#include <windows.h>
#include <iostream>

using namespace std;

typedef int (WINAPI *MSGBX) (HWND,LPCSTR, LPCSTR,UINT);


int main(){

	HMODULE hMod = LoadLibrary("User32.dll");
	if (!hMod) return 1;
	
	MSGBX mymsgbox = (MSGBX)GetProcAddress(hMod, "MessageBoxA");
	
	if (!mymsgbox) return 1;
	
	cout << "libreria cargada y funcion localizada, esperando el hook..." << endl;
	
	cin.get();
	
	mymsgbox(NULL, "no ok", "!!", 0);             // no interceptada
	MessageBox(NULL, "no ok", "!!", 0);           // interceptada
	
	return 0;
	
}

case DLL_PROCESS_ATTACH:
		
			CreateThread(0, 0, (LPTHREAD_START_ROUTINE)begin, 0, 0, 0);

LPTHREAD_START_ROUTINE begin()
{
			
			HMODULE hMod = GetModuleHandle(0);
			orig_box = (MSGBX) InterceptDllCall(hMod, "User32.dll", "MessageBoxA", (DWORD)&mymsgbox);
			MessageBox(NULL, "Inyectado", "adasd", 0);
			
			return 0;
			
}

void *InterceptDllCall( HMODULE hModule, char *szDllName, char *szFunctionName, DWORD pNewFunction )
{
	PIMAGE_DOS_HEADER pDosHeader;
	PIMAGE_NT_HEADERS pNTHeader;
	PIMAGE_IMPORT_DESCRIPTOR pImportDesc;
	PIMAGE_THUNK_DATA pThunk;
	DWORD dwOldProtect;
	DWORD dwOldProtect2;
	void *pOldFunction;

	if( !( pOldFunction = (void*) GetProcAddress( GetModuleHandle( szDllName ), szFunctionName ) ) )
	{
	MessageBox(NULL, "GetProcAddress failed", "Error", 0);
	return( NULL );
	}

	pDosHeader = ( PIMAGE_DOS_HEADER )hModule;
	if( pDosHeader->e_magic != IMAGE_DOS_SIGNATURE )
	{
	MessageBox(NULL, "dos_header failed", "Error", 0);
	return( NULL );
	}

	pNTHeader = MakePtr( PIMAGE_NT_HEADERS, pDosHeader, pDosHeader->e_lfanew );
	if( pNTHeader->Signature != IMAGE_NT_SIGNATURE
			|| ( pImportDesc = MakePtr( PIMAGE_IMPORT_DESCRIPTOR, pDosHeader, pNTHeader->OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_IMPORT].VirtualAddress ) ) == ( PIMAGE_IMPORT_DESCRIPTOR )pNTHeader )
	{
	MessageBox(NULL, "PIMAGE_NT_HEADERS failed", "Error", 0);
	return( NULL );
	}

	while( pImportDesc->Name )
	{
		char *szModuleName = MakePtr( char *, pDosHeader, pImportDesc->Name );
		if( !stricmp( szModuleName, szDllName ) )
		break;
		pImportDesc++;
	}
	if( pImportDesc->Name == NULL )
	{
	MessageBox(NULL, "pImportDesc failed", "Error", 0);
	return( NULL );
	}

	pThunk = MakePtr( PIMAGE_THUNK_DATA, pDosHeader, pImportDesc->FirstThunk );
	while( pThunk->u1.Function )
	{
		if( pThunk->u1.Function == ( DWORD )pOldFunction )
		{
			VirtualProtect( ( void * )&pThunk->u1.Function, sizeof( DWORD ), PAGE_EXECUTE_READWRITE, &dwOldProtect );
			pThunk->u1.Function = (DWORD)pNewFunction;
			VirtualProtect( ( void * )&pThunk->u1.Function, sizeof( DWORD ), dwOldProtect, &dwOldProtect2 );
			return( pOldFunction );
		}
		pThunk++;
	}
	return( NULL ); 
}