Scope Logic with Local Reasoning and Pre/Post-State Properties

Abstract

This paper presents an extension to Hoare logic for pointer program verification. Logic formulas with user-defined recursive functions are used to specify properties on the program states before/after program executions. Three basic functions are introduced to represents memory access, record-field access and array-element access. Some axioms are introduced to specify these basic functions in our logic. The concept Memory Scope Function (MSF) is introduced in our logic. Given a recursive function $f$, the MSF of $f$ computes the set of memory units accessed during the evaluation of $f$. A set of rules are given to derive the definition of this MSF syntactically from the definition of $f$. As MSFs are also recursive functions, they also have their MSFs. An axiom is given to specify that an MSF contains its MSF. Based on this axiom, local reasoning is supported with predicate variables. Pre-state terms are used to specify the relations between pre-states and post-states. People can use pre-state terms in post-conditions to represents the values on the pre-state. The axiom of assignment statements in Hoare's logic is modified to deal with pointers. The basic idea is that during the program execution, a recursive function is evaluated to the same value as long as no memory unit in its memory scope is modified. Another proof rule is added for memory allocation statements. We use a simple example to show that our logic can deal with pointer programs in this paper. In the appendix, the Shorre-Waite algorithm is proved using our logic. We also use the selection-sort program to show that our logic can be used to prove program with indirectly-specified components.