This is sysinfo/procfs_pidmax.c (Listing 12-1, page 228), an example program file from the book, The Linux Programming Interface.

The source code file is copyright 2010, Michael Kerrisk, and is licensed under the GNU Affero General Public License, version 3.

This page shows the "distribution" or "book" version of the file (why are there two versions?), or the differences between the two versions. You can switch between the views using the tabs below.

In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.

  Cover of The Linux Programming Interface

Function list (Bold in this list means a function is not static)

/* 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 (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`");



Download sysinfo/procfs_pidmax.c