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

NAME
       access, faccessat - check user's	permissions for	a file

SYNOPSIS
       #include	<unistd.h>

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

       #include	<fcntl.h>	    /* Definition of AT_* constants */
       #include	<unistd.h>

       int faccessat(int dirfd,	const char *pathname, int mode,	int flags);

   Feature Test	Macro Requirements for glibc (see feature_test_macros(7)):

       faccessat():
	   Since glibc 2.10:
	       _XOPEN_SOURCE >=	700 || _POSIX_C_SOURCE >= 200809L
	   Before glibc	2.10:
	       _ATFILE_SOURCE

DESCRIPTION
       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.

   faccessat()
       The  faccessat()	 system	 call  operates	in exactly the same way	as ac-
       cess(), except for the differences described here.

       If the pathname given in	pathname is relative, then it  is  interpreted
       relative	 to  the  directory  referred  to by the file descriptor dirfd
       (rather than relative to	the current working directory of  the  calling
       process,	as is done by access() for a relative pathname).

       If  pathname  is	relative and dirfd is the special value	AT_FDCWD, then
       pathname	is interpreted relative	to the current	working	 directory  of
       the calling process (like access()).

       If pathname is absolute,	then dirfd is ignored.

       flags  is  constructed  by ORing	together zero or more of the following
       values:

       AT_EACCESS
	      Perform access checks using the effective	user  and  group  IDs.
	      By default, faccessat() uses the real IDs	(like access()).

       AT_SYMLINK_NOFOLLOW
	      If  pathname  is a symbolic link,	do not dereference it: instead
	      return information about the link	itself.

       See openat(2) for an explanation	of the need for	faccessat().

RETURN VALUE
       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.

ERRORS
       access()	and faccessat()	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.

       ENAMETOOLONG
	      pathname is too long.

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

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

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

       access()	and faccessat()	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.

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

       The following additional	errors can occur for faccessat():

       EBADF  dirfd is not a valid file	descriptor.

       EINVAL Invalid flag specified in	flags.

       ENOTDIR
	      pathname is relative and dirfd is	a file descriptor referring to
	      a	file other than	a directory.

VERSIONS
       faccessat()  was	 added	to Linux in kernel 2.6.16; library support was
       added to	glibc in version 2.4.

CONFORMING TO
       access(): SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.

       faccessat(): POSIX.1-2008.

NOTES
       Warning:	Using these calls to check if a	user is	authorized to, for ex-
       ample, open a file before actually doing	so using open(2) creates a se-
       curity 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-
       LINK_NOFOLLOW.

       These calls return an error if any of the access	types in mode  is  de-
       nied, 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.

       These  calls  may not work correctly on NFSv2 filesystems with UID map-
       ping enabled, because UID mapping is done on the	server and hidden from
       the  client, which checks permissions.  (NFS versions 3 and higher per-
       form the	check on the server.)  Similar	problems  can  occur  to  FUSE
       mounts.

   C library/kernel ABI	differences
       The  raw	 faccessat() system call takes only the	first three arguments.
       The AT_EACCESS and AT_SYMLINK_NOFOLLOW flags are	 actually  implemented
       within  the glibc wrapper function for faccessat().  If either of these
       flags is	specified, then	the wrapper function employs fstatat(2)	to de-
       termine access permissions.

   Glibc notes
       On older	kernels	where faccessat() is unavailable (and when the AT_EAC-
       CESS and	AT_SYMLINK_NOFOLLOW flags are not specified), the glibc	 wrap-
       per  function  falls  back  to the use of access().  When pathname is a
       relative	pathname, glibc	constructs a pathname based  on	 the  symbolic
       link in /proc/self/fd that corresponds to the dirfd argument.

BUGS
       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,  these  calls  ignored  the	 effect	of the
       MS_NOEXEC flag if it was	used to	mount(2)  the  underlying  filesystem.
       Since kernel 2.6.20, the	MS_NOEXEC is honored

SEE ALSO
       chmod(2),  chown(2),  open(2),  setgid(2),  setuid(2), stat(2), euidac-
       cess(3),	credentials(7),	path_resolution(7), symlink(7)

COLOPHON
       This page is part of release 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-08-19			     ACCESS(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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