Is it legal to implement inheritance in C by casting pointers between one struct that is a subset of another rather than first member?

Question

Now I know I can implement inheritance by casting the pointer to a struct to the type of the first member of this struct.

However, purely a

Answer 1

This is more-or-less the same poor man's inheritance used by struct sockaddr, and it is not reliable with the current generation of compilers. The easiest way to demonstrate a problem is like this:

#include 
#include 
#include 

struct base
{
    double some;
    char space_for_subclasses[];
};
struct derived
{
    double some;
    int value;
};

double test(struct base *a, struct derived *b)
{
    a->some = 1.0;
    b->some = 2.0;
    return a->some;
}

int main(void)
{
    void *block = malloc(sizeof(struct derived));
    if (!block) {
        perror("malloc");
        return 1;
    }
    double x = test(block, block);
    printf("x=%g some=%g\n", x, *(double *)block);
    return 0;
}

If a->some and b->some were allowed by the letter of the standard to be the same object, this program would be required to print x=2.0 some=2.0, but with some compilers and under some conditions (it won't happen at all optimization levels, and you may have to move test to its own file) it will print x=1.0 some=2.0 instead.

Whether the letter of the standard does allow a->some and b->some to be the same object is disputed. See http://blog.regehr.org/archives/1466 and the paper it links to.