When Does `boost::asio::io_service::run()` Block and Unblock in Asynchronous Operations?

Confused when boost::asio::io_service run method blocks/unblocks

In Boost.Asio, the io_service::run() method is responsible for processing work and invoking handlers associated with the io_service. When a thread calls run(), it will typically block until:

• All work and handlers have been completed
• The io_service is stopped (via stop())
• An exception is thrown from a handler

Handler: A handler is a callback function that is invoked when an asynchronous operation completes.

In Example 3a:
The io_service::run() method blocks the worker threads until the Fibonacci calculations (handlers) have been completed. The io_service::work object ensures that the io_service always has work to do, preventing the worker threads from exiting. When the work is complete, the threads join.

In the Question Code:
io_service::run() blocks until the read operation completes. As soon as the read handler is invoked, io_service::run() returns.

Differences and Key Points:

• Example 3a uses io_service::work to prevent io_service::run() from returning immediately due to lack of work.
• The Question Code does not use io_service::work.
• Both examples rely on asynchronous operations to perform the Fibonacci calculations.
• In the Question Code, io_service::run() can return before asynchronous operations are complete (due to the handler being invoked).

Synchronous vs. Asynchronous

While the Question Code uses asynchronous operations, it behaves effectively like synchronous code because io_service::run() waits for the asynchronous operation to finish. Mixing synchronous and asynchronous operations can lead to complexity.

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