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 filereferred 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.
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.
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.
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