From:	Alex Vinokur <alexvn AT bigfoot DOT com>
Newsgroups:	alt.comp.lang.learn.c-c++,comp.os.msdos.djgpp
Subject:	Re: use of extern -- please help
Followup-To:	alt.comp.lang.learn.c-c++
Date:	Sun, 09 Sep 2001 08:44:06 +0200
Organization:	Scopus Network Technologies
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Alex Vinokur wrote:

> Luke Vanderfluit wrote:
>
> > Hi,
> >
> > I've been trying to work out how to use "extern"
> >
> > As far as I've been able to gather, I can compile 2 files seperately and
> > get one to access the others variables/functions.
> >
>
> [snip]
>
> It is necessary to careful with extern declarations in C.
> Here is an example.
>
>

[snip]

It is necessary to careful with extern declarations in C++ as well.

Here is a new example.

   ===============================
   ======== Part#1. Files ========
   ===============================
Note. There is an error in program below :
 - main.c contains extern declaration of function foo()
          that takes no parameters;
 - soo2.c : 1) contains no function definition,
            2) contains variable foo__Fv.


------ File main.c ------
extern int foo(void);
int main() {return foo();}
-------------------------

------ File soo2.c ------
int foo__Fv;
-------------------------

===========================================
gcc version 2.95.3 20010315/djgpp (release)
Windows98
  gpp : compiles and links C++ sources
===========================================

   ===============================
   ===== Part#2. C++-program =====
   === Compilation and Linkage ===
   ===============================
%gpp main.c soo2.c

<NOTE>C++ compiler/linker doesn't detect our error</NOTE>

   ===============================
   ===== Part#3. C++-program =====
   ========== Analysis ===========
   ===============================
%gpp -c main.c soo2.c
%nm main.o soo2.o

main.o:
00000020 b .bss
00000020 d .data
00000020 ? .eh_frame
00000000 t .text
00000020 ? ___FRAME_BEGIN__
00000000 t ___gnu_compiled_cplusplus
         U _foo__Fv
00000000 T _main
00000000 t gcc2_compiled.

soo2.o:
00000000 b .bss
00000000 d .data
00000004 ? .eh_frame
00000000 t .text
00000004 ? ___FRAME_BEGIN__
00000000 t ___gnu_compiled_cplusplus
00000000 B _foo__Fv
00000000 t gcc2_compiled.


<COMMENT>
We can see that both main.o and soo1.o contain
   the same symbol _foo__Fv.

So, a linker has no problem with these object files.
</COMMENT>


 -------------
So, C++ compiler doesn't detect our error.
Is it a bug in the C++ compiler? It seems it isn't.
Does that problem have s solution ?



  ===========================
  Alex Vinokur
    mailto:alexvn AT bigfoot DOT com
    mailto:alexvnw AT yahoo DOT com
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    http://go.to/alexv_math
  ===========================

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