NAME
access - check user's permissions for a file
SYNOPSIS
#include <unistd.h>
I int access(const char * pathname , int mode );
DESCRIPTION
R 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
R pathname .
If
pathname
is a symbolic link permissions of the file referred to by this
symbolic link are tested.
mode
is either the value
R F_OK
or a mask consisting of the bitwise OR of one or more of
R R_OK , W_OK and X_OK .
R 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
R 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
R 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
R pathname .
(See also
path_resolution(7).)
ELOOP
Too many symbolic links were encountered in resolving
R 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.
R 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
R access ()
returns an error if any of the access types in the requested call
fails, even if other types might be successful.
R 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.
R Warning:
Using
R 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,
R 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,
R access ()
honors this flag.
SEE ALSO
