WM_SYSKEYUP

The WM_SYSKEYUP message is posted to the window with the keyboard focus when the user releases a key that was pressed while the alt key was held down. It also occurs when no window currently has the keyboard focus; in this case, the WM_SYSKEYUP message is sent to the active window. The window that receives the message can distinguish between these two contexts by checking the context code in the lKeyData parameter.

WM_SYSKEYUP 
nVirtKey = (int) wParam;    // virtual-key code 
lKeyData = lParam;          // key data

Parameters

nVirtKey

Value of wParam. Specifies the virtual-key code of the key being released.

lKeyData

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.

Value	Description
0–15	Specifies the repeat count for the current message. The value is the number of times the keystroke is auto-repeated as a result of the user holding down the key. The repeat count is always one for a WM_SYSKEYUP message.
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 1 if the ALT key is down while the key is released; it is 0 if the WM_SYSKEYDOWN message is posted to the active window because no window has the keyboard focus.
30	Specifies the previous key state. The value is always 1 for a WM_SYSKEYUP message.
31	Specifies the transition state. The value is always 1 for a WM_SYSKEYUP message.

Return Values

An application should return zero if it processes this message.

Default Action

The DefWindowProc function sends a WM_SYSCOMMAND message to the top-level window if the f10 key or the alt key was released. The wParam parameter of the message is set to SC_KEYMENU.

Remarks

When the context code is zero, the message can be passed to the TranslateAccelerator function, which will handle it as though it were a normal key message instead of a character-key message. This allows accelerator keys to be used with the active window even if the active window does not have the keyboard focus.

For enhanced 101- and 102-key keyboards, extended keys are the right alt and ctrl keys on the main section of the keyboard; the ins, del, home, end, page up, page down and arrow keys in the clusters to the left of the numeric keypad; and the divide (/) and enter keys in the numeric keypad. Other keyboards may support the extended-key bit in the lKeyData parameter.

For non-U.S. enhanced 102-key keyboards, the right alt key is handled as a ctrl+alt key. The following table shows the sequence of messages that result when the user presses and releases this key.

Message	Virtual-key code
WM_KEYDOWN	VK_CONTROL
WM_KEYDOWN	VK_MENU
WM_KEYUP	VK_CONTROL
WM_SYSKEYUP	VK_MENU

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