Thread: problem with timer & signal

I write a timer which periodically send signal interrupt to implement function "contextswitch"
but something wrong with my code
it can print "inside thr_quantum", "inside timer", "inside interrupt"
but never print "inside contextswitch"
I cannot find out what's wrong with it
please help
Thank you very much!

The following is my code.

Code:

/*myhandler: contextswitch*/ 
void contextswitch(void) 
{ 
fprintf(stderr, "inside contextswitch\n"); 
      ............. 
      ............. 
} 

/* set up myhandler for SIGALRM */ 
static int th_interrupt(void) 
{ 
fprintf(stderr, "inside interrupt\n"); 
struct sigaction act; 
act.sa_handler = contextswitch; 
act.sa_flags = 0; 
return (sigemptyset(&act.sa_mask) || sigaction(SIGALRM , &act, NULL)); 
} 

/* set ITIMER_REAL for 5-ms intervals */ 
static int th_timer(time_t n) 
{ 
fprintf(stderr, "inside timer\n"); 
struct itimerval value; 
value.it_interval.tv_sec = 0; 
value.it_interval.tv_usec = n; 
value.it_value = value.it_interval; 
return (setitimer(ITIMER_REAL, &value, NULL)); 
} 


void thr_quantum(int n) 
{ 
fprintf(stderr, "inside quantum\n"); 
if (th_interrupt() == -1) { 
      perror("Cannot set up interrupt"); 
      exit( 1 ); 
  } 
if (th_timer(n) == -1) { 
      perror("Cannot set up timer"); 
      exit( 1 ); 
  } 
}