[MPlayer-dev-eng] 1-element hack for flexible arrays (was: [PATCH] Drop support for gcc-2.95)

[Uoti Urpala]

On Wed, 2006-07-12 at 19:44 -0400, Rich Felker wrote:
> The pointer could just as easily have come from:
> 
> struct blah *foo = malloc(size);
> int *bar = (int *)((char *)foo + offsetof(struct blah, a);
> 
> This pointer "bar" will have the SAME value as foo->a, where
> "int a[1];" is the last element of struct blah. Since both
> expressions bar[1] and foo->a[1] are the same, how do you justify your
> claim that the latter is invalid when the former is clearly valid?

In the latter case the compiler knows the array has size 1. Even if bar
and foo->a would have the same value as pointers it doesn't mean the
compiler can't use any other information. If it knows that foo->a is an
array of size N, then it can assume that in "foo->a[n]" n < N. I don't
think the argument "even if the pointer addition goes outside the
current array object, the memory could be interpreted as a larger array
object, and thus the addition in the small array is not undefined but
must match the result in the larger one" is valid.