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

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.

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).

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.