PostMessage

This function places a message in the message queue associated with the thread that created the specified window and then returns without waiting for the thread to process the message. Messages in a message queue are retrieved by calls to the GetMessage or PeekMessage function.

At a Glance

Header file:	Winuser.h
Windows CE versions:	1.0 and later

Syntax

BOOL PostMessage(HWND hWnd, UINT Msg, WPARAM wParam, LPARAM lParam);

Parameters

hWnd

[in] Handle to the window whose window procedure is to receive the message. Two values have special meanings:

Value	Description
HWND_BROADCAST	The message is posted to all top-level windows in the system, including disabled or invisible unowned windows, overlapped windows, and pop-up windows. The message is not posted to child windows.
NULL	The function behaves like a call to PostThreadMessage with the dwThreadId parameter set to the identifier of the current thread.

Msg

[in] Specifies the message to be posted.

wParam

[in] Specifies additional message-specific information.

lParam

[in] Specifies additional message-specific information.

Return Values

Nonzero indicates success. Zero indicates failure. To get extended error information, call GetLastError.

Remarks

Applications that need to communicate using HWND_BROADCAST should use the RegisterWindowMessage function to obtain a unique message for inter-application communication.

If you send a message in the range below WM_USER to the asynchronous message functions (PostMessage, SendNotifyMessage, and SendMessageCallback), its message parameters can not include pointers. Otherwise, the operation will fail. The functions will return before the receiving thread has had a chance to process the message and the sender will free the memory before it is used.