Linking C compiled static library to C++ Program - c++

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.