posix_access constants

Note:

Please note that some of these constants may not be available on every system.

POSIX_F_OK (int)
Check whether the file exists.
POSIX_R_OK (int)
Check whether the file exists and has read permissions.
POSIX_W_OK (int)
Check whether the file exists and has write permissions.
POSIX_X_OK (int)
Check whether the file exists and has execute permissions.