Waitable

Waitable is a term used in computer science to describe an object whose state can be waited on by one or more threads or tasks until it becomes signaled, ready, or completed. In practice, a waitable provides a mechanism to suspend execution and resume when a corresponding event occurs or a result becomes available. The concept spans both low-level operating system primitives and higher-level asynchronous abstractions.

In operating systems and threading libraries, waitables often refer to synchronization primitives that can be waited on. Examples include events, mutexes, semaphores, and timers in Windows and other OSes. These objects are designed so that a thread can block until the object signals, or until a timeout occurs, or until multiple waitable objects are considered. On some platforms, multiple waitable objects can be waited on simultaneously.

In asynchronous programming, waitables appear as futures, promises, tasks, or similar constructs that expose an interface for waiting or awaiting results. For instance, a future or task may be awaited to obtain its computed value, while a promise represents a value that will be produced later. Languages and frameworks differ in syntax, but the underlying idea is the same: enable a caller to suspend until the asynchronous operation completes.

Common usage patterns include blocking waits, timed waits, or non-blocking checks with callbacks or continuation. Important considerations when using waitables include potential for deadlocks or starvation, the need for timeouts and cancellation, and ensuring proper synchronization when multiple waiters are involved. See also: synchronization primitive, futures and promises, asynchronous programming, and event-driven design.