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aliasing between internal zero-length-arrays and other members


GCC silently (without -Wpedantic) accepts declarations of zero
length arrays that are followed by other members in the same
struct, such as in:

  struct A { char a, b[0], c; };

Is it intended that accesses to elements of such arrays that
alias other members be well-defined?

In my tests, GCC assumes that neither read nor write accesses
to elements of internal zero-length arrays alias other members,
so assuming those aren't bugs I wonder if the documentation
should be updated to make that clear and a warning added for
such declarations (and perhaps also accesses).

For example, the test in the following function is eliminated,
implying that GCC assumes that the access to p->b does not modify
p->c, even though with i set to 0 it would:

  void f (struct A *p, int i)
  {
    int x = p->c;
    p->b[i] = 1;
    if (x != p->c)
      __builtin_abort ();
  }

Martin


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