Which statement best distinguishes synchronous I/O from asynchronous I/O and describes typical use cases?

• A. Synchronous I/O blocks a thread until completion; asynchronous I/O allows progress while I/O happens, typically via callbacks, futures, or completion channels.
• B. Synchronous I/O blocks the thread; asynchronous I/O blocks when callbacks are used.
• C. Synchronous I/O uses callbacks for completion; asynchronous I/O does not use callbacks.
• D. Synchronous I/O never blocks; asynchronous I/O blocks the thread.

Answer: (D)

Blocking behavior and how completion is delivered differentiate synchronous from asynchronous I/O. In synchronous I/O, the thread that starts the operation is blocked until the I/O completes, so the next line of code runs only after you have the data or the write has finished. This makes the flow straightforward and easy to reason about, but it can waste threads that are simply waiting for slow I/O, which isn’t ideal when you need high concurrency.

In asynchronous I/O, you start the operation and immediately regain control, doing other work while the I/O proceeds in the background. When the operation finishes, you’re notified through a callback, a future/promise, or a completion channel, allowing you to handle the result without blocking the initiating thread. This pattern is especially valuable for servers handling many simultaneous connections or UI applications that must stay responsive, since it lets you overlap work and avoid idle waiting.

The idea that synchronous I/O never blocks is incorrect, and the notion that asynchronous I/O would inherently block is also inaccurate. The strength of asynchronous I/O lies in not blocking the initiating thread and delivering completion later via a signaling mechanism, whereas synchronous I/O blocks until completion.