Borland C++ linker error c file include

I'm trying to build a C++ program with borland C++ Builder V5.5. Everything works fine until I tryed to call a function defined in a c file. The compiler works finde but linking is not working. The linker is not able to find the function called from c++ file. The .obj file is gernerated and part of the project xml file. Do anybody now how I can solve this error. To be sure that there are no side effects, I start a new project just calling c function call in constructor. The Project structure looks the following:

Form1.h defines Constructor and includes header where the c function is defined

Form1.cpp implements Constructor and call the c function

test.h defines an void function "void abc();"

test.c includes test.h and implements an void function "void abc() {}"

Unresolved external "abc()" referenced from ...unit1.obj

Do anyone have an idea??

Here some code

#include <vcl.h>
#pragma hdrstop
#include "Multicopter_Model.h"
#include "Unit1.h"

#pragma package(smart_init)
#pragma resource "*.dfm"
TForm1 *Form1;
__fastcall TForm1::TForm1(TComponent* Owner)
        : TForm(Owner)
{
abcdefg();
return;
}

#ifndef RTW_HEADER_Multicopter_Model_h_
#define RTW_HEADER_Multicopter_Model_h_

void abcdefg();

#endif */

#include "Multicopter_Model.h"
/* Model initialize function */
void abcdefg()
{

}

Solution

See C++ FAQ Item [32.4] How can I modify my own C header files so it's easier to #include them in C++ code. (Read the whole of section 32 actually.). Use:

#ifndef RTW_HEADER_Multicopter_Model_h_
#define RTW_HEADER_Multicopter_Model_h_

#ifdef __cplusplus
 extern "C" {
#endif 

void abcdefg();

#ifdef __cplusplus
 }
#endif 
#endif */