How to wait until all child processes called by fork() complete?

Question

I am forking a number of processes and I want to measure how long it takes to complete the whole task, that is when all processes forked are completed. Please advise how to make the parent process wait until all child processes are terminated? I want to make sure that I stop the timer at the right moment.

Here is as a code I use:

#include <iostream>
#include <string>
#include <fstream>
#include <sys/time.h>
#include <sys/wait.h>

using namespace std;

struct timeval first,  second,  lapsed;
struct timezone tzp; 

int main(int argc, char* argv[])// query, file, num. of processes.
{

    int pCount = 5; // process count

    gettimeofday (&first, &tzp); //start time

    pid_t* pID = new pid_t[pCount];

    for(int indexOfProcess=0; indexOfProcess<pCount; indexOfProcess++)
    {
        pID[indexOfProcess]= fork();

        if (pID[indexOfProcess] == 0)                // child
        {
            // code only executed by child process

            // magic here

            // The End
            exit(0);
        }
        else if (pID[indexOfProcess] < 0)    // failed to fork
        {
            cerr << "Failed to fork" << endl;
            exit(1);
        }
        else                         // parent
        {
            // if(indexOfProcess==pCount-1) and a loop with waitpid??

            gettimeofday (&second, &tzp); //stop time
            if (first.tv_usec > second.tv_usec)
            {
                second.tv_usec += 1000000;
                second.tv_sec--;
            }

            lapsed.tv_usec = second.tv_usec - first.tv_usec;
            lapsed.tv_sec = second.tv_sec - first.tv_sec; 

            cout << "Job performed in " <<lapsed.tv_sec << " sec and " << lapsed.tv_usec    << " usec"<< endl << endl;

        }

    }//for

}//main

C: https://stackoverflow.com/questions/19461744/make-parent-wait-for-all-child-processes-to-finish — Ciro Santilli OurBigBook.com, Dec 13 '18 at 23:57

Steve Jessop · Accepted Answer · 2012-01-18T23:37:32.550

I'd move everything after the line "else //parent" down, outside the for loop. After the loop of forks, do another for loop with waitpid, then stop the clock and do the rest:

for (int i = 0; i < pidCount; ++i) {
    int status;
    while (-1 == waitpid(pids[i], &status, 0));
    if (!WIFEXITED(status) || WEXITSTATUS(status) != 0) {
        cerr << "Process " << i << " (pid " << pids[i] << ") failed" << endl;
        exit(1);
    }
}

gettimeofday (&second, &tzp); //stop time

I've assumed that if the child process fails to exit normally with a status of 0, then it didn't complete its work, and therefore the test has failed to produce valid timing data. Obviously if the child processes are supposed to be killed by signals, or exit non-0 return statuses, then you'll have to change the error check accordingly.

An alternative using wait:

while (true) {
    int status;
    pid_t done = wait(&status);
    if (done == -1) {
        if (errno == ECHILD) break; // no more child processes
    } else {
        if (!WIFEXITED(status) || WEXITSTATUS(status) != 0) {
            cerr << "pid " << done << " failed" << endl;
            exit(1);
        }
    }
}

This one doesn't tell you which process in sequence failed, but if you care then you can add code to look it up in the pids array and get back the index.

There's a _slight_ typo in the second piece of code above: the error signal's name in `if (done == -1) { ... }` should be `ECHILD`, not `ECHLD`. — Vicky Chijwani, Jan 18 '12 at 20:31

score 13 · Answer 2 · edited Apr 30 '17 at 03:45

13

The simplest method is to do

while(wait() > 0) { /* no-op */ ; }

This will not work if wait() fails for some reason other than the fact that there are no children left. So with some error checking, this becomes

int status;
[...]
do {
    status = wait();
    if(status == -1 && errno != ECHILD) {
        perror("Error during wait()");
        abort();
    }
} while (status > 0);

See also the manual page wait(2).

edited Apr 30 '17 at 03:45

heemayl

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answered Nov 11 '08 at 01:17

gnud

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you still need to account for the number of child processes – BCS Nov 11 '08 at 01:27
Nah. When there are no more children, wait will return -1 and errno will be set to ECHILD. Not optimal, but for his use it should be sufficient. – gnud Nov 11 '08 at 01:28
If it isn't desirable, then ignore the signals first. That's not unique to process handling-- – gnud Nov 12 '08 at 10:24

Chris Morley · Answer 3 · 2008-11-11T01:29:05.457

4

Call wait (or waitpid) in a loop until all children are accounted for.

In this case, all processes are synchronizing anyway, but in general wait is preferred when more work can be done (eg worker process pool), since it will return when the first available process state changes.

edited Nov 11 '08 at 01:29

answered Nov 11 '08 at 01:16

Chris Morley

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Could you please provide me with a code sample? I must admit I do not fully understand the flags I should use... – Kamil Zadora Nov 11 '08 at 01:18

score 3 · Answer 4 · answered Nov 11 '08 at 01:16

3

I believe the wait system call will accomplish what you are looking for.

answered Nov 11 '08 at 01:16

SoapBox

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2

Note that wait() only waits for one child process - therefore, you must call wait() as many times as there are children. Alternatively, you can use waitpid() to wait for a specific child, but again you'd have to do this for every child. – Adam Rosenfield Nov 11 '08 at 01:17

score 0 · Answer 5 · edited Nov 05 '16 at 04:09

0

for (int i = 0; i < pidCount; i++) {
    while (waitpid(pids[i], NULL, 0) > 0);
}

It won't wait in the right order, but it will stop shortly after the last child dies.

edited Nov 05 '16 at 04:09

Jim Jones

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answered Nov 11 '08 at 01:17

BCS

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How to wait until all child processes called by fork() complete?

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