Integrating User-Level Threads with Processes in Scsh

Scsh, the Scheme shell, enables concurrent system programming with portable user-level threads. In scsh, threads behave like processes in many ways: each thread receives its own set of process resources; like Unix processes, new threads can inherit resources from the creating thread. The combination of scsh's interface to the POSIX API with user-level threads creates a number of design and implementation challenges: Scsh's abstractions for managing process resources raise interesting modularity issues, particularly in connection with first-class continuations. Scsh also provides an interface to the fork system call; its implementation must avoid common pitfalls that arise with a user-level thread system. This paper describes the design and implementation of the relevant abstractions and discusses the implications for programming-language and system design.