This is sysinfo/procfs_pidmax.c (Listing 12-1, page 228), 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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/* procfs_pidmax.c

   This program demonstrates how to access a file in the /proc file system. It
   can be used to read and modify the /proc/sys/kernel/pid_max file (which is
   available only in Linux 2.6 and later).

   Usage: procfs_pidmax [new-pidmax]

        Displays the current maximum PID, and, if a command line
        argument is supplied, sets the maximum PID to that value.

        Note: privilege is required to change the maximum PID value.

   This program is Linux-specific.
#include <fcntl.h>
#include "tlpi_hdr.h"

#define MAX_LINE 100
main(int argc, char *argv[])
    int fd;
    char line[MAX_LINE];
    ssize_t n;

    fd = open("/proc/sys/kernel/pid_max", (argc > 1) ? O_RDWR : O_RDONLY);
    if (fd == -1)

    n = read(fd, line, MAX_LINE);
    if (n == -1)

    if (argc > 1)
        printf("Old value: ");
    printf("%.*s", (int) n, line);

    if (argc > 1) {
        if (lseek(fd, 0, SEEK_SET) == -1)

        if (write(fd, argv[1], strlen(argv[1])) != strlen(argv[1]))
            fatal("write() failed");

        system("echo /proc/sys/kernel/pid_max now contains "
               "`cat /proc/sys/kernel/pid_max`");



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