Global WH_CBT hook DLL is loaded into some processes only

Question

The main program calls the function SetHook in the wi.dll to install global WH_CBT hook.

bool WI_API SetHook()
{
    if (!g_hHook)
    {
        g_hHook = SetWindowsHookEx(WH_CBT, (HOOKPROC) CBTProc, g_hInstDll, 0);
    }

    return g_hHook != NULL;
}

I presume after installing global hook, wi.dll should be loaded into each process' address space. However wi.dll is loaded in to some processes only. For example, if I start Skype, MS Word I can see that wi.dll is loaded into these processes as well (using Process Explorer), however if I run Firefox, uTorrent, Adobe Reader then wi.dll is not loaded into these processes.

I'm using W7 64-bit, main program and wi.dll is 32-bit, all programs mentioned here is 32-bit programs as well.

Any ideas why that happens?

Thanks in advance.

Answer 1

The hook chain mechanism is not bulletproof and relies on everyone involved following the rules. If an application installs its own per-thread WH_CBT hook and does not call CallNextHookEx in its hook procedure, earlier hooks won't get called. See the MSDN docs for CallNextHookEx.