Advanced MFC Programming

Chapter 13. Adding Special Features to Application
function. As we can see, if macro __DLLIMPORT__ is defined, function LibMain(…), WEP(…) and
KeyboardProc(…) will be declared (they will be used only in the DLL). In this case, two other functions
SetKeyboardHook(…) and UnsetKeyboardHook() will be declared as import function. If the macro is not
defined, the two functions will be declared as export functions.
The reason for using macro EXTERN_C is that the DLL is built with C convention and our application is
built with C++ convention. To make them compatible, we must explicitly specify how to build the
functions in the DLL. In the sample, two functions are exported from the DLL: SetKeyboardHook(…) will
be used by the application to install hook; UnsetKeyboardHook() will be used to remove the hook.
In file “HookLib.c”, first two static variables are declared:
#pragma data_seg("SHARDATA")
static HWND g_hWnd=NULL;
static HHOOK g_hHook=NULL;
#pragma data_seg()
We use #pragma data_seg("SHARDATA") to specify that g_hWnd and g_hHook will be stored in
"SHARDATA" segment instead of being mapped to calling processes.
Function SetKeyboardHook(…) installs system wide keyboard hook by calling function
SetWindowsHookEx(…). When using this function, we must provide the instance handle of the DLL library
and the handle of the application’s mainframe window:
STDENTRY_(BOOL) SetKeyboardHook(HWND hWnd, HINSTANCE hInstance)
{
g_hHook=SetWindowsHookEx
(
WH_KEYBOARD,
KeyboardProc,
hInstance,
0
);
if(g_hHook == FALSE)
{
return FALSE;
}
g_hWnd=hWnd;
}
return TRUE;
The handle of application’s mainframe window is stalled in variable g_hWnd for later use. The handle
of the hook is stored in variable g_hHook and will be used in function UnsetKeyboardHook() to remove the
keyboard hook:
STDENTRY_(BOOL) UnsetKeyboardHook()
{
return UnhookWindowsHookEx(g_hHook);
}
Function KeyboardProc(…) is the hook procedure. When there is a keystroke, this function will be
called, and the keystroke information will be processed within this function:
LRESULT CALLBACK KeyboardProc(int code, WPARAM wParam, LPARAM lParam)
{
if(code < 0 || code != HC_ACTION)
{
return CallNextHookEx(g_hHookKeyboard, code, wParam, lParam);
}
if(code == HC_ACTION)
{
int nKeyState;
nKeyState=GetKeyState(VK_CONTROL);
if(lParam & 0x80000000 || lParam & 0x40000000)
{
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