Allows LibraryLink projects to call MLPrintf to print straight to a Mathematica notebook, using ordinary printf formatting. Super useful to debug LibraryLink projects live while working with them.

MLPrintf.c

#include <cstdarg>
#include <stdio.h>
#include <stdarg.h>

WolframLibraryData logLibData = NULL;

void MLPrintString(const char* str) {
	if (!logLibData) return;
	MLINK link = logLibData->getWSLINK(logLibData);
	MLPutFunction(link, "EvaluatePacket", 1);
	MLPutFunction(link, "CellPrint", 1);
	MLPutFunction(link, "Cell", 4);
	MLPutString(link, str);
	MLPutString(link, "Print");
	MLPutFunction(link, "Rule", 2);
	MLPutSymbol(link, "CellLabel");
	MLPutString(link, "LibraryLink");
	MLPutFunction(link, "Rule", 2);
	MLPutSymbol(link, "ShowCellLabel");
	MLPutString(link, "True");

	logLibData->processWSLINK(link);
	auto pkt = MLNextPacket(link);
	if (pkt == RETURNPKT) {
		MLNewPacket(link);
	}
}

void MLPrintf(const char *fmt, ...) {
	if (!logLibData) return;
	va_list args;
	va_start(args, fmt);
	char buffer[8192];
	vsnprintf(buffer, sizeof(buffer), fmt, args);
	MLPrintString(buffer);
	va_end(args);
}

void EnableLogging(WolframLibraryData libData) {
	logLibData = libData; 
}

void DisableLogging() { 
	logLibData = NULL; 
}