[fitsbits] FITS Binary Table Proposals

[Doug Tody]

Back when this was designed the intention was certainly that, for a single
BINTABLE, the heap must be in the same extension as the main table, and heap
offsets must point to data segments which are contained entirely within
the heap region.  If any part of a referenced heap segment is outside the
heap area that is an error.  Other than that anything is possible, e.g.,
arrays can be aliased (multiple table records can point to the same area
of the heap), or there can be gaps in the heap which do not contain valid
data, e.g., as a result of an update.    - Doug



On Fri, 12 Nov 2004, Arnold Rots wrote:

> 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.
> > ...
> > 
> --------------------------------------------------------------------------
> 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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