Re: AF_UNIX sockets: strange behaviour

[linux-os]

On Wed, 17 Nov 2004, Catalin Drula wrote:

> > Dick Johnson wrote:
> >
> > > Catalin Drula wrote:
> > >
> > > - there is a skb in the sk_receive_queue with a len of 13
> > > - 6 bytes are read from it
> > > - a skb with the remaining 7 bytes is requeued in sk_receive_queue
> > > - on the next call to unix_stream_recvmsg, the sk_receive_queue is
> > >  empty (!)
> > >
> > > Thus, this confirms the behaviour observed from userspace. Is this a
> > > bug? Who could be removing the skb from the receive_queue?
> >
> > If you need STREAM behavior I think you need to use recv(),
> > recvfrom(),or read().
> >
> > If you use recvmsg(), the "message" will be removed even it you
> > haven't read it all. Note in the 'man' page description:
> > "If the a message is too long to fit in the supplied buffer, excess
> > bytes may be discarded depending upon the type of socket the message
> > is being received from...
>
> At first I used read() and then I tried recv() as well.
>
> Catalin

My reading of the specs is that read() damn-well never throw
away anything that hasn't been read yet. If it does, then
it's a bug. That said, the bug may never be fixed because
(1) people may deny it's a bug, or (2) programs may rely
on it.

FYI, it the first condition, I've seen these kinds of bugs
mysteriously fixed later on even though many people denied they
were bugs.

I suggest you supply a big enough buffer to read the hole thing
and then tear it apart at the application level.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.9 on an i686 machine (5537.79 BogoMips).
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