0                  Indicates a sig argument that is outside the
                       range defined in the <signal.h> header file;
                       errno is set to EINVAL.
    sig                Indicates that SIG_IGN (ignore signal) has
                       been established as the action for the signal.
    x                  Indicates that signal, ssignal, or sigvec has
                       established an action function for the signal.
                       That function is called, and its return value
                       is returned by gsignal.