linux-kernel - Re: [PATCH v6 0/5] Add preadv & pwritev system calls.

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Date:	Mon, 19 Jan 2009 15:20:11 +0100
From:	Gerd Hoffmann <kraxel@...hat.com>
To:	Ulrich Drepper <drepper@...hat.com>
CC:	Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
	aarcange@...hat.com
Subject: Re: [PATCH v6 0/5] Add preadv & pwritev system calls.

Ulrich Drepper wrote:
> If they are in the kernel there is no reason not to export them from
> glibc.

Great.

> But I have a general comment about all kinds of read syscalls.
> If think they have been misdesigned from day one and if we are going to
> add new ones we might want to fix them.
> 
> The problem is that they don't allow for zero-copy operations in enough
> cases.  The kernel is not free to store the data wherever it wants even
> if the userlevel code is fine with that.

[ ... more text snipped ... ]

I do see the point in adding a interface like this ...

>    ssize_t readz (int fd, void *buf, size_t len, void **res)

... to help the kernel do zero-copy I/O.

I think system calls for vector I/O are *not* the right place for that
though.  Usually applications use vectored I/O because they *do* care
about the place the data is stored, because vectored I/O allows them to
avoid copying data within the application.

cheers,
  Gerd


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