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From: murat@fir.ucsb.edu (Karaorman, Murat)
Newsgroups: comp.lang.eiffel
Subject: Re: C and Eiffel ?
Message-ID: <12051@hub.ucsb.edu>
Date: 12 Jun 91 21:34:16 GMT
References: <702@mailgzrz.tu-berlin.de>
Sender: news@hub.ucsb.edu
Reply-To: murat@cs.ucsb.edu (Murat Karaorman)
Organization: University of California, Santa Barbara
Lines: 98

In article <702@mailgzrz.tu-berlin.de> duns1222@w203zrz.zrz.tu-berlin.de (Martin Dunschen) writes:
>Hello,
>
>We have a problem calling eiffel from C.
>
>We typed in the example as given in "Eiffel: The Language" Version 2.2, page
>221 (but we use version 2.3 level 4) and tried to create an executable 
>programm by adding an appropriate (?) "main.c". Of course we got errors 

That part of the documentation IS faulty, as I remember having bumped into
about one and a half years ago

>from "ld" saying, that there are undefined symbols. Okay, so we hacked in
>a toy class calling this given example. Compiled with "es" everything works
>fine. Now we want to use this in conjunction with eiffel classes compiled to C.
>To get these C-versions of the system's classes we generated a C-package by
>changing the SDF. 
>
>Is this necessary, or is there another way to use the C verions of the 
>system classes? 
>
>We ran "identify A" in the directory containing the C-routines.
>An archive was created.
>
>But what now? When we try to link everything together with a self written main
>"ld" complains about undefined symbols (eif_create, eif_rout, eif_attr, 
>MakeStr), which we found in the archive-file in __eifc.o (most of them)!
>
>To complete the confusion we add the sources of everything as a shell-archive.
>Please try it and tell us, what we do wrong.
[... stuff deleted  for brevity ...]

What you need to do is basicly the following:

1) create a C-package archive (using the standard identify, etc. stuff)
   and make it visible under a name you choose (do VISIBLE (Y): my_name 
   during C-package generation). This should create a my_name.a file.
2) write your C program that creates eiffel objects, sends them messages
   etc., this is similiar to the sample prgm in the documentation, EXCEPT
   before doing anything you need to start-up an associated eiffel runtime
   environment by calling 
		my_name_eiffel_init();
   before using any eiffel objects. 
2.1) In this C program you shouldn't use eif_rout ( ... )  to invoke a 
   method, but instead obtain a function pointer to the method and then
   invoke the method via this pointer. You can look at the example I give.
3) compile this C program, say test.c, using
                cc -o test test.c my_name.a 
   provided that you have the correct paths, etc.
4) (hopefully) test is runnable.
5) You don't need to create the C-package again as long as your C programs
   requirements for the Eiffel environment don't change.
6) Below is how I would modify  your test.c code that follows my guidelines. 
   The lines starting with >> are my insertions,
   The lines starting with << were in your original, they need to be removed,
   The rest is unchanged (except for inserting DATUM) from your original.

==============================test.c=======================================
  #include "/cad01/Eiffel/files/_eiffel.h"
  #include <stdio.h>

>> #include " ... rename.h "
>> #include " ... Eiffel_rout.h"

  extern OBJPTR eif_create ();
  extern DATUM eif_rout();
  extern int eif_attr ();

<<  int test ()
>>  main ()
  {
    OBJPTR obj;
>>  ROUT_PTR f_ptr;

>>  my_name_eiffel_init();
 
    obj = eif_create("string",(DATUM) 10);

>>  f_ptr = eif_rout (obj, "append");
>>  (* f_ptr)(obj, MakeStr("Hello world"));
 
<<  eif_rout(obj, "append", MakeStr("Hello world"));
    printf("Size of Eiffel string is: %d\n",(int32) eif_attr (obj, "size"));
    return 25;
  }

=================================end====================================


I hope this helps. Let me know if anything goes wrong. It has been a while
now, but I know that this stuff should eventually work.

good luck,

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