Re: pragma pack in external packages

["Pat Rogers" <progers@classwide.com>]

"Robert Quinn" <robert_s_quinn@yahoo.com> wrote in message
news:db500a4f.0204250849.61c0791e@posting.google.com...
> 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


In package Definitions, do something like this:

  type Bit is range 0 .. 1;

  for Bit'Size use 1;

  type Five_Bits is array( 1 .. 5 ) of Bit;

  for Five_Bits'Component_Size use 1;

  for Five_Bits'Size use 5;


That is, use 'Component_Size instead of pragma Pack, and take it out of the
package More_Definitions too.

I strongly suggest you think about the alignment issues for the record type, by
the way!  There is an attribute definition clause for that, too.

HTH,

---
Patrick Rogers                       Consulting and Training in:
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