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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
       access(), 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
       errno 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
       example,	 open  a file before actually doing so using open(2) creates a
       security hole, because the user might exploit the short	time  interval
       between	checking and opening the file to manipulate it.	 For this rea‐
       son, 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
       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,
       regardless 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.

   faccessat ()
       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
       determine access permissions.

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 3.65 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2014-02-21			     ACCESS(2)
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