access(2)



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,  ac-
       cess()  does not answer the "can I read/write/execute this file?" ques-
       tion.  It answers a slightly different question: "(assuming I'm  a  se-
       tuid  binary)  can  the  user  who  invoked  me read/write/execute this
       file?", which gives set-user-ID programs the possibility to prevent ma-
       licious  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  ac-
       cess(), 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  er-
       rno 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 ex-
       ample, open a file before actually doing so using open(2) creates a se-
       curity hole, because the user might exploit the short time interval be-
       tween 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() with the  flag  AT_SYM-
       LINK_NOFOLLOW.

       These  calls  return an error if any of the access types in mode is de-
       nied, 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.   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 de-
       termine 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 4.16 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)

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