ACCESS (2)
check user's permissions for a file
SYNOPSIS
#include <unistd.h>
int access(const char * pathname , int mode );
DESCRIPTION
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.
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
EACCES
The requested access would be denied to the file or search permission
is denied to one of the directories in
pathname .
EROFS
Write permission was requested for a file on a read-only filesystem.
EFAULT
pathname
points outside your accessible address space.
EINVAL
mode
was incorrectly specified.
ENAMETOOLONG
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.
ENOMEM
Insufficient kernel memory was available.
ELOOP
Too many symbolic links were encountered in resolving
pathname .
EIO
RESTRICTIONS
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.
CONFORMING TO
SVID, AT&T, POSIX, X/OPEN, BSD 4.3
SEE ALSO
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