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).

• (make-semaphore i)   creates and returns a new semaphore with the counter initially set to i. The i argument must be a fixnum integer, but it is optional; i defaults to 0.
• (semaphore? v)   returns #t if v is a semaphore created by make-semaphore or #f otherwise.
• (semaphore-post sema)   increments the semaphore's internal counter and returns #<void>.
• (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-callback sema f)   can only be used within applications that have a top-level event loop instead of a simple read-eval-print loop. (MrEd is such an application.)   The semaphore-callback procedure installs the thunk f to be invoked when sema's counter is positive, decrementing the counter as f is invoked. The f procedure is invoked in the thread that handles events -- usually the main thread -- in a single-threaded manner. The procedure is invoked only once and then the callback is removed. The return value of semaphore-callback is #<void>.
• (input-port-post-semaphore port semaphore)     installs a poll on port that increments sema's counter when a character is ready to be read from port. The semaphore is only incremented once and then the poll is removed. The port is polled by periodically evaluating   (char-ready? port). Since it is possible to read characters from port without incurring a post to sema, the input-port-post-semaphore procedure is most useful when port is read only after a hit on sema. A call to input-port-post-semaphore returns immediately with #<void>.