Kamil Zadora Kamil Zadora - 1 month ago 18
C++ Question

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

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

exit(0);// The End
}
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

Answer

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.