In my Eclipse C/C++ project there is this `undefined reference` error which doesn't go away, doesn't matter what I do. I already changed the link order, checked if all files are compiled and included, basically everything, which is recommended in the in the Internet, when facing this problem.

**Here's my program**:

[...] - means that there is more code, which doesn't relate.

*menulib.h*

```
[...]
void start_GUI( void );
[...]

```

*start\_GUI.c* - **Note this is a `.c`-File**

```
#include "menulib.h"

void start_GUI( void )
{
 [...]
}

```

*coreInterface.h*

```
#ifndef COREINTERFACE_H_
#define COREINTERFACE_H_

#include <stdint.h>
#include <stdlib.h>

#ifdef __cplusplus
/** only include the Core when compiled for C++ */
#include "Core.h"
#endif /* #ifdef CORE */

#ifdef __cplusplus
extern "C" {
#endif

void init_GUI( Core* core);

#ifdef __cplusplus
}
#endif

#endif /* COREINTERFACE_H_ */

```

*coreInterface.cpp* - **Note this is a `.cpp`-File**

```
#include "coreInterface.h"
#include "menulib.h"

void init_GUI( Core* core)
{
    gui_core =  core;
    start_GUI();            // <--- **error appears here** - calls a .c file
}

```

**Here's the error in the makefile** (the auto generated one from Eclipse):

```
g++ -L/home/PC/the_Project/menu_GUI -o "the_Project"  [...]  ./menu_GUI/coreInterface.o [...] ./menu_GUI/start_GUI.o  [...]   -lcurses
./menu_GUI/coreInterface.o: In function `init_GUI':
/home/PC/the_Project/Default/../menu_GUI/coreInterface.cpp:23: undefined reference to `start_GUI()'

```

I believe this is coming from merging the `.c` and `.cpp` files. Still I couldn't solve it. Has anyone an idea?

Kind Regards