This is procres/rlimit_nproc.c (Listing 36-3, page 758), 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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/* rlimit_nproc.c

   Experiment with maximum processes resource limit.

   Usage: rlimit_nproc hard-limit soft-limit

   NOTE: Only Linux and the BSDs support the RLIMIT_NPROC resource limit.
#include <sys/resource.h>
#include "print_rlimit.h"               /* Declaration of printRlimit() */
#include "tlpi_hdr.h"
main(int argc, char *argv[])
    struct rlimit rl;
    int j;
    pid_t childPid;

    if (argc < 2 || argc > 3 || strcmp(argv[1], "--help") == 0)
        usageErr("%s soft-limit [hard-limit]\n", argv[0]);

    printRlimit("Initial maximum process limits: ", RLIMIT_NPROC);

    /* Set new process limits (hard == soft if not specified) */

    rl.rlim_cur = (argv[1][0] == 'i') ? RLIM_INFINITY :
                                getInt(argv[1], 0, "soft-limit");
    rl.rlim_max = (argc == 2) ? rl.rlim_cur :
                (argv[2][0] == 'i') ? RLIM_INFINITY :
                                getInt(argv[2], 0, "hard-limit");
    if (setrlimit(RLIMIT_NPROC, &rl) == -1)

    printRlimit("New maximum process limits:     ", RLIMIT_NPROC);

    /* Create as many children as possible */

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

        case 0: _exit(EXIT_SUCCESS);            /* Child */

        default:        /* Parent: display message about each new child
                           and let the resulting zombies accumulate */
            printf("Child %d (PID=%ld) started\n", j, (long) childPid);


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