third_party/boost/asio/doc/overview/basics.qbk

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 / Copyright (c) 2003-2021 Christopher M. Kohlhoff (chris at kohlhoff dot com)
 /
 / Distributed under the Boost Software License, Version 1.0. (See accompanying
 / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 /]

[section:basics Basic Boost.Asio Anatomy]

Boost.Asio may be used to perform both synchronous and asynchronous operations on I/O
objects such as sockets. Before using Boost.Asio it may be useful to get a conceptual
picture of the various parts of Boost.Asio, your program, and how they work together.

As an introductory example, let's consider what happens when you perform a
connect operation on a socket. We shall start by examining synchronous
operations.

[$boost_asio/sync_op.png]

[*Your program] will have at least one [*I/O execution context], such as an
`boost::asio::io_context` object, `boost::asio::thread_pool` object, or
`boost::asio::system_context`. This [*I/O execution context] represents [*your
program]'s link to the [*operating system]'s I/O services.

  boost::asio::io_context io_context;

To perform I/O operations [*your program] will need an [*I/O object] such as a
TCP socket:

  boost::asio::ip::tcp::socket socket(io_context);

When a synchronous connect operation is performed, the following sequence of
events occurs:

1. [*Your program] initiates the connect operation by calling the [*I/O
object]:

  socket.connect(server_endpoint);

2. The [*I/O object] forwards the request to the [*I/O execution context].

3. The [*I/O execution context] calls on the [*operating system] to perform the
connect operation.

4. The [*operating system] returns the result of the operation to the
[*I/O execution context].

5. The [*I/O execution context] translates any error resulting from the
operation into an object of type `boost::system::error_code`. An `error_code` may be
compared with specific values, or tested as a boolean (where a `false` result
means that no error occurred). The result is then forwarded back up to the
[*I/O object].

6. The [*I/O object] throws an exception of type `boost::system::system_error` if the
operation failed. If the code to initiate the operation had instead been
written as:

  boost::system::error_code ec;
  socket.connect(server_endpoint, ec);

then the `error_code` variable `ec` would be set to the result of the
operation, and no exception would be thrown.

When an asynchronous operation is used, a different sequence of events occurs.

[$boost_asio/async_op1.png]

1. [*Your program] initiates the connect operation by calling the [*I/O
object]:

  socket.async_connect(server_endpoint, your_completion_handler);

where `your_completion_handler` is a function or function object with the
signature:

  void your_completion_handler(const boost::system::error_code& ec);

The exact signature required depends on the asynchronous operation being
performed. The reference documentation indicates the appropriate form for each
operation.

2. The [*I/O object] forwards the request to the [*I/O execution context].

3. The [*I/O execution context] signals to the [*operating system] that it
should start an asynchronous connect.

Time passes. (In the synchronous case this wait would have been contained
entirely within the duration of the connect operation.)

[$boost_asio/async_op2.png]

4. The [*operating system] indicates that the connect operation has completed
by placing the result on a queue, ready to be picked up by the [*I/O execution
context].

5. When using an `io_context` as the [*I/O execution context], [*your program]
must make a call to `io_context::run()` (or to one of the similar `io_context`
member functions) in order for the result to be retrieved. A call to
`io_context::run()` blocks while there are unfinished asynchronous operations,
so you would typically call it as soon as you have started your first
asynchronous operation.

6. While inside the call to `io_context::run()`, the [*I/O execution context]
dequeues the result of the operation, translates it into an `error_code`, and
then passes it to [*your completion handler].

This is a simplified picture of how Boost.Asio operates. You will want to delve
further into the documentation if your needs are more advanced, such as
extending Boost.Asio to perform other types of asynchronous operations.

[endsect]