Operational semantics: Concepts and their expression

We present a new approach to providing a structural operational semantics for imperative programming languages with concurrency and procedures. The approach is novel because we expose the building block operations—variable assignment and condition checking—in the labels on the transitions; these form the context-dependent behaviour of a program. Using this style results in two main advantages over standard formalisms for imperative programming language semantics: firstly, our individual transition rules are more concise, and secondly, we are able to more abstractly and intuitively describe the semantics of procedures, including by-value and by-reference parameters. Standard techniques in the literature tend to result in complex and hard-to-read rules for even simple language constructs when procedures and parameters are dealt with. Our semantics for procedures utilises the context-dependent behaviour in the transition label to handle variable name scoping, and defines the semantics of recursion without requiring additional rules. In contrast with Plotkin’s seminal structural operational semantics paper, we do not use locations to describe some of the more complex language constructs. Novel aspects of the abstract syntax include local states (in contrast to a single global store), which simplifies the reasoning about local variables, and a command for dynamically renaming variables (in contrast to mapping variables to locations), which simplifies the reasoning about the effect of procedures on by-reference parameters.

The aim of this paper is to make a contribution to (compositional) development methods for concurrent programs. In particular, it takes a fresh look at a number of familiar ideas including the problem of interference. Some subtle issues of observability–including granularity–are explored. Based on these points, the paper sets out some requirements for an approach to developing systems by “splitting atoms safely”.

The intention of this paper is to make a contribution to (compositional) development methods for concurrent programs. The topics touched on include interference, atomicity, observability and granularity. The paper sets out some requirements for an approach to developing systems by “splitting atoms safely”.