Re: Ada95 Streams Question

[dewar@cs.nyu.edu (Robert Dewar)]

Michael says:

The first problem arises when you try to exchange stream data files between
computers of different architectures. This is just not foreseen by the Ada 95
standard (see implementation advice in RM and current practice of GNAT).

  The *default* implementation of streams in GNAT indeed follows the
  IA in the RM. However GNAT is set up to make it very easy to choose
  some other implementation, and we have heterogenous distribution very
  much in mind.

  All primitive data is converted to and from stream format using a simple
  procedural interface (System.Stream_Attributes in files s-stratt.ad?) All
  you need to do to get an architecture independent view of streams is to
  replace this one unit (which has a very simple structure) with a different
  one using, e.g. XDR, for data representation.

  We intend to provide an XDR replacement for s-stratt.ad? in the near
  future with some kind of configuratoin pragma to select which one you
  want.

For the described setup it would be even more elegant to utilize an
implementation of the distributed system annex but as this builds on the
stream facility as well you face the same problem as mentioned above.
If you just look at the wording of this annex the big picture is a huge
monolithic program that is now broken into pieces (partitions)

  That's not what I see reading the wording in the annex! There is no
  huge monolithic program. What is called here a partition is what would
  normally be called a program, and a distributed Ada "program" is in fact
  a collection of partitions. Note in particular that with GNAT's source
  based approach, you do not even need to compile these programs on the
  same machine or in the same "library", consistency can be maintained
  purely at the source level.

  You are reading the annex far too narrowly!

and executed as a whole on a homogenous network of computers. That is
definitely not what I understand by distributed computing.

  Well distribution may be homogenous or heterogenous. The Ada model is
  certainly aimed at homogenous distribution, but extends fine to the
  hetergenous case (we first demonstrated hetergenous distribution using
  this annex nearly two years ago at Tri-Ada, running on Sun's and SGI's)

Although this could certainly be interpreted in a more open way, this is
exactly what is currently implemented for GNAT.

  We do interprete it very openly, look more closely. Have a look at
  s-stratt. Even if you can't replace this yourself (which is really
  easy to do), other people including us will provide replacements
  soon.