inline.cpp

#include "pch.h"
#include <iostream>
#include <Windows.h>

FARPROC messageBoxAddress = NULL;
SIZE_T bytesWritten = 0;
char messageBoxOriginalBytes[6] = {};

int __stdcall HookedMessageBox(HWND hWnd, LPCSTR lpText, LPCSTR lpCaption, UINT uType) {
	
	// print intercepted values from the MessageBoxA function
	std::cout << "Ohai from the hooked function\n";
	std::cout << "Text: " << (LPCSTR)lpText << "\nCaption: " << (LPCSTR)lpCaption << std::endl;
	
	// unpatch MessageBoxA
	WriteProcessMemory(GetCurrentProcess(), (LPVOID)messageBoxAddress, messageBoxOriginalBytes, sizeof(messageBoxOriginalBytes), &bytesWritten);
	
	// call the original MessageBoxA
	return MessageBoxA(NULL, lpText, lpCaption, uType);
}

int main()
{
	// show messagebox before hooking
	MessageBoxA(NULL, "hi", "hi", MB_OK);

	HINSTANCE library = LoadLibraryA("user32.dll");
	SIZE_T bytesRead = 0;
	
	// get address of the MessageBox function in memory
	messageBoxAddress = GetProcAddress(library, "MessageBoxA");

	// save the first 6 bytes of the original MessageBoxA function - will need for unhooking
	ReadProcessMemory(GetCurrentProcess(), messageBoxAddress, messageBoxOriginalBytes, 6, &bytesRead);
	
	// create a patch "push <address of new MessageBoxA); ret"
	void *hookedMessageBoxAddress = &HookedMessageBox;
	char patch[6] = { 0 };
	memcpy_s(patch, 1, "\x68", 1);
	memcpy_s(patch + 1, 4, &hookedMessageBoxAddress, 4);
	memcpy_s(patch + 5, 1, "\xC3", 1);

	// patch the MessageBoxA
	WriteProcessMemory(GetCurrentProcess(), (LPVOID)messageBoxAddress, patch, sizeof(patch), &bytesWritten);

	// show messagebox after hooking
	MessageBoxA(NULL, "hi", "hi", MB_OK);

	return 0;
}