Re: [boost] Access to unaligned (packed) structs

From: Sebastian Redl (sebastian.redl_at_[hidden])
Date: 2007-10-11 17:46:44

Roland Schwarz wrote:
> Altough I am not terribly skilled in standards interpretation I think 
> the following should be safe:
>
> union { mem[]; foo s; };
>
> Now I should be able to access this as mem[0], mem[1], ... from the
> writing side, e.g. from a function that fills my buffer, and later
> access it with s.a, s.b.
>   
Absolutely not. The only circumstance in which you may access a member
of a union that is not the one that has been most recently set is when
you access common, structurally conformant members of structs, i.e.

struct foo
{
  int i;
  float f;
};

struct bar
{
  int one;
  float two;
};

union foobar
{
  foo f;
  bar b;
};

foobar fb;
fb.f.i = 0;
fb.f.f = 1.1;

cout << fb.b.one; // Yep, valid.

The moment the types are not exactly equal (and two consecutive chars
are not the same as a char[2]) you're no longer allowed to assume anything.

Sebastian Redl