Apparently the original implementation from facebook was incorrect.

Fixed it, sadly this does mean we need to call vsnprintf twice.
Once to find out the size involved, a second time to apply.

common/StringUtil.cpp

@@ -15,9 +15,10 @@
  */
 
 #include "StringUtil.h"
+
 #include <string>
-#include <cstring> // for strncpy
 #include <cstdarg>
+#include <cstring> // for strncpy
 #include <stdexcept>
 
 #ifdef _WINDOWS
@@ -26,8 +27,10 @@
 	#define snprintf	_snprintf
 	#define strncasecmp	_strnicmp
 	#define strcasecmp  _stricmp
+
 #else
 	#include <stdlib.h>
+	#include <stdio.h>
 #endif
 
 #ifndef va_copy
@@ -37,53 +40,64 @@
 // original source: 
 // https://github.com/facebook/folly/blob/master/folly/String.cpp
 //
-void StringFormat(std::string& output, const char* format, ...) 
+void vStringFormat(std::string& output, const char* format, va_list args)
 {
-	va_list args;
-	// Tru to the space at the end of output for our output buffer.
-	// Find out write point then inflate its size temporarily to its
-	// capacity; we will later shrink it to the size needed to represent
-	// the formatted string.  If this buffer isn't large enough, we do a
-	// resize and try again.
+	va_list tmpargs;
 
-	const auto write_point = output.size();
-	auto remaining = output.capacity() - write_point;
-	output.resize(output.capacity());
+	va_copy(tmpargs,args);
+	int characters_used = vsnprintf(nullptr, 0, format, tmpargs);
+	va_end(tmpargs);
 
-	va_start(args, format);
-	int bytes_used = vsnprintf(&output[write_point], remaining, format,args);
-	va_end(args);
-	if (bytes_used < 0) {
-		
+	if (characters_used < 0) {
+		// Looks like we have an invalid format string.
+		// error out.
 		std::string errorMessage("Invalid format string; snprintf returned negative with format string: ");
 		errorMessage.append(format);
 		
 		throw std::runtime_error(errorMessage);
-	} 
-	else if ((unsigned int)bytes_used < remaining) {
-		// There was enough room, just shrink and return.
-		output.resize(write_point + bytes_used);
+	}
+	else if ((unsigned int)characters_used > output.capacity()) {
+		output.resize(characters_used+1);
+		va_copy(tmpargs,args);
+		characters_used = vsnprintf(&output[0], output.capacity(), format, tmpargs);
+		va_end(tmpargs);
+
+		if (characters_used < 0) {
+			// We shouldn't have a format error by this point, but I can't imagine what error we
+			// could have by this point. Still, error out and report it.
+			std::string errorMessage("Invalid format string or unknown vsnprintf error; vsnprintf returned negative with format string: ");
+			errorMessage.append(format);
+		
+			throw std::runtime_error(errorMessage);
+		}
 	} 
 	else {
-		output.resize(write_point + bytes_used + 1);
-		remaining = bytes_used + 1;
+		output.resize(characters_used + 1);
 
-		va_start(args, format);
-		bytes_used = vsnprintf(&output[write_point], remaining, format, args);
-		va_end(args);
-		
-		if ((unsigned int)(bytes_used + 1) != remaining) {
-			
-			std::string errorMessage("vsnprint retry did not manage to work with format string: ");
+		va_copy(tmpargs,args);
+		characters_used = vsnprintf(&output[0], output.capacity(), format, tmpargs);
+		va_end(tmpargs);
+
+		if (characters_used < 0) {
+			// We shouldn't have a format error by this point, but I can't imagine what error we
+			// could have by this point. still error out and report it.
+			std::string errorMessage("Invalid format string or unknown vsnprintf error; vsnprintf returned negative with format string: ");
 			errorMessage.append(format);
 		
 			throw std::runtime_error(errorMessage);
 		}
 		
-		output.resize(write_point + bytes_used);
 	}
 }
 
+void StringFormat(std::string& output, const char* format, ...)
+{
+	va_list args;
+	va_start(args, format);
+	vStringFormat(output,format,args);
+	va_end(args);
+}
+
 // normal strncpy doesnt put a null term on copied strings, this one does
 // ref: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/wcecrt/htm/_wcecrt_strncpy_wcsncpy.asp
 char* strn0cpy(char* dest, const char* source, uint32 size) {

common/StringUtil.h

@@ -17,8 +17,11 @@
 #define _STRINGUTIL_H_
 
 #include <string>
+#include <cstdarg>
 #include "types.h"
 
+
+void vStringFormat(std::string& output, const char* format, va_list args);
 void StringFormat(std::string& output, const char* format, ...);
 //////////////////////////////////////////////////////////////////////
 //

common/debug.cpp

@@ -2,6 +2,7 @@
 
 #include <iostream>
 #include <string>
+#include <cstdarg>
 
 #include <time.h>
 
@@ -12,10 +13,11 @@
 	#define vsnprintf	_vsnprintf
 	#define strncasecmp	_strnicmp
 	#define strcasecmp	_stricmp
+	
 #else
+	
 	#include <sys/types.h>
 	#include <unistd.h>
-	#include <stdarg.h>
 #endif
 
 #include "../common/StringUtil.h"
@@ -150,28 +152,36 @@ bool EQEMuLog::write(LogIDs id, const char *fmt, ...) {
 	time( &aclock ); /* Get time in seconds */
 	newtime = localtime( &aclock ); /* Convert time to struct */
 
-	if (dofile)
+	if (dofile) {
 #ifndef NO_PIDLOG
 		fprintf(fp[id], "[%02d.%02d. - %02d:%02d:%02d] ", newtime->tm_mon+1, newtime->tm_mday, newtime->tm_hour, newtime->tm_min, newtime->tm_sec);
 #else
 		fprintf(fp[id], "%04i [%02d.%02d. - %02d:%02d:%02d] ", getpid(), newtime->tm_mon+1, newtime->tm_mday, newtime->tm_hour, newtime->tm_min, newtime->tm_sec);
 #endif
+	}
 
-	va_list argptr, tmpargptr;
-	va_start(argptr, fmt);
+	va_list argptr,tmpargptr;
+	
 	if (dofile) {
-		va_copy(tmpargptr, argptr);
+		va_start(argptr, fmt);
+		va_copy(tmpargptr,argptr);
 		vfprintf( fp[id], fmt, tmpargptr );
+		va_end(tmpargptr);
 	}
 	if(logCallbackFmt[id]) {
 		msgCallbackFmt p = logCallbackFmt[id];
-		va_copy(tmpargptr, argptr);
+		va_start(argptr, fmt);
+		va_copy(tmpargptr,argptr);
 		p(id, fmt, tmpargptr );
+		va_end(tmpargptr);
 	}
-	
+
 	std::string outputMessage;
-	StringFormat(outputMessage, fmt, argptr);
-	
+	va_start(argptr, fmt);
+	va_copy(tmpargptr,argptr);
+	vStringFormat(outputMessage, fmt, tmpargptr);
+	va_end(tmpargptr);
+
 	if (pLogStatus[id] & 2) {
 		if (pLogStatus[id] & 8) {
 			
@@ -183,7 +193,6 @@ bool EQEMuLog::write(LogIDs id, const char *fmt, ...) {
 			std::cout << outputMessage;
 		}
 	}
-	va_end(argptr);
 	if (dofile)
 		fprintf(fp[id], "\n");
 	if (pLogStatus[id] & 2) {