Re: Help: Storage size

["David C. Hoos, Sr." <david.c.hoos.sr@ada95.com>]

Adrian Hoe wrote in message <7mfndb$mc8$1@nnrp1.deja.com>...
>In article <7mf7lp$c9g@hobbes.crc.com>,
>  "David C. Hoos, Sr." <david.c.hoos.sr@ada95.com> wrote:
>>
>> Adrian Hoe wrote in message <7mejue$9lk$1@nnrp1.deja.com>...
>> >If I have a C code like this:
>> >
>> >typedef unsigned short   CARD16;
>> >
>> >and
>> >
>> >typedef struct {
>> >   CARD8     reqType;
>> >   CARD16    length : 16;
>> >} tReq;
>> >
>> >How can the above struct (record) be implemented in Ada?
>> >
>> >Where CARD16 is define as follow in Ada:
>> >
>> >type CARD16 is new X.Unsigned_Short;
>> >
>> >
>> >Can anyone please help?
>> >--
>> Several problems here -- e.g.:
>>
>> 1. Where is the declaration for CARD8?  This will affect the result.
>
>CARD8 is declare as unsigned char;
>
>
>
>> 2. If #pragma pack is used, this will affect the result.
>
>#pragma pack is not used.
>
>
>
>> 3. If unsigned short is 16 bits on your system, the bit-field
>>    specification (:16) would appear to be redundant.
>
>Most platforms have 16 bits for unsigned short. The bit-field
>specification (e.g. :16) is used to ensure 16 bits mapping on other
>platform(s).
>
>
>
>> 4. Where is the declaration for X.Unsigned_Short?
>
>In X.ads, it is declare as
>
> subtype unsigned_short is interfaces.c.unsigned_short;
>
>
>
>> 5. Bit-field implementations are compiler-dependent, so from
>>    the code you supplied (without any compiler or platform
>>    identification) it is impossible to say for sure what is
>>    the memory layout of the tReq struct.
>
>I am using GNAT for Linux (PC) and intend to target the code to other
>platforms like Sun and Alpha.
>
>
>
>> If the memory layout of the C struct is known, writing the
>> Ada declaration is trivial.
>>
>>
>
>
>Let's assume the bit-field specification (:16 e.g.) is needed badly, how
>can I implement the C's struct in Ada?
>--

If you want to be assured that your types have the correct size,
then I would not recommend basing your types on
Interfaces.C.Unsigned_Short, because that type is dependent on the size
of C's unsigned short type for that platform.

How about starting from scratch -- e.g.:

   type Card8 is mod 2 ** 8;
   for Card8'size use 8;
   type Card16 is mod 2 ** 16;
   for Card16'size use 16;

   type Treq is record
      ReqType : Card8;
      Length  : Card16;
   end record;

   for Treq use record
      ReqType at 0 range 0 .. 7;
      Length  at 2 range 0 .. 15;
   end record;

   for Treq'size use 32;

Note that there is one byte of unocupied space between the
record components, consistent with the way gcc lays out
your struct, in the absence of #pragma pack.

Hope this helps.