PostMessageA function (winuser.h)

Article
02/09/2023

Places (posts) a message in the message queue associated with the thread that created the specified window and returns without waiting for the thread to process the message.

To post a message in the message queue associated with a thread, use the PostThreadMessage function.

Syntax

BOOL PostMessageA(
  [in, optional] HWND   hWnd,
  [in]           UINT   Msg,
  [in]           WPARAM wParam,
  [in]           LPARAM lParam
);

Parameters

[in, optional] hWnd

Type: HWND

A handle to the window whose window procedure is to receive the message. The following values have special meanings.

Value	Meaning
HWND_BROADCAST ((HWND)0xffff)	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.

Starting with Windows Vista, message posting is subject to UIPI. The thread of a process can post messages only to message queues of threads in processes of lesser or equal integrity level.

[in] Msg

Type: UINT

The message to be posted.

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: 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.

Remarks

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

Messages in a message queue are retrieved by calls to the GetMessage or PeekMessage function.

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 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.

Do not post the WM_QUIT message using PostMessage; use the PostQuitMessage function.

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

A message queue can contain at most 10,000 messages. This limit should be sufficiently large. If your application exceeds the limit, it should be redesigned to avoid consuming so many system resources. To adjust this limit, modify the following registry key.

HKEY_LOCAL_MACHINE
   SOFTWARE
      Microsoft
         Windows NT
            CurrentVersion
               Windows
                  USERPostMessageLimit

If the function fails, call GetLastError to get extended error information. GetLastError returns ERROR_NOT_ENOUGH_QUOTA when the limit is hit.

The minimum acceptable value is 4000.

Examples

The following example shows how to post a private window message using the PostMessage function. Assume you defined a private window message called WM_COMPLETE:

#define        WM_COMPLETE     (WM_USER + 0)

You can post a message to the message queue associated with the thread that created the specified window as shown below:

 WaitForSingleObject (pparams->hEvent, INFINITE) ;
 lTime = GetCurrentTime () ;
 PostMessage (pparams->hwnd, WM_COMPLETE, 0, lTime);

For more examples, see Initiating a Data Link.

Note

The winuser.h header defines PostMessage 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.

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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