 Delphi Tips
Create a system wide keyboard hook under Win32
The following example demonstrates creating a system wide windows hook under Win32. The example provides both the code for the system hook dll and an example application. The hook function that we will create will also demonstrate advanced coding techniques such as sharing global memory across process boundaries using memory mapped files, sending messages from the key hook function back to the originating application, and dynamic loading of a dll at runtime.
The example keyboard hook that we create will keep a count of the number of keystrokes a user enters on the keyboard. Further, we will demonstrate trapping the enter key, and passing a message back to the application that initiated the keyboard hook each time the enter key is pressed. Finally, we will demonstrate trapping the left arrow key and instead of letting it through to the current application, we will instead replace it with a right arrow keystroke. (Note: that this can cause much confusion to a unsuspecting user).
Library TheHook;
uses
Windows, Messages, SysUtils;
{Define a record for recording and passing information process wide}
type
PHookRec = ^ THookRec;
THookRec = Packed Record
TheHookHandle: HHOOK;
TheAppWinHandle: HWnd;
TheCtrlWinHandle: HWnd;
TheKeyCount: DWord;
end;
var
hObjHandle : THandle; {Variable for the file mapping object}
lpHookRec : PHookRec;
{Pointer to our hook record}
procedure MapFileMemory (dwAllocSize: DWord);
begin { MapFileMemory }
{Create a process wide memory mapped variable}
hObjHandle := CreateFileMapping ($FFFFFFFF, Nil, PAGE_READWRITE, 0,
dwAllocSize, 'HookRecMemBlock');
if (hObjHandle = 0) then
begin
MessageBox (0, 'Hook DLL', 'Could not create file map object', mb_Ok);
exit
end { (hObjHandle = 0) };
{Get a pointer to our process wide memory mapped variable}
lpHookRec := MapViewOfFile (hObjHandle, FILE_MAP_WRITE, 0, 0, dwAllocSize);
if (lpHookRec = Nil) then
begin
CloseHandle (hObjHandle);
MessageBox (0, 'Hook DLL', 'Could not map file', mb_Ok);
exit
end { (lpHookRec = Nil) }
end; { MapFileMemory }
procedure UnMapFileMemory;
begin { UnMapFileMemory }
{Delete our process wide memory mapped variable}
if (lpHookRec <> Nil) then
begin
UnMapViewOfFile (lpHookRec);
lpHookRec := Nil
end { (lpHookRec <> Nil) };
if (hObjHandle > 0) then
begin
CloseHandle (hObjHandle);
hObjHandle := 0
end { (hObjHandle > 0) }
end; { UnMapFileMemory }
function GetHookRecPointer : pointer
stdcall;
begin { GetHookRecPointer }
{Return a pointer to our process wide memory mapped variable}
Result := lpHookRec
end; { GetHookRecPointer }
{The function that actually processes the keystrokes for our hook}
function KeyBoardProc (code: Integer; wParam: Integer; lParam: Integer) :
Integer;
stdcall;
var
KeyUp : bool;
{Remove comments for additional functionability
IsAltPressed : bool;
IsCtrlPressed : bool;
IsShiftPressed : bool;
}
begin { KeyBoardProc }
Result := 0;
Case code Of
HC_ACTION:
begin
{We trap the keystrokes here}
{Is this a key up message?}
KeyUp := ((lParam and (1 shl 31)) <> 0);
(*Remove comments for additional functionability
{Is the Alt key pressed}
if ((lParam and (1 shl 29)) <> 0) then begin
IsAltPressed := TRUE;
end else begin
IsAltPressed := FALSE;
end;
{Is the Control key pressed}
if ((GetKeyState(VK_CONTROL) and (1 shl 15)) <> 0) then begin
IsCtrlPressed := TRUE;
end else begin
IsCtrlPressed := FALSE;
end;
{if the Shift key pressed}
if ((GetKeyState(VK_SHIFT) and (1 shl 15)) <> 0) then begin
IsShiftPressed := TRUE;
end else begin
IsShiftPressed := FALSE;
end;
*)
{if KeyUp then increment the key count}
if (KeyUp <> false) then
begin
inc (lpHookRec^.TheKeyCount)
end { (KeyUp <> false) };
Case wParam Of
{Was the enter key pressed?}
VK_RETURN:
begin
{if KeyUp}
if (KeyUp <> false) then
begin
{Post a bogus message to the window control in our app}
PostMessage (lpHookRec^.TheCtrlWinHandle, WM_KEYDOWN, 0, 0);
PostMessage (lpHookRec^.TheCtrlWinHandle, WM_KEYUP, 0, 0)
end { (KeyUp <> false) };
{if you wanted to swallow the keystroke then return -1}
{else if you want to allow the keystroke then return 0}
Result := 0;
exit
end; {VK_RETURN}
{if the left arrow key is pressed then lets play a joke!}
VK_LEFT:
begin
{if KeyUp}
if (KeyUp <> false) then
begin
{Create a UpArrow keyboard event}
keybd_event (VK_RIGHT, 0, 0, 0);
keybd_event (VK_RIGHT, 0, KEYEVENTF_KEYUP, 0)
end { (KeyUp <> false) };
{Swallow the keystroke}
Result := -1;
exit
end; {VK_LEFT}
end { case wParam }; {case wParam}
{Allow the keystroke}
Result := 0
end; {HC_ACTION}
HC_NOREMOVE:
begin
{This is a keystroke message, but the keystroke message}
{has not been removed from the message queue, since an}
{application has called PeekMessage() specifying PM_NOREMOVE}
Result := 0;
exit
end;
end { case code }; {case code}
if (code < 0) then
{Call the next hook in the hook chain}
Result := CallNextHookEx (lpHookRec^.TheHookHandle, code, wParam, lParam)
end; { KeyBoardProc }
procedure StartKeyBoardHook
stdcall;
begin { StartKeyBoardHook }
{if we have a process wide memory variable}
{and the hook has not already been set...}
if ((lpHookRec <> Nil) and (lpHookRec^.TheHookHandle = 0)) then
begin
{Set the hook and remember our hook handle}
lpHookRec^.TheHookHandle := SetWindowsHookEx (WH_KEYBOARD, @KeyBoardProc,
HInstance, 0)
end { ((lpHookRec <> Nil) and (lpHookRec^.TheHookHandle = 0)) }
end; { StartKeyBoardHook }
procedure StopKeyBoardHook
stdcall;
begin { StopKeyBoardHook }
{if we have a process wide memory variable}
{and the hook has already been set...}
if ((lpHookRec <> Nil) and (lpHookRec^.TheHookHandle <> 0)) then
begin
{Remove our hook and clear our hook handle}
if (UnHookWindowsHookEx (lpHookRec^.TheHookHandle) <> false) then
begin
lpHookRec^.TheHookHandle := 0
end { (UnHookWindowsHookEx (lpHookRec^.TheHookHandle) <> false) }
end { ((lpHookRec <> Nil) and (lpHookRec^.TheHookHandle <> 0)) }
end; { StopKeyBoardHook }
procedure DllEntryPoint (dwReason: DWord);
begin { DllEntryPoint }
Case dwReason Of
Dll_Process_Attach:
begin
{if we are getting mapped into a process, then get}
{a pointer to our process wide memory mapped variable}
hObjHandle := 0;
lpHookRec := Nil;
MapFileMemory (sizeof (lpHookRec^))
end;
Dll_Process_Detach:
begin
{if we are getting unmapped from a process then, remove}
{the pointer to our process wide memory mapped variable}
UnMapFileMemory
end;
end { case dwReason }
end; { DllEntryPoint }
Exports
KeyBoardProc name 'KEYBOARDPROC',
GetHookRecPointer name 'GETHOOKRECPOINTER',
StartKeyBoardHook name 'STARTKEYBOARDHOOK',
StopKeyBoardHook name 'STOPKEYBOARDHOOK';
begin
{Set our Dll's main entry point}
DLLProc := @DllEntryPoint;
{Call our Dll's main entry point}
DllEntryPoint (Dll_Process_Attach)
end.
Back to Index of Tips |
