How do I configure libinput devices from C code?

On wayland there is no configuration file for libinput. This is not usually a problem because desktop environments (such as Gnome) often offer a way to configure the devices. However, there is no way to enable middle click emulation for a clickpad device. By default (using button areas so that I can right click with the bottom right of the touchpad) a middle button area is also created. This often results in me clicking the middle button when I try to left click and middle click instead (causing something to be pasted). This middle click area can be disabled if middle emulation is enabled, however because Gnome does not provide a way to configure this I decided to try to build my own program to do so.

I have looked through libinput's API docs and examples (unfortunately I can't seem to find any examples of device configuration). The following code is what I have put together (compiled with command in comment).

/**
 * Simple program to setup libinput touchpad to use middle click emulation in wayland.
 *
 * To build install libinput-devel and libudev-devel
 * gcc -o middleemulation main.c `pkg-config --cflags --libs libinput libudev`
 * 
 * Copyright 2019 Marcus Behel
 *
 *Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 */

#include <string.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <libudev.h>
#include <libinput.h>

static int open_restricted(const char *path, int flags, void *user_data){
  int fd = open(path, flags);
  return fd < 0 ? -errno : fd;
}

static void close_restricted(int fd, void *user_data){
  close(fd);
}

const static struct libinput_interface interface = {
        .open_restricted = open_restricted,
        .close_restricted = close_restricted,
};

int main(void){

  struct libinput *li;
  struct libinput_event *ev;
  struct udev *udev = udev_new();
  int dev_count = 0, tp_count = 0;

  li = libinput_udev_create_context(&interface, NULL, udev);
  libinput_udev_assign_seat(li, "seat0");
  libinput_dispatch(li);

  while ((ev = libinput_get_event(li))) {
    if (libinput_event_get_type(ev) == LIBINPUT_EVENT_DEVICE_ADDED){
      dev_count++;

      double w = 0, h = 0;
      struct libinput_device *dev = libinput_event_get_device(ev);
      const char *name = libinput_device_get_name(dev);
      if(libinput_device_has_capability(dev, LIBINPUT_DEVICE_CAP_POINTER) &&
     libinput_device_get_size(dev, &w, &h) == 0){
    // Pointer with a size is a touchpad
    tp_count++;

    // This is a touchpad. Enable middle click emulation
    printf("Found touchpad: '%s'.\n", name);

    printf("Is middle click enabled: %s.\n", libinput_device_config_middle_emulation_get_enabled(dev) ? "true" : "false");

    if(libinput_device_config_middle_emulation_is_available(dev)){
      printf("Enabling middle emulation for device...");
      enum libinput_config_status err = libinput_device_config_middle_emulation_set_enabled(dev, LIBINPUT_CONFIG_MIDDLE_EMULATION_ENABLED);

      if(err == LIBINPUT_CONFIG_STATUS_SUCCESS){
        printf("Succeeded.\n");
      }else{
        printf("Failed.\n");
      }

      printf("Is middle click enabled: %s.\n\n", libinput_device_config_middle_emulation_get_enabled(dev) ? "true" : "false");

    }else{
      printf("Device does not support middle emulation.\n\n");
    }
      }
    }

    libinput_event_destroy(ev);
    libinput_dispatch(li);
  }

  libinput_unref(li);

  if(dev_count == 0){
    fprintf(stderr, "No libinput devices were found. Run this as root and make sure libinput driver is enabled.\n");
    return 1;
  }

  if(tp_count == 0){
    printf("No touchpads found on this system.");
  }

  return 0;
}

I have tested this on Ubuntu 18.04 (Dell Inspiron 5000) and on Fedora 30 (HP Pavilion 15). In both cases the program indicates success, however when running libinput list-devices it still shows that middle emulation is disabled and the behavior does not change.

Output from program (HP Pavilion)

Found touchpad: 'SynPS/2 Synaptics TouchPad'.
Is middle click enabled: false.
Enabling middle emulation for device...Succeeded.
Is middle click enabled: true.

Output from libinput list-devices after running the program

Device:           SynPS/2 Synaptics TouchPad
Kernel:           /dev/input/event4
Group:            9
Seat:             seat0, default
Size:             106x61mm
Capabilities:     pointer gesture
Tap-to-click:     disabled
Tap-and-drag:     enabled
Tap drag lock:    disabled
Left-handed:      disabled
Nat.scrolling:    disabled
Middle emulation: disabled
Calibration:      n/a
Scroll methods:   *two-finger edge 
Click methods:    *button-areas clickfinger 
Disable-w-typing: enabled
Accel profiles:   none
Rotation:         n/a

Solution

It looks like this would require a preload library to work as intended. The solution here: https://github.com/gaul/libinput-force-middle-click-emulation works for me.