path: root/manual/filesys.texi

@node File System Interface
@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{Input/Output on Streams} and @ref{Low-Level Input/Output}, 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.
@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}, int @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 less than or equal to zero.

@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 behavior equivalent
to GNU's @code{getcwd (0, 0)} using only the standard behavior of
@code{getcwd}:

@example
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 example

@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}.  The GNU library provides @code{getwd}
for backwards compatibility with BSD.  The @var{buffer} should be a
pointer to an array at least @code{PATH_MAX} bytes long.
@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}).
@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 the
files for which it contains entries, 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
@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{Input/Output 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 {struct Type} dirent
This is a structure type used to return information about directory
entries.  It contains the following members:

@table @code
@item char *d_name
This is the null-terminated file name component.

@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}.

@item size_t d_namlen
This is the length of the file name.
@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 syntax 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 Input/Output}.  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{..}.  @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:

@example
#include <stddef.h>
#include <stdio.h>
#include <sys/types.h>
#include <dirent.h>

main ()
@{   
  DIR *dp;
  struct dirent *ep;

  if (dp = opendir ("./")) @{
    while (ep = readdir (dp))
      puts (ep->d_name);
    closedir (dp);
    @}
  else
    perror ("couldn't open working directory");
@}
@end example

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.

@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, GNU
@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, GNU
@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.

Since a particular file exists within a single file system, all its
names must be in directories in that file system.  @code{link} reports
an error if you try to make a hard link to the file from another file
system.

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 syntax 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
The directory in which the new link is to be written is not writable.
@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 @code{LINK_MAX}; see @ref{File
System Parameters}.)
@c Can this really happen in GNU?

@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
Some implementations only allow privileged users to make links to
directories, and others prohibit this operation entirely.  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.
@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.  So does @code{link}, the function that makes a
hard link.

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.

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.

@c ??? Probably possible for
@c ??? most system calls, and therefore doesn't need to be listed here.
@c @item EFAULT
@c An invalid pointer argument was passed.

@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:

@example
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 readlink;
    free (buffer);
    size *= 2;
  @}
@}
@end example

A value of @code{-1} is returned in case of 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 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
@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}.
(These names are synonymous.)

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 syntax errors
(@pxref{File Name Errors}), the following @code{errno} error conditions are 
defined for this function:

@table @code
@item EACCESS
Write permission is denied for the directory from which the file is to be
removed.

@item EBUSY
This error indicates that the file is being used by the system in such a
way that it can't be unlinked.  Examples of situations where you might
see this error are 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.

@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 stdio.h
@comment ANSI
@deftypefun int remove (const char *@var{filename})
The @code{remove} function another name for @code{unlink}.
@code{remove} is the ANSI C name, whereas @code{unlink} is the POSIX
name.  The name @code{remove} is declared in @file{stdio.h}.
@pindex stdio.h
@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 EEXIST
@itemx ENOTEMPTY
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 prototype for this function is declared in the header file
@file{unistd.h}.
@pindex unistd.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}).

@c ??? Isn't this inconsistent?  Shouldn't this delete the old name
@c ??? while preserving the new name?
If @var{oldname} and @var{newname} are two names for the same file,
@code{rename} does nothing and reports success.

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 @code{rename} fails, it returns @code{-1}.  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
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.  For example,
a directory that is a mount point for a filesystem might have this
problem.

@item EEXIST
The directory @var{newname} isn't empty.

@item ENOTEMPTY
The directory @var{newname} isn't empty.

@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.
@c ??? Can this happen in GNU?

@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.)

@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.
@c ??? Can this happen in GNU?

@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 files
@file{sys/types.h} and @file{sys/stat.h}.
@pindex sys/stat.h
@pindex sys/types.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 {struct Type} 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.

@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.

@item nlink_t st_nlink
The number of 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.  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.

@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}.

@item st_nblocks
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:

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

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 st_blksize
The optimal block size for reading of writing this file.  You might use
this size for allocating the buffer space for reading of writing the
file.
@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 short 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{short 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 @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 syntax 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 Input/Output}.

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_stat} 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,

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

@noindent
is equivalent to:

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

@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.  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 operating system.
@c ??? Say what the GNU system does.

@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 syntax 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 systems, only privileged users can change the file owner; some
systems allow you to change the owner if you are currently the owner.

@strong{Incomplete:}  What does the GNU system do?

@xref{File System Parameters}, 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_stat} 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.  For
information about the file type code, @ref{Testing File Type}.

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.
@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{User/Group IDs of a Process}.

@comment sys/stat.h
@comment POSIX
@item S_ISGID
@vindex S_ISGID
This is the set-group-ID on execute bit, usually 02000.
@xref{User/Group IDs of a Process}.

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

@c ??? What does it mean to inhibit swapping?
@c ??? Does it mean the file is locked in core?
@c ??? Does this bit merely discourage swapping?
When set on an executable file, this bit inhibits swapping when that
executable is run.

On directories, this allows deletion of files in the directory only by
users who have write permission on the specific file (not anybody who has
write permission on the directory).
@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{User/Group IDs of a Process}.

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{mask} 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{mask} 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.

@pindex 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:

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

@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 ()
Return the current value of the file creation mask for the current
process.
@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.  There is actually no way to set the mode of a link, which is
always @code{-1}.

This function returns @code{0} if successful and @code{-1} if not.  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 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.
@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.

@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{User/Group IDs of a Process}), 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 syntax 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

When an existing file is opened, its attribute change time and
modification time fields are updated.  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 {struct Type} utimbuf
The @code{utimbuf} structure is used with the @code{utime} function to
specify new access and modification times for a file.  It contains at
least 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 syntax 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 syntax 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