UTF8ToString does not work when compiling for 64bit

[ptc-jpharvey]

Please include the following in your bug report:

Version of emscripten/emsdk:
3.1.72

Failing command line in full:
em++ -o test.html -sMEMORY64=1 -g -fexceptions test.cpp

At runtime the method says it can only take a number not a bigint
test.js:550
Aborted(Assertion failed: UTF8ToString expects a number (got bigint))
test.js:568
Uncaught RuntimeError: Aborted(Assertion failed: UTF8ToString expects a number (got bigint))
at abort (test.js:568:11)
at assert (test.js:286:5)
at UTF8ToString (test.js:1080:7)
at SetCanvasAsEventTarget (test.js:922:60)
at invoke_vj (test.js:1592:37)
at test.wasm.main (test.wasm:0xded)
at test.js:1697:49
at test.js:620:12
at callMain (test.js:2045:15)
at doRun (test.js:2095:23)

If bigint is allowed through or assertions are disabled then the code fails in UTF8ArrayToString due to mixing types (bigint & number) etc.

#include <stdio.h>

#include "emscripten.h"
#include <string>
EM_JS(void, SetCanvasAsEventTarget, (const char* canvasName),
    {
        var canvas = UTF8ToString(canvasName);
    }
);

int
main(int argc, char **argv)
{
    printf("Hello\n");
    std::string blah("foo");
    SetCanvasAsEventTarget(blah.c_str());
}

[kripken]

This is expected atm, hence that assertion on it:

emscripten/src/library_strings.js

Line 120 in 8046a49

assert(typeof ptr == 'number', `UTF8ToString expects a number (got ${typeof ptr})`);

However, I agree that this is confusing: in wasm64, a pointer is a bigint, and it would be nice for functions that receive pointers to handle that. @sbc100 what do you think?

[kripken]

@ptc-jpharvey for now you can work around this with UTF8ToString(Number(canvasName))

[sbc100]

The way emscripten works today is that it converts pointers to int64 (number) so the JS code can assume that pointers are int53 numbers. However we currently an automatic way to do this conversion for EM_JS function.

IIRC this is because we dont have any way to distinguish between pointers and non-pointer EM_JS arguments.

So for EM_JS and EM_ASM one currently needs to do this conversion to int64 yourself.

We should probably have more docs on how to do this..

[sbc100]

Here is the issue: #16975. Maybe we can close this as a dup of that?

[kripken]

Sure. I'll comment there.