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.
[mailto:email@example.com] 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 <firstname.lastname@example.org> 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.
Jan-Benedict Glaw email@example.com . +49-172-7608481
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