Linking C compiled static library to C++ Program

Question

I tried to link a static library (compiled with gcc) to a c++ program and I got 'undefined reference'. I used gcc and g++ version 4.6.3 on a ubuntu 12.04 server machine. For example, here is the simple library file for factorial method:

mylib.h

#ifndef __MYLIB_H_
#define __MYLIB_H_

int factorial(int n);

#endif

mylib.c

#include "mylib.h"

int factorial(int n)
{
    return ((n>=1)?(n*factorial(n-1)):1);
}

I created object for this mylib.c using gcc:

gcc -o mylib.o -c mylib.c

Again the static library was created from the object file using AR utility:

ar -cvq libfact.a mylib.o

I tested this library with a C program (test.c) and C++ program (test.cpp)

Both C and C++ program have the same body:

#include "mylib.h"
int main()
{
    int fact = factorial(5);
    return 0;
}

Assuming static library libfact.a is available in /home/test directory, I compiled my C program without any issues:

gcc test.c -L/home/test -lfact

However while testing C++ program, it threw a link error:

g++ test.cpp -L/home/test -lfact

test.cpp:(.text+0x2f): undefined reference to `factorial(int)'
collect2: ld returned 1 exit status

I even tried adding extern command in test.cpp:

extern int factorial(int n) //added just before the main () function

Still the same error.

• Can someone tell me what I am wrong here?
• Is there anything I missed while creating the static library?
• Do I have to add anything in my test.cpp to make it work?

Answer 1

The problem is that you haven't told your C++ program that factorial is written in C. You need to change your test.h header file. Like this

#ifndef __MYLIB_H_
#define __MYLIB_H_

#ifdef __cplusplus
extern "C" {
#endif

int factorial(int n);

#ifdef __cplusplus
}
#endif

#endif

Now your header file should work for both C and C++ programs. See here for details.

BTW names containing a double underscore are reserved for the compliler (so are names starting with an underscore and a capital letter) so #ifndef __MYLIB_H_ is illegal strictly speaking. I would change to #ifndef MYLIB_H #define MYLIB_H

Answer 2

While the accepted answer is absolutely correct, I thought I'd just add an observation. Some editors have trouble with the open / close brace, and will indent the entire extern "C" scope in the header. If mylib.h is a key header for a library, you might consider:

#if defined (__cplusplus)
#define _MYLIB_INIT_DECL extern "C" {
#define _MYLIB_FINI_DECL }
#else
#define _MYLIB_INIT_DECL
#define _MYLIB_FINI_DECL
#endif

All other headers in mylib library, e.g., mylib_aux.h, can be of the form:

#ifndef _MYLIB_AUX_H
#define _MYLIB_AUX_H

#include <mylib.h>

_MYLIB_INIT_DECL

... header content ...

_MYLIB_FINI_DECL

#endif /* _MYLIB_AUX_H */

Obviously, the names I'm using are arbitrary, but for multiple library headers, this approach has been useful to me.