[SOLVED] Using a c Shared Object in C++

toredo · 05-09-2012, 04:57 AM

Hello,

I'm just writing a bit c and c++, now a have a problem. I have a shared-object, compiled with gcc, and a program, compiled with g++. The program should call a function of the library.

But the linking doesn't work?

An example:
lib.c

Code:

#include "lib.h"

int kalk(int n1, int n2) {
    return n1 + n2;
}

lib.h

Code:

#ifndef __MY_LIB
#define __MY_LIB

int kalk(int n1, int n2);

#endif

test.cpp

Code:

#include <iostream>
#include "lib.h"

using namespace std;

int main(void) {
    cout << kalk(1, 2);
    return 0;
}

Then i compile the library with the following command:

Code:

gcc lib.c -shared -olib.so

And the program itself with:

Code:

g++ test.cpp lib.so -otest

but this compilation failes:

Code:

/tmp/ccoYXf6C.o: In function `main':
test.cpp:(.text+0x19): undefined reference to `kalk(int, int)'
collect2: ld returned 1 exit status

If i compile the library with g++, then it works:

Code:

g++ lib.c -shared -olib.so

it should be possible to use c-librarys in c++, shouldn't it be? I don't know. I think i did something wrong.

Thanks for your help

best regards
toredo

Nominal Animal · 05-09-2012, 05:16 AM

Have you tried telling the C++ compiler the header file is C and not C++? i.e., write lib.h as

Code:

#ifndef __MY_LIB
#define __MY_LIB
#ifdef __cplusplus
extern "C"
{
#endif

int kalk(int n1, int n2);

#ifdef __cplusplus
}
#endif
#endif

bigearsbilly · 05-09-2012, 05:46 AM

There is plenty on web search about this.

You know that C++ mangles function names, so basically it is inherently incompatible with .so libs
except by methods like the previous poster said.
(the caller doesn't know what the mangled name is in the callee)

Specifying as extern "C" means the name isn't mangled.

If you want to act on objects it will be quite a lot of hassle.
You need to keep it quite simple is my advice.

if you use nm on the object files you'll see the what the names mangle to.
nm is a useful tool if you don't know it well worth remembering.

toredo · 05-10-2012, 01:19 AM

Thank you for your help and the explanation.

It just worked and nm seems to be very usefull.

Thanks

Best regards
toredo

Nominal Animal · 05-10-2012, 06:21 AM

Quote:

Originally Posted by bigearsbilly

There is plenty on web search about this.

Although you are absolutely correct about C++ mangling function names, C++ and C are two completely different programming languages. I much prefer my explanation better.

There is, obviously, common ancestry between C++ and C, and you can compile most C code using a C++ compiler and get a working C++ binary (it will differ a lot from when compiled using a C compiler, most importantly it will require the C++ runtime), but that does not make them the same language, or even variations of the same language. It just makes most C compatible with C++.

I realise most programmers find that distinction unimportant. Working with embedded devices (or something like the Linux kernel) where the C++ runtime is unavailalbe, the distinction is very real and quite important.

I wish people (not you, Bigearsbilly, specifically; I do mean in general) would stop mixing the two languages; it's just confusing. Perhaps I should start intentionally conflating Perl with Scheme in retaliation?

Just kidding.

bigearsbilly · 05-10-2012, 08:53 AM

Quote:

Originally Posted by Nominal Animal

Although you are absolutely correct about C++ mangling function names, C++ and C are two completely different programming languages.

No I was referring to C++ name mangling and shared objects in particular.
It's a common problem.

I wasn't comparing C and C++.

But i forgive you