nrmj <maa1666@xxxxxxxx> writes:
> I have been googling a lot on the web about alias
> analysis and strict aliasing, but haven't found much
> about what is allowed or not when working with
> structures.
I don't think these questions have anything to do with gcc. I
recommend that you ask on some place like comp.std.c.
> 1) If I understand the C standard correctly, the
> compiler is free to assume that two pointers pointing
> to different types don't alias.
>
> If two struct have different tag name, like:
> struct s1 {int i;};
> struct s2 {int i;};
> struct s1 *p1;
> struct s2 *p2;
>
> p1 and p2 are considered by the compiler to point
> to different locations and don't alias, because p1
> points to type "struct s1" and p2 points to type
> "struct s2" which are different types, even if they
> have the same members, right ?
You have to watch out for the exception in C99 6.5.2.3 if there is a
visible union declaration which contains both structs. But otherwise,
yes, they are different types.
> 2) Is it safe to cast pointer to struct B to pointer
> to struct A, in order to fake inheritance, like in
> this code sniplet ?
No, that is not safe.
But it is safe to do
struct Color_Point { struct Point p; Color color; };
p = &my_color_point.p;
> 3) When programming with socket, the following cast to
> (struct sockaddr *) is very common.
> But is it really safe ?
>
> struct sockaddr_in my_addr;
> ...
> bind(sockfd, (struct sockaddr *)&my_addr,
> sizeof(struct sockaddr));
That is safe because the code does not dereference the resulting
pointer.
Ian
