Challenges in writing wrappers for C++ functions so that they can be used from C code

Answer 1

The easiest way to avoid undefined symbols while linking is to link with g++ (not gcc). You can still compile your .c file with gcc, though.

Also please use system at a time. The link error may go away if you run all your gcc and g++ commands on the same system (no matter the old or the new one).

Answer 2

To call a C++ function from C, you can't have mangled names. Remove the conditional test for __cplusplus where you do the extern "C". Even though your functions will be compiled by a C++ compiler, using extern "C" will cause it to avoid name-mangling.

Here is an example:

The C file.

/* a.c */
#include "test.h"

void call_cpp(void)
{
  cpp_func();
}

int main(void)
{
  call_cpp();
  return 0;
}

The header file.

/* test.h */
#ifndef TEST_H
#define TEST_H
extern "C" void cpp_func(void);
#endif

The CPP file.

// test.cpp
#include <iostream>
#include "test.h"

extern "C" void cpp_func(void)
{
  std::cout << "cpp_func" << std::endl;
}

The compiler command line.

g++ a.c test.cpp