Re: [Bug-tar] tar -tf quits before EOF?

[Tim Kientzle]

On Aug 7, 2010, at 11:12 PM, Paul Eggert wrote:
> On 08/06/10 22:42, Dustin J. Mitchell wrote:
>> My understanding is that tar should keep running until it gets EOF on
>> its input - that there's no explicit indication within a tarfile that
>> would cause tar to exit normally without reading to EOF.
> 
> That's incorrect.  A tar file ends with two blocks full of zeros; tar
> doesn't need to keep reading after that.  ...

To be pedantic, the end of a tar archive is marked by two
512-byte "records" of all zeros.  The term "block" is generally
used to refer to the I/O size (which traditionally defaults
to 20 records).

Historically, there's been a lot of variation in how readers
handle the end-of-archive:
* Some implementations stop reading when they see the
   first all-zero record.  If this happens at the end of a block,
   they won't read the next block.
* Some readers will try to drain pipes to avoid sending
   SIGPIPE to the writer.
* Some readers aggressively read ahead to try to maintain
   high throughput.

I believe GNU tar has changed its behavior a couple
of times.  star uses a FIFO and aggressively reads ahead
but I don't think it deliberately tries to drain pipes.
bsdtar does deliberately drain pipes.  It sounds like
Solaris tar also deliberately drains pipes.

> example, in the POSIX standard for tar format
> <http://www.opengroup.org/onlinepubs/9699919799/utilities/pax.html>.

Thanks for pointing this out; most people don't know that
there is a POSIX standard for the tar format, largely because
it's hidden away under "pax".

> You must be using Solaris /bin/tar for that.  I believe it reads past
> the two zero blocks.  GNU tar doesn't do that. ...
> arguably GNU tar's behavior is more useful.

I'm curious about why you think so.  I've always thought
that draining the pipe was the more useful behavior.

Tim