Add a MSVC / Windows compatible snprintf function (#7353)

Use sizeof where applicable for mt_snprintf

src/porting.cpp

@@ -50,6 +50,8 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "util/string.h"
 #include "settings.h"
 #include <list>
+#include <cstdarg>
+#include <cstdio>
 
 namespace porting
 {
@@ -661,6 +663,28 @@ void attachOrCreateConsole()
 #endif
 }
 
+int mt_snprintf(char *buf, const size_t buf_size, const char *fmt, ...)
+{
+	// https://msdn.microsoft.com/en-us/library/bt7tawza.aspx
+	//  Many of the MSVC / Windows printf-style functions do not support positional
+	//  arguments (eg. "%1$s"). We just forward the call to vsnprintf for sane
+	//  platforms, but defer to _vsprintf_p on MSVC / Windows.
+	// https://github.com/FFmpeg/FFmpeg/blob/5ae9fa13f5ac640bec113120d540f70971aa635d/compat/msvcrt/snprintf.c#L46
+	//  _vsprintf_p has to be shimmed with _vscprintf_p on -1 (for an example see
+	//  above FFmpeg link).
+	va_list args;
+	va_start(args, fmt);
+#ifndef _MSC_VER
+	int c = vsnprintf(buf, buf_size, fmt, args);
+#else  // _MSC_VER
+	int c = _vsprintf_p(buf, buf_size, fmt, args);
+	if (c == -1)
+		c = _vscprintf_p(fmt, args);
+#endif // _MSC_VER
+	va_end(args);
+	return c;
+}
+
 // Load performance counter frequency only once at startup
 #ifdef _WIN32