Change definition of true and false macros to be js friendly

[hoodmane]

No description provided.

[hoodmane]

@sbc100 I added a test but curiously it does not pass because stdbool.h is included from upstream/lib/clang/21/include/stdbool.h and not from cache/sysroot/include/stdbool.h.

[sbc100]

@sbc100 I added a test but curiously it does not pass because stdbool.h is included from upstream/lib/clang/21/include/stdbool.h and not from cache/sysroot/include/stdbool.h.

But you've seen this fail locally for you?

[hoodmane]

Yes, I've been using this in Pyodide for years:
https://github.com/pyodide/pyodide/blob/main/src/core/types.h#L12-L17

I just had to copy paste it into another project and figured it would be nice to upstream.

[sbc100]

Yes, I've been using this in Pyodide for years: https://github.com/pyodide/pyodide/blob/main/src/core/types.h#L12-L17

I just had to copy paste it into another project and figured it would be nice to upstream.

So when you see it fail I assume that failure is also coming from the clang version this header? If so, I guess this fix needs to happen upstream in clang instead?

[sbc100]

Another solution would be to using -std=c17 (or C++) in which case these macros won't get defined.

[hoodmane]

Yeah, I guess the system search path looks like:

.../emsdk-dev/upstream/lib/clang/21/include
.../emscripten/cache/sysroot/include

so the clang include files are preferred over the emscripten files.

[sbc100]

We could potentially inject a fix like your into our em_asm/em_js headers? I would certainly want to guard the fix behind if defined(true) && defined(false) though so it was a no-op in C++ / C-17

[hoodmane]

Makes sense to me, I did that.

[hoodmane]

Okay I added the comments you suggested.

[sbc100]

I wonder if its worth adding this without also adding something like EM_JS_MACROS.

I would imagine the best place for something like this would be alongside something like EM_JS_MACROS.

Is the idea that there could be other users out there who try to write EM_JS_MACROS and this will help them avoid the pain you went through trying to debug stuff?

[hoodmane]

Yeah. A few months ago, someone was asking how to make EM_JS_MACROS in an issue and I told them that this was the one tricky thing. I think it would make sense to add an EM_JS_MACROS macro at the same time. It is super useful if you have to do struct accesses:

struct MyStruct {
   void* field1;
   void* field2;
}

#define MyStruct_field1_offset 0
#define MyStruct_field2_offset 4

_Static_assert(offsetof(MyStruct, field1) == MyStruct_field1_offset);
_Static_assert(offsetof(MyStruct, field2) == MyStruct_field2_offset);

#define MyStruct_field1(x) HEAPU32[x/4 + MyStruct_field1_offset]
#define MyStruct_field2(x) HEAPU32[x/4 + MyStruct_field2_offset]

EM_JS_MACROS(void, f, (MyStruct* arg), {
   const field1 = MyStruct_field1(x);
   const field2 = MyStruct_field2(x);
});

Another common use case is for sharing bitflags between C and JS:

#define FLAG1 (1 << 0)
#define FLAG2 (1 << 1)
...

EMSCRIPTEN_KEEPALIVE int get_flags() {
    return FLAG1 | FLAG2;
}

EM_JS_MACROS(void, process_flags, (int flags), {
   if (flags & FLAG1) {
     // handle FLAG1
   }
   if (flags & FLAG2) {
     // handle FLAG2
   }
});

But the biggest reason for me is to inject standard error handling everywhere so that it's easier to implement a C calling convention in JS:

#define EM_JS_REF(ret, func_name, args, body...)                               \
  EM_JS_MACROS(ret WARN_UNUSED, func_name, args, {                             \
    try    /* intentionally no braces, body already has them */                \
      body /* <== body of func */                                              \
    catch (e) {                                                                \
        LOG_EM_JS_ERROR(func_name, e);                                         \
        Module.handle_js_error(e);                                             \
        return 0;                                                              \
    }                                                                          \
    errNoRet();                                                                \
  })

[hoodmane]

Opened #24272.