[fitsbits] FITS Binary Table Proposals

[Arnold Rots]

By golly, now it starts getting interesting!
If a negative THEAP points to the start of the data in a previous HDU,
you can include an entire table or image in another table's cell.

You could try to prevent this kind of behavior by requiring THEAP to
be positive, but that would not prevent anyone from making THEAP
point beyond the current extension, causing essentially the same
problem.  Non-discrimination requires that rather stricter and
preciser limits be put on THEAP in both directions (as Tom indicated),
if any limits are applied at all.

  - Arnold

William Pence wrote:
> ...
> > 2. Can THEAP be negative?   The possibility of a negative 'gap'
> > is not explicitly forbidden, but probably should be.   Or
> > do we want to allow reuse of data from the table as variable
> > length data.  One could make interesting structures...  Even if
> > we permit negative THEAP's (which I suspect will not be the case)
> > presumably it needs to be restricted to staying within the
> > current Data unit.
> 
> If THEAP is negative, it would be pointing a location before the start of 
> the main data table, i.e., to somewhere in the Binary table header, or even 
> to a previous extension in the FITS file.  It would be sensible to require 
> that THEAP be positive, but that will require a change to a different 
> section of the FITS Standard than what is being considered in this proposal.
> ...
> 
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Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head.cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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