access - check user's permissions for a file
#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.
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 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 [Toc] [Back]
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 [Toc] [Back]
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 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 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
stat(2), open(2), chmod(2), chown(2), setuid(2), setgid(2)
Linux 1998-01-13 ACCESS(2)
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