Subject: Re: read-sequence from socket
From: (Rob Warnock)
Date: Tue, 04 Nov 2008 20:45:04 -0600
Newsgroups: comp.lang.lisp
Message-ID: <>
David Reitter  <> wrote:
| I'm trying to read into a buffer from a socket using `read-sequence'.
| I'd like `read-sequence' to stop when there is no more data, and
| the :input-timeout argument to `make-socket' in the current CCL allows
| me to specify a timeout, upon which an error is signalled.  However,
| I'm unclear how I would then find out how many bytes were read -
| that's the normal return value of `read-sequence'.

You're going to find that doing this with READ-SEQUENCE is likely
to interact badly with the underlying operating system, since if
the timeout exception occurs in the middle of a read the "amount
already read" is not well-defined for many [most?] OSs.

What I would do instead in this case is write your own version
of READ-SEQUENCE which uses whatever underlying "select()/poll()"
calls [or equiv.] intermingled with native "read()"s (which will
be non-blocking if you only issue one when the "select()/poll()"
says there's something there [though if you're really paranoid
you might want to do an FIONREAD "ioctl()" or equiv. to be sure]),
so that the timeouts *only* ever occur during the "select()/poll()"
calls. In that case, you'll have a deterministic count of "amount
already read".


p.s. Even with the above, setting a static timeout value which
can properly handle unexpected delays [e.g. temporary routing
loops or route flapping] which can persist for several minutes
is considered by networking experts to be... "interesting", at
the very least. You have been warned.  ;-}

Rob Warnock			<>
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