Scheme 48 Manual | Contents | In Chapter: Threads
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Condition variables

Condition variables (defined in the condvars structure) allow threads perform condition synchronization: It allows threads to block, waiting for a specified condition--associated with a condition variable--to occur, and other threads to wake up the waiting threads when the condition is fulfilled.

Note that, in Scheme 48, condition variables work in conjunction with proposals, not with mutex locks or semaphores, as in most other implementations of this concept.

Make-condvar creates a condition variable. (The optional id argument is only for debugging purposes; the discloser for condition variables prints it out if present.) Condvar? is the predicate for condition variables.

Each condition variable has an associated value and a flag has-value? signalling if the condition has already occured. The accessor for flag is condvar-has-value?; set-condvar-has-value?! sets it. Both are provisional operations and go through the current proposal. Set-condvar-value! sets the value of the condition variable (provisionally), and condvar-value extracts it.

Maybe-commit-and-wait-for-condvar attempts to commit the current proposal. If the commit succeeds, it suspends the current thread and registers it with the condvar condition variable. Upon waking up again maybe-commit-and-wait-for-condvar returns #t, If the commit fails, maybe-commit-and-set-condvar returns #f.

Maybe-commit-and-set-condvar! sets the value of the condvar condition variable to value, (provisionally) sets the has-value? flag to #t, and then attempt to commit the current proposal. Upon success, it wakes up all suspended threads registered with condvar and returns #t, otherwise, it returns #f.

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