1 .\" This manpage is Copyright (C) 1992 Drew Eckhardt;
2 .\" 1993 Michael Haardt, Ian Jackson.
3 .\" and Copyright (C) 2007 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16 .\" manual page may be incorrect or out-of-date.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 2013-02-28 "Linux" "Linux Programmer's Manual"
45 access \- check real user's permissions for a file
48 .B #include <unistd.h>
50 .BI "int access(const char *" pathname ", int " mode );
54 checks whether the calling process can access the file
58 is a symbolic link, it is dereferenced.
62 specifies the accessibility check(s) to be performed,
63 and is either the value
65 .\" F_OK is defined as 0 on every system that I know of.
66 or a mask consisting of the bitwise OR of one or more of
67 .BR R_OK ", " W_OK ", and " X_OK .
69 tests for the existence of the file.
70 .BR R_OK ", " W_OK ", and " X_OK
71 test whether the file exists and grants read, write, and
72 execute permissions, respectively.
74 The check is done using the calling process's
76 UID and GID, rather than the effective IDs as is done when
77 actually attempting an operation (e.g.,
80 This allows set-user-ID programs to
81 easily determine the invoking user's authority.
83 If the calling process is privileged (i.e., its real UID is zero),
86 check is successful for a regular file if execute permission
87 is enabled for any of the file owner, group, or other.
89 On success (all requested permissions granted), zero is returned.
90 On error (at least one bit in
92 asked for a permission that is denied, or some other error occurred),
101 The requested access would be denied to the file, or search permission
102 is denied for one of the directories in the path prefix of
105 .BR path_resolution (7).)
108 Too many symbolic links were encountered in resolving
118 does not exist or is a dangling symbolic link.
121 A component used as a directory in
123 is not, in fact, a directory.
126 Write permission was requested for a file on a read-only file system.
133 points outside your accessible address space.
137 was incorrectly specified.
140 An I/O error occurred.
143 Insufficient kernel memory was available.
146 Write access was requested to an executable which is being
149 SVr4, 4.3BSD, POSIX.1-2001.
155 to check if a user is authorized to, for example,
156 open a file before actually doing so using
158 creates a security hole, because the user might exploit the short time
159 interval between checking and opening the file to manipulate it.
160 .BR "For this reason, the use of this system call should be avoided" .
161 (In the example just described,
162 a safer alternative would be to temporarily switch the process's
163 effective user ID to the real ID and then call
167 always dereferences symbolic links.
168 If you need to check the permissions on a symbolic link, use
171 .BR AT_SYMLINK_NOFOLLOW .
174 returns an error if any of the access types in
176 is denied, even if some of the other access types in
180 If the calling process has appropriate privileges (i.e., is superuser),
181 POSIX.1-2001 permits an implementation to indicate success for an
183 check even if none of the execute file permission bits are set.
184 .\" HPU-UX 11 and Tru64 5.1 do this.
185 Linux does not do this.
187 A file is only accessible if the permissions on each of the
188 directories in the path prefix of
190 grant search (i.e., execute) access.
191 If any directory is inaccessible, then the
193 call will fail, regardless of the permissions on the file itself.
195 Only access bits are checked, not the file type or contents.
196 Therefore, if a directory is found to be writable,
197 it probably means that files can be created in the directory,
198 and not that the directory can be written as a file.
199 Similarly, a DOS file may be found to be "executable," but the
201 call will still fail.
204 may not work correctly on NFS file systems with UID mapping enabled,
205 because UID mapping is done on the server and hidden from the client,
206 which checks permissions.
207 Similar problems can occur to FUSE mounts.
209 In kernel 2.4 (and earlier) there is some strangeness in the handling of
212 If all categories of execute permission are disabled
213 for a nondirectory file, then the only
215 test that returns \-1 is when
227 returns 0 for such files.
228 .\" This behavior appears to have been an implementation accident.
229 Early 2.6 kernels (up to and including 2.6.3)
230 also behaved in the same way as kernel 2.4.
232 In kernels before 2.6.20,
234 ignored the effect of the
236 flag if it was used to
238 the underlying file system.
252 .BR path_resolution (7)