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From: Andreas Almroth <andreasNO@SPAMalmroth.com>
Subject: Re: Importing C structs?
Date: Mon, 04 Aug 2003 15:33:52 +0100
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Date: 2003-08-04T15:33:52+01:00
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On Wed, 30 Jul 2003 23:50:26 +0000, tmora wrote:

>>   type Jpeg_Error_Mgr is record
>>      ...
>>      Msg_Parm            : System.Address;
> 
>   This is wrong.  The C has
> 
>>   union {
>>     int i[8];
>>     char s[JMSG_STR_PARM_MAX];
>>   } msg_parm;
>   which means msg_parm is JMSG_STR_PARM_MAX number of bytes, interpretable
> sometimes as a char array and sometimes as an int array followed by
> filler.  msg_parm is never an address.  To match the C, I suggest making

I really never have thought about the padding part, but I now see that
when using unions, the C compiler actually pad out to the largest type.
Quite logical actually...

When mapping unions I usually try to mimic the union in (in GNAT mind you)
as in order to get a clearer representation. I got this from "googling" a
bit a while ago;

type Union_Range is new Positive 1..2;
type Some_Union(X : Union_Range := 1) is record
  case X is
    when 1 =>
      I : Int_8_Array;
    when 2 =>
      S : Char_80_Array;
  end case;
end record;
pragma Unchecked_Union(Some_Union);

type Struct_With_Union is record
  X : Interfaces.C.Int;
  Y : Some_Union;
  Z : Interfaces.C.Double;
end record;
pragma Convention(C,Struct_With_Union);

This has worked quite well so far with GNAT, and no need for manual
conversions in the code.
I'm curious if this works with other Ada compilers?


> msg_parm an interfaces.c.char_array (the larger of the two objects),
> and then doing an unchecked_conversion on those (relatively rare)
> occasions when you need to treat it as an array of int's.  Here's
> a compilable example:
> 
> with interfaces.c;
> with ada.unchecked_conversion;
> procedure testluc is
> 
>   JMSG_STR_PARM_MAX: constant := 80;
>   subtype Msg_Parm_C_Type is Interfaces.C.Char_Array(1 .. JMSG_STR_PARM_MAX);
> 
>   subtype I_Part is Interfaces.C.Size_T range 1 .. 32;
>   subtype Msg_Parm_C_I_Part is Interfaces.C.Char_Array(I_Part);
>   type Msg_Parm_I_Type is array(1 .. 8) of Interfaces.C.Int;
> 
>   function To_I is new Ada.Unchecked_Conversion
>     (Source=>Msg_Parm_C_I_Part,
>      Target=>Msg_Parm_I_Type);
> 
>   function To_C is new Ada.Unchecked_Conversion
>     (Source=>Msg_Parm_I_Type,
>      Target=>Msg_Parm_C_I_Part);
> 
>   type Jpeg_Error_Mgr is record
>     msg_parm: Msg_Parm_C_Type;
>   end record;
> 
>   X : Jpeg_Error_Mgr;
>   X_I : Msg_Parm_I_Type;
>   N : Interfaces.C.Int;
> 
> begin
>   -- fetch ints from X.msg_parm
>   N := To_I(X.msg_parm(I_Part))(1);
>   N := To_I(X.msg_parm(I_Part))(2);
>   -- store ints into X.msg_parm
>   X_I(1) := 1;
>   X_I(8) := 2;
>   X.msg_parm(I_Part):=To_C(X_I);
> end testluc;


Cheers,
Andreas