printf override


Author: Dave
Date: 05.16.14 - 3:26pm



I have a C dll that was taken from a console app. It has a metric shit ton of printf statements included in it.

I now want to use this in a GUI app and redirect the printf calls to my windows interface. There are a couple ways to do this.

The first thought is to use a detours style hook at runtime hook WriteFile and look for STDOUT handle (0x07) and grab the text from there. This has the disadvantage of having to include an entire hooking library and disassembler in the code which is a lot of complexity and size. This would be the way to do it if I didnt have the source to the dll.

A second thought is to do some kind of pipe redirection for stdout. I havent researched this method so cant really comment.

Since I have the source to the dll though, and will be compiling it for my project I think I am going to use the following technique: (in MS Visual Studio)

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

//if you still need to use the original function this has to be done 
//before including the header where print is redefined..
typedef int (*realprintf)(const char*, ...);
realprintf real_printf = printf;

//this define must be in a global header file
#define	printf my_printf 

int my_printf(char* format, ...){
	
	char *ret = 0;
	char buf[1024]; //avoid malloc/free for short strings if we can
	bool alloced = false;

	if(!format) return 0;

	va_list args; 
	va_start(args,format); 
	int size = _vscprintf(format, args); 
	if(size==0){va_end(args); return 0;}

	if(size < 1020){
		ret = &buf[0];
	}else{
		alloced = true;
		size++; //for null
		ret = (char*)malloc(size+2);
		if(ret==0){va_end(args); return 0;}
	}

	_vsnprintf(ret, size, format, args);
	ret[size]=0; //explicitly null terminate
	va_end(args);

	//here is where you could forward the char* to a UI handler..
	MessageBoxA(0,ret,"Hooked printf!",0);
	
	if(alloced) free(ret);
	return 0;
}

void main(void){
	printf("this is my test!");
        real_printf("this works as normal")
}





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