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From: robert_s_quinn@yahoo.com (Robert Quinn)
Newsgroups: comp.lang.ada
Subject: pragma pack in external packages
Date: 25 Apr 2002 09:49:29 -0700
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Date: 2002-04-25T16:49:29+00:00
List-Id: <comp.lang.ada>

I'm trying to figure out something about raw storage.  I just
installed gnat and I'm trying to teach myself to represent bits of
data with Ada, but I hit a snag.  Basically I can't seem to "pragma
pack" the data type if I am using the data type in a package OTHER
than the declaration package.  e.g.:

Suppose I have an array of bits:

   package definitions is

      ONE_BIT_TYPE is range 0..1;
      for ONE_BIT_TYPE'SIZE use 1

      5_BIT_ARRAY_TYPE is array (1..5) of ONE_BIT;
      pragma pack (5_BIT_ARRAY_TYPE);

   end definitions;

It seems like my 5 bit array should now refer to 5 consecutive bits.
But now if I try to use this array in a record in an external package,
I get an error:

   package more_definitions is

      type my_record is record
         IMPORTANT_5_BIT_ARRAY : definitions.5_BIT_ARRAY_TYPE;
      end record
      
      pragma pack(my_record);
      
      for my_record use record
         IMPORTANT_5_BIT_ARRAY at 0 range 0 .. 4;
      end record;

   end more_definitions;

after compile I get:
ERROR: size for IMPORTANT_5_BIT_ARRAY too small, minimum allowed is 40

Note that I don't get this error if I move the record declaration
INTERNAL to the package where the array type is defined.  Only when I
use the array type in an external package does the compiler decide I
need a whole BYTE for each BIT of data!

Any ideas?
Thanks,
Robert Quinn

Note that I don't do any low-level programming in my job (is this what
you low level programmers do a lot of?), this is just a hobby.