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From: tmoran@acm.org
Newsgroups: comp.lang.ada
Subject: Re: Binding to C
References: <MxXSa.2290$l63.28380@newsfep4-glfd.server.ntli.net>
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>Also does anyone have any advice in general mapping to C.
 Look at some of the Windows binding source code at www.adapower.com
>For the jpeg_error_mgr struct, I mapped ptrs to functions to System.Address
>and made the struct a record.  Will this work, or will the corresponding
>type of the struct have to be a System.Address?
 You almost never need a System.Address.  Use access types.

>   type Jpeg_Error_Mgr is record
>      Error_Exit      : System.Address; -- addr of a handler
>      Emit_Message    : System.Address;
> ...
 Instead:

  type Error_Exit_Ptr is access
  procedure (cinfo   : access Decompress_Cinfo_Type);
  pragma Convention(C, Error_Exit_Ptr);

  type Emit_Message_Ptr is access
  procedure (cinfo    : access Decompress_Cinfo_Type;
             msg_level: Interfaces.C.Int);
  pragma Convention(C, Emit_Message_Ptr);
  ...

  procedure error_exit(cinfo   : access Decompress_Cinfo_Type);
  pragma Export(C, error_exit);

  procedure emit_message(cinfo    : access Decompress_Cinfo_Type;
                         msg_level: Interfaces.C.Int);
  pragma Export(C, Emit_Message);
  ...

  type Jpeg_Error_Mgr is record
    P_Error_Exit: Error_Exit_Ptr;     -- will be set := error_exit'access;
    P_Emit_Message: Emit_Message_Ptr; -- will be set := emit_message'access;
etc.