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Appendix D: The ILU Common Lisp Lightweight Process System
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Appendix D The ILU Common Lisp Lightweight Process
System
D.1 Introduction
Although it is not required by the specification, most Common Lisp implementations include a facility
for multiple, independent threads of control (often called lightweight processes ) within a single Lisp
environment. Unfortunately, this facility is not standardized across the various implementations. Although
the capabilities provided are very similar across implementations, the details of lightweight processes and the
interface to them differ significantly. This situation makes it difficult to write programs that use lightweight
processes and yet are portable across Common Lisp implementations.
Common Lisp ILU does not requires lightweight processes in order to function, but they are useful. In
particular, servers typically make heavy use of lightweight process facilities. The purpose of the ILU CL
Process Interface is to provide a standardized, portable interface to lightweight processes for use within the
ILU environment. This interface isolates ILU users from the differences in the various lightweight process
implementations and allows them to write programs that are portable across all implementations to which
the ILU CL Process Interface has been ported. At present, these implementations include Franz Allegro CL
4.1, and Lucid Common Lisp 3.0 (a.k.a., Sun Common Lisp).
This chapter explains how the ILU CL Process Interface works for ILU users. It begins with an overview
that describes the ILU CL Process model, followed by a listing of some functional capabilities of this model.
After brief discussions of the implementation architecture and general limitations of the ILU CL Process
Interface, the chapter presents an example of how to use the interface to define a simple shared FIFO queue.
Next, it lists all of the functions and macros necessary to use lightweight processes in the ILU environment.
The chapter concludes with a brief list of references.
To use the information in this chapter, you should be familiar with Common Lisp and with the notion
of processes and threads in an operating system. Familiarity with the UNIX process model would also be
helpful. (See the References section for recommendations on further reading.)
D.2 Overview Of The ILU CL Process Model
The ILU CL Process Interface features an interface to lightweight processes similar to that on the
Symbolics Lisp machine. In particular, within a single Lisp environment (which on stock hardware runs as a
single heavyweight UNIX process) there are multiple threads of control that can be scheduled independently.
These threads are called lightweight processes (or sometimes just processes ). Each lightweight process
contains its own run-time control and binding stack, but it shares the global data and program address space
with all other processes in the Lisp environment. Note that this arrangement differs from that of the UNIX
heavyweight process facility, where each process has its own address space as well as its own run-time
stack.