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RE: Socket problem?

To: "'Jan-Benedict Glaw'" <>, <>
Subject: RE: Socket problem?
From: "Gill" <>
Date: Mon, 24 May 2004 09:25:43 +0100
Importance: Normal
In-reply-to: <>
Organization: Exterity
Original-recipient: rfc822;
Hi -

To clarify, more specifically select() returns a positive value, and a
subsequent call to FD_ISSET() returns TRUE for one of the sockets the app is
listening on.  Then, app calls recvfrom() which returns (sometimes) -1.
Problem occurs sporadically...I'm seeing it every hundred or so packets at
the moment, but it varies.


-----Original Message-----
[] On Behalf Of Jan-Benedict Glaw
Sent: 22 May 2004 17:07
Subject: Re: Socket problem?

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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