0                  Successful completion.
    -1                 Indicates an error. The function sets errno to
                       one of the following values:

                       o  EINVAL - The value of the uid argument
                          is invalid and not supported by the
                          implementation.

                       o  EPERM - The process does not have
                          appropriate privileges and uid does not
                          match the real user ID or the saved set-
                          user-ID.