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Date: Tue, 6 Apr 2010 14:30:02 +0100
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] [PATCH 1/5] char: Let the caller know how many	bytes
	were written in case of incomplete writes
Message-ID: <20100406133002.GA24003@shareable.org>
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To: Gerd Hoffmann <kraxel@redhat.com>
Cc: Amit Shah <amit.shah@redhat.com>, qemu-devel@nongnu.org, Paul Brook <paul@codesourcery.com>, Juan Quintela <quintela@redhat.com>

Gerd Hoffmann wrote:
> On 04/06/10 13:05, Amit Shah wrote:
> >On (Tue) Apr 06 2010 [12:21:52], Gerd Hoffmann wrote:
> >>On 04/06/10 11:58, Amit Shah wrote:
> >>>>>It would certainly be beneficial for consumers of virtio-serial to be
> >>>>>notified of -EAGAIN so that the guest can be throttled till the chardev
> >>>>>catches up with the data being sent.
> >>>>
> >>>>EAGAIN should only ever occur if no bytes are written.
> >>>
> >>>Right. That, or just return 0 and let the caller handle the situation?
> >>
> >>Go with the usual unix semantics instead of creating something new.
> >>
> >>When something was written -- return the number of bytes.  Caller has to
> >>compare with the length passed in to figure whenever it was a partial
> >>write or not.
> >>
> >>When nothing was written -- return the error.
> >
> >Unless Paul wants to never return recoverable error messages as he
> >mentioned in his first mail.
> 
> Return value "0" usually means end-of-file, I would not use that for 
> something else too.  We have to agree on something though ...

Which is why EAGAIN was invented.

-- Jamie