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Casting an array to a record

Casting an array to a record

Casting an array to a record

I'm trying to do the following C code in Ada

CODE --> Code in C

char x[MAXX];
struct yyy { ... } *y;
y = (struct yyy*) x;
I'm writing a comms interface.  The problem I've got is that the data is coming in as a stream of bytes, which I store in an array.  After examining bytes 2 and 3, I know what type of record it is.  How do I now cast or copy this array to a record?

I tried using unchecked_conversion but it doesn't like variable sized arrays.

RE: Casting an array to a record

Hi xwb,

This should help you:

with Ada.Unchecked_Conversion;

procedure Test is

   -- Define types to reprsent data coming in
   type ByteType is range 0..2**8-1;
   for ByteType'Size use 8;
   type DataType is array (Natural range <>) of ByteType;
   pragma Pack (DataType );
   -- Example of types that might be needed in the application
   type ModeType is (On , Off , Test);
   for ModeType use (On   => 2#0000_0000# ,
                     Off  => 2#0001_0000# ,
                     Test => 2#0011_1000# );
   for ModeType'Size use 8;                   
   type HeightType is range 0..2**8-1 ;
   for HeightType'size use 8;

   -- Define record type and lay it out using representation
   -- Could use a variant record if it is dependant on the data comming in.   
   type RecordType is
      Mode   : ModeType ;
      Height : HeightType ;
   end Record ;
   for RecordType use
      Mode   at 0 range 0..7 ;
      Height at 0 range 8..15;
   end record ;

   TheData   :  DataType := (0 => 0 ,  
                             1 => 22);
   TheRecord :  RecordType ;                                           

   if TheData (1) = 22
        --  Constrain the array type to the correct length
        subtype ConstrainedType is DataType (TheData'Range);

        function Convert is new Ada.Unchecked_Conversion (Source => ConstrainedType,
                                                          Target => RecordType );
        -- Convert to record format
        TheRecord := Convert (S => TheData);

        if TheRecord.Mode'Valid  -- Ada95 only
           -- Safe to use Mode
        end if;
      end ConstrainAndUseData;
   end if;

end Test;

Derek Russell

RE: Casting an array to a record

This is a fairly common problem, especially when dealing with unconstrained types.  

Instead of checking the individual bytes, you could define a variant record, provide representation and do the checking and conversion in 1 go.

If you'd like to chat further, let me know.


Derek Russell

RE: Casting an array to a record

Thanks - I can see what you're doing.  Once it has worked out what type it is, it is casting to the correct type.  The eureka line is


subtype ConstrainedType is DataType (TheData'Range);
I tried all sorts of combinations with unchecked_conversion but it just wouldn't compile.

RE: Casting an array to a record

Compiler: gnat - basically gcc 4.1.2.
Code snippet: don't worry about it.  Your eureka line fixed it.

It is on one of those write only machines.  We can put stuff in but it is really difficult getting anything out.  Floppies and memory sticks don't work.  We have to burn DVDs and get them declassified.  Really not worth the effort.

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