access - check user's permissions for a file
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 sys-
tem object) whose name is pathname. If pathname is a sym-
bolic link permissions of the file referred to by this sym-
bolic link are tested.
mode is a mask consisting of one or more of R_OK, W_OK, X_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. Simi-
larly, a DOS file may be found to be "executable," but the
execve(2) call will still fail.
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.
EACCES The requested access would be denied to the file or
search permission is denied to one of the direc-
tories in pathname.
EROFS Write permission was requested for a file on a
EFAULT pathname points outside your accessible address
EINVAL mode was incorrectly specified.
pathname is too long.
ENOENT A directory component in pathname would have been
accessible but does not exist or was a dangling sym-
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 resolv-
EIO An I/O error occurred.
access returns an error if any of the access types in the
requested call fails, even if other types might be success-
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 secu-
rity hole, because the user might exploit the short time
interval between checking and opening the file to manipulate
SVID, AT&T, POSIX, X/OPEN, BSD 4.3
stat(2), open(2), chmod(2), setuid(2), setgid(2).