Re: Packing Record Structures in Ada

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

Randall says

<<When compiler A sends a message out across a bus, it will send
  |1.........20|21......32|   1st quad-word
   first field   second
  |1...10|11............32|   2nd quad-word
    2nd      third field

However when the message is received, using compiler B it gets
  |1.....12|13..........32|   1st quad-word
    2nd        1st field
  |1...........22|23....32|   2nd quad-word
    3rd field      2nd
(this is due to the way data is sent across the bus, 32 bits at a
time)
>>


If this is an endianness difference then the differences are just what
one would expect. If not, then one of the compilers is behaving in a very
peculiar manner. You might want to say what machines and compilers are
involved here to understand things better. I assume you have rep clauses
for the two records? If not, then of course all bets are off, and the
problem is yours (to expect any default layout to be applicable between
two separate compilers is plain wrong).

Note that if you use the distribution support in GNAT, all such issues
are handled automatically, and indeed you can deal with the situation
in a completely general manner, compiler independently, by using the
stream facility in Ada 95.

THe stream facility in Ada 95 is one of its very powerful features. I have
the impression that a lot of Ada 95 programmers are not sufficiently aware
of this very important capability, and its wide applicability to solving
problems of this nature, either through explicit use, or implicit use via
the distribution annex.