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. F

Answer 1

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.

Answer 2

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