SHIM: A Deterministic Approach to Programming with Threads

Concurrent programming languages should be a good fit for embedded systems because they match the intrinsic parallelism of their architectures and environments. Unfortunately, most concurrent programming formalisms are prone to races and nondeterminism, despite the presence of mechanisms such as monitors. In this paper, we propose SHIM, the core of a concurrent language with disciplined shared variables that remains deterministic, meaning the behavior of a program is independent of the scheduling of concurrent operations. SHIM does not sacrifice power or flexibility to achieve this determinism. It supports both synchronous and asynchronous paradigms—loosely and tightly synchronized threads—the dynamic creation of threads and shared variables, recursive procedures, and exceptions. We illustrate our programming model with examples including breadth-first-search algorithms and pipelines. By construction, they are race-free. We provide the formal semantics of SHIM and a preliminary implementation.