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:
	       _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.  Similarly, for the root user,  the	 check
       uses the set of permitted capabilities rather than the set of effective
       capabilities; and for non-root users, the check uses an	empty  set  of
       capabilities.

       This  allows  set-user-ID  programs  and capability-endowed programs to
       easily determine	 the  invoking	user's	authority.   In	 other	words,
       access()	 does  not  answer  the	 "can I read/write/execute this file?"
       question.  It answers a slightly different question: "(assuming	I'm  a
       setuid  binary)	can  the  user	who invoked me read/write/execute this
       file?", which gives set-user-ID programs	 the  possibility  to  prevent
       malicious  users	 from causing them to read files which users shouldn't
       be able to read.

       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() 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.

   C library/kernel 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
       determine 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 flag 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 4.10 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
       https://www.kernel.org/doc/man-pages/.



Linux				  2016-03-15			     ACCESS(2)