Re: pragma pack in external packages

[Stephen Leake <stephen.a.leake.1@gsfc.nasa.gov>]

This compiles with GNAT 3.15a:

package definitions is

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

   type MY_5_BIT_ARRAY_TYPE is array (1..5) of ONE_BIT_TYPE;
   pragma pack (MY_5_BIT_ARRAY_TYPE);


end definitions;

with Definitions;
package More_Definitions is

   type my_record is record
      IMPORTANT_5_BIT_ARRAY : Definitions.MY_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;



robert_s_quinn@yahoo.com (Robert Quinn) writes:

> 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.

-- 
-- Stephe