When Does `boost::asio::io_service::run()` Block?

When Does boost::asio::io_service run Blocking/Unblocking

The blocking behavior of io_service::run() stems from its handling of work and handlers. It remains blocked until:

• All work has been completed
• There are no more handlers left to execute
• The io_service has been stopped via stop()

Deciphering the Phrase "No More Handlers to Be Dispatched"

When io_service::run() blocks until "no more handlers to be dispatched," it signifies that there are no outstanding tasks in the form of registered callbacks.

Behavior in Example Code

Example 3a: Blocking

In the first example (3a), the io_service blocks within run() until all three posted Fibonacci calculation handlers have executed.

Detailed Execution Flow:

1. io_service::work is created to prevent immediate return from run().
2. Worker threads are launched to run the io_service loop.
3. Handlers for Fibonacci calculations are posted.
4. io_service::work is reset, allowing run() to exit.
5. The worker threads join, indicating task completion.

Client Code: Asynchronous but Effectively Synchronous

The client code appears to use asynchronous I/O (async_receive), but it effectively behaves synchronously:

Execution Flow:

1. async_receive() initiates an asynchronous operation.
2. io_service::run() blocks until the operation completes or an error occurs.
3. Control returns to the calling thread, and the handler is executed.

Synchronous Counterpart:

The equivalent synchronous operation would be:

boost::asio::error_code error;
std::size_t bytes = socket.receive(buffer, error);
// Execute handler with error and bytes

General Guideline: Separate Synchronous and Asynchronous Operations

Mixing synchronous and asynchronous operations can introduce complexity. Consider avoiding this practice. The Boost.Asio documentation offers valuable insights on the benefits of asynchronous programming.

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