15.2.9 Stopping and unstopping processes

This section describes a typical way of using process-stop and process-unstop.

Suppose a pool of "worker" processes is managed by a "manager" process. A process in the worker pool marks itself as available for work, and then calls process-stop. The manager process later finds a worker process that is marked as available for work, puts the work in a place known to the worker process, and then calls process-unstop on the worker process.

For this scheme to work properly, the check of whether the worker is available needs to include a call to process-stopped-p. Otherwise, it is possible for the following sequence of events to occur:

1. A worker marks itself as available.
2. The manager process finds the worker and gives it the work.
3. The manager process calls process-unstop on the worker.
4. The worker process proceeds and calls process-stop, and never wakes up.

To guard against this possibility, then the manager should call process-stopped-p when finding the worker in the second step above. Alternatively, it could check the result of process-unstop.

LispWorks User Guide and Reference Manual - 21 Dec 2011