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: Thu, 06 Sep 2001 15:34:33 +0200
Organization: Scopus Network Technologies
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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.

   ===============================
   ======== Part#1. Files ========
   ===============================
Note#1. There is an error in program below :
 - foo() in main.c takes no parameters
 - foo() in soo1.c takes one parameter
Note#2. See also Part#6

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

------ File soo1.c ------
int foo (int a)
{
  return a;
}
-------------------------



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


   ===============================
   ====== Part#2. C-program ======
   === Compilation and Linkage ===
   ===============================
%gcc main.c soo1.c

<COMMENT> No problem </COMMENT>

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


   ===============================
   ===== Part#3. C++-program =====
   === Compilation and Linkage ===
   ===============================
%gpp main.c soo1.c
main.o(.text+0x7):main.c: undefined reference to `foo(void)'
collect2: ld returned 1 exit status

<NOTE>C++-compiler/linker does detect our error</NOTE>


   ===============================
   ====== Part#4. C-program ======
   ========== Analysis ===========
   ===============================
%gcc -c main.c soo1.c
%nm main.o soo1.o ### nm - list symbols from object files

main.o:
00000020 b .bss
00000020 d .data
00000000 t .text
00000000 t ___gnu_compiled_c
         U _foo
00000000 T _main
00000000 t gcc2_compiled.

soo1.o:
00000020 b .bss
00000020 d .data
00000000 t .text
00000000 t ___gnu_compiled_c
00000000 T _foo
00000000 t gcc2_compiled.

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

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


   ===============================
   ===== Part#5. C++-program =====
   ========== Analysis ===========
   ===============================
%gpp -c main.c soo1.c
%nm main.o soo1.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.

soo1.o:
00000020 b .bss
00000020 d .data
00000020 ? .eh_frame
00000000 t .text
00000020 ? ___FRAME_BEGIN__
00000000 t ___gnu_compiled_cplusplus
00000000 T _foo__Fi
00000000 t gcc2_compiled.

<COMMENT>
We can see that both main.o and soo1.o don't contain
   the same symbol _foo.

main.o contains symbol _foo__Fv
   (because foo() in main.c takes no parameters)

soo1.o contains symbol _foo__Fi
   (because foo() in soo1.c takes one int-parameter)

So, a linker does have a problem with these object files.
</COMMENT>


   ===============================
   ====== Part#6. C-program ======
   ============ Style ============
   ===============================

Style used in Part#1 is correct but not recommended.
Here is a recommended style (using header file)

------ File hoo1.h ------
int foo (int a);
-------------------------

------ File main.c ------
#include "hoo1.h"
int main()
{
  return foo();
}
-------------------------

------ File soo1.c ------
#include "hoo1.h"
int foo (int a)
{
  return a;
}
-------------------------

%gcc main.c soo1.c
main.c: In function `main':
main.c:4: too few arguments to function `foo'

<COMMENT>
So, a compiler (not a linker) detects our error.
</COMMENT>


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