SendMessageCallbackA function (winuser.h)

Sends the specified message to a window or windows. It calls the window procedure for the specified window and returns immediately if the window belongs to another thread. After the window procedure processes the message, the system calls the specified callback function, passing the result of the message processing and an application-defined value to the callback function.


BOOL SendMessageCallbackA(
  [in] HWND          hWnd,
  [in] UINT          Msg,
  [in] WPARAM        wParam,
  [in] LPARAM        lParam,
  [in] SENDASYNCPROC lpResultCallBack,
  [in] ULONG_PTR     dwData


[in] hWnd

Type: HWND

A handle to the window whose window procedure will receive the message. If this parameter is HWND_BROADCAST ((HWND)0xffff), the message is sent to all top-level windows in the system, including disabled or invisible unowned windows, overlapped windows, and pop-up windows; but the message is not sent to child windows.

[in] Msg

Type: UINT

The message to be sent.

For lists of the system-provided messages, see System-Defined Messages.

[in] wParam


Additional message-specific information.

[in] lParam


Additional message-specific information.

[in] lpResultCallBack


A pointer to a callback function that the system calls after the window procedure processes the message. For more information, see SendAsyncProc.

If hWnd is HWND_BROADCAST ((HWND)0xffff), the system calls the SendAsyncProc callback function once for each top-level window.

[in] dwData


An application-defined value to be sent to the callback function pointed to by the lpCallBack parameter.

Return value

Type: BOOL

If the function succeeds, the return value is nonzero.

If the function fails, the return value is zero. To get extended error information, call GetLastError.


If the target window belongs to the same thread as the caller, then the window procedure is called synchronously, and the callback function is called immediately after the window procedure returns. If the target window belongs to a different thread from the caller, then the callback function is called only when the thread that called SendMessageCallback also calls GetMessage, PeekMessage, or WaitMessage.

If you send a message in the range below WM_USER to the asynchronous message functions (PostMessage, SendNotifyMessage, and SendMessageCallback), its message parameters cannot 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.

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

The system only does marshalling for system messages (those in the range 0 to (WM_USER-1)). To send other messages (those >= WM_USER) to another process, you must do custom marshalling.


The winuser.h header defines SendMessageCallback as an alias which automatically selects the ANSI or Unicode version of this function based on the definition of the UNICODE preprocessor constant. Mixing usage of the encoding-neutral alias with code that not encoding-neutral can lead to mismatches that result in compilation or runtime errors. For more information, see Conventions for Function Prototypes.


Requirement Value
Minimum supported client Windows 2000 Professional [desktop apps only]
Minimum supported server Windows 2000 Server [desktop apps only]
Target Platform Windows
Header winuser.h (include Windows.h)
Library User32.lib
DLL User32.dll
API set ext-ms-win-ntuser-message-l1-1-0 (introduced in Windows 8)

See also


Messages and Message Queues