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[Frama-c-discuss] green postcondition in spite of invalid loop invariant

  • Subject: [Frama-c-discuss] green postcondition in spite of invalid loop invariant
  • From: Christoph.Weber at first.fraunhofer.de (Christoph Weber)
  • Date: Wed, 17 Dec 2008 14:03:25 +0100
Hello,

I have following annotated algorithm:

/*@
requires 0 <= length;
requires \valid_range (a, 0, length-1);
requires \valid_range (dest, 0, length-1);
requires disjoint_arrays(a, dest, length);
ensures \result == length - nb_occ{Old}(a, 0, length, value);
ensures \forall integer k; 0 <= k < \result ==> dest[k] != value;

*/
int remove_copy_array (int* a, int length, int* dest, int value )
{
int index_a = 0;
int index_dest = 0;
/*@
loop invariant 0 <= index_a <= length;
loop invariant index_dest <= index_a;
loop invariant 0 <= index_dest <= length;

loop invariant \forall integer k; 0 <= k < index_dest ==> dest[k] != value;
loop invariant index_dest == index_a - nb_occ{Pre}(a, 0, index_a, value);

*/
for ( ; index_a != length; ++index_a)
if (a[index_a] != value)
{
dest[index_dest] = a[index_a];
++index_dest;
}
return index_dest;

}

The thing I am curious about is, that I get a green bullet for

ensures \result == length - nb_occ{Old}(a, 0, length, value);

while the related loop invariant gets stuck. 
Now I wonder why is that

Cheers 
Christoph
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