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Re: Socket problem?

Subject: Re: Socket problem?
From: Jan-Benedict Glaw <>
Date: Sat, 22 May 2004 18:06:35 +0200
In-reply-to: <000001c43fd3$2c25a350$2000a8c0@gillpc>
Original-recipient: rfc822;
References: <000001c43fd3$2c25a350$2000a8c0@gillpc>
User-agent: Mutt/
On Sat, 2004-05-22 09:02:48 +0100, Gill <>
wrote in message <000001c43fd3$2c25a350$2000a8c0@gillpc>:
> Hi -
> Has anyone come across a socket problem where the user app calls select() on
> a set of sockets, which returns, indicating that data is waiting...then
> subsequent recvfrom() call returns -1 indicating that there's nothing
> there??

Read the select() manpage again. Upon successful return, it's guaranteed
that the very next read() on a file descriptor contained in the result
set won't block. Typically, it's the case after data arrived, but
returning -1 *immediately* (with no blocking) is okay, too.

> I'm using linux v2.4.2, IPv4, and the ethernet driver is pcnet32.  We're
> receiving a UDP stream.

Quite aged version, though perfectly correct on select() behaviour.

> I'm trying to check for dropped packets.  /proc/net/snmp indicates a number
> of UDP InErrors (~1 per second).  However, not yet sure whether this is a
> consequence of the problem above, or cause of it.

Neither - nor. You've got a small thinko in your application. Albeit
that: update your kernel version... It most probably contains a number
of known root exploits.


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