SIGCHLD

While I was trying to write a Process Monitoring "Process" for my current employer ( well actually it is Process Monitoring Daemon ) I was breaking my head for almost 1 hour before figuring out what is really going on. I knew SIGCHLD is ignored by default. But I was under impression since I block the signal and wait on that signal specifically I should get the signal. Well it turns out that I won't. You won't either...! You need to specifically install a signal handler before sigwaiting on SIGCHLD. If you remove the sigaction from the below program the while(1) loop will never get to see the SIGCHLD when a child terminates.

#include 
#include 
#include 

static void handle_signal(int signum)
{
  printf("got signal %d", signum);
}


main()
{
  sigset_t sig_set;
  struct sigaction  sa;
  int signum;
  int status;
  int pid;
  pthread_t tsig;


  sa.sa_handler=handle_signal;
  sa.sa_flags=0;
  sigemptyset(&sa.sa_mask);
  sigaction(SIGCHLD, &sa, NULL);

  pthread_sigmask(SIG_BLOCK, &sig_set, NULL);

  pid = fork();
  if ( pid == 0 ) {
      if (execve( "test.sh",
                    NULL,NULL ) < 0) {
           perror("execve");
       }
  } else  {
    while (1) {
     sigwait(&sig_set,&signum);
     if ( signum == SIGCHLD ) {
        int pid = wait(&status);
     }
    }
  }
}