Re: AfxBeginThread question
On Mon, 07 Jan 2008 22:48:44 -0800, Zapanaz
<http://joecosby.com/code/mail.pl> wrote:
I have an application where I use AfxBeginThread() to create a worker
thread to do some heavy work. Within the thread function, I use
PostMessage() to pass messages back to the application.
I am wondering what happens if the user closes the application while
the thread is still working. I assumed I would want to kill the
thread during program shutdown/cleanup, but off hand I don't see a way
to do that.
But I'm not sure if I need to kill the thread. I ran the application
and intentionally started the thread doing a very long job, then
closed the application long before it finished. Viewing "processes"
in the task manager, the thread appeared to stop working (it uses a
lot of CPU when it is working) and there don't appear to be any stray
processes remaining.
See:
http://members.cox.net/doug_web/threads.htm
Also when I viewed processes while the thread was running (and the
application was still open), the CPU went up on the application
project, MyApplication.exe, rather than some separate process. The
application other than the thread wasn't doing anything, so apparently
the thread is "inside" the application process from Windows' point of
view.
Where else would it be? A thread lives within the process that created it
(CreateRemoteThread notwithstanding). If you look up "processes and
threads" in MSDN, you should find a decent explanation of what a running
"application" is; in brief, it's a process, and a process contains one or
more threads.
Anyway ... so, is it safe to count on this? That when the application
shuts down, the thread will be shut down also? That appears to be how
it works.
In general, it's unsafe; see the referenced web page for more.
For that matter, it isn't really a problem if the thread runs to
completion after the application shuts down, the only concern I have
is that it will keep calling PostMessage() to post messages back to
the calling program. Is that a problem? If I post a message (a
custom message which only the calling application is going to receive)
and there is nobody listening for it *, is Windows going to queue up a
bunch of messages that never go anywhere, and so eat up memory or
something?
If the target thread doesn't exist, the message will be discarded, and
PostMessage will return an error. It would not be great design to post
messages that have nowhere to go. There seems to be some confusion about
what an "application" is. Do the MSDN search I mentioned earlier and learn
about processes and threads, then read the web page I linked to understand
the process shutdown issues WRT running secondary threads.
--
Doug Harrison
Visual C++ MVP