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# [Frama-c-discuss] Specification Examples

```Hi,

First, just to remind everbody of the example we're talking about:

/*@ predicate disjoint_arrays(int *a, int *b, integer i) =
@   \forall integer k1, k2;
@      0 <= k1 < i && 0 <= k2 < i ==> a + k1 != b + k2;
@*/

/*@ requires 0 < n;
@ requires \valid_range(a, 0, n-1) && \valid_range(b, 0, n-1);
@ requires disjoint_arrays(a, b, n);
@ ensures  \forall int k; 0 <= k < n ==> a[k] == b[k];
@*/
void array_cpy(int* a, int n, int* b)
{
/*@ loop invariant 0 <= i <= n;
@ loop invariant \forall int m; 0 <= m < i  ==> a[m] == b[m];
@*/
for (int i = 0; i < n; i++)
a[i] = b[i];
}

> But if I am using the Pre Label I don't meet the ensures requirements, or
> at least I don't get a green bullet.
>

as I told you, the normal way of treating separation in Jessie is to divide
heap into regions automatically, which will become the default in the next
release. With only one region for memory, your example is proved with any of
Z3, Alt-Ergo and Simplify (not Yices though).

To prove instead that the value of [a] after copying is the same as the
value of [b] before, you need to strengthen the loop invariant like follows,
to assess that all of [b] does not change during the loop.

/*@ requires 0 < n;
@ requires \valid_range(a, 0, n-1) && \valid_range(b, 0, n-1);
@ requires disjoint_arrays(a, b, n);
@ ensures  \forall int k; 0 <= k < n ==> a[k] == \at(b[k],Pre);
@*/
void array_cpy(int* a, int n, int* b)
{
/*@ loop invariant 0 <= i <= n;
@ loop invariant \forall int m; 0 <= m < n  ==> b[m] == \at(b[m],Pre);
@ loop invariant \forall int m; 0 <= m < i  ==> a[m] == b[m];
@*/
for (int i = 0; i < n; i++)
a[i] = b[i];
}

With these annotations, Z3, ALt-Ergo and Simplify still prove your function
(not Yices).

>
> Currently I'm stuck with an other Problem. Within some annotated string.h
> examples I found a dereferenced \result.
>
> Attempts to apply it to my example will result in the following error.
>
> Fatal error: exception Assert_failure("src/jessie/interp.ml", 336, 14)
>
> do I get it due to my Hydrogen version ?
>

Most probably, yes. There was such a problem with derefencing \result and it
was solved. The best way to track this kind of problem is to fill in a bug
report on the Frama-C public BTS, so that you get a precise answer when the
bug gets fixed.

HTH.

--
Yannick
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