C question

Suppose you had some declarations:

typedef struct {
int datum;
void (*func) (void* s, int count, ...);
} A;

typedef struct {
int datum;
void (*func) (void* s, int count, ...);
int anotherdatum;
} B;



and a pointer to a B struct (call it bp).  You pass bp to a function expecting a pointer to an A, so you typecast it: somefunc((A*)bp);.  Call the formal parameter to somefunc foo.  That's cool (as I understand it), because the two structs share the same initial layout.  Within somefunc, the following line: foo->func((void*)foo, 0);.  Within the particular function that is bp's func, s is cast to be a pointer to B.  My question: will s be able to access anotherdatum?

Comments

[tom]

Good question. I have no idea. It's been a few years since I had to deal with C -- I've grown soft & weak.

[ben wolfson]

Answer: yes. It seems one can write brief programs to test these things.

[ben wolfson]

In retrospect, I think this is more or less exactly how Python implements its built-in objects at the C level.

[tweedledopey]

It makes sense that it would allow you to access bp->anotherdatum. My guess, though, is that it is one of those "undefined" behaviours that always works out right (i.e., if there were a lot of memory usage, it might not work out well).