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

[Paul Eggert]

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

> A bit of
> experimentation with /dev/zero on my own systems lends credence to
> this understanding, but of course does not prove it:
>   (tar -cf - ChangeLog; cat /dev/zero) |  tar -tf -
>   ChangeLog
> (this hangs forever here, and truss or strace shows tar reading a lot
> of 0's without any _exit(0) action..)

You must be using Solaris /bin/tar for that.  I believe it reads past
the two zero blocks.  GNU tar doesn't do that.  On my Solaris 10 host:

  $ cd /etc
  $ (/usr/sfw/bin/gtar -cf - passwd; cat /dev/zero) | /usr/sfw/bin/gtar -tf -
  passwd
  $ /usr/sfw/bin/gtar --version | head -1
  tar (GNU tar) 1.23

Notice that /bin/tar, even though it reads past the zero blocks, doesn't
do anything useful after that:

  $ cd /etc
  $ (/bin/tar -cf - passwd; /bin/tar -cf - motd) | /bin/tar -tf -
  passwd

so arguably GNU tar's behavior is more useful.  Anyway, Amanda
shouldn't rely on either behavior, obviously, if it wants to be
portable.