C and C++ sources should be compiled separatelly, but we can link them together. Because of the different strategy to generate external names in C and C++, we use extern "C".

// this is a c source, compile with gcc

struct Y
{
    int y_;
};

int cfunc( double x, struct Y y, double z)
{
    return  y.y_;
}

///////////////////////////////////////////
// this is a cpp source, compile with g++

#include <iostream>

using namespace std;

struct Y
{
    int y_;
    virtual void f() {};    // this makes the problem!
};

extern "C" int cfunc( double x, struct Y y, double z);

int main()
{
    Y y;
    y.y_ = 1;
    cout << cfunc(3.14, y, 4.14) << endl;   // bad!

    return 0;
}

We can use - and frequently we should use - C and C++ programs together.

// C++ header for C files:
extern "C"
{
    int     f(int);
    double  g(double, int);
    // ...
}

// C++ header for C/C++ files:
#ifdef __cplusplus
extern "C"
{
#endif
    int     f(int);
    double  g(double, int);
    // ...
#ifdef __cplusplus
}
#endif