linux-kernel - Re: SMP read() stopping at memory page boundaries

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Date:	Wed, 20 Jun 2007 19:57:02 +0300
From:	Timo Sirainen <tss@....fi>
To:	Ray Lee <ray-lk@...rabbit.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: SMP read() stopping at memory page boundaries

On Wed, 2007-06-20 at 09:22 -0700, Ray Lee wrote:
> On 6/20/07, Timo Sirainen <tss@....fi> wrote:
> > On Wed, 2007-06-20 at 17:52 +0300, Timo Sirainen wrote:
> > > Sometimes read() returns only 4096 bytes. I'm locking the file, so I
> > > don't think this should ever happen, right?
> 
> man 2 read
> 
> read() is always allowed to return less than you asked for. You need
> to go back and ask for the rest. That's why people wrap read() and
> write() in loops, and use those wrapped versions instead.

I thought this wouldn't happen when reading from local filesystems.
Anyway, my real program was doing that, and it was still seeing partial
data. But looks like I can't reproduce it in my test program with two
pread()s, so I'll have to do some more debugging.


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