PostMessage消息发出后不等消息函数处理完就执行它的下一语句   
 SendMessage消息发出后阻塞等消息函数处理完执行它的下一语句

The PostMessage function 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 associate with a thread, use the PostThreadMessage function.

BOOL PostMessage(
  HWND hWnd,      // handle to destination window
  UINT Msg,       // message
  WPARAM wParam,  // first message parameter
  LPARAM lParam   // second message parameter
);

Parameters

hWnd

[in] Handle to the window whose window procedure is to receive the message. The following values have special meanings.

Value	Meaning
HWND_BROADCAST	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.

Msg

[in] Specifies the message to be posted.

wParam

[in] Specifies additional message-specific information.

lParam

[in] Specifies additional message-specific information.