Making an object from a C dll in C++

Question

I have a C++ program which I call multiple C++ Dlls in it. Usually I make a simple class in the Dlls with a constructor and a destructor and do some initialization in the constructors. Then in the main program I make multiple objects from each Dlls' class and use them in multiple threads.

MyNamespcae::MyDllClass * MyObj = new MyNamespcae::MyDllClass(/*...inputs...*/);  # I make an object for each thread seperately

Now I have a C Dll that I'm calling in the main c++ program. I exported the C functions with __declspec(dllexport) and use them directly in the main program. The problem here is that I have some global variables in the C Dll, so I can not use the Dll in multiple threads.

So my question is how can I do some object-oriented-like method (like what we do in C++) in the C dll to call it in C++, from multiple threads? (Note My question is about what I need to do in C not c++)

Answer 1

The link shared by Lorinczy Zsigmond which is: http://lzsiga.users.sourceforge.net/oop.html#S0004 solved my problem. I repeat it's content here for more convenience:

Header-file: /* mytype.h */

#ifndef MYTYPE_H
#define MYTYPE_H

typedef struct MyType MyType; /* opaque */

MyType *MyType_Alloc (int par);
void    MyType_Release (MyType *mp);

int MyType_Operation1 (MyType *mp, int par1, int par2);

#endif

source code: /* mytype.c */

#include <stdlib.h>
#include <string.h>

#include "mytype.h"

struct MyType {
    int somedata;
};

MyType *MyType_Alloc (int par) {
    MyType *mp= malloc (sizeof *mp);
    mp->somedata= par;
    return mp;
}

void MyType_Release (MyType *mp) {
    memset (mp, 0, sizeof *mp);
    free (mp);
}

int MyType_Operation1 (MyType *mp, int par1, int par2) {
    return mp->somedata + par1 + par2;
}

test:/* mytypetest.c */

#include <stdio.h>

#include "mytype.h"

int main (void) {
    MyType *mp= MyType_Alloc (20200000);
    int sum= MyType_Operation1 (mp, 100, 25);
    printf ("sum=%d\n", sum);
    MyType_Release (mp);
    return 0;
}