What are the rules for user defined types for providing __sig in js library

[k1nder10]

I have a js library. I use mergeInto() in it. I have to describe function signature for emscripten compiler but I don't know the rules for user-defined types.

void foo(myClass1 m1, myClass2 m2, emscripten::EM_VAL);
foo__sig: 'v???'

[kripken]

Passing classes by value is tricky. The JS library API is very low-level, so you'd need to find out how the C++ ABI for such classes ends up implemented in wasm imports. That might not be easy and is not recommended. Instead, using embind at a high level is probably best.

JS library methods are good when your types are simple:

• v - void (no returned value)
• i - i32
• j - i64
• f - f32
• d - f64
• p - pointer (usually i32, but i64 in wasm64)

[k1nder10]

What about std::string? Can I use 'p' sign for it?

[k1nder10]

Can you have a look at the convertJsFunctionToWasm() in the out .js file produced by emcc? There's no check for 'p' at all. So, if one uses it, the error occurs.

The code from that function:
if (typeof WebAssembly.Function == "function") {
var typeNames = {
'i': 'i32',
'j': 'i64',
'f': 'f32',
'd': 'f64'
};
var type = {
parameters: [],
results: sig[0] == 'v' ? [] : [typeNames[sig[0]]]
};

[k1nder10]

To enable using of 'p' I added it to typeCodes&typeNames objects in the out .js file though it has no difference between 'i'. It seems to be working. Can it break sth in the future? Are there any pitfalls?

var typeCodes = {
'p': 0x7f, // i32
'i': 0x7f, // i32
'j': 0x7e, // i64
'f': 0x7d, // f32
'd': 0x7c, // f64
};

[kripken]

Atm we use p internally in JS library function signatures, but not in runtime stuff like addFunction, as you ran into. I don't think we have plans to change that. But you are modifying internals in ways that might change. It would be safer to use embind.

[sbc100]

convertJsFunctionToWasm() should always contain a p handler. If yours doesn't maybe you are using an older version of emscripten?:

emscripten/src/runtime_functions.js

Lines 68 to 78 in 8818da2

	var typeCodes = {
	'i': 0x7f, // i32
	#if MEMORY64
	'p': 0x7e, // i64
	#else
	'p': 0x7f, // i32
	#endif
	'j': 0x7e, // i64
	'f': 0x7d, // f32
	'd': 0x7c, // f64
	};

emscripten/src/runtime_functions.js

Lines 22 to 32 in 8818da2

	var typeNames = {
	'i': 'i32',
	'j': 'i64',
	'f': 'f32',
	'd': 'f64',
	#if MEMORY64
	'p': 'i64',
	#else
	'p': 'i32',
	#endif
	};

[k1nder10]

Yes, I used 3.1.8. Today I updated emsdk to 3.1.13 which supports 'p' for pointers. Thanks!