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-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 process has appropriate privileges, an implementation may
indicate success for
X_OK even if none of the execute file permission bits are set.
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.
