Spawning multiple processes

• Reaping multiple child processes:
  • Of course, the parent is allowed to call fork multiple times, provided it eventually reaps the child processes after they exit.
  • If we want to reap child processes as they exit without concern for the ordering in which they fork, then this does the trick:
  • Check this out (code is in lecture examples folder processes/reap-as-they-exit.c and can also be viewed right here)

  int main(int argc, char *argv[]) {
    for (size_t i = 0; i < kNumChildren; i++) {
      pid_t pid = fork();
      exitIf(pid == -1, kForkFail, stderr, "Fork function failed.\n");
      if (pid == 0) exit(110 + i);
    }
  
    while (true) {
      int status;
      pid_t pid = waitpid(-1, &status, 0);
      if (pid == -1) break;
      if (WIFEXITED(status)) {
        printf("Child %d exited: status %d\n", pid, WEXITSTATUS(status));
      } else {
        printf("Child %d exited abnormally.\n", pid);
      }
    }
  
    exitUnless(errno == ECHILD, kWaitFail, stderr, "waitpid failed.\n");
    return 0;
  }

  • Note we feed a -1 as the first argument to waitpid. That -1 states we want to hear about any child as it exits.
  • Eventually, all children exit (normally or not) and waitpid correctly returns -1 to signal that all child processes have ended.
    When waitpid returns -1, it sets a global variable called errno to the constant ECHILD as a signal that -1 was returned because all child processes terminated. Interestingly enough, that's the "error" we want.