@comment POSIX.1
@deftypefun int mkdir (const char *@var{filename}, mode_t @var{mode})
The @code{mkdir} function creates a new, empty directory whose name is
@var{filename}.

The argument @var{mode} specifies the file permissions for the new
directory file.  @xref{Permission Bits}, for more information about
this.

A return value of @code{0} indicates successful completion, and
@code{-1} indicates failure.  In addition to the usual file name syntax
errors (@pxref{File Name Errors}), the following @code{errno} error
conditions are defined for this function:

@table @code
@item EACCES
Write permission is denied for the parent directory in which the new
directory is to be added.

@item EEXIST
A file named @var{filename} already exists.

@item EMLINK
The parent directory has too many links.

Well-designed file systems never report this error, because they permit
more links than your disk could possibly hold.  However, you must still
take account of the possibility of this error, as it could result from
network access to a file system on another machine.

@item ENOSPC
The file system doesn't have enough room to create the new directory.

@item EROFS
The parent directory of the directory being created is on a read-only
file system, and cannot be modified.
@end table

To use this function, your program should include the header file
@file{sys/stat.h}.
@pindex sys/stat.h
@end deftypefun

@node File Attributes
@section File Attributes

@pindex ls
When you issue an @samp{ls -l} shell command on a file, it gives you
information about the size of the file, who owns it, when it was last
modified, and the like.  This kind of information is called the
@dfn{file attributes}; it is associated with the file itself and not a
particular one of its names.

This section contains information about how you can inquire about and
modify these attributes of files.

@menu
* Attribute Meanings::          The names of the file attributes, 
                                 and what their values mean.
* Reading Attributes::          How to read the attributes of a file.
* Testing File Type::           Distinguishing ordinary files,
                                 directories, links... 
* File Owner::                  How ownership for new files is determined,
                                 and how to change it.
* Permission Bits::             How information about a file's access
                                 mode is stored. 
* Access Permission::           How the system decides who can access a file.
* Setting Permissions::         How permissions for new files are assigned,
                                 and how to change them.
* Testing File Access::         How to find out if your process can
                                 access a file. 
* File Times::                  About the time attributes of a file.
@end menu

@node Attribute Meanings
@subsection What the File Attribute Values Mean
@cindex status of a file
@cindex attributes of a file
@cindex file attributes

When you read the attributes of a file, they come back in a structure
called @code{struct stat}.  This section describes the names of the
attributes, their data types, and what they mean.  For the functions
to read the attributes of a file, see @ref{Reading Attributes}.

The header file @file{sys/stat.h} declares all the symbols defined
in this section.
@pindex sys/stat.h

@comment sys/stat.h
@comment POSIX.1
@deftp {Data Type} {struct stat}
The @code{stat} structure type is used to return information about the
attributes of a file.  It contains at least the following members:

@table @code
@item mode_t st_mode
Specifies the mode of the file.  This includes file type information
(@pxref{Testing File Type}) and the file permission bits
(@pxref{Permission Bits}).

@item ino_t st_ino
The file serial number, which distinguishes this file from all other
files on the same device.

@item dev_t st_dev
Identifies the device containing the file.  The @code{st_ino} and
@code{st_dev}, taken together, uniquely identify the file.  The
@code{st_dev} value is not necessarily consistent across reboots or
system crashes, however.

@item nlink_t st_nlink
The number of hard links to the file.  This count keeps track of how
many directories have entries for this file.  If the count is ever
decremented to zero, then the file itself is discarded as soon as no
process still holds it open.  Symbolic links are not counted in the
total.

@item uid_t st_uid
The user ID of the file's owner.  @xref{File Owner}.

@item gid_t st_gid
The group ID of the file.  @xref{File Owner}.

@item off_t st_size
This specifies the size of a regular file in bytes.  For files that
are really devices and the like, this field isn't usually meaningful.
For symbolic links, this specifies the length of the file name the link
refers to.

@item time_t st_atime
This is the last access time for the file.  @xref{File Times}.

@item unsigned long int st_atime_usec
This is the fractional part of the last access time for the file.
@xref{File Times}.

@item time_t st_mtime
This is the time of the last modification to the contents of the file.
@xref{File Times}.

@item unsigned long int st_mtime_usec
This is the fractional part of the time of last modification to the
contents of the file.  @xref{File Times}.

@item time_t st_ctime
This is the time of the last modification to the attributes of the file.
@xref{File Times}.

@item unsigned long int st_ctime_usec
This is the fractional part of the time of last modification to the
attributes of the file.  @xref{File Times}.

@c !!! st_rdev
@item unsigned int st_blocks
This is the amount of disk space that the file occupies, measured in
units of 512-byte blocks.

The number of disk blocks is not strictly proportional to the size of
the file, for two reasons: the file system may use some blocks for
internal record keeping; and the file may be sparse---it may have
``holes'' which contain zeros but do not actually take up space on the
disk.

You can tell (approximately) whether a file is sparse by comparing this
value with @code{st_size}, like this:

@smallexample
(st.st_blocks * 512 < st.st_size)
@end smallexample

This test is not perfect because a file that is just slightly sparse
might not be detected as sparse at all.  For practical applications,
this is not a problem.

@item unsigned int st_blksize
The optimal block size for reading of writing this file, in bytes.  You
might use this size for allocating the buffer space for reading of
writing the file.  (This is unrelated to @code{st_blocks}.)
@end table
@end deftp

  Some of the file attributes have special data type names which exist
specifically for those attributes.  (They are all aliases for well-known
integer types that you know and love.)  These typedef names are defined
in the header file @file{sys/types.h} as well as in @file{sys/stat.h}.
Here is a list of them.

@comment sys/types.h
@comment POSIX.1
@deftp {Data Type} mode_t
This is an integer data type used to represent file modes.  In the
GNU system, this is equivalent to @code{unsigned int}.
@end deftp

@cindex inode number
@comment sys/types.h
@comment POSIX.1
@deftp {Data Type} ino_t
This is an arithmetic data type used to represent file serial numbers.
(In Unix jargon, these are sometimes called @dfn{inode numbers}.)
In the GNU system, this type is equivalent to @code{unsigned long int}.
@end deftp

@comment sys/types.h
@comment POSIX.1
@deftp {Data Type} dev_t
This is an arithmetic data type used to represent file device numbers.
In the GNU system, this is equivalent to @code{int}.
@end deftp

@comment sys/types.h
@comment POSIX.1
@deftp {Data Type} nlink_t
This is an arithmetic data type used to represent file link counts.
In the GNU system, this is equivalent to @code{unsigned short int}.
@end deftp

@node Reading Attributes
@subsection Reading the Attributes of a File

To examine the attributes of files, use the functions @code{stat},
@code{fstat} and @code{lstat}.  They return the attribute information in
a @code{struct stat} object.  All three functions are declared in the
header file @file{sys/stat.h}.

@comment sys/stat.h
@comment POSIX.1
@deftypefun int stat (const char *@var{filename}, struct stat *@var{buf})
The @code{stat} function returns information about the attributes of the
file named by @w{@var{filename}} in the structure pointed at by @var{buf}.

If @var{filename} is the name of a symbolic link, the attributes you get
describe the file that the link points to.  If the link points to a
nonexistent file name, then @code{stat} fails, reporting a nonexistent
file.

The return value is @code{0} if the operation is successful, and @code{-1}
on failure.  In addition to the usual file name errors
(@pxref{File Name Errors}, the following @code{errno} error conditions
are defined for this function:

@table @code
@item ENOENT
The file named by @var{filename} doesn't exist.
@end table
@end deftypefun

@comment sys/stat.h
@comment POSIX.1
@deftypefun int fstat (int @var{filedes}, struct stat *@var{buf})
The @code{fstat} function is like @code{stat}, except that it takes an
open file descriptor as an argument instead of a file name.
@xref{Low-Level I/O}.

Like @code{stat}, @code{fstat} returns @code{0} on success and @code{-1}
on failure.  The following @code{errno} error conditions are defined for
@code{fstat}:

@table @code
@item EBADF
The @var{filedes} argument is not a valid file descriptor.
@end table
@end deftypefun

@comment sys/stat.h
@comment BSD
@deftypefun int lstat (const char *@var{filename}, struct stat *@var{buf})
The @code{lstat} function is like @code{stat}, except that it does not
follow symbolic links.  If @var{filename} is the name of a symbolic
link, @code{lstat} returns information about the link itself; otherwise,
@code{lstat} works like @code{stat}.  @xref{Symbolic Links}.
@end deftypefun

@node Testing File Type
@subsection Testing the Type of a File

The @dfn{file mode}, stored in the @code{st_mode} field of the file
attributes, contains two kinds of information: the file type code, and
the access permission bits.  This section discusses only the type code,
which you can use to tell whether the file is a directory, whether it is
a socket, and so on.  For information about the access permission,
@ref{Permission Bits}.

There are two predefined ways you can access the file type portion of
the file mode.  First of all, for each type of file, there is a 
@dfn{predicate macro} which examines a file mode value and returns
true or false---is the file of that type, or not.  Secondly, you can
mask out the rest of the file mode to get just a file type code.
You can compare this against various constants for the supported file
types.

All of the symbols listed in this section are defined in the header file
@file{sys/stat.h}.
@pindex sys/stat.h

The following predicate macros test the type of a file, given the value
@var{m} which is the @code{st_mode} field returned by @code{stat} on
that file:

@comment sys/stat.h
@comment POSIX
@deftypefn Macro int S_ISDIR (mode_t @var{m})
This macro returns nonzero if the file is a directory.
@end deftypefn

@comment sys/stat.h
@comment POSIX
@deftypefn Macro int S_ISCHR (mode_t @var{m})
This macro returns nonzero if the file is a character special file (a
device like a terminal).
@end deftypefn

@comment sys/stat.h
@comment POSIX
@deftypefn Macro int S_ISBLK (mode_t @var{m})
This macro returns nonzero if the file is a block special file (a device
like a disk).
@end deftypefn

@comment sys/stat.h
@comment POSIX
@deftypefn Macro int S_ISREG (mode_t @var{m})
This macro returns nonzero if the file is a regular file.
@end deftypefn

@comment sys/stat.h
@comment POSIX
@deftypefn Macro int S_ISFIFO (mode_t @var{m})
This macro returns nonzero if the file is a FIFO special file, or a
pipe.  @xref{Pipes and FIFOs}.
@end deftypefn

@comment sys/stat.h
@comment GNU
@deftypefn Macro int S_ISLNK (mode_t @var{m})
This macro returns nonzero if the file is a symbolic link.
@xref{Symbolic Links}.
@end deftypefn

@comment sys/stat.h
@comment GNU
@deftypefn Macro int S_ISSOCK (mode_t @var{m})
This macro returns nonzero if the file is a socket.  @xref{Sockets}.
@end deftypefn

An alterate non-POSIX method of testing the file type is supported for
compatibility with BSD.  The mode can be bitwise ANDed with
@code{S_IFMT} to extract the file type code, and compared to the
appropriate type code constant.  For example,

@smallexample
S_ISCHR (@var{mode})
@end smallexample

@noindent
is equivalent to:

@smallexample
((@var{mode} & S_IFMT) == S_IFCHR)
@end smallexample

@comment sys/stat.h
@comment BSD
@deftypevr Macro int S_IFMT
This is a bit mask used to extract the file type code portion of a mode
value.
@end deftypevr

These are the symbolic names for the different file type codes:

@table @code
@comment sys/stat.h
@comment BSD
@item S_IFDIR
@vindex S_IFDIR
This macro represents the value of the file type code for a directory file.

@comment sys/stat.h
@comment BSD
@item S_IFCHR
@vindex S_IFCHR
This macro represents the value of the file type code for a
character-oriented device file.

@comment sys/stat.h
@comment BSD
@item S_IFBLK
@vindex S_IFBLK
This macro represents the value of the file type code for a block-oriented
device file.

@comment sys/stat.h
@comment BSD
@item S_IFREG
@vindex S_IFREG
This macro represents the value of the file type code for a regular file.

@comment sys/stat.h
@comment BSD
@item S_IFLNK
@vindex S_IFLNK
This macro represents the value of the file type code for a symbolic link.

@comment sys/stat.h
@comment BSD
@item S_IFSOCK
@vindex S_IFSOCK
This macro represents the value of the file type code for a socket.

@comment sys/stat.h
@comment BSD
@item S_IFIFO
@vindex S_IFIFO
This macro represents the value of the file type code for a FIFO or pipe.
@end table

@node File Owner
@subsection File Owner
@cindex file owner
@cindex owner of a file
@cindex group owner of a file

Every file has an @dfn{owner} which is one of the registered user names
defined on the system.  Each file also has a @dfn{group}, which is one
of the defined groups.  The file owner can often be useful for showing
you who edited the file (especially when you edit with GNU Emacs), but
its main purpose is for access control.

The file owner and group play a role in determining access because the
file has one set of access permission bits for the user that is the
owner, another set that apply to users who belong to the file's group,
and a third set of bits that apply to everyone else.  @xref{Access
Permission}, for the details of how access is decided based on this
data.

When a file is created, its owner is set from the effective user ID of
the process that creates it (@pxref{Process Persona}).  The file's group
ID may be set from either effective group ID of the process, or the
group ID of the directory that contains the file, depending on the
system where the file is stored.  When you access a remote file system,
it behaves according to its own rule, not according to the system your
program is running on.  Thus, your program must be prepared to encounter
either kind of behavior, no matter what kind of system you run it on.

@pindex chown
@pindex chgrp
You can change the owner and/or group owner of an existing file using
the @code{chown} function.  This is the primitive for the @code{chown}
and @code{chgrp} shell commands.

@pindex unistd.h
The prototype for this function is declared in @file{unistd.h}.

@comment unistd.h
@comment POSIX.1
@deftypefun int chown (const char *@var{filename}, uid_t @var{owner}, gid_t @var{group})
The @code{chown} function changes the owner of the file @var{filename} to
@var{owner}, and its group owner to @var{group}.

Changing the owner of the file on certain systems clears the set-user-ID
and set-group-ID bits of the file's permissions.  (This is because those
bits may not be appropriate for the new owner.)  The other file
permission bits are not changed.

The return value is @code{0} on success and @code{-1} on failure.
In addition to the usual file name errors (@pxref{File Name Errors}), 
the following @code{errno} error conditions are defined for this function:

@table @code
@item EPERM
This process lacks permission to make the requested change.

Only privileged users or the file's owner can change the file's group.
On most file systems, only privileged users can change the file owner;
some file systems allow you to change the owner if you are currently the
owner.  When you access a remote file system, the behavior you encounter
is determined by the system that actually holds the file, not by the
system your program is running on.

@xref{Options for Files}, for information about the
@code{_POSIX_CHOWN_RESTRICTED} macro.

@item EROFS
The file is on a read-only file system.
@end table
@end deftypefun

@comment unistd.h
@comment BSD
@deftypefun int fchown (int @var{filedes}, int @var{owner}, int @var{group})
This is like @code{chown}, except that it changes the owner of the file
with open file descriptor @var{filedes}.

The return value from @code{fchown} is @code{0} on success and @code{-1}
on failure.  The following @code{errno} error codes are defined for this
function:

@table @code
@item EBADF
The @var{filedes} argument is not a valid file descriptor.

@item EINVAL
The @var{filedes} argument corresponds to a pipe or socket, not an ordinary
file.

@item EPERM
This process lacks permission to make the requested change.  For
details, see @code{chmod}, above.

@item EROFS
The file resides on a read-only file system.
@end table
@end deftypefun

@node Permission Bits
@subsection The Mode Bits for Access Permission

The @dfn{file mode}, stored in the @code{st_mode} field of the file
attributes, contains two kinds of information: the file type code, and
the access permission bits.  This section discusses only the access
permission bits, which control who can read or write the file.
@xref{Testing File Type}, for information about the file type code.

All of the symbols listed in this section are defined in the header file
@file{sys/stat.h}.
@pindex sys/stat.h

@cindex file permission bits
These symbolic constants are defined for the file mode bits that control
access permission for the file:

@table @code
@comment sys/stat.h
@comment POSIX.1
@item S_IRUSR
@vindex S_IRUSR
@comment sys/stat.h
@comment BSD
@itemx S_IREAD
@vindex S_IREAD
Read permission bit for the owner of the file.  On many systems, this
bit is 0400.  @code{S_IREAD} is an obsolete synonym provided for BSD
compatibility.

@comment sys/stat.h
@comment POSIX.1
@item S_IWUSR
@vindex S_IWUSR
@comment sys/stat.h
@comment BSD
@itemx S_IWRITE
@vindex S_IWRITE
Write permission bit for the owner of the file.  Usually 0200.
@w{@code{S_IWRITE}} is an obsolete synonym provided for BSD compatibility.

@comment sys/stat.h
@comment POSIX.1
@item S_IXUSR
@vindex S_IXUSR
@comment sys/stat.h
@comment BSD
@itemx S_IEXEC
@vindex S_IEXEC
Execute (for ordinary files) or search (for directories) permission bit
for the owner of the file.  Usually 0100.  @code{S_IEXEC} is an obsolete
synonym provided for BSD compatibility.

@comment sys/stat.h
@comment POSIX.1
@item S_IRWXU
@vindex S_IRWXU
This is equivalent to @samp{(S_IRUSR | S_IWUSR | S_IXUSR)}.

@comment sys/stat.h
@comment POSIX.1
@item S_IRGRP
@vindex S_IRGRP
Read permission bit for the group owner of the file.  Usually 040.

@comment sys/stat.h
@comment POSIX.1
@item S_IWGRP
@vindex S_IWGRP
Write permission bit for the group owner of the file.  Usually 020.

@comment sys/stat.h
@comment POSIX.1
@item S_IXGRP
@vindex S_IXGRP
Execute or search permission bit for the group owner of the file.
Usually 010.

@comment sys/stat.h
@comment POSIX.1
@item S_IRWXG
@vindex S_IRWXG
This is equivalent to @samp{(S_IRGRP | S_IWGRP | S_IXGRP)}.

@comment sys/stat.h
@comment POSIX.1
@item S_IROTH
@vindex S_IROTH
Read permission bit for other users.  Usually 04.

@comment sys/stat.h
@comment POSIX.1
@item S_IWOTH
@vindex S_IWOTH
Write permission bit for other users.  Usually 02.

@comment sys/stat.h
@comment POSIX.1
@item S_IXOTH
@vindex S_IXOTH
Execute or search permission bit for other users.  Usually 01.

@comment sys/stat.h
@comment POSIX.1
@item S_IRWXO
@vindex S_IRWXO
This is equivalent to @samp{(S_IROTH | S_IWOTH | S_IXOTH)}.

@comment sys/stat.h
@comment POSIX
@item S_ISUID
@vindex S_ISUID
This is the set-user-ID on execute bit, usually 04000. 
@xref{How Change Persona}.

@comment sys/stat.h
@comment POSIX
@item S_ISGID
@vindex S_ISGID
This is the set-group-ID on execute bit, usually 02000.
@xref{How Change Persona}.

@cindex sticky bit
@comment sys/stat.h
@comment BSD
@item S_ISVTX
@vindex S_ISVTX
This is the @dfn{sticky} bit, usually 01000.

On a directory, it gives permission to delete a file in the directory
only if you own that file.  Ordinarily, a user either can delete all the
files in the directory or cannot delete any of them (based on whether
the user has write permission for the directory).  The same restriction
applies---you must both have write permission for the directory and own
the file you want to delete.  The one exception is that the owner of the
directory can delete any file in the directory, no matter who owns it
(provided the owner has given himself write permission for the
directory).  This is commonly used for the @file{/tmp} directory, where
anyone may create files, but not delete files created by other users.

Originally the sticky bit on an executable file modified the swapping
policies of the system.  Normally, when a program terminated, its pages
in core were immediately freed and reused.  If the sticky bit was set on
the executable file, the system kept the pages in core for a while as if
the program were still running.  This was advantageous for a program
likely to be run many times in succession.  This usage is obsolete in
modern systems.  When a program terminates, its pages always remain in
core as long as there is no shortage of memory in the system.  When the
program is next run, its pages will still be in core if no shortage
arose since the last run.

On some modern systems where the sticky bit has no useful meaning for an
executable file, you cannot set the bit at all for a non-directory.
If you try, @code{chmod} fails with @code{EFTYPE}; 
@pxref{Setting Permissions}.

Some systems (particularly SunOS) have yet another use for the sticky
bit.  If the sticky bit is set on a file that is @emph{not} executable,
it means the opposite: never cache the pages of this file at all.  The
main use of this is for the files on an NFS server machine which are
used as the swap area of diskless client machines.  The idea is that the
pages of the file will be cached in the client's memory, so it is a
waste of the server's memory to cache them a second time.  In this use
the sticky bit also says that the filesystem may fail to record the
file's modification time onto disk reliably (the idea being that noone
cares for a swap file).
@end table

The actual bit values of the symbols are listed in the table above
so you can decode file mode values when debugging your programs.
These bit values are correct for most systems, but they are not
guaranteed.

@strong{Warning:} Writing explicit numbers for file permissions is bad
practice.  It is not only nonportable, it also requires everyone who
reads your program to remember what the bits mean.  To make your
program clean, use the symbolic names.

@node Access Permission
@subsection How Your Access to a File is Decided
@cindex permission to access a file
@cindex access permission for a file
@cindex file access permission

Recall that the operating system normally decides access permission for
a file based on the effective user and group IDs of the process, and its
supplementary group IDs, together with the file's owner, group and
permission bits.  These concepts are discussed in detail in
@ref{Process Persona}.

If the effective user ID of the process matches the owner user ID of the
file, then permissions for read, write, and execute/search are
controlled by the corresponding ``user'' (or ``owner'') bits.  Likewise,
if any of the effective group ID or supplementary group IDs of the
process matches the group owner ID of the file, then permissions are
controlled by the ``group'' bits.  Otherwise, permissions are controlled
by the ``other'' bits.

Privileged users, like @samp{root}, can access any file, regardless of
its file permission bits.  As a special case, for a file to be
executable even for a privileged user, at least one of its execute bits
must be set.

@node Setting Permissions
@subsection Assigning File Permissions

@cindex file creation mask
@cindex umask
The primitive functions for creating files (for example, @code{open} or
@code{mkdir}) take a @var{mode} argument, which specifies the file
permissions for the newly created file.  But the specified mode is
modified by the process's @dfn{file creation mask}, or @dfn{umask},
before it is used.

The bits that are set in the file creation mask identify permissions
that are always to be disabled for newly created files.  For example, if
you set all the ``other'' access bits in the mask, then newly created
files are not accessible at all to processes in the ``other''
category, even if the @var{mode} argument specified to the creation 
function would permit such access.  In other words, the file creation
mask is the complement of the ordinary access permissions you want to
grant.

Programs that create files typically specify a @var{mode} argument that
includes all the permissions that make sense for the particular file.
For an ordinary file, this is typically read and write permission for
all classes of users.  These permissions are then restricted as
specified by the individual user's own file creation mask.

@findex chmod
To change the permission of an existing file given its name, call
@code{chmod}.  This function ignores the file creation mask; it uses
exactly the specified permission bits.

@pindex umask
In normal use, the file creation mask is initialized in the user's login
shell (using the @code{umask} shell command), and inherited by all
subprocesses.  Application programs normally don't need to worry about
the file creation mask.  It will do automatically what it is supposed to
do.

When your program should create a file and bypass the umask for its
access permissions, the easiest way to do this is to use @code{fchmod}
after opening the file, rather than changing the umask.

In fact, changing the umask is usually done only by shells.  They use
the @code{umask} function.

The functions in this section are declared in @file{sys/stat.h}.
@pindex sys/stat.h

@comment sys/stat.h
@comment POSIX.1
@deftypefun mode_t umask (mode_t @var{mask})
The @code{umask} function sets the file creation mask of the current
process to @var{mask}, and returns the previous value of the file
creation mask.

Here is an example showing how to read the mask with @code{umask}
without changing it permanently:

@smallexample
mode_t
read_umask (void)
@{
  mask = umask (0);
  umask (mask);
@}
@end smallexample

@noindent
However, it is better to use @code{getumask} if you just want to read
the mask value, because that is reentrant (at least if you use the GNU
operating system).
@end deftypefun

@comment sys/stat.h
@comment GNU
@deftypefun mode_t getumask (void)
Return the current value of the file creation mask for the current
process.  This function is a GNU extension.
@end deftypefun

@comment sys/stat.h
@comment POSIX.1
@deftypefun int chmod (const char *@var{filename}, mode_t @var{mode})
The @code{chmod} function sets the access permission bits for the file
named by @var{filename} to @var{mode}.

If the @var{filename} names a symbolic link, @code{chmod} changes the
permission of the file pointed to by the link, not those of the link
itself.

This function returns @code{0} if successful and @code{-1} if not.  In
addition to the usual file name errors (@pxref{File Name
Errors}), the following @code{errno} error conditions are defined for
this function:

@table @code
@item ENOENT
The named file doesn't exist.

@item EPERM
This process does not have permission to change the access permission of
this file.  Only the file's owner (as judged by the effective user ID of
the process) or a privileged user can change them.

@item EROFS
The file resides on a read-only file system.

@item EFTYPE
@var{mode} has the @code{S_ISVTX} bit (the ``sticky bit'') set,
and the named file is not a directory.  Some systems do not allow setting the
sticky bit on non-directory files, and some do (and only some of those
assign a useful meaning to the bit for non-directory files).

You only get @code{EFTYPE} on systems where the sticky bit has no useful
meaning for non-directory files, so it is always safe to just clear the
bit in @var{mode} and call @code{chmod} again.  @xref{Permission Bits},
for full details on the sticky bit.
@end table
@end deftypefun

@comment sys/stat.h
@comment BSD
@deftypefun int fchmod (int @var{filedes}, int @var{mode})
This is like @code{chmod}, except that it changes the permissions of
the file currently open via descriptor @var{filedes}.

The return value from @code{fchmod} is @code{0} on success and @code{-1}
on failure.  The following @code{errno} error codes are defined for this
function:

@table @code
@item EBADF
The @var{filedes} argument is not a valid file descriptor.

@item EINVAL
The @var{filedes} argument corresponds to a pipe or socket, or something
else that doesn't really have access permissions.

@item EPERM
This process does not have permission to change the access permission of
this file.  Only the file's owner (as judged by the effective user ID of
the process) or a privileged user can change them.

@item EROFS
The file resides on a read-only file system.
@end table
@end deftypefun

@node Testing File Access
@subsection Testing Permission to Access a File
@cindex testing access permission
@cindex access, testing for
@cindex setuid programs and file access

When a program runs as a privileged user, this permits it to access
files off-limits to ordinary users---for example, to modify
@file{/etc/passwd}.  Programs designed to be run by ordinary users but
access such files use the setuid bit feature so that they always run
with @code{root} as the effective user ID.
 
Such a program may also access files specified by the user, files which
conceptually are being accessed explicitly by the user.  Since the
program runs as @code{root}, it has permission to access whatever file
the user specifies---but usually the desired behavior is to permit only
those files which the user could ordinarily access.

The program therefore must explicitly check whether @emph{the user}
would have the necessary access to a file, before it reads or writes the
file.

To do this, use the function @code{access}, which checks for access
permission based on the process's @emph{real} user ID rather than the
effective user ID.  (The setuid feature does not alter the real user ID,
so it reflects the user who actually ran the program.)

There is another way you could check this access, which is easy to
describe, but very hard to use.  This is to examine the file mode bits
and mimic the system's own access computation.  This method is
undesirable because many systems have additional access control
features; your program cannot portably mimic them, and you would not
want to try to keep track of the diverse features that different systems
have.  Using @code{access} is simple and automatically does whatever is
appropriate for the system you are using.

@code{access} is @emph{only} only appropriate to use in setuid programs.
A non-setuid program will always use the effective ID rather than the
real ID.

@pindex unistd.h
The symbols in this section are declared in @file{unistd.h}.

@comment unistd.h
@comment POSIX.1
@deftypefun int access (const char *@var{filename}, int @var{how})
The @code{access} function checks to see whether the file named by
@var{filename} can be accessed in the way specified by the @var{how}
argument.  The @var{how} argument either can be the bitwise OR of the
flags @code{R_OK}, @code{W_OK}, @code{X_OK}, or the existence test
@code{F_OK}.

This function uses the @emph{real} user and group ID's of the calling
process, rather than the @emph{effective} ID's, to check for access
permission.  As a result, if you use the function from a @code{setuid}
or @code{setgid} program (@pxref{How Change Persona}), it gives
information relative to the user who actually ran the program.

The return value is @code{0} if the access is permitted, and @code{-1}
otherwise.  (In other words, treated as a predicate function,
@code{access} returns true if the requested access is @emph{denied}.)

In addition to the usual file name errors (@pxref{File Name
Errors}), the following @code{errno} error conditions are defined for
this function:

@table @code
@item EACCES
The access specified by @var{how} is denied.

@item ENOENT
The file doesn't exist.

@item EROFS
Write permission was requested for a file on a read-only file system.
@end table
@end deftypefun

These macros are defined in the header file @file{unistd.h} for use
as the @var{how} argument to the @code{access} function.  The values
are integer constants.
@pindex unistd.h

@comment unistd.h
@comment POSIX.1
@deftypevr Macro int R_OK
Argument that means, test for read permission.
@end deftypevr

@comment unistd.h
@comment POSIX.1
@deftypevr Macro int W_OK
Argument that means, test for write permission.
@end deftypevr

@comment unistd.h
@comment POSIX.1
@deftypevr Macro int X_OK
Argument that means, test for execute/search permission.
@end deftypevr

@comment unistd.h
@comment POSIX.1
@deftypevr Macro int F_OK
Argument that means, test for existence of the file.
@end deftypevr

@node File Times
@subsection File Times

@cindex file access time
@cindex file modification time
@cindex file attribute modification time
Each file has three timestamps associated with it:  its access time,
its modification time, and its attribute modification time.  These
correspond to the @code{st_atime}, @code{st_mtime}, and @code{st_ctime}
members of the @code{stat} structure; see @ref{File Attributes}.  

All of these times are represented in calendar time format, as
@code{time_t} objects.  This data type is defined in @file{time.h}.
For more information about representation and manipulation of time
values, see @ref{Calendar Time}.
@pindex time.h

Reading from a file updates its access time attribute, and writing
updates its modification time.  When a file is created, all three
timestamps for that file are set to the current time.  In addition, the
attribute change time and modification time fields of the directory that
contains the new entry are updated.

Adding a new name for a file with the @code{link} function updates the
attribute change time field of the file being linked, and both the
attribute change time and modification time fields of the directory
containing the new name.  These same fields are affected if a file name
is deleted with @code{unlink}, @code{remove}, or @code{rmdir}.  Renaming
a file with @code{rename} affects only the attribute change time and
modification time fields of the two parent directories involved, and not
the times for the file being renamed.

Changing attributes of a file (for example, with @code{chmod}) updates
its attribute change time field.

You can also change some of the timestamps of a file explicitly using
the @code{utime} function---all except the attribute change time.  You
need to include the header file @file{utime.h} to use this facility.
@pindex utime.h

@comment time.h
@comment POSIX.1
@deftp {Data Type} {struct utimbuf}
The @code{utimbuf} structure is used with the @code{utime} function to
specify new access and modification times for a file.  It contains the
following members:

@table @code
@item time_t actime
This is the access time for the file.

@item time_t modtime
This is the modification time for the file.
@end table
@end deftp

@comment time.h
@comment POSIX.1
@deftypefun int utime (const char *@var{filename}, const struct utimbuf *@var{times})
This function is used to modify the file times associated with the file
named @var{filename}.

If @var{times} is a null pointer, then the access and modification times
of the file are set to the current time.  Otherwise, they are set to the
values from the @code{actime} and @code{modtime} members (respectively)
of the @code{utimbuf} structure pointed at by @var{times}.  

The attribute modification time for the file is set to the current time
in either case (since changing the timestamps is itself a modification
of the file attributes).

The @code{utime} function returns @code{0} if successful and @code{-1}
on failure.  In addition to the usual file name errors
(@pxref{File Name Errors}), the following @code{errno} error conditions
are defined for this function:

@table @code
@item EACCES
There is a permission problem in the case where a null pointer was
passed as the @var{times} argument.  In order to update the timestamp on
the file, you must either be the owner of the file, have write
permission on the file, or be a privileged user.

@item ENOENT
The file doesn't exist.

@item EPERM
If the @var{times} argument is not a null pointer, you must either be
the owner of the file or be a privileged user.  This error is used to
report the problem.

@item EROFS
The file lives on a read-only file system.
@end table
@end deftypefun

Each of the three time stamps has a corresponding microsecond part,
which extends its resolution.  These fields are called
@code{st_atime_usec}, @code{st_mtime_usec}, and @code{st_ctime_usec};
each has a value between 0 and 999,999, which indicates the time in
microseconds.  They correspond to the @code{tv_usec} field of a
@code{timeval} structure; see @ref{High-Resolution Calendar}.

The @code{utimes} function is like @code{utime}, but also lets you specify
the fractional part of the file times.  The prototype for this function is
in the header file @file{sys/time.h}.
@pindex sys/time.h

@comment sys/time.h
@comment BSD
@deftypefun int utimes (const char *@var{filename}, struct timeval @var{tvp}@t{[2]})
This function sets the file access and modification times for the file
named by @var{filename}.  The new file access time is specified by
@code{@var{tvp}[0]}, and the new modification time by
@code{@var{tvp}[1]}.  This function comes from BSD.

The return values and error conditions are the same as for the @code{utime}
function.
@end deftypefun

@node Making Special Files
@section Making Special Files
@cindex creating special files
@cindex special files

The @code{mknod} function is the primitive for making special files,
such as files that correspond to devices.  The GNU library includes
this function for compatibility with BSD.

The prototype for @code{mknod} is declared in @file{sys/stat.h}.
@pindex sys/stat.h

@comment sys/stat.h
@comment BSD
@deftypefun int mknod (const char *@var{filename}, int @var{mode}, int @var{dev})
The @code{mknod} function makes a special file with name @var{filename}.
The @var{mode} specifies the mode of the file, and may include the various
special file bits, such as @code{S_IFCHR} (for a character special file)
or @code{S_IFBLK} (for a block special file).  @xref{Testing File Type}.

The @var{dev} argument specifies which device the special file refers to.
Its exact interpretation depends on the kind of special file being created.

The return value is @code{0} on success and @code{-1} on error.  In addition
to the usual file name errors (@pxref{File Name Errors}), the
following @code{errno} error conditions are defined for this function:

@table @code
@item EPERM
The calling process is not privileged.  Only the superuser can create
special files.

@item ENOSPC
The directory or file system that would contain the new file is full
and cannot be extended.

@item EROFS
The directory containing the new file can't be modified because it's on
a read-only file system.

@item EEXIST
There is already a file named @var{filename}.  If you want to replace
this file, you must remove the old file explicitly first.
@end table
@end deftypefun

@node Temporary Files
@section Temporary Files

If you need to use a temporary file in your program, you can use the
@code{tmpfile} function to open it.  Or you can use the @code{tmpnam}
function make a name for a temporary file and then open it in the usual
way with @code{fopen}.

The @code{tempnam} function is like @code{tmpnam} but lets you choose
what directory temporary files will go in, and something about what
their file names will look like.

These facilities are declared in the header file @file{stdio.h}.
@pindex stdio.h

@comment stdio.h
@comment ANSI
@deftypefun {FILE *} tmpfile (void)
This function creates a temporary binary file for update mode, as if by
calling @code{fopen} with mode @code{"wb+"}.  The file is deleted
automatically when it is closed or when the program terminates.  (On
some other ANSI C systems the file may fail to be deleted if the program
terminates abnormally).
@end deftypefun

@comment stdio.h
@comment ANSI
@deftypefun {char *} tmpnam (char *@var{result})
This function constructs and returns a file name that is a valid file
name and that does not name any existing file.  If the @var{result}
argument is a null pointer, the return value is a pointer to an internal
static string, which might be modified by subsequent calls.  Otherwise,
the @var{result} argument should be a pointer to an array of at least
@code{L_tmpnam} characters, and the result is written into that array.

It is possible for @code{tmpnam} to fail if you call it too many times.
This is because the fixed length of a temporary file name gives room for
only a finite number of different names.  If @code{tmpnam} fails, it
returns a null pointer.
@end deftypefun

@comment stdio.h
@comment ANSI
@deftypevr Macro int L_tmpnam
The value of this macro is an integer constant expression that represents
the minimum allocation size of a string large enough to hold the
file name generated by the @code{tmpnam} function.
@end deftypevr

@comment stdio.h
@comment ANSI
@deftypevr Macro int TMP_MAX
The macro @code{TMP_MAX} is a lower bound for how many temporary names
you can create with @code{tmpnam}.  You can rely on being able to call
@code{tmpnam} at least this many times before it might fail saying you
have made too many temporary file names.

With the GNU library, you can create a very large number of temporary
file names---if you actually create the files, you will probably run out
of disk space before you run out of names.  Some other systems have a
fixed, small limit on the number of temporary files.  The limit is never
less than @code{25}.
@end deftypevr

@comment stdio.h
@comment SVID
@deftypefun {char *} tempnam (const char *@var{dir}, const char *@var{prefix})
This function generates a unique temporary filename.  If @var{prefix} is
not a null pointer, up to five characters of this string are used as a
prefix for the file name.  The return value is a string newly allocated
with @code{malloc}; you should release its storage with @code{free} when
it is no longer needed.

The directory prefix for the temporary file name is determined by testing
each of the following, in sequence.  The directory must exist and be
writable.

@itemize @bullet
@item
The environment variable @code{TMPDIR}, if it is defined.

@item
The @var{dir} argument, if it is not a null pointer.

@item
The value of the @code{P_tmpdir} macro.

@item
The directory @file{/tmp}.
@end itemize

This function is defined for SVID compatibility.
@end deftypefun
@cindex TMPDIR environment variable

@comment stdio.h
@comment SVID
@c !!! are we putting SVID/GNU/POSIX.1/BSD in here or not??
@deftypevr {SVID Macro} {char *} P_tmpdir
This macro is the name of the default directory for temporary files.
@end deftypevr

Older Unix systems did not have the functions just described.  Instead
they used @code{mktemp} and @code{mkstemp}.  Both of these functions
work by modifying a file name template string you pass.  The last six
characters of this string must be @samp{XXXXXX}.  These six @samp{X}s
are replaced with six characters which make the whole string a unique
file name.  Usually the template string is something like
@samp{/tmp/@var{prefix}XXXXXX}, and each program uses a unique @var{prefix}.

@strong{Note:} Because @code{mktemp} and @code{mkstemp} modify the
template string, you @emph{must not} pass string constants to them.
String constants are normally in read-only storage, so your program
would crash when @code{mktemp} or @code{mkstemp} tried to modify the
string.

@comment unistd.h
@comment Unix
@deftypefun {char *} mktemp (char *@var{template})
The @code{mktemp} function generates a unique file name by modifying
@var{template} as described above.  If successful, it returns
@var{template} as modified.  If @code{mktemp} cannot find a unique file
name, it makes @var{template} an empty string and returns that.  If
@var{template} does not end with @samp{XXXXXX}, @code{mktemp} returns a
null pointer.
@end deftypefun

@comment unistd.h
@comment BSD
@deftypefun int mkstemp (char *@var{template})
The @code{mkstemp} function generates a unique file name just as
@code{mktemp} does, but it also opens the file for you with @code{open}
(@pxref{Opening and Closing Files}).  If successful, it modifies
@var{template} in place and returns a file descriptor open on that file
for reading and writing.  If @code{mkstemp} cannot create a
uniquely-named file, it makes @var{template} an empty string and returns
@code{-1}.  If @var{template} does not end with @samp{XXXXXX},
@code{mkstemp} returns @code{-1} and does not modify @var{template}.
@end deftypefun

Unlike @code{mktemp}, @code{mkstemp} is actually guaranteed to create a
unique file that cannot possibly clash with any other program trying to
create a temporary file.  This is because it works by calling
@code{open} with the @code{O_EXCL} flag bit, which says you want to
always create a new file, and get an error if the file already exists.