0.0.0.4 1.4.4 Threads.

Dividing a computation into concurrent processes (or threads of control) is a fundamental method of separating concerns. For example, suppose you are programming a terminal emulator with a blinking cursor: the most satisfactory way to separate the cursor blinking code from the rest of the program is to make it a separate thread. Or suppose you are augmenting a program with a new module that communicates over a buffered channel. Without threads, the rest of the program will be blocked whenever the new module blocks on its buffer, and conversely, the new module will be unable to service the buffer whenever any other part of the program blocks. If this is unacceptable (as it almost always is) there is no way to add the new module without finding and modifying every statement of the program that might block. These modifications destroy the structure of the program by introducing undesirable dependencies between what would otherwise be independent modules.

The provisions for threads in Modula-2 are weak, amounting essentially to coroutines. Hoare's monitors [Hoare] are a sounder basis for concurrent programming. Monitors were used in Mesa, where they worked well; except that the requirement that a monitored data structure be an entire module was irksome. For example, it is often useful for a monitored data structure to be an object instead of a module. Mesa relaxed this requirement, made a slight change in the details of the semantics of Hoare's Signal primitive, and introduced the Broadcast primitive as a convenience [Lampson]. The Mesa primitives were simplified in the Modula-2+ design, and the result was successful enough to be incorporated with no substantial changes in Modula-3.

A threads package is a tool with a very sharp edge. A common programming error is to access a shared variable without obtaining the necessary lock. This introduces a race condition that can lie dormant throughout testing and strike after the program is shipped. Theoretical work on process algebra has raised hopes that the rendezvous model of concurrency may be safer than the shared memory model, but the experience with Ada, which adopted the rendezvous, lends at best equivocal support for this hope--Ada still allows shared variables, and apparently they are widely used.