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ACCESS(2)		   Linux Programmer's Manual		     ACCESS(2)

       access -	check real user's permissions for a file

       #include	<unistd.h>

       int access(const	char *pathname,	int mode);

       access()	 checks	 whether the calling process can access	the file path-
       name.  If pathname is a symbolic	link, it is dereferenced.

       The mode	specifies the accessibility check(s) to	be performed,  and  is
       either the value	F_OK, or a mask	consisting of the bitwise OR of	one or
       more of R_OK, W_OK, and X_OK.  F_OK tests  for  the  existence  of  the
       file.   R_OK,  W_OK,  and  X_OK test whether the	file exists and	grants
       read, write, and	execute	permissions, respectively.

       The check is done using the calling process's real UID and GID,	rather
       than the	effective IDs as is done when actually attempting an operation
       (e.g., open(2)) on the file.  This allows set-user-ID programs to  eas-
       ily determine the invoking user's authority.

       If the calling process is privileged (i.e., its real UID	is zero), then
       an X_OK check is	successful for a regular file if execute permission is
       enabled for any of the file owner, group, or other.

       On  success (all	requested permissions granted, or mode is F_OK and the
       file exists), zero is returned.	On error (at least  one	 bit  in  mode
       asked  for  a  permission  that is denied, or mode is F_OK and the file
       does not	exist, or some other error occurred), -1 is returned, and  er-
       rno is set appropriately.

       access()	shall fail if:

       EACCES The requested access would be denied to the file,	or search per-
	      mission is denied	for one	of the directories in the path	prefix
	      of pathname.  (See also path_resolution(7).)

       ELOOP  Too many symbolic	links were encountered in resolving pathname.

	      pathname is too long.

       ENOENT A	component of pathname does not exist or	is a dangling symbolic

	      A	component used as a directory in pathname is not, in  fact,  a

       EROFS  Write  permission	 was  requested	for a file on a	read-only file

       access()	may fail if:

       EFAULT pathname points outside your accessible address space.

       EINVAL mode was incorrectly specified.

       EIO    An I/O error occurred.

       ENOMEM Insufficient kernel memory was available.

	      Write access was requested to an executable which	is being  exe-

       SVr4, 4.3BSD, POSIX.1-2001.

       Warning:	 Using access()	to check if a user is authorized to, for exam-
       ple, open a file	before actually	doing so using open(2) creates a secu-
       rity  hole,  because the	user might exploit the short time interval be-
       tween checking and opening the file to manipulate it.  For this reason,
       the  use	 of  this system call should be	avoided.  (In the example just
       described, a safer alternative  would  be  to  temporarily  switch  the
       process's effective user	ID to the real ID and then call	open(2).)

       access()	 always	dereferences symbolic links.  If you need to check the
       permissions on a	symbolic link, use faccessat(2)	with the flag  AT_SYM-

       access()	returns	an error if any	of the access types in mode is denied,
       even if some of the other access	types in mode are permitted.

       If the calling process has appropriate privileges (i.e.,	is superuser),
       POSIX.1-2001  permits an	implementation to indicate success for an X_OK
       check even if none of the execute file permission bits are set.	 Linux
       does not	do this.

       A file is accessible only if the	permissions on each of the directories
       in the path prefix of pathname grant search (i.e., execute) access.  If
       any  directory  is  inaccessible, then the access() call	will fail, re-
       gardless	of the permissions on the file itself.

       Only access bits	are checked, not the file type	or  contents.	There-
       fore,  if  a  directory is found	to be writable,	it probably means that
       files can be created in the directory, and not that the	directory  can
       be  written  as a file.	Similarly, a DOS file may be found to be "exe-
       cutable," but the execve(2) call	will still fail.

       access()	may not	work correctly on NFS file systems  with  UID  mapping
       enabled,	 because UID mapping is	done on	the server and hidden from the
       client, which checks permissions.  Similar problems can occur  to  FUSE

       In  kernel  2.4 (and earlier) there is some strangeness in the handling
       of X_OK tests for superuser.  If	all categories of  execute  permission
       are  disabled for a nondirectory	file, then the only access() test that
       returns -1 is when mode is specified as just X_OK; if R_OK or  W_OK  is
       also  specified in mode,	then access() returns 0	for such files.	 Early
       2.6 kernels (up to and including	2.6.3) also behaved in the same	way as
       kernel 2.4.

       In  kernels before 2.6.20, access() ignored the effect of the MS_NOEXEC
       flag if it was used to mount(2) the underlying file system.  Since ker-
       nel 2.6.20, access() honors this	flag.

       chmod(2),   chown(2),   faccessat(2),  open(2),	setgid(2),  setuid(2),
       stat(2),	euidaccess(3), credentials(7), path_resolution(7)

       This page is part of release 3.53 of the	Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found	at

Linux				  2013-04-16			     ACCESS(2)


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