This is procexec/fork_whos_on_first.c (Listing 24-5, page 526), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* 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"
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:

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

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



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