Waiting for all child processes before parent resumes execution UNIX

Question

In my program I am forking (in parallel) child processes in a finite while loop and doing exec on each of them. I want the parent process to resume execution (the point after this while loop ) only after all children have terminated. How should I do that?

i have tried several approaches. In one approach, I made parent pause after while loop and sent some condition from SIGCHLD handler only when waitpid returned error ECHILD(no child remaining) but the problem I am facing in this approach is even before parent has finished forking all processes, retStat becomes -1

    void sigchld_handler(int signo) {
        pid_t pid;
        while((pid= waitpid(-1,NULL,WNOHANG)) > 0);
        if(errno == ECHILD) {
         retStat = -1;
        }
    }

    **//parent process code**
    retStat = 1;
    while(some condition) {
       do fork(and exec);
    }

    while(retStat > 0)
        pause();
//This is the point where I want execution to resumed only when all children have finished

Answer 1

I think you should use the waitpid() call. It allows you to wait for "any child process", so if you do that the proper number of times, you should be golden.

If that fails (not sure about the guarantees), you could do the brute-force approach sitting in a loop, doing a waitpid() with the NOHANG option on each of your child PIDs, and then delaying for a while before doing it again.

Answer 2

Instead of calling waitpid in the signal handler, why not create a loop after you have forked all the processes as follows:

while (pid = waitpid(-1, NULL, 0)) {
   if (errno == ECHILD) {
      break;
   }
}

The program should hang in the loop until there are no more children. Then it will fall out and the program will continue. As an additional bonus, the loop will block on waitpid while children are running, so you don't need a busy loop while you wait.

You could also use wait(NULL) which should be equivalent to waitpid(-1, NULL, 0). If there's nothing else you need to do in SIGCHLD, you can set it to SIG_IGN.