Description

   The ualarm function causes the SIGALRM signal to be generated for
   the calling process after the number of real-time microseconds
   specified by useconds has elapsed. When the interval argument is
   nonzero, repeated timeout notification occurs with a period in
   microseconds specified by interval. If the notification signal
   SIGALRM is not intercepted or is ignored, the calling process is
   terminated.

   If you call a combination of ualarm and setitimer functions, and
   the AST status is disabled, the return value is invalid.

   If you call a combination of ualarm and setitimer functions, and
   the AST status is enabled, the return value is valid.

   This is because you cannot invoke an AST handler to clear the
   previous value of the timer when ASTs are disabled or invoked
   from a handler that was invoked at AST level.

                                  NOTE

      Interactions between ualarm and either alarm, or sleep are
      unspecified.

   See also setitimer.