Open a GTK 4 window with bun:ffi

GTK 4 is a C library, so bun:ffi can drive it directly: create a GtkApplication, connect its activate signal to a JSCallback, and run the GLib main loop with g_application_run. GLib calls the callback synchronously on the JavaScript thread. No npm package or native addon is needed.

import { dlopen, FFIType, JSCallback } from "bun:ffi";

const { ptr: p, i32, u32, u64, void: none } = FFIType;
// Linux names; on Windows use libgtk-4-1.dll, libgobject-2.0-0.dll, libgio-2.0-0.dll.
const { symbols: gtk } = dlopen("libgtk-4.so.1", {
  gtk_application_new: { args: [p, u32], returns: p },
  gtk_application_window_new: { args: [p], returns: p },
  gtk_window_set_title: { args: [p, p], returns: none },
  gtk_window_set_default_size: { args: [p, i32, i32], returns: none },
  gtk_window_set_child: { args: [p, p], returns: none },
  gtk_window_present: { args: [p], returns: none },
  gtk_label_new: { args: [p], returns: p },
});
const { symbols: gobject } = dlopen("libgobject-2.0.so.0", {
  g_signal_connect_data: { args: [p, p, p, p, p, u32], returns: u64 },
  g_object_unref: { args: [p], returns: none },
});
const { symbols: gio } = dlopen("libgio-2.0.so.0", {
  g_application_run: { args: [p, i32, p], returns: i32 },
});

const cstr = (s: string) => Buffer.from(s + "\0", "utf8");
const title = cstr("Hello from Bun");
const text = cstr("GTK 4 through bun:ffi");

const onActivate = new JSCallback(
  app => {
    const window = gtk.gtk_application_window_new(app);
    gtk.gtk_window_set_title(window, title);
    gtk.gtk_window_set_default_size(window, 640, 400);
    gtk.gtk_window_set_child(window, gtk.gtk_label_new(text));
    gtk.gtk_window_present(window);
  },
  { args: [p, p], returns: none },
);

const G_APPLICATION_NON_UNIQUE = 1 << 5;
const app = gtk.gtk_application_new(cstr("sh.bun.Example"), G_APPLICATION_NON_UNIQUE);
gobject.g_signal_connect_data(app, cstr("activate"), onActivate.ptr, null, null, 0);
const status = gio.g_application_run(app, 0, null); // returns when the last window closes
gobject.g_object_unref(app);
onActivate.close();
process.exit(status);

bun window.ts

GTK, GLib, GObject and GIO are separate libraries. On Linux, dlsym on the GTK handle would also find GLib symbols, but GetProcAddress on Windows does not, so open each one. On Windows, install GTK with MSYS2 (pacman -S mingw-w64-ucrt-x86_64-gtk4) and pass absolute paths under C:\msys64\ucrt64\bin, or add that directory to PATH. On Linux, GTK needs a display (DISPLAY or WAYLAND_DISPLAY); xvfb-run works in CI.

g_application_run blocks the JavaScript thread, so timers, promises and I/O callbacks do not run while the window is open. Use GLib sources such as g_timeout_add with a JSCallback for periodic work inside the loop.

Tested platforms

The fixture prints window created and window closed with exit code 0 on:

• Windows 11: GTK 4.24.1 from MSYS2 UCRT64
• Ubuntu 26.04: GTK 4.22.4 under xvfb-run

The Linux images are scripts/aphrody/gui/ubuntu.Dockerfile and scripts/aphrody/gui/alpine.Dockerfile.

A fuller version with a key controller for Escape, g_timeout_add, and --title, --width, --height and --timeout options lives in the Bun repository at test/js/bun/ffi/gtk-window.fixture.ts.

See Docs > Runtime > FFI for the full bun:ffi reference.