How does emscripten_console_xxx work?

[msqr1]

I follow the function hierarchy for it, and it just calls printf_core. How does it skip the fs subsystem? It calls the function printf calls?

[brendandahl]

The non formatting versions directly call console.log etc here.

[msqr1]

What about the format version? If it calls printf then what advantage does it offer over regular printf

[brendandahl]

The format versions use special compiler format attributes that don't actually go through printf.

[sbc100]

The formatting is done to a string, rather than to stdout (using sprintf). The result of the formatting is that sent directly to JS:

emscripten/system/lib/libc/emscripten_console.c

Lines 13 to 28 in d9aed28

	static void vlogf(const char* fmt, va_list ap, void (*callback)(const char*)) {
	va_list ap2;
	va_copy(ap2, ap);
	size_t len = vsnprintf(0, 0, fmt, ap2);
	va_end(ap2);
	char* buf = alloca(len + 1);
	vsnprintf(buf, len + 1, fmt, ap);
	callback(buf);
	}
	void emscripten_console_logf(const char* fmt, ...) {
	va_list ap;
	va_start(ap, fmt);
	vlogf(fmt, ap, &emscripten_console_log);
	va_end(ap);
	}

If you want even more low level logging (that can also bypass the console object and write directly to stdout/stderr on node) you can also use emscripten_out / emscripten_err / emscripten_dbg and the f versions of those 3. Those function correspond directly the emscripten err / out / dbg helpers in JavaScript.

[msqr1]

So the format version just format on a string, and send that string to the normal version

[sbc100]

You can just use the actual printf function.. that writes to stdout. The stdout of emscripten will go to the console by default by you can override it by providing Module['print'] yourself and then you can do whatever you want with it. See https://emscripten.org/docs/api_reference/module.html#module-object for how to provide Module overrides like this.

[sbc100]

If just want the output to go the DevTools console then you don't need to do anything, and printf should just work.

[msqr1]

So stdout is an abstract object, it will be console on the browser, and the system stdout in node,... emscripten_console_xxx is only for browser, and it bypass the abstract stdout.

[sbc100]

Exactly.

Although nodejs also supports console to the emscripten_console_xx APIs works fine there too.

[msqr1]

Oh great! Thanks very much!