From: Tanaka Akira <akr@...17n.org>
Date: 2005-08-10T12:37:08+09:00
Subject: Re: event driven framework for ruby

In article <42F97309.3040901@path.berkeley.edu>,
  Joel VanderWerf <vjoel@path.berkeley.edu> writes:

>> * read from socket: works well by default. (O_NONBLOCK is not required.)
>>   (UDP packet with wrong checksum on Linux 2.6 is an exception.)
>
> Can you say a bit more about what happens differently on Linux 2.6 when
> a UDP packet has a bad checksum? Is it a ruby behavior or a linux behavior?
>
> (I'm working with wireless UDP on linux 2.6, having migrated from QNX,
> so it sounds important to me...)

Linux 2.6's select notify readability by a UDP packet with a bad checksum.
But read discards it and waits next packet with a correct checksum if
O_NONBLOCK is clear.  If the next packet is not available, the read
hangs.  Since O_NONBLOCK avoids the hang, it is possible to care the
problem in application level.

Linux-Kernel Archive: UDP recvmsg blocks after select(), 2.6 bug?
http://www.ussg.iu.edu/hypermail/linux/kernel/0410.0/1372.html

Debian Bug report logs - #275585 - /usr/sbin/inetd: UDP builtins can be used to hang inetd
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=275585&archive=yes

select(2):
       Under Linux, select may report a socket file descriptor as  "ready  for
       reading",  while  nevertheless a subsequent read blocks. This could for
       example happen when data has arrived but  upon  examination  has  wrong
       checksum  and  is discarded. There may be other circumstances.  Thus it
       may be safer to use O_NONBLOCK on sockets that should not block.

[ruby-talk:148436] Nonblocking Sockets
-- 
Tanaka Akira