Date: Tue, 22 Feb 2011 12:51:33 -0500
From: John Baldwin <jhb@freebsd.org>
To: freebsd-hackers@freebsd.org
Cc: Alexander Best <arundel@freebsd.org>, Garrett Cooper <gcooper@freebsd.org>
Subject: Re: seeking into /dev/{null,zero}
Message-ID: <201102221251.33717.jhb@freebsd.org>
In-Reply-To: <AANLkTimwOVub6XxZqgCmHPnLbekJrEbYQBzxUr%2BUrcR5@mail.gmail.com>
References: <20110222141112.GA98964@freebsd.org> <AC6674AB7BC78549BB231821ABF7A9AE970EA06474@EMBX01-WF.jnpr.net> <AANLkTimwOVub6XxZqgCmHPnLbekJrEbYQBzxUr%2BUrcR5@mail.gmail.com>
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On Tuesday, February 22, 2011 11:46:05 am Garrett Cooper wrote:
> (Please bottom post)
>=20
> On Tue, Feb 22, 2011 at 8:31 AM, Andrew Duane <aduane@juniper.net> wrote:
> > I thought seeking past EOF was valid; writing something creates a file=
=20
with a hole in it. I always assumed that was standard semantics.
>=20
> That's with SET_HOLE/SET_DATA though, correct? If so, outside of
> that functionality I would assume relatively standard POSIX semantics.
Err, no, you can always seek past EOF and then call write(2) to extend a fi=
le=20
(it does an implicit ftruncate(2)). SEEK_HOLE and SEEK_DATA are different,=
=20
they are just used to discover sparse regions within a file.
=46rom the manpage:
The lseek() system call allows the file offset to be set beyond the end
of the existing end-of-file of the file. If data is later written at
this point, subsequent reads of the data in the gap return bytes of ze=
ros
(until data is actually written into the gap).
=2D-=20
John Baldwin
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