README.md

State modifications in a view function

StarkNet provides the @view decorator to signal that a function should not make state modifications. However, this is not currently enforced by the compiler. Developers should take care when designing view functions but also when calling functions in other contracts as they may result in unexpected behavior if they do include state modifications accidentally.

Example

Consider the following function that's declared as a @view. It may have originally been intended as an actual view function but was later repurposed to fetch a nonce and also increment it in the process to ensure a nonce is never repeated when building a signature.

@view
func bad_get_nonce{syscall_ptr: felt*, pedersen_ptr: HashBuiltin*, range_check_ptr}() -> (
    nonce: felt
) {
    let (user) = get_caller_address();
    let (nonce) = user_nonces.read(user);
    user_nonces.write(user, nonce + 1);

    return (nonce);
}

Mitigations

• Carefully review all @view functions, including those called in 3rd party contracts, to ensure they don't modify state unexpectedly.

External Examples