text: #include <sys/types.h>
  #include <sys/stat.h>
  #include <fcntl.h>
  #include <unistd.h>
  #include <fcntl.h>           
  #include <sys/stat.h>
  #include <stdio.h>
  #include <signal.h>
  #include <sys/time.h>
  #include <sys/resource.h>
  #include <stdio.h>
  #include <sys/ipc.h>
  #include <sys/sem.h>   
  #include <stdlib.h>    
  
  void f()
  {
  	printf("Hello world!\n");
  	
  }
  
  int main()
  {	
  	int ret;
  	struct itimerval value;
  	value.it_value.tv_sec=10;
  	value.it_value.tv_usec=10; 
  	ret=setitimer(ITIMER_PROF,&value,0);
  	printf("%d\n",ret);
  	signal(14,f);
  	sleep(10);
  }
  
  //when it is in both kernel and user mode
  