Rust x GameMaker example

This is an example of how to make native extensions for GameMaker in Rust!

Points of interest

Export macro

This lets you do

export! {
	fn rust_get_string() -> GMStr {
		rets("This string comes from Rust!")
	}
	fn rust_add_numbers(left: f64, right: f64) -> f64 {
		left + right
	}
}

instead of

#[unsafe(no_mangle)]
extern "C" fn rust_get_string() -> GMStr {
	rets("This string comes from Rust!")
}
#[unsafe(no_mangle)]
extern "C" fn rust_add_numbers(left: f64, right: f64) -> f64 {
	left + right
}

similar to how I always have a dllx "decorator" in my C++ extensions.

Receiving and returning numbers

This one's easy, you specify f64 for argument types and/or return type and that's it.

Receiving and returning strings

This one's quirkier due to FFI philosophy differences between languages - Rust generally expects (CString.into_raw) the returned C strings to be freed by the caller program while GM makes a copy of the returned string without freeing it.

This means that you must store the string you're about to return somewhere so that it does not get freed on function exit.

For this reason I'm including gm::gets (create a Rust String from a GM string) and gm::rets (convert a Rust &str to a GM string for immediate return) helper functions.

Working with buffers

On GM side you mark the argument as a "string" (pointer, really), pass a buffer_get_address to the native extension, and that's it! This is an actual pointer to buffer data so you can also write to the buffer, but note that you'll need to call buffer_set_used_size afterwards if you're populating a freshly made buffer with data.

Mac OS support

Compile a DyLib/SO and add it as a separate Proxy File to the extension using the GameMaker IDE.

I don't have a functioning Mac to test this.

Special thanks

• Jacob for answering my general "and how do you do X here" Rust questions
• Dashiell for explaining the particularities of Rust's static and a few other bits