Verifying leftPad() in Whiley

The leftPad(string,int) function simply pads a string up to a given size by inserted spaces at the beginning. For example, leftPad(“hello”,8) produces ” hello”. This little function shot to fame in 2016 when a developer pulled all his modules from NPM, of which one provided the leftPad() functionality. There were two basic issues causing . . . → Read More: Verifying leftPad() in Whiley

Program Specification in Practice?

Recently, as part of our Programming Languages Reading group, we looked at the paper “Contracts in Practice” by Estler et al., (see here for a copy).  This is quite an interesting paper and the authors perform an empirical investigation as to how contracts are used by programmers in practice.  They dig out some data . . . → Read More: Program Specification in Practice?

Encoding C Strings in Whiley

In this post, we’re going to consider representing the classic C string in Whiley. This turns out to be useful as we can then try to verify properties about functions which operate on C strings (e.g. strlen(), strcpy(), etc). If you’re not familiar with C strings, then the main points are:

Roughly speaking, C . . . → Read More: Encoding C Strings in Whiley

Verifying Software with Whiley

A couple of weeks back, I gave a presentation to the Wellington Java User Group. The talk provides a useful introduction to verifying software in Whiley, and shows a bunch of interesting examples. Anyway, you can see the presentation here:

Wellington Java User Group Presentation from John Hurst on Vimeo.

. . . → Read More: Verifying Software with Whiley

Introductory Lecture on Verification in Whiley

We’ve started using Whiley again in my second year course Formal Foundations of Programming. The aim of this course is to introduce students into a range of techniques related to software correctness. So far, we’ve looked at some static analysis and model checking tools.

Anyway, last week I gave an introductory lecture on writing . . . → Read More: Introductory Lecture on Verification in Whiley

Verification with Data from Untrusted Sources

Recently, I was listening to the latest edition of the Illegal Argument podcast, and it turns out they were discussing Whiley! (about 103:16 minutes in). The discussion was about how verification interacts with data from an untrusted source (e.g. from a database, network connection, etc). In particular, whether or not we can safely feed . . . → Read More: Verification with Data from Untrusted Sources

Loop Variant Relations

Proving that a loop always terminates is a common requirement when verifying software. The usual approach to doing this is to provide a loop variant function. This is typically an integer expression which decreases on every iteration of the loop. Consider the following loop:

function contains([int] items, int item) => bool: int i = . . . → Read More: Loop Variant Relations

Understanding Ghost Variables in Software Verification

Verification tools often support the use of ghost variables to help in the verification process. A ghost variable is not needed for the program to execute, and will not be compiled into object code. Typically, they are declared with a ghost modifier or similar, but are otherwise identical to normal program variables (i.e. they . . . → Read More: Understanding Ghost Variables in Software Verification

Loop Invariants and Do/While Statements

Recently, I encountered what I thought was a bug in the Whiley Compiler. The issue related to the current treatment of do/while loops and loop invariants. Having now spent a fair bit of time researching the issue, the answer is not so clear.

The problem manifested itself when I tried to verify the following . . . → Read More: Loop Invariants and Do/While Statements

Loop invariants and Break Statements

In this article, I’ll look at some interesting issues relating to the use of break statements within loops, and how this affects the idea of a loop invariant. For some general background on writing loop invariants in Whiley, see my previous post.  To recap the main points, here’s a simple function which requires a . . . → Read More: Loop invariants and Break Statements