Forks and suspends

suspend.c

#define _XOPEN_SOURCE 700
#define _POSIX_C_SOURCE 200809L
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <sys/types.h>

// This was a school exercise to help us understand signals.
// I'm putting it out here for future reference,
// though we have been warned:
// waiting for child processes is better done with `wait()`
// sigsuspend() was just to show us "hey this thing exists".

void pid(int i, pid_t pids[]) {
  pids[i] = getpid();
}

void sigchld(int sig) {
  printf("[%d]: SIGCHLD (%d)\n",getpid(),sig);
}

void done() {
  printf("[%d] - DONE\n",getpid());
}

int main(int argc, char *argv[])
{
  if(argc < 2) {
    fprintf(stderr,"Wrong number of arguments. Provide a number of processes.");
    exit(-1);
  }
  sigset_t msk,omsk;
  sigemptyset(&msk); sigaddset(&msk,SIGCHLD); sigaddset(&msk,SIGINT);

  int i = 0;
  int N = atoi(argv[1]);
  pid_t pids[N+1];
  pid(i,pids);
  for (i = 1; i <= N; ++i) {
    int p = fork();
    if(p == -1) {
      perror("FORK ERROR");
    } else if(p != 0) {

      struct sigaction act;
      act.sa_handler = &sigchld;
      act.sa_flags = SA_NOCLDSTOP;
      //i'm your father
      if (sigaction(SIGCHLD,&act,NULL) < 0){
        perror("sigaction");
        exit(EXIT_FAILURE);
      }
      printf("[%d] I am a father! Mask: %d\n",getpid(),&msk);
      if(sigprocmask(SIG_BLOCK,&msk,&omsk) < 0){
        perror("block");
        exit(EXIT_FAILURE);
      }
      printf("[%d] I am sleeping. Mask: %d (%d)\n",getpid(),&msk,&omsk);
      sleep(1);
      printf("[%d] I am awake! Mask: %d (%d)\n",getpid(),&msk,&omsk);
      sigsuspend(&omsk);
      printf("[%d] I haz signal!\n",getpid());
      if(sigprocmask(SIG_UNBLOCK,&msk,NULL) < 0){
        perror("unblock");
        exit(EXIT_FAILURE);
      }
      printf("[%d] I just unblocked. Mask: %d (%d)\n",getpid(),&msk,&omsk);
      break;
    } else {
      pid(i,pids);
    }
  }
  i -= 1;
  // i is important only at the end of the chain,
  // where we want it to be `N`, not `N+1`

  if (getpid() == pids[0]) {
    //i'm the father
    printf("[%d] I'm the father\n",getpid());
    done();
    exit(0);
  } else if(i == N) {
    //i'm the youngun
    for (i = 0; i <= N; ++i) {
      printf("PID %d -> %d\n",i,pids[i]);
    }
    done();
    exit(0);
  } else {
    printf("[%d] I'm the middle man.\n",getpid());
    //i'm the middleman
    printf("%d > %d\n",getppid(),getpid());
    done();
    exit(0);
  }

  return -2;
}