PostThreadMessage

The PostThreadMessage function places (posts) a message in the message queue of the specified thread and then returns without waiting for the thread to process the message.

BOOL PostThreadMessage(
  DWORD idThread, // thread identifier
  UINT Msg,       // message to post
  WPARAM wParam,  // first message parameter
  LPARAM lParam   // second message parameter
);
 

Parameters

idThread

Thread identifier of the thread to which the message will 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 Win32 USER or GDI functions. For more information, see the Remarks section.

Msg

Specifies the type of message to be posted.

wParam

Specifies additional message-specific information.

lParam

Specifies additional message-specific information.

Return Values

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.

Remarks

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

• Call PostThreadMessage. If it fails, call the Sleep function and call PostThreadMessage again. Repeat until PostThreadMessage succeeds.
• 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(&msg, NULL, WM_USER, WM_USER, PM_NOREMOVE) to force the system to create the message queue. 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.

QuickInfo

  Windows NT: Requires version 3.1 or later.
  Windows: Requires Windows 95 or later.
  Windows CE: Requires version 1.0 or later.
  Header: Declared in winuser.h.
  Import Library: Use user32.lib.
  Unicode: Implemented as Unicode and ANSI versions on Windows NT.

See Also

Messages and Message Queues Overview, Message and Message Queue Functions, GetCurrentThreadId, GetMessage, GetWindowThreadProcessId, MSG, PeekMessage, PostMessage, Sleep, WaitForSingleObject