Name
access — check user's permissions for a file
Synopsis
#include <unistd.h>
int
access( |
const char * |
pathname, |
| |
int |
mode); |
DESCRIPTION
access() checks whether the
calling process would be allowed to read, write or test for
existence of the file (or other file system object) whose
name is pathname. If
pathname is a
symbolic link permissions of the file referred to by this
symbolic link are tested.
mode 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.
R_OK, W_OK and X_OK
request checking whether the file exists and has read, write
and execute permissions, respectively. F_OK just requests checking for the
existence of the file.
The tests depend on the permissions of the directories
occurring in the path to the file, as given in pathname, and on the
permissions of directories and files referred to by symbolic
links encountered on the way.
The check is done with the calling process's real UID and GID, rather than
with the effective IDs as is done when actually attempting an
operation. This is to allow set-user-ID programs to easily
determine the invoking user's authority.
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.
If the calling process has appropriate privileges, an
implementation may indicate success for X_OK even if none of the execute file
permission bits are set.
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 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
access() returns an error if
any of the access types in the requested call fails, even if
other types might be successful.
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.
![[Warning]](../stylesheet/warning.png) |
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.
|
Linux Notes
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() honours 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)
This manpage is Copyright (C) 1992 Drew Eckhardt;
1993 Michael Haardt, Ian Jackson.
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 (faith@cs.unc.edu)
Modified 1994-08-21 by Michael Chastain (mec@shell.portal.com):
Removed note about old kernel (pre-1.1.44) using wrong id on path.
Modified 1996-03-18 by Martin Schulze (joey@infodrom.north.de):
Stated more clearly how it behaves with symbolic links.
Added correction due to Nick Duffek (nsd@bbc.com), aeb, 960426
Modified 1996-09-07 by Michael Haardt:
Restrictions for NFS
Modified 1997-09-09 by Joseph S. Myers <jsm28@cam.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 <rl@hellgate.ch>
Modified 2004-06-23 by Michael Kerrisk
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