[Qt-interest] how to trace the Unix signal?

[zhu minjue]

I want to do some specified operations in my Qt application when system
reboot.I notice that connect the 'unixSignal' in Qt can do this work.But
actually I have not seen any response.My code likes this:
class BsApp: public QApplication
{
	Q_OBJECT
public:
	BsApp(int &argc, char **argv): QApplication (argc,argv)
	{
		watchUnixSignal(SIGINT,true);
		watchUnixSignal(SIGTERM,true);
		watchUnixSignal(SIGHUP,true);		

connect(this,SIGNAL(unixSignal(int)),this,SLOT(sighandler(int)));
	}
	virtual ~BsApp(){}
public slots:
	void sighandler(int nr)
	{
		/*output the nr value to local file in order to trace */
                ...
		switch(nr) 
		{
			case SIGINT:
			case SIGTERM:
				exit(0);
				break;
			case SIGHUP:
				break;
		}
	}
};
I run my UI application,reboot the system.after the system turn on,there
is no any output.But if I explicitly send the 'TERM' signal in Shell
terminal:
$ kill -15 5323
The Qt UI application do receive the 'SIGTERM' signal and output the
result.
Does anyone know why this happen?any suggestion?