GetAsyncKeyState function (winuser.h)

Determines whether a key is up or down at the time the function is called, and whether the key was pressed after a previous call to GetAsyncKeyState.

Syntax

SHORT GetAsyncKeyState(
  [in] int vKey
);

Parameters

[in] vKey

Type: int

The virtual-key code. For more information, see Virtual Key Codes.

You can use left- and right-distinguishing constants to specify certain keys. See the Remarks section for further information.

Return value

Type: SHORT

If the function succeeds, the return value specifies whether the key was pressed since the last call to GetAsyncKeyState, and whether the key is currently up or down. If the most significant bit is set, the key is down, and if the least significant bit is set, the key was pressed after the previous call to GetAsyncKeyState. However, you should not rely on this last behavior; for more information, see the Remarks.

The return value is zero for the following cases:

• The current desktop is not the active desktop
• The foreground thread belongs to another process and the desktop does not allow the hook or the journal record.

Remarks

The GetAsyncKeyState function works with mouse buttons. However, it checks on the state of the physical mouse buttons, not on the logical mouse buttons that the physical buttons are mapped to. For example, the call GetAsyncKeyState(VK_LBUTTON) always returns the state of the left physical mouse button, regardless of whether it is mapped to the left or right logical mouse button. You can determine the system's current mapping of physical mouse buttons to logical mouse buttons by calling GetSystemMetrics(SM_SWAPBUTTON).

which returns TRUE if the mouse buttons have been swapped.

Although the least significant bit of the return value indicates whether the key has been pressed since the last query, due to the preemptive multitasking nature of Windows, another application can call GetAsyncKeyState and receive the "recently pressed" bit instead of your application. The behavior of the least significant bit of the return value is retained strictly for compatibility with 16-bit Windows applications (which are non-preemptive) and should not be relied upon.

You can use the virtual-key code constants VK_SHIFT, VK_CONTROL, and VK_MENU as values for the vKey parameter. This gives the state of the SHIFT, CTRL, or ALT keys without distinguishing between left and right.

You can use the following virtual-key code constants as values for vKey to distinguish between the left and right instances of those keys.

These left- and right-distinguishing constants are only available when you call the GetKeyboardState, SetKeyboardState, GetAsyncKeyState, GetKeyState, and MapVirtualKey functions.

Example

while (GetMessage(&msg, nullptr, 0, 0))
{
    if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }

    switch (msg.message)
    {
    case WM_KEYDOWN:
        if ((GetAsyncKeyState(VK_ESCAPE) & 0x01) && bRunning)
        {
            Stop();
        }
        break;
    }
}

Example from Windows Classic Samples on GitHub.

Requirements

See also

• GetAsyncKeyState
• GetKeyState
• GetKeyboardState
• GetSystemMetrics
• MapVirtualKey
• SetKeyboardState
• Keyboard Input