#include <stdio.h>

typedef int (*Callback)(void *, int);
struct Closure { Callback call; void *env; };
struct Offset { int amount; };

static int add_offset(void *env, int x) {
    const struct Offset *offset = env;
    return x + offset->amount;
}

int main(void) {
    struct Offset offset = { 7 };
    struct Closure closure = { add_offset, &offset };
    printf("%d\n", closure.call(closure.env, 5));
    return 0;
}