How do I modify keys in a KeyboardProc hooking procedure? - windows

Question

I can use the SetWindowsHookEx function to hook a custom KeyboardProc. In the hooked procedure I can return 1 to block the keyboard input from reaching the application, however what I'm trying to do is actually modify the data. E.g. the user hits the key A and I'd like to replace that with key B.
But any modifications I make to the WPARAM and LPARAM arguments do not seem to have any effect in the application, it will still receive the original keys. E.g. to show a little pseudocode:
LRESULT KeyboardProc(int nCode, WPARAM wParam, LPARAM lParam)
{
memset(&wParam, 0, sizeof(WPARAM));
memset(&lParam, 0, sizeof(LPARAM));
// call the next procedure
return CallNextHookEx(keyHook, nCode, wParam, lParam);
}
No matter what modifications I make to the two parameters it will not have an effect, once the app reads the WM_KEYDOWN message it will receive the original keys as they were hit.
Is there any way I can actually make modifications to the keys?

Answer 1

The WH_KEYBOARD_LL hook enables you to monitor keyboard input events about to be posted in a thread input queue.
From: https://learn.microsoft.com/en-us/windows/win32/winmsg/about-hooks?redirectedfrom=MSDN#wh_debug
This means it is read-only and the params can not be modified.

Answer 2

The solution is to use SendInput, but only when lParam - which should be type-cast to KBDLLHOOKSTRUCT*) - has the LLKHF_INJECTED flag in its flags field.
Here's some D language code copied verbatim from one of my projects:
extern(Windows)
LRESULT LowLevelKeyboardProc(int code, WPARAM wParam, LPARAM lParam)
{
auto kbs = cast(KBDLLHOOKSTRUCT*)lParam;
// generate a new key event only if this key event was user-generated.
if (!(kbs.flags & LLKHF_INJECTED))
{
// Alt == toggle key binding
if (kbs.flags & LLKHF_ALTDOWN)
useKeyMap ^= 1;
if (!useKeyMap)
return CallNextHookEx(keyHook_LL, code, wParam, lParam);
INPUT input;
input.type = INPUT_KEYBOARD;
input.ki.dwFlags = (wParam == WM_KEYDOWN || wParam == WM_SYSKEYDOWN) ? 0 : KEYEVENTF_KEYUP;
// replace the key, must be in range 1 to 254
input.ki.wVk = keyMap.get(cast(Key)kbs.vkCode, cast(Key)kbs.vkCode);
SendInput(1, &input, INPUT.sizeof);
return -1;
}
return CallNextHookEx(keyHook_LL, code, wParam, lParam);
}