Date: Wed, 23 Feb 2011 16:15:20 -0800
From: Garrett Cooper <yanegomi@gmail.com>
To: Alexander Best <arundel@freebsd.org>
Cc: freebsd-hackers@freebsd.org
Subject: Re: seeking into /dev/{null,zero}
Message-ID: <AANLkTimX%2B64jws3M-=eE7zPbHvTcQBORryHg=wuBd7=N@mail.gmail.com>
In-Reply-To: <20110223233612.GA46124@freebsd.org>
References: <20110222141112.GA98964@freebsd.org> <AC6674AB7BC78549BB231821ABF7A9AE970EA06474@EMBX01-WF.jnpr.net> <AANLkTimwOVub6XxZqgCmHPnLbekJrEbYQBzxUr%2BUrcR5@mail.gmail.com> <201102221251.33717.jhb@freebsd.org> <3A287E45-A369-4C7A-BA8E-A205679AC0BC@gmail.com> <20110223233612.GA46124@freebsd.org>
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On Wed, Feb 23, 2011 at 3:36 PM, Alexander Best <arundel@freebsd.org> wrote= : > On Wed Feb 23 11, Garrett Cooper wrote: >> On Feb 22, 2011, at 9:51 AM, John Baldwin wrote: >> >> > On Tuesday, February 22, 2011 11:46:05 am Garrett Cooper wrote: >> >> (Please bottom post) >> >> >> >> On Tue, Feb 22, 2011 at 8:31 AM, Andrew Duane <aduane@juniper.net> wr= ote: >> >>> I thought seeking past EOF was valid; writing something creates a fi= le >> > with a hole in it. I always assumed that was standard semantics. >> >> >> >> =A0 =A0That'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 file >> > (it does an implicit ftruncate(2)). =A0SEEK_HOLE and SEEK_DATA are dif= ferent, >> > they are just used to discover sparse regions within a file. >> > >> > From the manpage: >> > >> > =A0 =A0 The lseek() system call allows the file offset to be set beyon= d the end >> > =A0 =A0 of the existing end-of-file of the file. =A0If data is later w= ritten at >> > =A0 =A0 this point, subsequent reads of the data in the gap return byt= es of zeros >> > =A0 =A0 (until data is actually written into the gap). >> >> =A0 =A0 =A0 You're correct. Linux (Fedora 13) isn't POSIX compliant (thi= s is from the official POSIX text): >> >> The lseek ( ) function shall allow the file offset to be set beyond the = end of the existing data in the file. If data is later written at this poin= t, subsequent reads of data in the gap shall return bytes with the value 0 = until data is actually written into the gap. > > so except for reading from /dev/zero freebsd also isn't posix compliant, = right? Huh...? Please better explain what you mean here. Thanks, -Garrett
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