The WM_LBUTTONDOWN message is posted when the user presses the left mouse button while the cursor is in the client area of a window. If the mouse is not captured, the message is posted to the window beneath the cursor. Otherwise, the message is posted to the window that has captured the mouse.

A window receives this message through its WindowProc function.

Syntax

WM_LBUTTONDOWN

    WPARAM wParam
    LPARAM lParam;
    

Parameters

wParam

Indicates whether various virtual keys are down. This parameter can be one or more of the following values.

MK_CONTROL

The CTRL key is down.

MK_LBUTTON

The left mouse button is down.

MK_MBUTTON

The middle mouse button is down.

MK_RBUTTON

The right mouse button is down.

MK_SHIFT

The SHIFT key is down.

MK_XBUTTON1

Windows 2000/XP: The first X button is down.

MK_XBUTTON2

Windows 2000/XP: The second X button is down.

lParam

The low-order word specifies the x-coordinate of the cursor. The coordinate is relative to the upper-left corner of the client area.

The high-order word specifies the y-coordinate of the cursor. The coordinate is relative to the upper-left corner of the client area.

Return Value

If an application processes this message, it should return zero.

Remarks

Use the following code to obtain the horizontal and vertical position:

xPos = GET_X_LPARAM(lParam); 
yPos = GET_Y_LPARAM(lParam); 

You can also use the MAKEPOINTS macro to convert the lParam parameter to a POINTS structure.

To detect that the ALT key was pressed, check whether GetKeyState(VK_MENU) < 0. Note, this must not be GetAsyncKeyState.

Notification Requirements

Minimum DLL Version	None
Header	Declared in Winuser.h, include Windows.h
Minimum operating systems	Windows 95, Windows NT 3.1

See Also

Mouse Input Overview, GET_X_LPARAM, GET_Y_LPARAM, GetCapture, GetKeyState, MAKEPOINTS, POINTS, SetCapture, WM_LBUTTONDBLCLK, WM_LBUTTONUP