| NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON | The Linux Programming Interface |
ACCESS(2) Linux Programmer's Manual ACCESS(2)
access - check real user's permissions for a file
#include <unistd.h>
int access(const char *pathname, int mode);
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.
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.
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.
SVr4, 4.3BSD, POSIX.1-2001.
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 "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.
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.
chmod(2), chown(2), faccessat(2), open(2), setgid(2), setuid(2), stat(2),
euidaccess(3), credentials(7), path_resolution(7)
This page is part of release 3.32 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)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface