SendMessageA function (winuser.h)

Article
02/09/2023

Sends the specified message to a window or windows. The SendMessage function calls the window procedure for the specified window and does not return until the window procedure has processed the message.

To send a message and return immediately, use the SendMessageCallback or SendNotifyMessage function. To post a message to a thread's message queue and return immediately, use the PostMessage or PostThreadMessage function.

Syntax

LRESULT SendMessageA(
  [in] HWND   hWnd,
  [in] UINT   Msg,
  [in] WPARAM wParam,
  [in] LPARAM lParam
);

Parameters

[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.

Message sending is subject to UIPI. The thread of a process can send messages only to message queues of threads in processes of lesser or equal integrity level.

[in] Msg

Type: UINT

The message to be sent.

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

[in] wParam

Type: WPARAM

Additional message-specific information.

[in] lParam

Type: LPARAM

Additional message-specific information.

Return value

Type: LRESULT

The return value specifies the result of the message processing; it depends on the message sent.

Remarks

When a message is blocked by UIPI the last error, retrieved with GetLastError, is set to 5 (access denied).

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.

If the specified window was created by the calling thread, the window procedure is called immediately as a subroutine. If the specified window was created by a different thread, the system switches to that thread and calls the appropriate window procedure. Messages sent between threads are processed only when the receiving thread executes message retrieval code. The sending thread is blocked until the receiving thread processes the message. However, the sending thread will process incoming nonqueued messages while waiting for its message to be processed. To prevent this, use SendMessageTimeout with SMTO_BLOCK set. For more information on nonqueued messages, see Nonqueued Messages.

An accessibility application can use SendMessage to send WM_APPCOMMAND messages to the shell to launch applications. This functionality is not guaranteed to work for other types of applications.

Examples

For an example, see Displaying Keyboard Input.

Note

The winuser.h header defines SendMessage as an alias that 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 is not encoding-neutral can lead to mismatches that result in compilation or runtime errors. For more information, see Conventions for Function Prototypes.

Requirements

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)

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