# Open a COSMIC (libcosmic) window with bun:ffi

[libcosmic](https://github.com/pop-os/libcosmic) is the Rust toolkit of the COSMIC desktop (Pop!_OS), built on iced and winit. Export a function from a Rust `cdylib` that runs a `cosmic::Application`, and call it from `bun:ffi`. The event loop runs on the calling thread, which is the JavaScript thread.

```toml Cargo.toml icon="file-code"
[package]
name = "cosmic-window"
version = "0.1.0"
edition = "2024"

[lib]
crate-type = ["cdylib"]

[dependencies.libcosmic]
git = "https://github.com/pop-os/libcosmic"
default-features = false
features = ["winit", "tokio"]

# Wayland and X11 backends exist on Linux only.
[target.'cfg(target_os = "linux")'.dependencies.libcosmic]
git = "https://github.com/pop-os/libcosmic"
default-features = false
features = ["winit", "tokio", "wayland", "x11"]
```

```rust src/lib.rs icon="file-code"
use cosmic::app::{Core, Settings, Task};
use cosmic::iced::Size;
use cosmic::{Element, executor, widget};

struct Window {
    core: Core,
}

impl cosmic::Application for Window {
    type Executor = executor::Default;
    type Flags = ();
    type Message = ();
    const APP_ID: &'static str = "sh.bun.Example";

    fn core(&self) -> &Core {
        &self.core
    }
    fn core_mut(&mut self) -> &mut Core {
        &mut self.core
    }
    fn init(core: Core, _flags: ()) -> (Self, Task<()>) {
        (Window { core }, Task::none())
    }
    fn view(&self) -> Element<'_, ()> {
        widget::text::body("libcosmic through bun:ffi").into()
    }
}

#[unsafe(no_mangle)]
pub extern "C" fn cosmic_window_run(width: u32, height: u32) -> i32 {
    let settings = Settings::default()
        .size(Size::new(width as f32, height as f32))
        .transparent(false);
    match cosmic::app::run::<Window>(settings, ()) {
        Ok(()) => 0,
        Err(_) => 1,
    }
}
```

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

const { symbols } = dlopen(`./target/release/${process.platform === "win32" ? "" : "lib"}cosmic_window.${suffix}`, {
  cosmic_window_run: { args: [FFIType.u32, FFIType.u32], returns: FFIType.i32 },
});
process.exit(symbols.cosmic_window_run(640, 400)); // returns when the window closes
```

```sh terminal icon="terminal"
cargo build --release
bun window.ts
```

Building libcosmic needs `pkg-config`, `cmake` and the xkbcommon, Wayland, fontconfig and freetype development headers (on Ubuntu 24.04 and Pop!_OS 24.04: `libxkbcommon-dev libwayland-dev libfontconfig-dev libfreetype-dev libexpat1-dev`). With `default-features = false`, iced renders in software with tiny-skia, so the window also opens under `xvfb-run` in CI. `.transparent(false)` keeps the window on a 24-bit visual: softbuffer rejects the 32-bit ARGB visual that a transparent window gets under Xvfb.

Callbacks into JavaScript must stay on the thread that called into the library. If you pass a [`JSCallback`](https://bun.aphrody.com/docs/runtime/ffi#callbacks) to the `cdylib`, call it only from that thread, for example from `Application::init` after checking `std::thread::current().id()`.

## Tested platforms

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

- Windows 11: winit/Win32 backend, MSVC toolchain
- Ubuntu 26.04: X11 backend under `xvfb-run`

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

---

A fuller version with a `JSCallback`, `--timeout` (an iced time subscription that returns `cosmic::iced::exit()`), and `--title`, `--width` and `--height` options lives in the Bun repository at [`test/js/bun/ffi/cosmic-window.fixture.ts`](https://github.com/aphrody-labs/bun/blob/main/test/js/bun/ffi/cosmic-window.fixture.ts) with its crate in [`test/js/bun/ffi/cosmic-window`](https://github.com/aphrody-labs/bun/tree/main/test/js/bun/ffi/cosmic-window). To show a libcosmic window without writing Rust, `bun:cosmic` runs one in a separate helper process.

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