NAME

       access - check real user's permissions for a file

SYNOPSIS

       #include <unistd.h>

       int access(const char *pathname, int mode);

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.

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() 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() 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.

CONFORMING TO

       SVr4, 4.3BSD, POSIX.1-2001.

NOTES

       Warning: Using access() 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.

       access()  returns 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.

       access() 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.

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, access() ignored the effect of the MS_NOEXEC flag if it was  used  to  mount(2)
       the underlying filesystem.  Since kernel 2.6.20, access() honors this flag.

SEE ALSO

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

COLOPHON

       This page is part of release 3.54 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                                              2013-09-13                                          ACCESS(2)