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From: resander <kresander@gmail.com>
Newsgroups: comp.lang.ada
Subject: How to access this package written in C?
Date: Wed, 21 Apr 2010 09:43:02 -0700 (PDT)
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Date: 2010-04-21T09:43:02-07:00
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I am working on software that manipulates SQL databases via a GUI. It
is written in C or C++ without classes and compiled on Linux. Would
also want it to run on Windows eventually. This software has
essentially one entry procedure:

  int guidb ( int appno , int recaddress );  // in C

which returns an outcome as an int, takes an int appno which specifies
application and an address of a record in recaddress. The latter is
used for passing the memory address of any recordtype variable and is
cast to a byte address inside guidb.

guidb uses appno to locate field-offsets and length of the recordtype
in tables built by initialisation functions for all recordtypes that
take part.

Typical use in C/C++:

typedef struct { int prodcode; float price; char descr [50]... }
PRODUCT;
typedef struct { bool bitesyou; int speciescode; char name[20] ... }
ANIMAL ;

void getproduct ( PRODUCT * pout , int * resultout )
   {
   *resultout = guidb ( 0 , (int)pout ) ;
   }

void addtozoo ( ANIMAL * a , int * resultout )
   {
   *resultout = guidb ( 1 , (int)a ) ;
   }

PRODUCT prod ;
ANIMAL addthis ;
int outcome ;

//  use as...

  getproduct ( &prod , &outcome );
  // get addthis animal data then
  addtozoo ( &addthis , &outcome );

All wrappers like getproduct and addtozoo are automatically generated
from other specs. That is easy in C by using fldoffset macro to get
field offsets and sizeof(rectypename) to get record length and a cast
that changes the interpretation of address to an int.

Desired use in Ada (same as above):

type PRODUCT is RECORD int prodcode; float price; char descr
[50]...END RECORD;

procedure getproduct ( p : out PRODUCT ; result : out int ) is
begin
   result = guidb ( 0 , address_of(p) how ??? ) ;
end

prod : PRODUCT;
outcome : int ;

  getproduct ( prod , outcome );


To be able to use the guidb with Ada I need to know how to:

 - get the memory address carried (how?) by the record formal
parameter
 - obtain field offsets for a record type
 - obtain length of a record object/variable

How to do this in in Ada? If it cannot be done, can the interface
above be changed to make it possible?

Ken