use externref in c code

externref.cpp

// __externref_t is a opaque pointer that will be translated to wasm externref.

__attribute__((import_name("receive_object")))
__externref_t receive_object(void);

__attribute__((import_name("send_object")))
void send_object(__externref_t);

int main() {
    send_object(receive_object());
    return 0;
}

funcref.cpp

// tested on emscripten 3.1.37

using fp_t = void (*__funcref)() ;
using main_t = int (*__funcref)();

int main() {
	return 0;
}

// call funcref from c
// with emcc option -O0, invalid code will be generated.
// extern "C" is essential, as funcrefs cannot be mangled.

__attribute__((used))
extern "C" void importFuncRef(fp_t a) {
	(*a)();
}

// this code causes compiler crash
// funcref pointer is in address space 20, but function pointers cannot be converted.

// __attribute__((used))
// extern "C" main_t exportMainAsFuncRef() {
// 	return (main_t)&main;
// }

// this asm code is required to call funcref

__asm(
".globl __funcref_call_table\n"
"__funcref_call_table:\n"
"  .tabletype __funcref_call_table, funcref, 1\n"
);

Heh this looked promising but doesn't seem to compile with optimisations yet.

Nvm, got it to work.