FreeRTOS provides methods for multiple threads or tasks, mutexes, semaphores and software timers. Tasks are synchronized using mutexes or semaphores as in a conventional operating system. Various low-level mechanisms can be used to aid this, including locks, semaphores, mutexes and queues. Fetch-and-add can be used to implement concurrency control structures such as mutex locks and semaphores. Message queues, remote procedure calls, notifications, semaphores, and lock sets. Rodos supports typical features of realtime operatingsystems, like threads and semaphores. Hydraulic telegraph semaphores date back to the 4th century BCE Greece. Some semaphores would allow only one thread or process in the code section. Hoare demonstrated their equivalence to semaphores. Careful programming (often using mutual exclusion, semaphores, etc.) is required to prevent this. Conductors are informed by classical light semaphores. Likewise, telephone communication was problematic, with many positions using line-of-sight optical semaphores for communication. On one of these hills is one of those precious jobs, called "semaphores". The functions wait() and signal() are functions provided by the semaphores. The colour lights show the same light patterns the semaphores show at night.