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Re: [Bug-tar] format v7 unexpected header fields

From: Joerg Schilling
Subject: Re: [Bug-tar] format v7 unexpected header fields
Date: Thu, 12 Nov 2015 12:16:36 +0100
User-agent: Heirloom mailx 12.5 7/5/10

Mario Aichinger <address@hidden> wrote:

> Last week I did a research about the v7 format. I did this, because I
> wanted to push my programming skills and thought, why not to program a tar
> extractor. So I started reading about the most basic tar format which is
> obviously v7. The first thing I programed was the part which tries to find
> out which version of archive is passed to my program (v7, star, ustar, gnu,
> posix, pax, ...). My understanding was and is that v7 tar's have a 256 byte
> padding (null bytes) at the end of their headers, and an empty  magic
> header field. So these are the things I test against. After I implemented
> this in my program I created some test archives. Of course with tar and the
> option --format=v7. But my program failed to detect them as v7 tar's. So I
> opened them (the archives) in a text editor (kate) and found two fields in
> the header filled with zeros. This fields where located after the linkname
> header field.

gtar is not very clean with format switching. Last time I checked, it did even 
create gnu long name headers when other formats have been selected.

I recommend you to rather use "star H=v7tar"


 EMail:address@hidden                    (home) Jörg Schilling D-13353 Berlin
       address@hidden (work) Blog: http://schily.blogspot.com/
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