Cigarette Smokers Problem - Argument

Argument

Patil's argument was that Edsger Dijkstra's semaphore primitives were limited. He used the cigarette smokers problem to illustrate this point by saying that it cannot be solved with semaphores. However, Patil placed heavy constraints on his argument:

  1. The agent code is not modifiable.
  2. The solution is not allowed to use conditional statements or an array of semaphores.

With these two constraints, a solution to the cigarette smokers problem is impossible.

The first restriction makes sense, as Downey says in The Little Book of Semaphores, because if the agent represents an operating system, it would be unreasonable or impossible to modify it every time a new application came along. However, as David Parnas points out, the second restriction makes almost any nontrivial problem impossible to solve:

It is important, however, that such an investigation not investigate the power of these primitives under artificial restrictions. By artificial we mean restrictions which cannot be justified by practical considerations. In this author's opinion, restrictions prohibiting either conditionals or semaphore arrays are artificial. On the other hand, prohibition of "busy waiting" is quite realistic.

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