Member Avatar for wildplace

so i fork n process:

for(i=0; i<numOfProcess; i++)
    pid == fork();
    if(pid == -1)
        //error handling
    if(pid == 0){
        //child, do something
        exit(0);
    }
    else{ //parent
        //i want to wait till all children are done before continue..how?
        //i tried wait(NULL), and waitpid(pid, NULL, 0)
        //but both wont work..?

    //...........
    //something done in parent
    }
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  • Latest Post Latest Post by histrungalot

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Generated

To block the parent until all children have finished, start all children first, then reap them all. wait/waitpid returns one child at a time, so you must loop until there are no children left. Also handle EINTR (signals) and be aware that if SIGCHLD is ignored or SA_NOCLDWAIT is set, reaping is disabled and wait/waitpid will fail with ECHILD even though children ran.

Example of a robust reap-any-child loop after the fork loop:

int remaining = numOfProcess;
int status;
while (remaining > 0) {
    pid_t w = waitpid(-1, &status, 0);   // -1 waits for any child
    if (w == -1) {
        if (errno == EINTR) continue;    // interrupted by a signal, retry
        if (errno == ECHILD) break;      // no children (possibly due to SIGCHLD ignored)
        perror("waitpid");
        break;
    }
    --remaining;
    if (WIFEXITED(status)) {
        int code = WEXITSTATUS(status);  // collect exit code if needed
        (void)code;
    } else if (WIFSIGNALED(status)) {
        int sig = WTERMSIG(status);      // child died by signal
        (void)sig;
    }
}

If you need to wait for specific children, store each pid_t returned by fork() and call waitpid(pid, ...) for each. Do not call wait inside the fork loop if you want children to run concurrently. See waitpid(2) and POSIX wait for semantics, and signal(7) for SIGCHLD behavior and ECHILD edge cases.

Member Avatar for histrungalot

This works. You have to wait for all started.

#include <sys/types.h>
#include <sys/wait.h>
#include <cstdlib>
#include <iostream>
#include <unistd.h>
using namespace std;

int main(){
  int pid,i,numOfProcess(10);

  for(i=0; i<numOfProcess; i++){
    pid = fork();    // <-------- You had a == not = 
    if(pid == -1){   // <-------- You didn't have a open {, close }
        //error handling
    }
    else if(pid == 0){
        //child, do something
        cout << "Child " << i << " sleeping(" << i << ")" << endl;
        sleep(i);
        exit(0);
    }
    else{ //parent
        //i want to wait till all children are done before continue..how?
        //i tried wait(NULL), and waitpid(pid, NULL, 0)
        //but both wont work..?
      cout << "Started proc(" << i << ")" << endl;
    //...........
    //something done in parent
    }
  }
  cout << "Waiting" << endl;
  // Need to wait for all
  for(i=0; i<numOfProcess; i++){
    wait(NULL);
    cout << "Got " << i+1 << " done" << endl;
  }
  return 0;  
}

Output

$ ./a.out
Started proc(0)
Started proc(1)
Started proc(2)
Started proc(3)
Started proc(4)
Started proc(5)
Started proc(6)
Started proc(7)
Started proc(8)
Started proc(9)
Waiting
Child 6 sleeping(6)
Child 4 sleeping(4)
Child 8 sleeping(8)
Child 2 sleeping(2)
Child 0 sleeping(0)
Got 1 done
Child 5 sleeping(5)
Child 7 sleeping(7)
Child 9 sleeping(9)
Child 3 sleeping(3)
Child 1 sleeping(1)
Got 2 done
Got 3 done
Got 4 done
Got 5 done
Got 6 done
Got 7 done
Got 8 done
Got 9 done
Got 10 done
$
Edited by histrungalot because: Added more
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