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), 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] 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.

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 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 "executable," 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 3.24 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/.


This manpage is Copyright (C) 1992 Drew Eckhardt;
                              1993 Michael Haardt, Ian Jackson.
and Copyright (C) 2007 Michael Kerrisk <mtk.manpagesgmail.com>

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
professionally.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.

Modified 1993-07-21 Rik Faith (faithcs.unc.edu)
Modified 1994-08-21 by Michael Chastain (mecshell.portal.com):
  Removed note about old kernel (pre-1.1.44) using wrong id on path.
Modified 1996-03-18 by Martin Schulze (joeyinfodrom.north.de):
  Stated more clearly how it behaves with symbolic links.
Added correction due to Nick Duffek (nsdbbc.com), aeb, 960426
Modified 1996-09-07 by Michael Haardt:
  Restrictions for NFS
Modified 1997-09-09 by Joseph S. Myers <jsm28cam.ac.uk>
Modified 1998-01-13 by Michael Haardt:
  Using access is often insecure
Modified 2001-10-16 by aeb
Modified 2002-04-23 by Roger Luethi <rlhellgate.ch>
Modified 2004-06-23 by Michael Kerrisk
2007-06-10, mtk, various parts rewritten, and added BUGS section.