From: Bill Kelly <billk@...>
Date: 2009-07-09T01:46:33+09:00
Subject: Re: How to accept a varying amount of UDP packets using recvfrom


From: "Randy General" <randygeneral@gmail.com>
>
> I'm currently creating an application that can query a game-server for
> its status, total players, hostname, etc. I send out a challenge packet,
> and in return I receive between 1 and 3 UDP packets.
> 
> Is there an effective way to deal with receiving a varying amount of UDP
> packets? If I do a recvfrom for two packets, the process will time out
> if only a single packet arrives.

Generally, use select to specifiy the desired timeout.

However, it's a little more complicated with ruby and UDP, such
that one may end up needing to wrap the recvfrom in a
timeout { } block anyway.  (On some operating systems, if the
UDP packet has a checksum error, select can return ready,
but recvfrom will block.  (And in many (all?) versions of ruby,
setting fcntl O_NONBLOCK does not help due to ruby's
internals.))

This rubyforge project has code for receiving one-or-more UDP
packets from a Quake2 server:

http://rubyforge.org/scm/?group_id=288

Unfortunately, it doesn't seem possible to browse the CVS 
repository via the web anymore, but you can still check out the
source via anonymous CVS as described on that page.

E.g.

cvs -d:pserver:anonymous@rubyforge.org:/var/cvs/dorkbuster login
cvs -z3 -d:pserver:anonymous@rubyforge.org:/var/cvs/dorkbuster co dorkbuster

The relevant source file is: dorkbuster/dbcore/q2rcon.rb

Here's the innermost receive method: http://pastebin.com/f716b381d


Hope this helps,

Bill