Function LowLevelKeybdProc

source/hook.cpp:157–224

LRESULT CALLBACK LowLevelKeybdProc(int aCode, WPARAM wParam, LPARAM lParam)
{
	if (aCode != HC_ACTION)  // MSDN docs specify that both LL keybd & mouse hook should return in this case.
		return CallNextHookEx(g_KeybdHook, aCode, wParam, lParam);

	KBDLLHOOKSTRUCT &event = *(PKBDLLHOOKSTRUCT)lParam;  // For convenience, maintainability, and possibly performance.

	// Change the event to be physical if that is indicated in its dwExtraInfo attribute.
	// This is done for cases when the hook is installed multiple times and one instance of
	// it wants to inform the others that this event should be considered physical for the
	// purpose of updating modifier and key states:
	if (event.dwExtraInfo == KEY_PHYS_IGNORE)
		event.flags &= ~LLKHF_INJECTED;
	else if (event.dwExtraInfo == KEY_BLOCK_THIS)
		return TRUE;

	// Make all keybd events physical to try to fool the system into accepting CTRL-ALT-DELETE.
	// This didn't work, which implies that Ctrl-Alt-Delete is trapped at a lower level than
	// this hook (folks have said that it's trapped in the keyboard driver itself):
	//event.flags &= ~LLKHF_INJECTED;

	// Note: Some scan codes are shared by more than one key (e.g. Numpad7 and NumpadHome).  This is why
	// the keyboard hook must be able to handle hotkeys by either their virtual key or their scan code.
	// i.e. if sc were always used in preference to vk, we wouldn't be able to distinguish between such keys.

	bool key_up = (wParam == WM_KEYUP || wParam == WM_SYSKEYUP);
	vk_type vk = (vk_type)event.vkCode;
	sc_type sc = (sc_type)event.scanCode;
	if (vk && !sc) // Might happen if another app calls keybd_event with a zero scan code.
		sc = vk_to_sc(vk);
	// MapVirtualKey() does *not* include 0xE0 in HIBYTE if key is extended.  In case it ever
	// does in the future (or if event.scanCode ever does), force sc to be an 8-bit value
	// so that it's guaranteed consistent and to ensure it won't exceed SC_MAX (which might cause
	// array indexes to be out-of-bounds).  The 9th bit is later set to 1 if the key is extended:
	sc &= 0xFF;
	// Change sc to be extended if indicated.  But avoid doing so for VK_RSHIFT, which is
	// apparently considered extended by the API when it shouldn't be.  Update: Well, it looks like
	// VK_RSHIFT really is an extended key, at least on WinXP (and probably be extension on the other
	// NT based OSes as well).  What little info I could find on the 'net about this is contradictory,
	// but it's clear that some things just don't work right if the non-extended scan code is sent.  For
	// example, the shift key will appear to get stuck down in the foreground app if the non-extended
	// scan code is sent with VK_RSHIFT key-up event:
	if ((event.flags & LLKHF_EXTENDED)) // && vk != VK_RSHIFT)
		sc |= 0x100;

	// The below must be done prior to any returns that indirectly call UpdateKeybdState() to update
	// modifier state.
	// Update: It seems best to do the below unconditionally, even if the OS is Win2k or WinXP,
	// since it seems like this translation will add value even in those cases:
	// To help ensure consistency with Windows XP and 2k, for which this hook has been primarily
	// designed and tested, translate neutral modifier keys into their left/right specific VKs,
	// since beardboy's testing shows that NT4 receives the neutral keys like Win9x does:
	switch (vk)
	{
	case VK_SHIFT:   vk = (sc == SC_RSHIFT)   ? VK_RSHIFT   : VK_LSHIFT; break;
	case VK_CONTROL: vk = (sc == SC_RCONTROL) ? VK_RCONTROL : VK_LCONTROL; break;
	case VK_MENU:    vk = (sc == SC_RALT)     ? VK_RMENU    : VK_LMENU; break;
	}