有關signal的函數（像SIG_IGN等等）

http://www.cplusplus.com/reference/csignal/signal/

func

A pointer to a function. This may either be a function defined by the programmer or one of the following predefined functions:

SIG_DFL	Default handling: The signal is handled by the default action for that particular signal.
SIG_IGN	Ignore Signal: The signal is ignored.

If a function, it should follow the following prototype:
void handler_function ( int parameter );