What is asio::io_service?

Question

I am studying asynchronous I/O. i knows about select(), poll(), and epoll(). What I'm curious about is asynchronous I/O when using boost::asio. as a result of investigation, io_service is used. Which one of select(), poll(), and epoll?

Answer 1

asio::io_service is outdated.

In newer boost versions it is replaced by asio::io_context.

io_context (formly io_service) main purpose executing ansynchrounous operations for object in the asio library, for ex. socket, acceptor, etc, according documentation:

...Synchronous operations on I/O objects implicitly run the io_context object for an individual operation. The io_context functions run(), run_one(), run_for(), run_until(), poll() or poll_one() must be called for the io_context to perform asynchronous operations on behalf of a C++ program. Notification that an asynchronous operation has completed is delivered by invocation of the associated handler. Handlers are invoked only by a thread that is currently calling any overload of run(), run_one(), run_for(), run_until(), poll() or poll_one() for the io_context....

Bascially, you send the tasks to be executed to the io_context (async_... functions), and io_context is reponsible to execute those tasks ( probably on thread or thread pool) and call the provided callback (=handler).

It normally uses internally some version of task-stealing for efficient task distribution on threads in a thread pool.