C Opaque Structure

api.c

#include <stdlib.h>

#include "api.h"
#include "internal.h"

api* create_api() {
    internal* in = malloc(sizeof(internal));

    in->the_api.x = 1;
    in->the_api.y = 2;
    in->z = 3;
    
    return &in->the_api;
}

int get_z(api* the_api) {
    internal* in = (internal*)the_api;

    return in->z;
}

api.h

#ifndef API_H
#define API_H

typedef struct {
    int x;
    int y;
} api;

api* create_api();
int get_z(api*);

#endif /* API_H */

internal.h

#ifndef INTERNAL_H
#define INTERNAL_H

#include "api.h"

typedef struct {
    api the_api;
    int z;
} internal;

#endif /* INTERNAL_H */

main.c

#include <stdio.h>

#include "api.h"

int main(int argc, char **argv) {
    api* the_api = create_api();

    printf("x = %d\n", the_api->x);
    printf("y = %d\n", the_api->y);
    // There is no z in api
    //printf("z = %d\n", the_api->z);
    printf("z = %d\n", get_z(the_api));

    return 0;
}