Originální popis anglicky:
access - check user's permissions for a file
Návod, kniha: Linux Programmer's Manual
#include <unistd.h>
int access(const char *pathname, int mode);
access checks whether the 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 a mask consisting of one or more of
R_OK,
W_OK,
X_OK and
F_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 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-UID 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 process has appropriate privileges, an implementation may indicate
success for
X_OK even if none of the execute file permission bits are
set.
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(2).)
- ELOOP
- Too many symbolic links were encountered in resolving
pathname.
- ENAMETOOLONG
- pathname is too long.
- ENOENT
- A directory component in pathname would have been
accessible but does not exist or was 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.
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.
Using
access to check if a user is authorized to e.g. 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.
SVID, AT&T, POSIX, X/OPEN, BSD 4.3
chmod(2),
chown(2),
open(2),
path_resolution(2),
setgid(2),
setuid(2),
stat(2)
