Keyboard hook

Question

Hello,

First of all, excuse my languange, since English is no my native lang.

I try to hook the keyboard with SetWindowsHookEx() func, but something strange happened, please take a look at this code:

...
HINSTANCE hInst;
...
HHOOK g_hKbHook;  // Hook handle
HWND g_hEdit;     // Handle to Edit control
wchar_t *g_pBuff;   // pointer to buffer in global
...

void ToggleKbHook(HWND hWnd)
{
...
g_hKbHook = ::SetWindowsHookEx(WH_KEYBOARD, reinterpret_cast<HOOKPROC>(KeybHookProc), hInst, ::GetCurrentThreadId());
...
}
...
LRESULT CALLBACK KeybHookProc(int nCode, WPARAM wParm, LPARAM lParm)
{
	UINT mapRes = 0;
	switch (static_cast<unsigned int>(wParm))
	{
	case VK_SHIFT:
		::lstrcat(g_pBuff, L"[SHIFT]");
		break;
	case VK_RETURN:
		::lstrcat(g_pBuff, L"[ENTER]");
		break;
	case VK_TAB:
		::lstrcat(g_pBuff, L"[TAB]");
		break;	
	case VK_ESCAPE:
		::lstrcat(g_pBuff, L"[ESC]");
		break;
	case VK_CAPITAL:
		::lstrcat(g_pBuff, L"[CAPS]");
		break;
	default:
		mapRes = ::MapVirtualKey(static_cast<UINT>(wParm), MAPVK_VK_TO_CHAR);
		if (mapRes > 0){
			wchar_t res[2];
			res[0] = static_cast<wchar_t>(mapRes);
			res[1] = L'\0';
			::lstrcat(g_pBuff, &res[0]);
		} else if (mapRes == 0) {
			::lstrcat(g_pBuff, L"[UNKNOWN_KEY]");
		} else {

		}
		break;
	}
	if (g_hEdit){
		if (::lstrlen(g_pBuff) > 0){
			::SetWindowText(g_hEdit, reinterpret_cast<LPWSTR>(g_pBuff));
		}
	}

	return ::CallNextHookEx(g_hKbHook, nCode, wParm, lParm);
}

When I put one or more breakpoint inside function KeybHokkProc(), the programm running as I expected, Edit control display text properly, but when I remove the breakpoints Edit control show "double characters", it mean when I press character A in keyboard, the Edit control show double character A (AA).

Why is this happen ?

Thanks before

Answer 1

I read this link over and over again:

http://msdn.microsoft.com/en-us/library/ms644990(VS.85).aspx

The page say that WH_KEYBOARD is in thread and global scope, and thread scope is called first than global scope. It mean hook procedure is called twice, right ? And somehow debugging system block one of them, I am not sure which one.

If you know someting about this, please confirm.

Answer 2

Hi,

I know this is late. However, I recently came across this issue. Turns out that the callback runs twice, once for WM_KEYDOWN and once for WM_KEYUP. Hence what I did was only process one of the callbacks, by putting all the logic in an if condition:

if(wParm == WM_KEYDOWN || wParm == WM_SYSKEYDOWN) { //or WM_KEYUP and WM_SYSKEYUP

//your switch statement here.

}

Hope this helps.