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# [Frama-c-discuss] Jessie - problem with an example

• Subject: [Frama-c-discuss] Jessie - problem with an example
• From: kerstin.hartig at first.fraunhofer.de (Kerstin Hartig)
• Date: Wed, 9 Sep 2009 10:31:23 +0200

```Hello,

I've got a question regarding a specific example I am working on.
Below you can see a simplified part of it, because it's part of a more
complex scene.
It should actually only show that data->b <= data->a and that both are
always positive.
Running it in Jessie (still in Lithium version, int-model exact) alt-ergo
can't prove that data->b is always positive.
Does maybe anyone have an idea why it is not possible to prove that? Because
I don't see a way for b to increase negatively.

Your help would be very much appreciated,

Kerstin

-------------------------------
struct MyData
{
int a;
int b;
};

/*@ predicate is_positive(integer a) = (a >= 0);
*/
/*@ ensures is_positive(\result);
*/
int something() ;

/*@ requires \valid(data);
requires is_positive(data->a);
requires is_positive(data->b);
requires data->b <= data->a;
behavior no:
assumes data->a > data->b + 10;
assigns \nothing;
behavior yes:
assumes data->a <= data->b + 10;
assigns data->b;
ensures data->b <= data->a;
*/
void refresh(struct MyData* data)
{
if (data->a <= data->b + 10)
{
data->b = data->a;
}
}

/*@ requires \valid(data);
requires is_positive(data->a);
requires is_positive(data->b);
requires data->a >= data->b;
assigns data->a;
*/
void refresh_loop(struct MyData* data)
{
int x = 0;
/*@ loop invariant \valid(data);
loop invariant is_positive(data->a);
loop invariant is_positive(data->b);
loop invariant data->a >= data->b;
*/
while(1)
{
x = something();
//@ assert is_positive(x);
data->a = data->a + x;   // increased by just any positive number
//@ assert is_positive(data->a);
//@ assert is_positive(data->b);
//@ assert data->a >= data->b;
refresh(data);
//@ assert is_positive(data->a);
//@ assert data->a >= data->b;
}
}

```