Exception handling in a multicontext environment.

Abstract

This dissertation proposes a new mechanism for dealing with exceptional program conditions in an environment composed of multiple context objects. Mechanisms for dealing with exceptions--unusual situations occurring during the execution of programs--are widely known and traditionally applied to sequential programs located within a single context object. This dissertation extends the mechanism to concurrent and parallel programs in which exceptions are signalled from one context object to another; consequently, multiple exceptions may occur and must be resolved concurrently. The mechanism developed in this dissertation is embedded within the framework of the Ada programming language, although this work is applicable to any high-level programming language or system which supports the concepts of exceptions and concurrency at the language level. Ada's exception propagation mechanism is first modified to prevent the propagation of unexpected exceptions and then extended to allow exceptions to be propagated to tasks separate from the one in which the exception is raised. Subsequently, a new signal mechanism is added which augments Ada's exception h and ling termination model with a resumption model, and associates parameters with exceptions. Exceptions may be signalled to separate tasks, and it is shown how the signal mechanism supports the resume, retry, transfer, terminate, and propagate handler responses. The increase in conceptual power afforded by the new mechanism has a concomitant cost in added complexity. Several problems arising directly from this added complexity are identified and proposed solutions are given. The major part of the dissertation is concerned with the justification, development, and illustration of these solutions; it is composed of an investigation of the problems encountered by multi-context exception h and ling mechanisms, a survey and analysis of previous and existing mechanisms, the development and an analysis of the components of the new mechanism, and a series of examples illustrating the efficacy of the mechanism.