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 sigrelse 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.