Re: [Bug-tar] Add option to include the contents of FIFO:s

[Tim Kientzle]

The tar format has to know the size of the entry
before it can write the header that goes before
the data.

With your proposal, there's no way for tar to
know the size of the data without actually
reading all of the data.  To work around this,
tar would have to copy the data to a temporary
file itself.  So you wouldn't really gain
anything over using a temporary file.

In some cases, you might have a programmatic
way to determine the size in advance.  If this
is true for your application, you might consider
writing a custom archiving program based on
the libarchive library, which was designed to
make it very easy to create tar archives with
dynamically-generated content.  (If you cannot
determine the size in advance, libarchive now
has support for writing zip format, which does
not require knowing the size in advance.)

http://libarchive.googlecode.com/ has full
details, including example code.

Cheers,

Tim Kientzle

Alexander T wrote:
> Hi,
>
> This is my first message to this list, so hi everyone and I hope that
> this is the right place to post. I would like to submit a feature
> request: to add a flag for including the contents of FIFO:s, like
> --dereference, but for named pipes.
>
> The problem this would solve is tar:ing multiple large program outputs
> without using intermediate files, something like
>
> mkdir bck
> mkfifo bck/1.dump
> mkfifo bck/2.dump
> dbdump db1 > bck/1.dump &
> dbdump db2 > bck/2.dump &
> tar -c?f bck.tar bck
>
> It is currently of course possible to come up with some workarounds,
> but they have drawbacks and this would IMHO be a very nice addition.
>
> There are some google results with people having the same problems:
> http://www.unix.com/unix-advanced-expert-users/30365-tar-fifos.html
> ,
> http://stackoverflow.com/questions/2021873/linux-adding-named-files-to-a-zip-archive-from-a-pipe/2022012#2022012
> and
> http://www.linuxquestions.org/questions/programming-9/make-tarziprar-play-nice-with-named-pipes-593392/
>
>
> Best Regards,
>
> Alexander T