How to wait for all the children to terminate before the parent moves on?

问题

I'm doing some parallel programming (multiprocessing) and I need the parent to:

1) Fork several children

2) AFTER all the children have been created, simply WAIT for all of them to terminate

3) After all the children are terminated, do some other work.

This is what I tried:

 int main(int argc, char **argv[])
{

  int j, i;
  pid_t children[3];
  int pid;

  // Fork the processes
  for(j = 0; j < 3; j++){
    if((children[j] = fork()) == 0){
      // Child process
      for(i = 0; i < 2; i++){
        printf("child %d printing: %d\n", j, i);
      }
    }else {
        // In parent now
        while (pid = waitpid(-1, NULL, 0)) {
            if (errno == ECHILD) {
                break;
            }
        }

        printf("all children terminated. in parent now\n");
    }
  }

  return 0;
}

Doesn't give the correct output. "all children terminated. in parent now" gets printed out several times, even before all the children are dead. Also, for each process, I should see only 2 outputs, but I see more. Any help would be appreicated.

回答1:

Is this more what you are trying to achieve? I've just set a simple count for each child with a delay to increase the parallelisation visibility.

#include <sys/wait.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>

int main(int argc, char **argv[])
{
  int i, j, k;
  int pid;

  // Fork the processes
  for(j = 0; j < 3; j++)
  {
    if (fork() == 0)
    {
        printf("starting child %d\n", j);
        // Child process - do your child stuff
        for (i = 0; i < 5; ++i)
        {
          for (k = 0; k < 10000000; ++k);
          printf("child %d printing: %d\n", j, i);
        }
         printf("child %d ending\n", j);
        // then just quit
        exit(0);
    }
  }

  j = 1;
  while (wait(NULL) > 0)
  {
    printf("%d child completed\n", j++);
  }

  // In parent now
  printf("all children terminated. in parent now\n");

  return 0;
}

The output I get is

starting child 0
starting child 1
child 0 printing: 0
starting child 2
child 1 printing: 0
child 0 printing: 1
child 2 printing: 0
child 1 printing: 1
child 0 printing: 2
child 0 printing: 3
child 2 printing: 1
child 1 printing: 2
child 1 printing: 3
child 0 printing: 4
child 0 ending
child 2 printing: 2
1 child completed
child 1 printing: 4
child 1 ending
2 child completed
child 2 printing: 3
child 2 printing: 4
child 2 ending
3 child completed
all children terminated. in parent now

Ken

回答2:

Each child is executing the loop and forking. You need to break out of the j loop after printing output. Since those grandchildren are not children of the original parent, and the first generation children are never waiting for them, the order of output is unspecified. Also, you need to bring the waitpid out of the j loop and not execute it until all the children have been forked.

来源：https://stackoverflow.com/questions/13465284/how-to-wait-for-all-the-children-to-terminate-before-the-parent-moves-on

标签

fork

multiprocessing

wait