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;

score 1 · Accepted Answer · answered Sep 27 '22 at 21:42

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).

Notice that WP provides some facilities to detect such a problem. In particular, the option `-wp-smoke-tests` detects dead code. On your example, if we remove that assertion, we get: `[Failed] (Doomed) typed_foo_wp_smoke_dead_code_s7 (Qed) [wp] a.c:11: Warning: Failed smoke-test`. (If the assertion is kept this is not the case, the assertin is interpreted by WP as "I now that this code is dead, please do not warn". — Ksass'Peuk, Sep 28 '22 at 07:17

How to assert that a point is unreachable?

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