compatible types vs. strict aliasing rules

Question

It is a common strategy in C to cast one type to another type, relying on the fact that the layout of a C struct has certain guarantees. Libraries such as GLib rely on this to implement object-oriented like inheritance. Basically:

struct Base
{
  int x;
  int y;
};

struct Derived
{
  struct Base b;
  int z;
};

This enables a Base* pointer to be assigned to the address of a Derived object.

But I'm also aware of the "strict aliasing" rule, which is the implicit assumption by the compiler that different-type pointers can't point to the same address. (This enables the compiler to perform certain optimizations.)

So, how are these two things reconciled? Many C libraries, include Glib, CPython, etc., use the above strategy to cast between types. Are they all simply compiling with flags like no-strict-aliasing?

Answer 1

There's no violation of strict aliasing in this case. struct Derived contains a struct Base. This sort of behaviour is explicitly allowed by the language standard. From C11 6.7.2.1 Structure and union specifiers, paragraph 15:

A pointer to a structure object, suitably converted, points to its initial member (or if that member is a bit-field, then to the unit in which it resides), and vice versa.