This function uses libudev to robustly detect all keyboard input event devices (/dev/input/eventX). Based on /usr/lib/udev/findkeyboards script.

detect_kbd_devices.c

#include <libudev.h>
#define MAX_KBD_DEVICES 10

const char** detect_kbd_devices()
{
  const char **devnodes = malloc(MAX_KBD_DEVICES);
  struct udev *udev;
  struct udev_enumerate *enumerate;
  struct udev_list_entry *devices, *dev_list_entry;
  struct udev_device *dev;
  int i = 0;

  udev = udev_new();
  if (!udev) {
    printf("Can't create udev.\n");
    exit(1);
  }
  enumerate = udev_enumerate_new(udev);
  udev_enumerate_add_match_property(enumerate, "ID_INPUT_KEYBOARD", "1");
  udev_enumerate_scan_devices(enumerate);
  devices = udev_enumerate_get_list_entry(enumerate);
  udev_list_entry_foreach(dev_list_entry, devices) {
    const char *path, *devnode;
    path = udev_list_entry_get_name(dev_list_entry);
    dev = udev_device_new_from_syspath(udev, path);
    devnode = udev_device_get_devnode(dev);
    if (devnode) {
      printf("Found keyboard input device: %s\n", devnode);
      devnodes[i] = strdup(devnode);
      i++;
    }
    udev_device_unref(dev);
  }
  udev_enumerate_unref(enumerate);
  udev_unref(udev);
  return devnodes;
}