Ada records and byte order

[karlran1234@yahoo.com (Karl Ran)]

Hi there,

I've a question about Ada records:

I have to read a binary file from disk which was written by another program.

The record size is fixed: 3 bytes
The file structure looks like this:
      

|byte 1 | |byte 2 | |byte 3 |   |byte 4...
aaaa.aaaa.bbbb.bbbb.bbbb.cccc | aaaa.a.....
M       L M            L M  L
S       S S            S S  S
B       B B            B B  B

a:  8 bits
b: 12 bits
c:  4 bits

Here is the code ...

procedure test is

   subtype BYTE     is Integer range 0 .. 2 **   8 - 1;
   subtype WORD12   is integer range 0 .. 2 **  12 - 1;
   subtype WORD4    is integer range 0 .. 2 **   4 - 1;

   type My_rec is
      record
         A : BYTE;
         B : WORD12;
         C : WORD4;
      end record;
   
   for My_rec use
      record
         A at 0 range 0 ..  7;
         B at 1 range 0 .. 11;
         C at 2 range 0 ..  3;
      end record;   

   Abc : My_Rec; 

begin
   Abc.b := 16#123#;
   ...
end test;
   
... which fails to compile whith GNAT on a (low-endian) i386:
test.adb:31:10: component "C" overlaps "B" at line 30
         
Is there an Ada like (TM) solution for this kind of (byte order) problem?

   
Thanks,
Karl