64bit (OSX and *nix) vsnprintf trashes va_list args. Must va_start between requesting length and performing expansion

gistfile1.cpp

std::string string_format(const char* s, ...)
{
	va_list args;
	va_start(args, s);
	
	std::string result;
	
	int len = vsnprintf(nullptr, 0, s, args);
	if (len > 0)
	{
		// va_end/va_start hack to fix *nix 64bit versions inc Mac OS X
		va_end(args);
		va_start(args, s);
		
		scope_array<char> tmp(len+1);
		vsnprintf(tmp.get(), tmp.size(), s, args);
		result = tmp;
	}
	va_end(args);

	return result;
}

Workaround Source
http://www.linuxquestions.org/questions/programming-9/vsnprintf-is-broken-301332