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 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.   This
       allows set-user-ID programs to easily 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  inter‐
       preted 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, facces‐
              sat() 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  permission  is  denied
              for  one of the directories in the path prefix of pathname.  (See also path_resolu‐
              tion(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 executed.

       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  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_SYMLINK_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 per‐
       mits 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 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 determine access permissions.

   Glibc notes
       On older kernels where faccessat() is unavailable (and when  the  AT_EACCESS  and  AT_SYM‐
       LINK_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.

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), euidaccess(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)