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

From: Paul Eggert
Subject: Re: [Bug-tar] tar -tf quits before EOF?
Date: Sat, 07 Aug 2010 23:12:38 -0700
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv: Gecko/20100713 Thunderbird/3.0.6

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

> 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 -
  $ /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 -

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

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