Operational semantics for signal handling

Signals are a lightweight form of interprocess communication in Unix. When a process receives a signal, the control flow is interrupted and a previously installed signal handler is run. Signal handling is reminiscent both of exception handling and concurrent interleaving of processes. In this paper, we investigate different approaches to formalizing signal handling in operational semantics, and compare them in a series of examples. We find the big-step style of operational semantics to be well suited to modelling signal handling. We integrate exception handling with our big-step semantics of signal handling, by adopting the exception convention as defined in the Definition of Standard ML. The semantics needs to capture the complex interactions between signal handling and exception handling.Comment: In Proceedings EXPRESS/SOS 2012, arXiv:1208.244

Compiling concurrency correctly: cutting out the middle man

The standard approach to proving compiler correctness for concurrent languages requires the use of multiple translations into an intermediate process calculus. We present a simpler approach that avoids the need for such an intermediate language, using a new method that allows us to directly establish a bisimulation between the source and target languages. We illustrate the technique on two small languages, using the Agda system to present and formally verify our compiler correctness proofs

Flag-based big-step semantics

Structural operational semantic specifications come in different styles: small-step and big-step. A problem with the big-step style is that specifying divergence and abrupt termination gives rise to annoying duplication. We present a novel approach to representing divergence and abrupt termination in big-step semantics using status flags. This avoids the duplication problem, and uses fewer rules and premises for representing divergence than previous approaches in the literature

Calculating Correct Compilers II: Return of the Register Machines

In ‘Calculating Correct Compilers’ (Bahr & Hutton, 2015), we developed a new approach to calculating compilers directly from specifications of their correctness. Our approach only required elementary reasoning techniques and has been used to calculate compilers for a wide range of language features and their combination. However, the methodology was focused on stack-based target machines, whereas real compilers often target register-based machines. In this article, we show how our approach can naturally be adapted to calculate compilers for register machines

Monadic compiler calculation (functional pearl)

Bahr and Hutton recently developed a new approach to calculating correct compilers directly from specifications of their correctness. However, the methodology only considers converging behaviour of the source language, which means that the compiler could potentially produce arbitrary, erroneous code for source programs that diverge. In this article, we show how the methodology can naturally be extended to support the calculation of compilers that address both convergent and divergent behaviour simultaneously , without the need for separate reasoning for each aspect. Our approach is based on the use of the partiality monad to make divergence explicit, together with the use of strong bisimilarity to support equational-style calculations, but also generalises to other forms of effect by changing the underlying monad

Denotational Semantics of Mobility in Unifying Theories of Programming (UTP)

UTP promotes the unification of programming theories and has been used successfully for giving denotational semantics to Imperative Programming, CSP process algebra, and the Circus family of programming languages, amongst others. In this thesis, we present an extension of UTP-CSP (the UTP semantics for CSP) with the concept of mobility. Mobility is concerned with the movement of an entity from one location (the source) to another (the target). We deal with two forms of mobility: • Channel mobility, concerned with the movement of links between processes, models networks with a dynamic topology; and • Strong process mobility, which requires to suspend a running process first, and then move both its code and its state upon suspension, and finally resume the process on the target upon reception. Concerning channel mobility: • We model channels as concrete entities in CSP, and show that it does not affect the underlying CSP semantics. • A requirement is that a process may not own a channel prior to receiving it. In CSP, the set of channels owned by a process (called its interface) is static by definition. We argue that making the interface variable introduces a paradox. We resolve this by introducing a new concept: the capability of a process, and show how it relates to the interface. We then define channel mobility as the operation that changes the interface of a process, but not its capability. We also provide a functional link between static CSP and its mobile version. Concerning strong mobility, we provide: • The first extension of CSP with jump features, using the concept of continuations. • A novel semantics for the generic interrupt (a parallel-based interrupt operator), using the concept of Bulk Synchronous Parallelism. We then define strong mobility as a specific interrupt operator in which the interrupt routine migrates the suspended program

Under consideration for the Journal of Functional Programming 1 What is the Meaning of These Constant Interruptions?

Asynchronous exceptions, or interrupts, are important for writing robust, modular programs, but are traditionally viewed as being difficult from a semantic perspective. In this article we present a simple, formally justified, semantics for interrupts. Our approach is to show how a high-level semantics for interrupts can be justified with respect to a low-level implementation, by means of a compiler and its correctness theorem. In this manner we obtain two different perspectives on the problem, formally shown to be equivalent, which gives greater confidence in the correctness of our semantics.