g_patch_hhook = SetWindowsHookEx(WH_GETMESSAGE, patch_GetMsgProc,
        g_hinstance, client_thread_id);

    if(g_patch_hhook == 0)
    {
        api_error("Failed to install hook in client");
        return false;
    }

HHOOK SetWindowsHookEx(

    int idHook,   // type of hook to install
    HOOKPROC lpfn,   // address of hook procedure
    HINSTANCE hMod,   // handle of application instance
    DWORD dwThreadId    // identity of thread to install hook for 
   );
An error may occur if the hMod parameter is NULL and the dwThreadId parameter is zero or specifies the identifier of a thread created by another process.