On Wed, Feb 23, 2011 at 3:53 PM, Alexander Best <arun...@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 <adu...@juniper.net> wrote:
>> >>> I thought seeking past EOF was valid; writing something creates a file
>> > with a hole in it. I always assumed that was standard semantics.
>> >>
>> >> 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
>> > file
>> > (it does an implicit ftruncate(2)). SEEK_HOLE and SEEK_DATA are different,
>> > they are just used to discover sparse regions within a file.
>
> on the other hand POSIX says:
>
> "The behavior of lseek() on devices which are incapable of seeking is
> implementation-defined.
> The value of the file offset associated with such a device is undefined."
>
> ...if we define /dev/{zero,null} as incapable of seeking the current
> implementation should be ok.
>
>> >
>> > From 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
>> > zeros
>> > (until data is actually written into the gap).
>>
>> You're correct. Linux (Fedora 13) isn't POSIX compliant (this 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 point,
>> subsequent reads of data in the gap shall return bytes with the value 0
>> until data is actually written into the gap.
Yes, but look at how it's defined by POSIX before you do that
(yes, there's a section for null/zero IIRC).
Thanks,
-Garrett
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