Using UTF8ToString with Wasm64 about emscripten

[sbc100]

With -sWASM64 we convert all incoming pointers to number (technically a 53 bit integer which we refer to as int53 internally) when they arrive on the JS side. In other words, from JS code, pointers should still be number.

How are you receiving the pointer value that you are passing to UTF8ToString?

[tbuchs]

That's very interesting, maybe it's just my fault. I tried to narrow it down to a very simple PoC:

main.cpp

#include <vector>

extern "C" {
extern int testWasm64ToString(const char **pUrls);
}

int main() {
    std::vector<const char*> v = {"abc", "abc"};
    return testWasm64ToString(v.data());   
}

helper.js

(function() {
	var Test = {
  		testWasm64ToString: function(data) {
        var result = UTF8ToString(data);
        return 0;
      },
    };
    mergeInto(LibraryManager.library, Test);
  })();

I compiled the program with: em++ main.cpp -sMEMORY64=1 --js-library helper.js -o test.html

Thank you for your ongoing support!

[sbc100]

The fix here would be to add a __sig entry to your testWasm64ToString function in the JS library.

Something like testWasm64ToString__sig: 'ip'. This says the return value is an i (integer) and the first and only argument is ap (pointer). This will enable emscripten to automatically convert the incoming bigint value to an integer.

[tbuchs]

Thanks, that works perfectly!