WM_SYSKEYUP
nVirtKey = (int) wParam; /* virtual-key code */
lKeyData = lParam; /* key data */
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.
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. The value is the number of times the keystroke is 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 a function key or a key on the numeric key pad. The value is 1 if it is an extended key; otherwise, it is 0. |
| 25–28 | Reserved. |
| 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. |
An application should return zero if it processes this message.
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 system-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 IBM Enhanced 101- and 102-key keyboards, extended keys are the right ALT and the right CONTROL keys on the main section of the keyboard; the INSERT, DELETE, HOME, END, PAGE UP, PAGE DOWN and DIRECTION keys in the clusters to the left of the numeric key pad; and the divide (/) and ENTER keys in the numeric key pad. Some other keyboards may support the extended-key bit in the lKeyData parameter.
For non-USA Enhanced 102-key keyboards, the right ALT key is handled as a CONTROL-ALT key. The following shows the sequence of messages which result when the user presses and releases this key:
| 1 | WM_KEYDOWN | VK_CONTROL |
| 2 | WM_KEYDOWN | VK_MENU |
| 3 | WM_KEYUP | VK_CONTROL |
| 4 | WM_SYSKEYUP | VK_MENU |
TranslateAccelerator, WM_SYSKEYDOWN