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From: Maciej Sobczak <see.my.homepage@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: API design problem - buffer scatter I/O
Date: Wed, 26 Nov 2008 00:34:18 -0800 (PST)
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Date: 2008-11-26T00:34:18-08:00
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On 24 Lis, 18:16, tmo...@acm.org wrote:

> > The problem is that this is a single, continuous array.

> =A0 Is this an actual problem?

Not im my case, really. Actually, I made up the I/O part because I
didn't want to expose the real problem case. I have still got an
excellent feedback that allowed me to fill the blanks in what I do and
it really helped. :-)

We can still argue (for the sake of technical discussion) that the
single buffer can be a problem even for real I/O, because it *might*
imply the use of dynamic memory, which for some reasons we might want
to avoid. Having a set of static buffers and custom scheme of
associating them with the fragments of the data stream might be an
interesting solution.
Of course, you could still argue that in the case of real I/O it might
be done with just a sequence of separate I/O calls for each involved
buffer, so again scatter operation is not necessary, but then the
counterargument would be for example atomicity of the whole data
transfer. And so on.

>=A0In most systems copying the bytes from
> an IO buffer into different places will be a small percentage of the
> time taken by the actual IO, so the simplicity of a single IO buffer
> is worth the modest cost.

Yes.
But then, the I/O might be an interprocess communication based on
shared memory where "copying bytes" is the only factor and therefore
not at all a small percentage. And so on. ;-)

--
Maciej Sobczak * www.msobczak.com * www.inspirel.com

Database Access Library for Ada: www.inspirel.com/soci-ada