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Date:	Sun, 13 Jun 1999 10:43:28 +0200
From:	Corinna Vinschen <corinna AT vinschen DOT de>
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To:	cygwin AT sourceware DOT cygnus DOT com
Subject:	Re: cygwin tar to tape?
References:	<s7318bc6 DOT 078 AT experian DOT com> <3731B674 DOT 63588B38 AT cityweb DOT de> <375E670E DOT 84E188E8 AT ericsson DOT com> <37617756 DOT 14E1EAF2 AT vinschen DOT de> <37630EDB DOT F2A51CCC AT bigfoot DOT com>

Jim & Jenn Dumser wrote:
> 
> On Fri, 11 Jun 1999 22:53:42 +0200, Corinna Vinschen
> <corinna AT vinschen DOT de> wrote:
> > You must(!) know the blocking (U*Xes typically uses 5K blocking) and
> > you have to set it with my `mt' command from ftp site
> 
> But the "normal" tar has a blocking option. Okay, WinNT won't
> automagically determine block size; even the Unixes I've worked with
> will only figure it out if you tell it to use a block size bigger than
> the actual size. So why won't the standard POSIX (or whatever) calls in
> GNU tar correctly set the block size in cygwin? Why do we need a

You mix up two different things. The blocking factor of the windows
tape driver is not the responsibility of the POSIX layer. The standard
gnu tar has the -b option as you mentioned but this only sets the
size of the blocks that are given to the `write' call (take a look into
the sources). It wouldn't be correct, if a user command may set the
blocking factor of a device.

> specially hacked version of tar/cpio/dd/mt/etc.? I would think the

It only tries an additional `ioctl' for convenience of windows users.
It's not an official patch that is needed for working with cygwin.

As I said, you may, but you don't have to use it. The tape driver may
be set to any blocking factor, the standard gnu tar will work.

> correct way to address this problem would be to hide the details of the
> "Windows way" behind the standard "Unix way." Isn't that the idea of a
> Unix-emulation layer?

So far as it is possible, but POSIX doesn't tell, that tar changes
the drivers settings on demand.

> BTW - While it is not a big deal in this case and I appreciate your
> help, my original message was sent directly to you, Corinna, and not to
> the list. Since you did not ask for my permission, to copy my personal
> message to this list is not in the best of form.

If you think your problems are mysteries, you should tell it.
On the other hand, it's a general problem that is of interest for the
mailing list. It's the `bazaar', do you remember?

Regards,
Corinna


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