Return_Values

   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.