Posted to the window with the keyboard focus when the user presses the F10 key (which activates the menu bar) or holds down the ALT key and then presses another key. It also occurs when no window currently has the keyboard focus; in this case, 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 lParam parameter.

#define WM_SYSKEYDOWN                   0x0104



The virtual-key code of the key being pressed. See Virtual-Key Codes.


The repeat count, scan code, extended-key flag, context code, previous key-state flag, and transition-state flag, as shown in the following table.

Bits Meaning
0-15 The repeat count for the current message. The value is the number of times the keystroke is autorepeated as a result of the user holding down the key. If the keystroke is held long enough, multiple messages are sent. However, the repeat count is not cumulative.
16-23 The scan code. The value depends on the OEM.
24 Indicates 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 The context code. The value is 1 if the ALT key is down while the key is pressed; it is 0 if the WM_SYSKEYDOWN message is posted to the active window because no window has the keyboard focus.
30 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 The transition state. The value is always 0 for a WM_SYSKEYDOWN message.

For more detail, see Keystroke Message Flags.

Return value

An application should return zero if it processes this message.


The DefWindowProc function examines the specified key and generates a WM_SYSCOMMAND message if the key is either TAB or ENTER.

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.

Because of automatic repeat, 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 enhanced 101- and 102-key keyboards, enhanced 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 lParam parameter.

This message is also sent whenever the user presses the F10 key without the ALT key.


Requirement Value
Minimum supported client
Windows 2000 Professional [desktop apps only]
Minimum supported server
Windows 2000 Server [desktop apps only]
Winuser.h (include Windows.h)

See also