func (w WaitStatus) StopSignal() syscall.Signal {
	if !w.Stopped() {
		return -1
	}
	return syscall.Signal(w>>shift) & 0xFF
}

func (w WaitStatus) TrapCause() int { return -1 }

//sys	wait4(pid int32, statusp *_C_int, options int, rusage *Rusage) (wpid int32, err error)

func Wait4(pid int, wstatus *WaitStatus, options int, rusage *Rusage) (int, error) {
	var status _C_int
	rpid, err := wait4(int32(pid), &status, options, rusage)