// call_cc.cpp   calls a C program which calls back to this C++ program
//               must be compiled with call_cc.c or at least linked with
//               the object file from the C program

extern "C" {                 // function prototype for a C function
  char call_c(int x, float y);
}

#include <iostream>
using namespace std;

int main()
{
  char c;
  int x=5;
  float y = 1.5;

  cout << "in C++ about to call a c: function, call_c" << endl;
  c = call_c(x, y);
  cout << "in C++: returned from call_c with " << c << endl;
  cout << "in C++: x = " << x << "  y = " << y << endl;
  return 0;
}

extern "C" {                // a C++ functiona callable be a C function
#include <stdio.h>          // probably should use C rather than C++

char call_cc(int x, float y)
{
  try     // can use C++ in C callable function
  {       // but must load C++ and C libraries
    cout << "in C++ extern \"C\" test that C++ allowed" << endl;
  } catch(...) {printf("some exception thrown in C++\n");}

  printf("in C++ called from c: x = %d, y = %g \n", x, y);
  return 'q';
}
}

// output of execution is:
// in C++ about to call a c: function, call_c
// in c: about to call a C++ function
// in C++ extern "C" test that C++ allowed
// in C++ called from c: x = 5, y = 1.5 
// in c: returned from call_cc with q 
// in c: x = 5, y = 1.5 
// in C++: returned from call_c with a
// in C++: x = 5  y = 1.5