Return_Values

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

                      o  EINVAL - The value of the ruid or euid
                         argument is invalid or out of range.

                      o  EPERM - The current process does not have
                         the IMPERSONATE privilege, and either an
                         attempt was made to change the effective
                         user ID to a value other than the real user
                         ID or the saved set-user-ID, or an attempt
                         was made to change the real user ID to a
                         value not permitted by the implementation.