[fitsbits] 64-bit integers

[William Pence]

It appears that most software environments (e.g., C/C++, Fortran, Java, IDL, 
  Tcl/tk, ...) now support 64-bit integers.  Can anyone cite a case where 
this is not true?  Note that C compilers can define this type differently, 
so it might be declared as 'long long' (on many 32-bit systems), 'long' (on 
many newer 64-bit computers), or '__int64' (on early versions of Microsoft 
Visual C++).

In the case of CFITSIO, the new 3.0 beta release now requires that the C 
compiler have a 64-bit integer data type.  This version has only been out 
for about a month, but so far no one has reported any problems compiling it.

Here are a couple other secondary issues about 64-bit integer support that 
I'm aware of:

- some languages only support signed 64-bit integers, and do not natively 
support unsigned 64-bit integers (e.g., Fortran and Java, I think).  In 
these systems integer values greater than 2**63 would appear as negative 
values.   Software applications that must support the full range of unsigned 
64-bit integers could probably be written to test for any negative values, 
and then reinterpret the value as the correct unsigned value.

- many libraries written in C (including CFITSIO) rely on the 'cfortran.h' 
macros, written by Burkhard Burow, to provide the machine-independent 
Fortran interface to the library routines.  cfortran.h does not support 
64-bit integers, so, for example, Fortran programmers currently cannot use 
CFITSIO to directly read or write 64-bit integers in FITS file.  It is still 
possible to read/write the 64-bit integer FITS values cast as double 
precision floating point variables in the Fortran program, but this is not a 
generally acceptable alternative.  The eventual solution to this problem 
will be to add support for the 64-bit integer data type to cfortran.h, 
however these macros are VERY complicated, so it will require someone with a 
lot of expertise in C and Fortran to do this.

Bill Pence


Mark Calabretta wrote:
>>Given the past controversies about adding 64-bit integer support to FITS, it 
>>is somewhat remarkable that there has been so little discussion of this now. 
> 
> I support the proposal, though with one reservation: how will cfitsio
> (or your favourite FITS reader) load a BITPIX=64 image onto a 32-bit
> computer?  Are we assuming the availability of "long long int" in C
> (as is gcc), INTEGER*8 (or whatever it is) in FORTRAN, and similarly
> in other languages?  Will coercing these values to double precision be
> considered acceptable if 64-bit integers are not available?

-- 
____________________________________________________________________
Dr. William Pence                          William.D.Pence at nasa.gov
NASA/GSFC Code 662         HEASARC         +1-301-286-4599 (voice)
Greenbelt MD 20771                         +1-301-286-1684 (fax)