CBT Hook receiving only some events - c++

Question

I created a DLL to hook some events using a CBT hook. It seems to work only for the windows created by the process launching the hook...
My system is Windows 7 x64, but the behaviour is the same also on the x32.
This is the code (sorry but I'm no C++ expert):
#include "windows.h"
extern "C"
{
static LRESULT CALLBACK CbtProcCb(int nCode, WPARAM wParam, LPARAM lParam);
HINSTANCE g_hDll = NULL;
HWND g_hNotifyWin = NULL;
DWORD g_uNotifyMsg = NULL;
HHOOK g_hHook = NULL;
__declspec(dllexport) HHOOK SetCbtHook(HWND hWnd, LPCWSTR lStrMsg, DWORD threadId)
{
g_hNotifyWin = hWnd;
g_uNotifyMsg = RegisterWindowMessage(lStrMsg);
g_hHook = SetWindowsHookEx(WH_CBT, (HOOKPROC)CbtProcCb, g_hDll, threadId);
return g_hHook;
}
__declspec(dllexport) int UnsetCbtHook()
{
if ( !g_hHook )
return 0;
return UnhookWindowsHookEx(g_hHook);
}
}
static LRESULT CALLBACK CbtProcCb(int nCode, WPARAM wParam, LPARAM lParam)
{
SendNotifyMessage(g_hNotifyWin, g_uNotifyMsg, nCode, wParam); // Send nCode to check the received event
return CallNextHookEx(g_hHook, nCode, wParam, lParam);
}
BOOL APIENTRY DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpvReserved)
{
if ( fdwReason == DLL_PROCESS_ATTACH )
g_hDll = hinstDLL;
return true;
}
Any hints?

Answer 1

When Windows installs globals hook, the DLL implementing the hook function is often loaded in other processes. In those processes, the g_hNotifyWin and g_uNotifyMsg global variables are NULL. That global variables are only not NULL in the context of the process in which the SetCbtHook call took place.
You must have a way to retrieve the correct value for g_hNotifyWin and 'g_uNotifyMsg', in an arbitrary process.
Add:
const char * g_pszClassName = "MyClassName";
const char * g_pszRegisteredMsg = "MyMessage";
to your DLL, and replace "MyClassName" by the class name of the g_hNotifyWin window. See the RegisterClass call in your EXE. Update also "MyMessage".
Then, use the following CbtProcCb function:
static LRESULT CALLBACK CbtProcCb(int nCode, WPARAM wParam, LPARAM lParam)
{
if ( nCode >= 0 ) {
if ( g_hNotifyWin == NULL ) g_hNotifyWin = FindWindow( g_pszClassName, NULL );
if ( g_uNotifyMsg == 0) g_uNotifyMsg = RegisterWindowMessage(g_pszRegisteredMsg);
if ( g_hNotifyWin && g_uNotifyMsg )
SendNotifyMessage(g_hNotifyWin, g_uNotifyMsg, nCode, wParam); // Send nCode to check the received event
}
return CallNextHookEx(NULL, nCode, wParam, lParam); // first arg useless see MSDN
}
EDIT:
As you noted in comment, same problem for g_uNotifyMsg See updated function. You could set up those variables in DllMain, indeed.
The first argument to CallNextHookEx may be NULL.

Answer 2

I guess your ThreadId is not zero, so hook is attached only to the calling process / thread respectively. Read documentation:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms644990(v=vs.85).aspx