Re: [Bug-ddrescue] Appending zeroes by seek is not working

[Mark]

Hi,

On Sat, December 13, 2014 05:28, Pavel Roskin wrote:
> I tried adding zeroes to the end of a file using ddrescue, but it failed.
>
> With stock ddrescue 1.17 from Fedora 20:
>
> /usr/bin/ddrescue -o 2048 -s 0 /dev/null foo
> ddrescue: Nothing to do.
>
> With locally compiled ddrescue 1.19:
>
> /usr/local/bin/ddrescue -o 2048 -s 0 /dev/null foo
> ddrescue: Empty domain.
>
> In either case, the file foo is not extended.
>
> "Empty domain" sounds more cryptic than "Nothing to do". I think it's a
> regression.
>
> Yes, I know that I can copy 1 byte from /dev/zero and subtract 1 from the
> output position, but I don't want any workarounds. I know that I can use
> dd. But I want to use ddrescue, as I know its options and it's doing the
> right thing most of the time.
>
> I think copying 0 bytes is just fine if the output position is specified.
>
> Here's a related but different issue.
>
> rm foo
> ddrescue -o 2048 -s 1 /dev/null foo
> ls -l foo
>
> The resulting file "foo" is 0 bytes long. Apparently, the seek is never
> performed because /dev/null is empty. I believe the seek should be
> performed if it's explicitly requested on the command line.

For the last point, you can instead do
ddrescue -o 2048 -s 1 /dev/zero foo
which I think works how you expect. You can't read anything from dev/null.


More generally, you can use a command like:
truncate --size=12345 myfile
to extend myfile with (a hole which reads back as) zeros.

Or you can use dd as you mentioned:
dd if=/dev/zero of=myfile bs=1 seek=12345 count=0

However, while the file-extending behaviour of both methods works on
Linux, it isn't guaranteed for other OSes; see the ftruncate() and write()
specifications at
http://pubs.opengroup.org/onlinepubs/7908799/xsh/ftruncate.html
http://pubs.opengroup.org/onlinepubs/009695399/functions/write.html

To be guaranteed to work on other OSes, you can use dd (or ddrescue) with
count=1 to write a single zero byte at the new end of the file. So the
above example would become
dd if=/dev/zero of=myfile bs=1 seek=12344 count=1


Mark