Description

   The sighold, sigrelse, and sigignore functions provide simplified
   signal management:

   o  The sighold function adds signal to the calling process's
      signal mask.

   o  The sigrelse function removes signal from the calling
      process's signal mask.

   o  The sigignore function sets the disposition of signal to SIG_
      IGN.

   The sighold function, in conjunction with sigrelse and sigpause,
   can be used to establish critical regions of code that require
   the delivery of a signal to be temporarily deferred.

   Upon success, the sigignore function returns a value of 0.
   Otherwise, a value of -1 is returned, and errno is set to
   indicate the error.

                                  NOTE

      These interfaces are provided for compatibility only. New
      programs should use sigaction and sigprocmask to control the
      disposition of signals.