Re: SetWindowsHookEx and WH_MOUSE

=?Utf-8?B?VHJlY2l1cw==?= <>
Tue, 5 Jun 2007 10:56:02 -0700
Thank you, Igor.

If you wouldn't mind, may I ask you more questions regarding DLLs.


"Igor Tandetnik" wrote:

Trecius <> wrote:

I've a question regarding SetWindowsHookEx with WH_MOUSE.

I have created a DLL that sets a hook, which captures the
WM_LBUTTONDOWN only ONCE. When it determines that the user clicked
the button, the hook should be released. Here's my code...

HHOOK g_hHook;
HANDLE g_hEvent;
HINSTANCE g_hInstance;
POINT g_pt;

POINT WINAPI HookWindow(HWND hWndToHook)
 g_hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);

 g_hHook = SetWindowsHookEx(WH_MOUSE, (HOOKPROC)MouseProc,
g_hInstance, GetWindowThreadProcessId(hWndToHook, NULL));

 WaitForSingleObject(g_hEvent, INFINITE);
 MessageBox(NULL, "Point acquired!", "Acquired!", MB_OK);

 return g_pt;

You appear to be installing a hook into a different process. This causes
your DLL to be loaded into that process. Now, a DLL has a separate copy
of global variables in every process it is loaded into. If you debug
your hook proc, you will find that no global variables are actually set.

First, in DllMain(HINSTANCE hinstDLL ...), is the hInstance the instance of
the DLL? MSDN says so, but I'm just confirming it.

Second, as I see it, when I have a global variable, the variable is global
ONLY to the process with which it was called unless declared within a
#pragma. For example, let's use my g_hEvent variable. If I have two
programs running, and each program calls LoadLibrary(), then I will have TWO
SEPARATE g_hEvents. If delcared in a #pragma, INITIALIZED, and #pragma
comment(linker, ...) then ALL references of the DLL will share those
variables contained in the #pragma section. So if one reference of the DLL
changes a variable, then it will be reflected in all other references. Is
this correct?

See KB article KB100634 "How to specify both shared data and non-shared
data in a DLL in Visual C++". But realize that:

a) g_hInstance should not be shared this way - in fact, it should not be
shared at all, it is only meaningful within a process.

You're telling me not to include g_hInstance in the #pragma
data_seg("shared"), correct? Well, does it really matter? If my first
question holds true -- that hinstDLL is the instance of the DLL -- then all
programs that load the DLL will have the same hinstDLL, so in a sense, it
could be shared? Am I wrong here?

b) g_hEvent cannot be shared this way - event handles are only valid
within a single process. You need to use a named event, or some other
IPC mechanism.

Again, are you telling me not to put g_hEvent in the #pragma data_seg?
Second, are you telling me to use CreateEvent(NULL, TRUE, FALSE, "MyEvent")
instead of CreateEvent(NULL, TRUE, FALSE, NULL)? In this case, the event
will be opened in the other process as it is now named.

c) You have a race condition. Multiple mouse events may arrive into your
hook before the other process reacts to the signalled event and
uninstalls the hook.

With best wishes,
    Igor Tandetnik

With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925

Lastly, Igor, I would like to ask WHAT is being injected into the other
process when I call SetWindowsHookEx().

SetWindowsHookEx has the following arguments...

SetWindowsHookEx(idHook, lpfn, hMod, dwThreadId)

So when I call SetWindowsHookEx(...), am I injecting ONLY the lpfn, or am I
injecting the ENTIRE DLL?

Thank you once again, Igor.


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