PostThreadMessageA function (winuser.h)

Posts a message to the message queue of the specified thread. It returns without waiting for the thread to process the message.


BOOL PostThreadMessageA(
  [in] DWORD  idThread,
  [in] UINT   Msg,
  [in] WPARAM wParam,
  [in] LPARAM lParam


[in] idThread


The identifier of the thread to which the message is to be posted.

The function fails if the specified thread does not have a message queue. The system creates a thread's message queue when the thread makes its first call to one of the User or GDI functions. For more information, see the Remarks section.

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

This thread must have the SE_TCB_NAME privilege to post a message to a thread that belongs to a process with the same locally unique identifier (LUID) but is in a different desktop. Otherwise, the function fails and returns ERROR_INVALID_THREAD_ID.

This thread must either belong to the same desktop as the calling thread or to a process with the same LUID. Otherwise, the function fails and returns ERROR_INVALID_THREAD_ID.

[in] Msg

Type: UINT

The type of message to be posted.

[in] wParam


Additional message-specific information.

[in] 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. GetLastError returns ERROR_INVALID_THREAD_ID if idThread is not a valid thread identifier, or if the thread specified by idThread does not have a message queue. GetLastError returns ERROR_NOT_ENOUGH_QUOTA when the message limit is hit.


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

The thread to which the message is posted must have created a message queue, or else the call to PostThreadMessage fails. Use the following method to handle this situation.

  • Create an event object, then create the thread.
  • Use the WaitForSingleObject function to wait for the event to be set to the signaled state before calling PostThreadMessage.
  • In the thread to which the message will be posted, call PeekMessage as shown here to force the system to create the message queue.

    PeekMessage(&msg, NULL, WM_USER, WM_USER, PM_NOREMOVE)

  • Set the event, to indicate that the thread is ready to receive posted messages.
The thread to which the message is posted retrieves the message by calling the GetMessage or PeekMessage function. The hwnd member of the returned MSG structure is NULL.

Messages posted by PostThreadMessage are not associated with a window. As a general rule, messages that are not associated with a window cannot be dispatched by the DispatchMessage function. Therefore, if the recipient thread is in a modal loop (as used by MessageBox or DialogBox), the messages will be lost. To intercept thread messages while in a modal loop, use a thread-specific hook.

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.

There is a limit of 10,000 posted messages per message queue. 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.

         Windows NT

The minimum acceptable value is 4000.


The winuser.h header defines PostThreadMessage 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

Other Resources