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copied!<ol> <li><p>The appropriate parent process should wait (using <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/wait.html" rel="nofollow"><code>wait()</code> or <code>waitpid()</code></a> or a platform-specific variant) for its children to die before exiting.</p></li> <li><p>Concurrent execution requires the scheduler to get a chance to run. You can force the issue with appropriate sleep (micro-sleep, <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/nanosleep.html" rel="nofollow">nano-sleep</a>) operations. Some system calls will help (so <code>fflush(0)</code> might help).</p></li> </ol> <hr> <pre><code>#include <stdio.h> #include <sys/wait.h> #include <time.h> #include <unistd.h> int main(void) { int pid, i; struct timespec tw = { .tv_sec = 0, .tv_nsec = 10000000 }; pid = fork(); switch(pid) { case -1: printf("Fork error"); break; case 0: printf("First child is born, my pid is %d\n", getpid()); for(i=1; i<10; i++) { printf("First child executes iteration %d\n", i); nanosleep(&tw, 0); } printf("First child dies quietly.\n"); break; default: printf("Parent process is born, my pid is %d\n", getpid()); pid = fork(); switch(pid) { case -1: printf("Fork error"); break; case 0: printf("Second child is born, my pid is %d\n", getpid()); for(i=1; i<10; i++) { printf("Second child executes iteration %d\n", i); nanosleep(&tw, 0); } printf("Second child dies quietly.\n"); break; default: printf("Parent process waiting for children.\n"); int corpse; int status; while ((corpse = waitpid(0, &status, 0)) > 0) printf("Child %d died with exit status 0x%.4X\n", corpse, status); printf("Parent process dies quietly.\n"); break; } } return 0; } </code></pre> <p>Example output:</p> <pre><code>Parent process is born, my pid is 46624 First child is born, my pid is 46625 First child executes iteration 1 Parent process waiting for children. Second child is born, my pid is 46626 Second child executes iteration 1 First child executes iteration 2 Second child executes iteration 2 First child executes iteration 3 Second child executes iteration 3 First child executes iteration 4 Second child executes iteration 4 Second child executes iteration 5 First child executes iteration 5 Second child executes iteration 6 First child executes iteration 6 Second child executes iteration 7 First child executes iteration 7 Second child executes iteration 8 First child executes iteration 8 Second child executes iteration 9 First child executes iteration 9 First child dies quietly. Second child dies quietly. Child 46625 died with exit status 0x0000 Child 46626 died with exit status 0x0000 Parent process dies quietly. </code></pre> <p>Note that the 10 millisecond delays almost force alternating execution. Without something similar, you are stuck with the idiosyncrasies of your system and its scheduler, and many a modern machine is just too fast!</p>

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