Re: Help: Storage size

[Adrian Hoe <byhoe@my-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?
--
Adrian BY, Hoe
--------------


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