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Date:	Wed, 06 Jun 2007 15:55:26 +0200
From:	chris schlund <>
To:	Rick Jones <>
Subject: Re: Socket hangs in read() when putting interface down

>> This may be ok on a major interface like eth0 but it is a problem when
>> using a ppp interface via GSM data or GPRS connection.
>> Killing the pppd while the socket waits in read() produces the same
>> behaviour. Also a broken connection looks the same.
>> It is reproducable on several 2.6 kernel versions including the lastest
>> stable 2.6.21.
>> On my understanding "ifconfig down" should lead into a read() error.
> Why?  Can't traffic for the endpoint arrive via some other interface on
> the system?  I would think that getting out of a blocking read()
> requires the application to either have a timer running (and so get out
> of read() with EINTR) or to have keepalives enabled on the TCP
> connection (I'm assuming TCP for an AF_INET, SOCK_STREAM although I
> suppose it could be SCTP)
> rick jones

Of course, you are right.
I am so focused to my small embedded world, that I simply have forgotten
how IP-traffic works in big systems (to clarify my first post: I'm using

Your hint with the self produced signal to get out of read() was very
helpful for me. Thats what I was missing.

I also learned another think, while reading the list FAQ:
PEBCAC (=Problem exists between chair and computer)
Obviously, thats what best described me!

Thank you very much!

Chris Schlund

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