Nonblocking

Nonblocking is a term used in computing to describe operations or systems that can start a task and return control to the caller without forcing the caller to wait for the task to complete. A nonblocking operation may complete immediately, or it may complete later while the caller continues with other work, often signaling completion through events, callbacks, or futures.

Nonblocking I/O refers to input/output operations that return immediately rather than blocking the executing thread. This is achieved through nonblocking file descriptors, asynchronous I/O, or event-driven architectures. Common techniques include multiplexing systems with select, poll, or epoll, and handling results via callbacks, futures, or promises. Nonblocking I/O improves responsiveness and scalability, especially in servers handling many concurrent connections or high-latency resources.

In concurrent programming, nonblocking synchronization and algorithms avoid blocking on locks. They aim to allow progress for at least one thread without requiring others to wait. Categories include wait-free, lock-free, and obstruction-free algorithms, which rely on atomic primitives such as compare-and-swap. While nonblocking approaches can improve throughput and avoid deadlock, they often introduce greater design complexity and require careful reasoning about progress guarantees and potential livelock.

Nonblocking concepts also appear in hardware and networking. A nonblocking switch fabric, for example, is capable of routing any admissible set of input signals to outputs without internal blocking, given sufficient resources and proper scheduling. In networking, nonblocking paths refer to configurations that support requested bandwidth without contention under defined conditions.

Examples of nonblocking usage include Java NIO, Node.js’s event-driven model, Python’s asyncio, and Rust’s async/await, as well as system calls and flags that enable nonblocking behavior (such as O_NONBLOCK on Unix-like systems).