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

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 opera-
       tion (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  permis-
       sion is enabled for any of the file owner, group, or other.

RETURN VALUE

       On  success  (all  requested  permissions  granted),  zero  is returned.  On error (at least one bit in mode asked for a permission that is
       denied, 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 file system.

       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 only accessible 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 "exe-
       cutable," but the execve(2) call will still fail.

       access() may not work correctly on NFS file systems with UID mapping enabled, because UID mapping is done on the server and hidden from the
       client, which checks permissions.

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 file system.  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 3.44 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								    2010-10-24								 ACCESS(2)