From:	Laurence Withers <lwithers AT lwithers DOT demon DOT co DOT uk>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Calling C++ functions from C functions: how?
Date:	Mon, 11 Jan 1999 19:40:58 +0000
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--- Original Message ---
From: to comp.os.msdos.djgpp <axlq AT unicorn DOT us DOT com>
Time: Mon, 11 Jan 1999, 02:50:56

>I have a library that contains several C modules and two C++ modules.
>The C modules end in .c and the C++ modules end in .cc.  I'm using RHIDE
>to build everything.  No special compiler options.
>
>I also have a test program in C that calls these functions.
>
>When I attempt to link my program to the library, the linker complains
>that two functions called by the test program are undefined.  These two
>functions happen to be the C++ ones in the library.
[snip]

As Eli has said, this occurs due to name mangling. One possible solution
to this is to provide C wrapper functions for anything in C++. For
example:

// obj.cc //////////////////////////////////////////////////////////////

// C++ function
int cplusplus_tester_function(int a, int b)
{
    return (a + b);
}

//
// C wrapper
//
extern "C" {
  int c_tester_function(int a, int b)
  {
      return cplusplus_tester_function(a, b);
  }
}

// end of obj.cc ///////////////////////////////////////////////////////

// main.c //////////////////////////////////////////////////////////////

#include <stdio.h>

int c_tester_function(int, int);

int main(void)
{
    int a = 2, b = 3;
    printf("Adding %d and %d gives: %d\n", a, b,
      c_tester_function(a, b));
    return 0;
}

// end of main.c ///////////////////////////////////////////////////////

Now compile and run:

19:30:49.54-> gxx -o obj.o -c obj.cc
 
19:36:10.58-> ar rs obj.a obj.o
 
19:36:15.02-> gcc -o main.o -c main.c
 
19:36:59.62-> gcc -o test.exe main.o obj.a
 
19:37:07.48-> test
Adding 2 and 3 gives: 5

As you can see, the C wrapper function has to be surrounded by an
extern "C" { /* ... */ } clause.

This might seem inefficient, but if you compile with the 
-finline-functions optimisation, I believe the call will be optimised
away (not sure though), effectively solving your problem.

Well, I hope this helps you. Bye for now,
-- 
Laurence Withers, mailto:lwithers AT lwithers DOT demon DOT co DOT uk
Integrated Peripherals Operating System Project Leader || OPES Project
Projects' homepage is at: http://www.lwithers.demon.co.uk/       Leader

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