How to assert that a point is unreachable?

Question

For Frama-C and the WP plugin, how can a user assert that a point in a program is unreachable?

Looking for something like:

//@ assert \unreachable;

Answer 1

You can use //@ assert \false; for an assertion that a point is unreachable because:

a (provably) unreachable assertion is always proved to be true

via Introduction to C program proof with Frama-C and its WP plugin by Allan Blanchard.

e.g. For a given file main.c:

/*@
  assigns \nothing;
*/
int foo(const int input) {
    int result = 0;

    if (input > 0) {
        result += 1;

        if (input < 0) {
            // This assertion will be proved successfully
            // because this location is provably unreachable.
            //@ assert \false;

            result += 1;
        }
    }

    return result;
}

To verify that the assertion is proved:

$ frama-c -wp main.c
[kernel] Parsing main.c (with preprocessing)
[wp] Warning: Missing RTE guards
[wp] 3 goals scheduled
[wp] Proved goals:    3 / 3
  Qed:             3

The above is using frama-c version 25.0 (Manganese).