/*************************************************************************\
*                  Copyright (C) Michael Kerrisk, 2026.                   *
*                                                                         *
* This program is free software. You may use, modify, and redistribute it *
* under the terms of the GNU General Public License as published by the   *
* Free Software Foundation, either version 3 or (at your option) any      *
* later version. This program is distributed without any warranty.  See   *
* the file COPYING.gpl-v3 for details.                                    *
\*************************************************************************/

/* fork_whos_on_first.c

   Parent repeatedly creates a child, and then processes both race to be the
   first to print a message. (Each child terminates after printing its message.)
   The results of running this program give us an idea of which of the two
   processes--parent or child--is usually scheduled first after a fork().

   Whether the child or the parent is scheduled first after fork() has
   changed a number of times across different kernel versions.
*/
#include <sys/wait.h>
#include "tlpi_hdr.h"

int
main(int argc, char *argv[])
{
    int numChildren, j;
    pid_t childPid;

    if (argc > 1 && strcmp(argv[1], "--help") == 0)
        usageErr("%s [num-children]\n", argv[0]);

    numChildren = (argc > 1) ? getInt(argv[1], GN_GT_0, "num-children") : 1;

    setbuf(stdout, NULL);               /* Make stdout unbuffered */

    for (j = 0; j < numChildren; j++) {
        switch (childPid = fork()) {
        case -1:
            errExit("fork");

        case 0:
            printf("%d child\n", j);
            _exit(EXIT_SUCCESS);

        default:
            printf("%d parent\n", j);
            wait(NULL);                 /* Wait for child to terminate */
            break;
        }
    }

    exit(EXIT_SUCCESS);
}