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(tar.info)Checksumming

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Checksumming Problems
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   SunOS and HP-UX `tar' fail to accept archives created using GNU
`tar' and containing non-ASCII file names, that is, file names having
characters with the eight bit set, because they use signed checksums,
while GNU `tar' uses unsigned checksums while creating archives, as per
POSIX standards.  On reading, GNU `tar' computes both checksums and
accept any.  It is somewhat worrying that a lot of people may go around
doing backup of their files using faulty (or at least non-standard)
software, not learning about it until it's time to restore their
missing files with an incompatible file extractor, or vice versa.

   GNU `tar' compute checksums both ways, and accept any on read, so
GNU tar can read Sun tapes even with their wrong checksums.  GNU `tar'
produces the standard checksum, however, raising incompatibilities with
Sun.  That is to say, GNU `tar' has not been modified to _produce_
incorrect archives to be read by buggy `tar''s.  I've been told that
more recent Sun `tar' now read standard archives, so maybe Sun did a
similar patch, after all?

   The story seems to be that when Sun first imported `tar' sources on
their system, they recompiled it without realizing that the checksums
were computed differently, because of a change in the default signing
of `char''s in their compiler.  So they started computing checksums
wrongly.  When they later realized their mistake, they merely decided
to stay compatible with it, and with themselves afterwards.
Presumably, but I do not really know, HP-UX has chosen that their `tar'
archives to be compatible with Sun's.  The current standards do not
favor Sun `tar' format.  In any case, it now falls on the shoulders of
SunOS and HP-UX users to get a `tar' able to read the good archives
they receive.