Signal Program using parent-child in UNIX

Let us now write a program that communicates between child and parent processes using kill() and signal().

fork() creates the child process from the parent. The pid can be checked to decide whether it is the child(==0) or the parent (pid = child process id).

The parent can then send messages to child using the pid and kill().

The child picks up these signals with signal() and calls appropriate functions. 

   An example of communicating process using signals is sig_talk.c:  
 /* sig_talk.c --- Example of how 2 processes can talk */  
 /* to each other using kill() and signal() */  
 /* We will fork() 2 process and let the parent send a few */  
 /* signals to it`s child */  
 /* cc sig_talk.c -o sig_talk */  
 #include <stdio.h>  
 #include <signal.h>  
 void sighup(); /* routines child will call upon sigtrap */  
 void sigint();  
 void sigquit();  
 main()  
 { int pid;  
  /* get child process */  
   if ((pid = fork()) < 0) {  
     perror("fork");  
     exit(1);  
   }  
   if (pid == 0)  
    { /* child */  
     signal(SIGHUP,sighup); /* set function calls */  
     signal(SIGINT,sigint);  
     signal(SIGQUIT, sigquit);  
     for(;;); /* loop for ever */  
    }  
  else /* parent */  
    { /* pid hold id of child */  
     printf("\nPARENT: sending SIGHUP\n\n");  
     kill(pid,SIGHUP);  
     sleep(3); /* pause for 3 secs */  
     printf("\nPARENT: sending SIGINT\n\n");  
     kill(pid,SIGINT);  
     sleep(3); /* pause for 3 secs */  
     printf("\nPARENT: sending SIGQUIT\n\n");  
     kill(pid,SIGQUIT);  
     sleep(3);  
    }  
 }  
 void sighup()  
 { signal(SIGHUP,sighup); /* reset signal */  
   printf("CHILD: I have received a SIGHUP\n");  
 }  
 void sigint()  
 { signal(SIGINT,sigint); /* reset signal */  
   printf("CHILD: I have received a SIGINT\n");  
 }  
 void sigquit()  
 { printf("My DADDY has Killed me!!!\n");  
  exit(0);  
 }