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Programming Windows®, Fifth Edition by Charles Petzold

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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 ...

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