Understanding Synchronous, Asynchronous, Blocking, and Non-Blocking I/O

Many articles discuss sync/async/blocking/non‑blocking but often miss the core definitions; this piece starts by correcting the common misconception that a request is called synchronous merely because the code waits for a response, emphasizing that the waiting is a consequence of the synchronous nature of the call.

Synchronization means that multiple tasks cannot proceed simultaneously; they must execute one after another, often waiting for previous results or resources, similar to a queue where each person is served in order.

Asynchrony, by contrast, allows multiple tasks to progress independently without waiting for each other, akin to several cars moving on different lanes at the same time.

Blocking refers to a state where progress is halted because an obstacle (e.g., a resource or condition) prevents movement, while non‑blocking means the operation can continue because no such obstacle exists.

The article combines these dimensions into four possible states: synchronous‑blocking, synchronous‑non‑blocking, asynchronous‑blocking, and asynchronous‑non‑blocking, illustrating each with traffic‑flow analogies and mapping them to thread behavior.

In the context of I/O, the process consists of two phases: waiting for data (the I/O wait) and copying data between kernel and user space. Blocking I/O occurs when a user thread is involved in either phase, causing it to wait; non‑blocking I/O lets the thread continue because the kernel notifies it only after data is ready.

According to the earlier definitions, synchronous I/O always blocks because execution cannot continue until data is received, while asynchronous I/O never blocks because the thread proceeds while the kernel handles the wait and copy phases.

The article concludes that synchronous I/O is equivalent to synchronous‑blocking I/O, and that asynchronous I/O can be either asynchronous‑blocking (thread waits during copy) or asynchronous‑non‑blocking (thread never waits), emphasizing the mutual exclusivity of sync and non‑blocking concepts.