PostThreadMessage

This function places a message in the message queue of the specified thread and then returns without waiting for the thread to process the message.

At a Glance

Header file:	Winuser.h
Windows CE versions:	1.0 and later

Syntax

BOOL PostThreadMessage(DWORD idThread, UINT Msg, WPARAM wParam, LPARAM lParam);

Parameters

idThread

[in] 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

[in] Specifies the type of message to be posted.

wParam

[in] Specifies additional message-specific information.

lParam

[in] Specifies additional message-specific information.

Return Values

Nonzero indicates success. Zero indicates failure. 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:

υCall PostThreadMessage. If it fails, call the Sleep function and call PostThreadMessage again. Repeat until PostThreadMessage succeeds.
υCreate 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.