Re: SendMessage() will block if message queue is not empty?
On Fri, 18 Jul 2008 06:24:45 -0700 (PDT), Faisal <faisalm83@gmail.com>
wrote:
For the interthread case, the sending thread will block until the target
thread enters a receptive state, which includes calling GetMessage,
PeekMessage, WaitMessage, etc, and perhaps surprisingly, SendMessage.
Windows will then switch to the target thread and allow it to process the
message. The sending thread remains blocked until the target thread returns
from the window procedure or calls ReplyMessage. This blocking behavior and
thread context switching can lead to deadlock if you aren't careful, but
the danger is often exaggerated since most people don't seem to know that a
thread stuck waiting on an interthread SendMessage call can still process
messages sent to it;
This means that the thread is not blocked and It can process other
messages. right?
While waiting for the interthread SendMessage call to complete, the sending
thread blocks, except that it can process messages sent to it by other
threads.
So sending message to another thread is like PostMessage?
No, PostMessage posts a message to the relevant message queue and returns
immediately. It never blocks. A thread waiting for its interthread
SendMessage call to complete will not process posted messages, because it's
blocked and not running its message loop.
How can we ensure that other message won't get processed until the
sent message is returned.
The SendMessage documentation says:
http://msdn.microsoft.com/en-us/library/ms644950.aspx
<q>
If the specified window was created by the calling thread, the window
procedure is called immediately as a subroutine. If the specified window
was created by a different thread, the system switches to that thread and
calls the appropriate window procedure. Messages sent between threads are
processed only when the receiving thread executes message retrieval code.
The sending thread is blocked until the receiving thread processes the
message. However, the sending thread will process incoming nonqueued
messages while waiting for its message to be processed. To prevent this,
use SendMessageTimeout with SMTO_BLOCK set. For more information on
nonqueued messages, see Nonqueued Messages.
</q>
So using SendMessageTimeout with SMTO_BLOCK prevents the sending thread
from processing messages sent to it while it's waiting on its
SendMessageTimeout call to complete.
--
Doug Harrison
Visual C++ MVP