1 .\" This manpage is Copyright (C) 1992 Drew Eckhardt;
2 .\" and Copyright (C) 1993 Michael Haardt, Ian Jackson.
3 .\" and Copyright (C) 2004, 2006, 2007, 2014 Michael Kerrisk <mtk.manpages@gmail.com>
5 .\" %%%LICENSE_START(VERBATIM)
6 .\" Permission is granted to make and distribute verbatim copies of this
7 .\" manual provided the copyright notice and this permission notice are
8 .\" preserved on all copies.
10 .\" Permission is granted to copy and distribute modified versions of this
11 .\" manual under the conditions for verbatim copying, provided that the
12 .\" entire resulting derived work is distributed under the terms of a
13 .\" permission notice identical to this one.
15 .\" Since the Linux kernel and libraries are constantly changing, this
16 .\" manual page may be incorrect or out-of-date. The author(s) assume no
17 .\" responsibility for errors or omissions, or for damages resulting from
18 .\" the use of the information contained herein. The author(s) may not
19 .\" have taken the same level of care in the production of this manual,
20 .\" which is licensed free of charge, as they might when working
23 .\" Formatted or processed versions of this manual, if unaccompanied by
24 .\" the source, must acknowledge the copyright and authors of this work.
27 .\" Modified 1993-07-21 Rik Faith (faith@cs.unc.edu)
28 .\" Modified 1994-08-21 by Michael Chastain (mec@shell.portal.com):
29 .\" Removed note about old kernel (pre-1.1.44) using wrong id on path.
30 .\" Modified 1996-03-18 by Martin Schulze (joey@infodrom.north.de):
31 .\" Stated more clearly how it behaves with symbolic links.
32 .\" Added correction due to Nick Duffek (nsd@bbc.com), aeb, 960426
33 .\" Modified 1996-09-07 by Michael Haardt:
34 .\" Restrictions for NFS
35 .\" Modified 1997-09-09 by Joseph S. Myers <jsm28@cam.ac.uk>
36 .\" Modified 1998-01-13 by Michael Haardt:
37 .\" Using access is often insecure
38 .\" Modified 2001-10-16 by aeb
39 .\" Modified 2002-04-23 by Roger Luethi <rl@hellgate.ch>
40 .\" Modified 2004-06-23 by Michael Kerrisk
41 .\" 2007-06-10, mtk, various parts rewritten, and added BUGS section.
43 .TH ACCESS 2 2014-02-21 "Linux" "Linux Programmer's Manual"
45 access, faccessat \- check user's permissions for a file
48 .B #include <unistd.h>
50 .BI "int access(const char *" pathname ", int " mode );
52 .BR "#include <fcntl.h> " "/* Definition of AT_* constants */"
53 .B #include <unistd.h>
55 .BI "int faccessat(int " dirfd ", const char *" pathname ", int " \
56 mode ", int " flags );
60 Feature Test Macro Requirements for glibc (see
61 .BR feature_test_macros (7)):
70 _XOPEN_SOURCE\ >=\ 700 || _POSIX_C_SOURCE\ >=\ 200809L
80 checks whether the calling process can access the file
84 is a symbolic link, it is dereferenced.
88 specifies the accessibility check(s) to be performed,
89 and is either the value
91 .\" F_OK is defined as 0 on every system that I know of.
92 or a mask consisting of the bitwise OR of one or more of
93 .BR R_OK ", " W_OK ", and " X_OK .
95 tests for the existence of the file.
96 .BR R_OK ", " W_OK ", and " X_OK
97 test whether the file exists and grants read, write, and
98 execute permissions, respectively.
100 The check is done using the calling process's
102 UID and GID, rather than the effective IDs as is done when
103 actually attempting an operation (e.g.,
106 This allows set-user-ID programs to
107 easily determine the invoking user's authority.
109 If the calling process is privileged (i.e., its real UID is zero),
112 check is successful for a regular file if execute permission
113 is enabled for any of the file owner, group, or other.
117 system call operates in exactly the same way as
119 except for the differences described here.
121 If the pathname given in
123 is relative, then it is interpreted relative to the directory
124 referred to by the file descriptor
126 (rather than relative to the current working directory of
127 the calling process, as is done by
129 for a relative pathname).
139 is interpreted relative to the current working
140 directory of the calling process (like
150 is constructed by ORing together zero or more of the following values:
153 Perform access checks using the effective user and group IDs.
156 uses the real IDs (like
159 .B AT_SYMLINK_NOFOLLOW
162 is a symbolic link, do not dereference it:
163 instead return information about the link itself.
167 for an explanation of the need for
170 On success (all requested permissions granted, or
174 and the file exists), zero is returned.
175 On error (at least one bit in
177 asked for a permission that is denied, or
181 and the file does not exist, or some other error occurred),
184 is set appropriately.
192 The requested access would be denied to the file, or search permission
193 is denied for one of the directories in the path prefix of
196 .BR path_resolution (7).)
199 Too many symbolic links were encountered in resolving
209 does not exist or is a dangling symbolic link.
212 A component used as a directory in
214 is not, in fact, a directory.
217 Write permission was requested for a file on a read-only filesystem.
226 points outside your accessible address space.
230 was incorrectly specified.
233 An I/O error occurred.
236 Insufficient kernel memory was available.
239 Write access was requested to an executable which is being
242 The following additional errors can occur for
247 is not a valid file descriptor.
250 Invalid flag specified in
257 is a file descriptor referring to a file other than a directory.
260 was added to Linux in kernel 2.6.16;
261 library support was added to glibc in version 2.4.
264 SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.
271 Using these calls to check if a user is authorized to, for example,
272 open a file before actually doing so using
274 creates a security hole, because the user might exploit the short time
275 interval between checking and opening the file to manipulate it.
276 .BR "For this reason, the use of this system call should be avoided" .
277 (In the example just described,
278 a safer alternative would be to temporarily switch the process's
279 effective user ID to the real ID and then call
283 always dereferences symbolic links.
284 If you need to check the permissions on a symbolic link, use
287 .BR AT_SYMLINK_NOFOLLOW .
289 These calls return an error if any of the access types in
291 is denied, even if some of the other access types in
295 If the calling process has appropriate privileges (i.e., is superuser),
296 POSIX.1-2001 permits an implementation to indicate success for an
298 check even if none of the execute file permission bits are set.
299 .\" HPU-UX 11 and Tru64 5.1 do this.
300 Linux does not do this.
302 A file is accessible only if the permissions on each of the
303 directories in the path prefix of
305 grant search (i.e., execute) access.
306 If any directory is inaccessible, then the
308 call will fail, regardless of the permissions on the file itself.
310 Only access bits are checked, not the file type or contents.
311 Therefore, if a directory is found to be writable,
312 it probably means that files can be created in the directory,
313 and not that the directory can be written as a file.
314 Similarly, a DOS file may be found to be "executable," but the
316 call will still fail.
319 may not work correctly on NFSv2 filesystems with UID mapping enabled,
320 because UID mapping is done on the server and hidden from the client,
321 which checks permissions. (NFS versions 3 and higher perform the check on
323 Similar problems can occur to FUSE mounts.
329 system call takes only the first three arguments.
333 .B AT_SYMLINK_NOFOLLOW
334 flags are actually implemented within the glibc wrapper function for
336 If either of these flags is specified, then the wrapper function employs
338 to determine access permissions.
340 In kernel 2.4 (and earlier) there is some strangeness in the handling of
343 If all categories of execute permission are disabled
344 for a nondirectory file, then the only
346 test that returns \-1 is when
358 returns 0 for such files.
359 .\" This behavior appears to have been an implementation accident.
360 Early 2.6 kernels (up to and including 2.6.3)
361 also behaved in the same way as kernel 2.4.
363 In kernels before 2.6.20,
364 these calls ignored the effect of the
366 flag if it was used to
368 the underlying filesystem.
369 Since kernel 2.6.20, the
381 .BR path_resolution (7),
384 This page is part of release 3.67 of the Linux
387 A description of the project,
388 information about reporting bugs,
389 and the latest version of this page,
391 \%http://www.kernel.org/doc/man\-pages/.