Redirecting keycodes in the evdev layer

redirect.c

/* In linux, the evdev subsystem already has an indirection layer for
 * keyboard keys. It allows to remap the evdev scancode it gets from
 * the keyboard to another scancode (keycode). */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

#include <linux/input.h>

int
main(void)
{
    int fd;
    int ret;
    unsigned scancode_keycode[2];

    /* Open the desired keyboard device. The paths can be different on
     * different systems.
     * You need root for this (but with systemd I think you can get the
     * fd without root if you are the active user). */
    fd = open("/dev/input/event0", O_CLOEXEC);
    if (fd < 0) {
        fprintf(stderr, "couldn't open device: %s\n", strerror(-fd));
        goto err;
    }

    /* This is how you get the current keycode. Normally it's == scancode.
     * You can see the full list of possible keycodes/scancodes in
     * /usr/include/linux/input.h. */
    scancode_keycode[0] = KEY_1;
    scancode_keycode[1] = 0;
    ret = ioctl(fd, EVIOCGKEYCODE, scancode_keycode);
    if (ret < 0) {
        fprintf(stderr, "couldn't get keycode: %s\n", strerror(-ret));
        goto err_fd;
    }

    /* This is how you set a new keycode.
     * To revert change this back to KEY_1. */
    scancode_keycode[0] = KEY_1;
    scancode_keycode[1] = KEY_2;
    ret = ioctl(fd, EVIOCSKEYCODE, scancode_keycode);
    if (ret < 0) {
        fprintf(stderr, "couldn't set keycode: %s\n", strerror(-ret));
        goto err_fd;
    }

    return 0;

err_fd:
    close(fd);
err:
    return 1;
}