how

    An integer value that indicates how to change the set of masked
    signals. Use one of the following values:

    SIG_BLOCK     The resulting set is the union of the current set
                  and the signal set pointed to by the set argument.
    SIG_UNBLOCK   The resulting set is the intersection of the
                  current set and the complement of the signal set
                  pointed to by the set argument.
    SIG_SETMASK   The resulting set is the signal set pointed to by
                  the set argument.

 set

    The signal set. If the value of the set argument is:

    o  Not NULL - It points to a set of signals used to change the
       currently blocked set.

    o  NULL - The value of the how argument is not significant, and
       the process signal mask is unchanged, so you can use the call
       to inquire about currently blocked signals.

 o_set

    A non-NULL pointer to the location where the signal mask in
    effect at the time of the call is stored.