/*************************************************************************\
*                  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.                                    *
\*************************************************************************/

/* 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

int
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)
        errExit("open");

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

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

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

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

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

    exit(EXIT_SUCCESS);
}