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From: tmoran@acm.org
Newsgroups: comp.lang.ada
Subject: Re: Importing C structs?
References: <ddWVa.1313$Kx1.14581@newsfep4-glfd.server.ntli.net>
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>   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
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;