Re: Deaf CAsyncSocket on Windows Service.

"Scott McPhillips [MVP]" <org-dot-mvps-at-scottmcp>
Mon, 19 Jan 2009 22:20:51 -0500
"Bill Brehm >" <<don't want spam> wrote in message


I think there is no way to find out before the Receive() call how much
data is waiting to be received, right? So I have to select an arbitrary
buffer size to read the data into. What happens if the amount of data I
read out is not all the data that's in their waiting to be read? Will I
get a second OnReceive() call for the data that came in already that I
failed to read out?

As you point out, there is no way you can know how much data is waiting to
be received. The notifications are designed with this in mind. Yes, you
will get another OnReceive call if more data is available. That is the
meaning of the section I quoted from the WSAAsyncSelect page. (CAsyncSocket
is a wrapper around WSAAsyncSelect and related functions.)

My problem (in another branch of this thread) is that I seem to lose the
OnReceive() call rather than getting extra. I don't know if I am getting
extra calls (I will check this when i'm back at work soon) but if I am,
they would just break out with the WSAEWOULDBLOCK condition and cause no
harm in my case.

I will test with a single call to Receive() inside of OnReceive() and see
if the problem goes away. But I would like to understand why the error is

You can lose an OnReceive call if you pump messages before returning from
OnReceive. Even a MessageBox call can cause this. As a check, try putting
in some temporary code to guard against reentry into OnReceive.

Scott McPhillips [VC++ MVP]

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