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

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

NAME         top

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

SYNOPSIS         top

       #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         top

       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 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         top

       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         top

       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.

       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         top

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

CONFORMING TO         top

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

       faccessat(): POSIX.1-2008.

NOTES         top

       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 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 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         top

       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         top

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

COLOPHON         top

       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)