Semaphores

A semaphore is a value that is used to synchronize MzScheme threads. Each semaphore has an internal counter; when this counter is zero, the semaphore can block a thread's execution (through semaphore-wait) until another thread increments the counter (using semaphore-post). The maximum value for a semaphore's internal counter is platform-specific, but always at least 10000. MzScheme's semaphores have the usual single-threaded access for reliable synchronization.

• (make-semaphore [init-k]) creates and returns a new semaphore with the counter initially set to init-k, which defaults to 0. If init-k is larger than a semaphore's maximum internal counter value, the exn:application:mismatch exception is raised.
• (semaphore? v) returns #t if v is a semaphore created by make-semaphore, #f otherwise.
• (semaphore-post sema) increments the semaphore's internal counter and returns void. If the semaphore's internal counter has already reached its maximum value, the exn:misc exception is raised.
• (semaphore-wait sema) blocks until the internal counter for semaphore sema is non-zero. When the counter is non-zero, it is decremented and semaphore-wait returns void.
• (semaphore-try-wait? sema) is like semaphore-wait, but semaphore-try-wait? never blocks execution. If sema's internal counter is zero, semaphore-try-wait? returns #f immediately without decrementing the counter. If sema's counter is positive, it is decremented and #t is returned.
• (semaphore-wait/enable-break sema) is like semaphore-wait, but breaking is enabled (see section 8.6) while waiting on sema. If breaking is disabled when semaphore-wait/enable-break is called, then either the semaphore's counter is decremented or the exn:misc:user-break exception is raised, but not both.