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


   #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)):

       Since glibc 2.10:
           _POSIX_C_SOURCE >= 200809L
       Before glibc 2.10:


   access()  checks  whether  the  calling  process  can  access  the file
   pathname.  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

   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.

   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

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

          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().


   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.


   access() and faccessat() shall fail if:

   EACCES The  requested  access  would  be  denied to the file, or search
          permission 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

   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.

          Write access was requested  to  an  executable  which  is  being

   The following additional errors can occur for faccessat():

   EBADF  dirfd is not a valid file descriptor.

   EINVAL Invalid flag specified in flags.

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


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


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

   faccessat(): POSIX.1-2008.


   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
   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

   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.
   Therefore,  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
   "executable," but the execve(2) call will still fail.

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

   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_EACCESS  and AT_SYMLINK_NOFOLLOW flags are not specified), the glibc
   wrapper 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.


   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.


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


   This  page  is  part of release 4.09 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

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