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Date:   Wed, 20 Jan 2021 23:02:54 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     David Laight <David.Laight@aculab.com>
Cc:     'Linus Torvalds' <torvalds@linux-foundation.org>,
        Christoph Hellwig <hch@lst.de>,
        Johannes Berg <johannes@sipsolutions.net>,
        Oliver Giles <ohw.giles@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: Splicing to/from a tty
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On Wed, Jan 20, 2021 at 10:25:56PM +0000, David Laight wrote:

> I also wonder if pread/pwrite with offset == 0 should be valid
> on things where the offset makes no sense.
> 
> I'm rather surprised the offset isn't just silently ignored
> for devices where seeking is non-sensical.
> You might want to error it for mag tapes, but not pipes,
> ttys, sockets etc.
> 
> I really can't remember what SYSV, Solaris or NetBSD do.

... nor can you be arsed to RTFPOSIX.   Why am I not surprised?

In https://pubs.opengroup.org/onlinepubs/9699919799/functions/write.html
(located by arcane action known as googling for pwrite POSIX):
==============================
The pwrite() function shall fail if:

[EINVAL]
	The file is a regular file or block special file, and the offset argument is negative.
The file offset shall remain unchanged.
[ESPIPE]
	The file is incapable of seeking.
==============================