The WM_KEYDOWN message is posted to the window with the keyboard focus when a nonsystem key is pressed. A nonsystem key is a key that is pressed when the ALT key is not pressed.
nVirtKey = (int) wParam; // virtual-key code
lKeyData = lParam; // key data
Value of wParam. Specifies the virtual-key code of the nonsystem key.
Value of lParam. Specifies the repeat count, scan code, extended-key flag, context code, previous key-state flag, and transition-state flag, as shown in the following table:
0-15 Specifies the repeat count. The value is the number of times the keystroke is repeated as a result of the user holding down the key.
16-23 Specifies the scan code. The value depends on the original equipment manufacturer (OEM).
24 Specifies whether the key is an extended key, such as the right-hand ALT and CTRL keys that appear on an enhanced 101- or 102-key keyboard. The value is 1 if it is an extended key; otherwise, it is 0.
25-28 Reserved; do not use.
29 Specifies the context code. The value is always 0 for a WM_KEYDOWN message.
30 Specifies the previous key state. The value is 1 if the key is down before the message is sent, or it is 0 if the key is up.
31 Specifies the transition state. The value is always 0 for a WM_KEYDOWN message.
An application should return zero if it processes this message.