/* sigExample.c
 *
 * Neal Nelson 2009.11.04
 * Modified code from Nutt p91 illustrating signal handling
 *
 * Note that signals don't queue, so there is a race condition between
 *   the parent sending SIGUSR2 and the parent sending SIGTERM
 *   I guess the SIGTERM overwrites the SIGUSR2 without the sleep.
 *   See the code below.
 *
 */

#include <stdio.h>
#include <unistd.h>     /* for pause() */
#include <signal.h>

/* prototype must occur before sig_handler is referenced */
static void sig_handler(int);

main()
{ int i, parent_pid, child_pid, status;

  /* prepare the sig_handler routine to catch SIGUSR1 and SIGUSR2 */
  if (signal(SIGUSR1, sig_handler) == SIG_ERR)
    printf("Parent: Unable to create handler for SIGUSR1\n");

  if (signal(SIGUSR2, sig_handler) == SIG_ERR)
    printf("Parent: Unable to create handler for SIGUSR2\n");

  parent_pid = getpid();
  if ((child_pid = fork()) == 0)      /* child */
  { printf("Child: raising SIGUSR1 signal to parent\n");  
    kill(parent_pid, SIGUSR1);
    for (;;) pause();                 /* child busy wait for SIGUSR2 */  
  } else                              /* parent */
  { printf("Parent: raising SIGUSR2 signal to child\n");  
    kill(child_pid, SIGUSR2);

    /* Without the sleep child doesn't catch interrupt before termination */
    sleep(1);

    printf("Parent: terminating child ...\n");
    kill(child_pid, SIGTERM);         /* raise the SIGTERM signal to child */
    wait(&status);                    /* parent wait for child termination */
    printf("done.\n");
  }
}

static void sig_handler(int signo)
{
  switch(signo)
  { case SIGUSR1:
      printf("Parent: Received SIGUSR1 signal from child\n");
      break;
    case SIGUSR2:
      printf("Child: Received SIGUSR2 signal from parent\n");
      break;
    default: break; /* should never get here */
  }
  return; 
}