Additionally export malloc & free

[akowi-sknobloch]

Hey,
I've written a simple 2d render engine in rust using the skia-safe crate.
Now i want to make a simple web based editor for my engine.
As a starting point i copied the wasm example from the rust-skia repo: https://github.com/rust-skia/rust-skia/tree/master/wasm-example

I've first tried to extend the example by implementing loading a typeface.
My Rust function:

#[unsafe(no_mangle)]
pub extern "C" fn load_typeface(ptr: *const u8, len: usize) {
    let data = unsafe { std::slice::from_raw_parts(ptr, len) };

    println!("font buffer size: {} bytes", len);

    println!("font first few bytes: {:?}", &data);

    let typeface_bytes = skia_safe::Data::new_copy(data);
    let font_manager = skia_safe::FontMgr::new();
    let skia_typeface = font_manager.new_from_data(&typeface_bytes, None);

    let typeface = skia_typeface.unwrap();
}

The function call in JavaScript:

const fontRes = await fetch("/web/comic-sans-ms.ttf");
const fontData = await fontRes.arrayBuffer();

const ptr = RustSkia._malloc(fontData.byteLength);
RustSkia.HEAPU8.set(new Uint8Array(fontData), ptr);
RustSkia._load_typeface(ptr, fontData.byteLength);
RustSkia._free(ptr);

Here I hit my first roadblock, as _malloc is does not exist in my module.
With a bit of help from ChatGPT I got it working my modifying the emcc flags in the Makefile like this:

BUILD = EMCC_CFLAGS="\
	-s MODULARIZE=1 \
	-s EXPORT_NAME=createRustSkiaModule \
	-s ERROR_ON_UNDEFINED_SYMBOLS=0 \
	-s MAX_WEBGL_VERSION=2 \
	-s EXPORTED_FUNCTIONS=['_malloc','_free','_load_typeface','_init','_draw_circle','_resize_surface'] \
	-s EXPORTED_RUNTIME_METHODS=['ccall','cwrap','GL','getValue','HEAPU8'] \
" EMSDK=$(EMSDK) cargo build --target wasm32-unknown-emscripten

Now my typeface loading works but the init function of the example did break with the following error:

Uncaught (in promise) RuntimeError: null function or function signature mismatch
    at skia_wasm.wasm.gl::GetIntegerv::habdf0b8e0c3d8ff9 (skia_wasm.wasm:0x10c35)
    at invoke_vii (skia-wasm.js:9153:29)
    at skia_wasm.wasm.skia_wasm::create_gpu_state::h80bce5124c32a570 (skia_wasm.wasm:0xe1d9)
    at invoke_vi (skia-wasm.js:9142:29)
    at skia_wasm.wasm.init (skia_wasm.wasm:0xec13)
    at Object._init (skia-wasm.js:648:12)
    at main.js:45:26

When explicitly exporting malloc and my functions using EXPORTED_FUNCTIONS, the GL functions needed for skia are no longer available.
I need the automatic export of the GL functions + malloc & free.
Is there any way to tell emcc to not set the exported functions but to add them?
Or is there an alternative way to using malloc for transfering complex data to wasm?

[sbc100]

No, there is currently way to make settings like -sEXPORTED_FUNCTIONS additive. There is an old PR to change this behaviour but there was some disagreement about how best to do it: #19938.

Are you claiming that when you don't specify your list of EXPORTED_FUNCTIONS that the skia GL functions are exported somehow? Unless skia is able to somehow add -sEXPORTED_FUNCTIONS to your link command I doubt that. The error you are does not look like that lack of an export to me, but more like maybe null function pointer.

(Also FYI a simpler way to do EXPORTED_FUNCTIONS and EXPORTED_RUNTIME_METHODS is to just use a simple comma separated list: e.g. -s EXPORTED_FUNCTIONS=_malloc,_free

[akowi-sknobloch]

As I said my test Project was based on the wasm-example of the skia-safe project.
The example does work but does not support malloc & free and does not have any entry in the EXPORTED_FUNCTIONS.
When I add malloc and free to the EXPORTED_FUNCTIONS, the init function of the example fails with the error I posted.

But I already found a workaround by simply implementing my own functions for allocating and freeing memory in rust.
Therefore I no longer need to add malloc & free to the EXPORTED_FUNCTIONS and everything works.