Programming Windows®, Fifth Edition

Keystroke Messages

When you press a key, Windows places either a WM_KEYDOWN or WM_SYSKEYDOWN message in the message queue of the window with the input focus. When you release a key, Windows places either a WM_KEYUP or WM_SYSKEYUP message in the message queue.

	Key Pressed	Key Released
Nonsystem Keystroke:	WM_KEYDOWN	WM_KEYUP
System Keystroke:	WM_SYSKEYDOWN	WM_SYSKEYUP

Usually the up and down messages occur in pairs. However, if you hold down a key so that the typematic (autorepeat) action takes over, Windows sends the window procedure a series of WM_KEYDOWN (or WM_SYSKEYDOWN) messages and a single WM_KEYUP (or WM_SYSKEYUP) message when the key is finally released. Like all queued messages, keystroke messages are time-stamped. You can retrieve the relative ...