WM_SYSKEYDOWN
wVkey = wParam; /* virtual-key code */
dwKeyData = lParam; /* key data */The WM_SYSKEYDOWN message is sent to the window with the input focus when the user holds down the ALT key and then presses another key. If no window currently has the input focus, the WM_SYSKEYDOWN 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 dwKeyData parameter.
wVkey
Value of wParam. Specifies the virtual-key code of the key being pressed.
dwKeyData
Value of lParam. Specifies the repeat count, scan code, extended key, context code, previous key state, and key-transition state, as shown in the following table:
| Bit | 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. |
| 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 keypad. The value is 1 if it is an extended key; otherwise, it is 0. |
| 25–26 | Not used. |
| 27–28 | Used internally by Windows. |
| 29 | Specifies the context code. The value is 1 if the ALT key is held down while the key is pressed; otherwise, the value is 0. |
| 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 key-transition state. The value is 1 if the key is being released, or it is 0 if the key is being pressed. |
For WM_SYSKEYDOWN messages, the value of bit 29 (context code) is 1 if the ALT key is down while the key is pressed; it is 0 if the message is sent to the active window because no window has the input focus. The value of bit 31 (key-transition state) is 0.
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 input focus.
Because of the autorepeat feature, more than one WM_SYSKEYDOWN message may occur before a WM_SYSKEYUP message is sent. The previous key state (bit 30) can be used to determine whether the WM_SYSKEYDOWN message indicates the first down transition or a repeated down transition.
For IBM Enhanced 101- and 102-key keyboards, enhanced keys are the right ALT key and the right CTRL key 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 division (/) and ENTER keys on the numeric keypad. Some other keyboards may support the extended-key bit in the lParam parameter.