# Open a Qt 6 window with bun:ffi

Qt is a C++ library with no C ABI, so `bun:ffi` cannot call it directly. Write a small `extern "C"` shim, compile it into a shared library with your C++ compiler, and load that with `dlopen`. The shim owns `QApplication::exec()`, and calls a [`JSCallback`](https://bun.aphrody.com/docs/runtime/ffi#callbacks) synchronously on the JavaScript thread.

```cpp shim.cpp icon="file-code"
#include <QApplication>
#include <QLabel>

extern "C" int bun_qt_window_run(const char* title, int width, int height, void (*on_created)(int, int)) {
    static int argc = 1;
    static char arg0[] = "bun";
    static char* argv[] = {arg0, nullptr};
    QApplication app(argc, argv);
    QLabel window(QStringLiteral("Qt 6 through bun:ffi"));
    window.setWindowTitle(QString::fromUtf8(title));
    window.resize(width, height);
    window.show();
    on_created(width, height);
    return app.exec(); // returns when the last window closes
}
```

```sh terminal icon="terminal"
c++ -std=c++20 -shared -fPIC -o libshim.so shim.cpp $(pkg-config --cflags --libs Qt6Widgets)
```

```ts window.ts icon="/icons/typescript.svg"
import { dlopen, FFIType, JSCallback } from "bun:ffi";

const { symbols } = dlopen("./libshim.so", {
  bun_qt_window_run: { args: [FFIType.ptr, FFIType.i32, FFIType.i32, FFIType.ptr], returns: FFIType.i32 },
});

const onCreated = new JSCallback((width: number, height: number) => console.log(`window ${width}x${height}`), {
  args: [FFIType.i32, FFIType.i32],
  returns: FFIType.void,
});

const status = symbols.bun_qt_window_run(Buffer.from("Hello from Bun\0"), 640, 400, onCreated.ptr);
onCreated.close();
process.exit(status);
```

```sh terminal icon="terminal"
bun window.ts
```

TinyCC, which powers [`cc`](https://bun.aphrody.com/docs/runtime/ffi#cc) in `bun:ffi`, compiles C only, so a C++ shim needs the system compiler. On Windows, [MSYS2](https://www.msys2.org) provides both the compiler and Qt (`pacman -S mingw-w64-ucrt-x86_64-{gcc,pkgconf,qt6-base}`); add `C:\msys64\ucrt64\bin` to `PATH` so the Qt DLLs are found when the shim loads. On Linux without a display, set `QT_QPA_PLATFORM=offscreen`.

`QApplication::exec()` blocks the JavaScript thread, so timers, promises and I/O callbacks do not run while the window is open. Use a `QTimer` in the shim for periodic work inside the loop.

## Tested platforms

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

- Windows 11: Qt 6.11.2 and GCC from MSYS2 UCRT64
- Ubuntu 26.04: Qt 6.10.2 under `xvfb-run`

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

---

A fuller version that builds and caches the shim, closes on Escape or after `--timeout`, and accepts `--title`, `--width` and `--height` lives in the Bun repository at [`test/js/bun/ffi/qt-window.fixture.ts`](https://github.com/aphrody-labs/bun/blob/main/test/js/bun/ffi/qt-window.fixture.ts) with the shim in [`qt-window.shim.cpp`](https://github.com/aphrody-labs/bun/blob/main/test/js/bun/ffi/qt-window.shim.cpp). For a KDE Kirigami window, see [Open a KDE Kirigami window](https://bun.aphrody.com/docs/guides/runtime/kde-window).

See [Docs > Runtime > FFI](https://bun.aphrody.com/docs/runtime/ffi) for the full `bun:ffi` reference.