nowait

Nowait is a term used in computing to indicate that an operation should not block the caller. When NOWAIT is specified, a function returns immediately, either with a result or with an indication that the operation could not be completed at once. The concept is central to non-blocking I/O, asynchronous programming, and parallel execution.

Common implementations include NOWAIT or NO_WAIT flags (and related constants such as O_NONBLOCK). In POSIX-like systems, non-blocking I/O is enabled on file descriptors with the O_NONBLOCK flag set via open or fcntl. In networking, non-blocking sockets may use MSG_DONTWAIT with send or recv, or operate in a non-blocking mode; if no data is ready or no connection is available, the call returns with a specific error such as EAGAIN or EWOULDBLOCK rather than blocking.

Non-blocking operations often require additional logic to determine when the resource is ready, using mechanisms such as select, poll, epoll, or kqueue. They may also involve handling partial results and retrying or restructuring the workflow into an event-driven or state-machine design.

Nowait is also used in some libraries and APIs to label asynchronous operations, sometimes with an explicit wait or test step later to complete the operation. In parallel and distributed frameworks, non-blocking variants exist that begin an operation and require a subsequent synchronization call to finish.

Warnings include the potential for busy-waiting, partial data, or higher programming complexity. Proper error handling, readiness detection, and synchronization are essential when using non-blocking NOWAIT semantics.