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Date: 2 Jul 91 17:09:02 GMT
From: cis.ohio-state.edu!zaphod.mps.ohio-state.edu!usc!petunia!nwebre@ucbvax.Be
rkeley.EDU  (Neil Webre)
Subject: Re: What should this do?
Message-ID: <2870b32e.58b0@polyslo.CalPoly.EDU>
List-Id: <comp.lang.ada>

You might use the following program to get a glimpse of what your Ada system
is doing with mutable records. It's not completely definitive but gives a 
good clue. Sorry for the global variables - this is one off quick and
dirty.

PS to Mike Feldman - Re your question on what Alsys does in this case.
This was run on Alsys Ada on AIX/370, IBM 3090/400. It appears that they
allocate maximum space (20 bytes) and have the string bob about in it.
Good for speed, bad for space utilization


-- Check Alsys techniques for mutable records.
with text_io; use text_io;
with system;  -- Alsys version has a procedure "image" that makes
              -- addresses into strings.
procedure main is 
   subtype index is integer range 0..20;
   type rec(len : index := 0) is record
      data : string(1..len);
   end record;
   S : rec;
procedure print is
begin
   put_line("S's address is " & system.image(S'address) & " hex");
   put_line("For S.len =" & integer'image(S.len) & ',' & 
           integer'image(S'size/8) & " bytes are used."); new_line;
end print;
begin
   S := (0, "");
   print;

   S := (10,"0123456789");
   print;

   S := (20,"01234567890123456789");
   print;

   S := (0,"");
   print;
end main;

Results of execution:

S's address is 0024BAFC hex
For S.len = 0, 7 bytes are used.

S's address is 0024BAFC hex
For S.len = 10, 17 bytes are used.

S's address is 0024BAFC hex
For S.len = 20, 27 bytes are used.

S's address is 0024BAFC hex
For S.len = 0, 7 bytes are used.


Neil Webre
Cal Poly, San Luis Obispo, CA
Department of Computer Science.