faccessat(2) — Linux manual page


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)):

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

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.  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.

       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().

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 is too long.

       ENOENT A component of pathname does not exist or is a dangling
              symbolic link.

              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.

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

       access() always dereferences symbolic links.  If you need to check
       the permissions on a symbolic link, use faccessat() 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 fails, 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 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

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 flag 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 5.08 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

Linux                            2016-03-15                        ACCESS(2)

Pages that refer to this page: strace(1)creat(2)open(2)openat(2)syscalls(2)eaccess(3)euidaccess(3)credentials(7)signal-safety(7)symlink(7)