cast_struct.c

#include <stdio.h>

struct base {
    int foo;
};

struct derived {
    int foo;
    char *bar;
};

void foobar(void *ptr)
{
    ((struct base *) ptr)->foo = 100;
}

int main(int argc, char **argv)
{
    struct derived d = {1, "derived"};
    printf("before: %d\n", d.foo);
    foobar(&d);
    printf("after:  %d\n", d.foo);
    return 0;
}