Waiting for an answer
Imagine asking a question on a phone call and waiting for the other person to answer before doing anything else. The answer marks the point where you can continue. A text message works differently: you can send it, do something else, and read the reply later. section 11.7
A synchronous call keeps control until an I/O operation completes. It does not return early, so the next step must wait. That completion becomes a synchronization point: work that depends on the result can proceed afterward. section 11.8 section 11.7
Synchronous and blocking describe related but distinct ideas. Synchronous describes the application’s flow of control during I/O; blocking describes how an application asks the operating system to access a device. A blocking I/O operation also waits to return until it completes, but the terms describe different levels. section 11.8
The practical question is what happens while the caller waits:
- If tasks must exchange data in order, a synchronous exchange makes that wait explicit. Pipes are one way to support synchronous communication between producer and consumer processes. section 11.7 section 5.6
- If many long-running tasks can do other work, waiting for every exchange may leave resources idle. An asynchronous call returns immediately and makes its result available later; it is an alternative, not an automatic improvement for every application. section 12.2 section 13.11
