A synchronization mechanism used to control access to a shared resource by [Thread]s. A synchronization semaphore which can be used to synchronize multiple [Thread]s. Initialized to zero on creation. Be careful to avoid deadlocks. For a binary version, see [Mutex]. $DOCS_URL/tutorials/performance/using_multiple_threads.html $DOCS_URL/tutorials/performance/thread_safe_apis.html Lowers the [Semaphore], allowing one more thread in. Like [method wait], but won't block, so if the value is zero, fails immediately and returns [code]false[/code]. If non-zero, it returns [code]true[/code] to report success. Waits for the [Semaphore], if its value is zero, blocks until non-zero.