[/
 / Copyright (c) 2003-2018 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 Asio Anatomy]

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

[$sync_op.png]

[*Your program] will have at least one [*io_context] object. The [*io_context]
represents [*your program]'s link to the [*operating system]'s I/O services.

  asio::io_context io_context;

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

  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 [*io_context].

3. The [*io_context] calls on the [*operating system] to perform the connect
operation.

4. The [*operating system] returns the result of the operation to the
[*io_context].

5. The [*io_context] translates any error resulting from the operation into an
object of type `asio::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 `asio::system_error` if the
operation failed. If the code to initiate the operation had instead been
written as:

  asio::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.

[$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 asio::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 [*io_context].

3. The [*io_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.)

[$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 [*io_context].

5. [*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 [*io_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 Asio operates. You will want to delve
further into the documentation if your needs are more advanced, such as
extending Asio to perform other types of asynchronous operations.

[endsect]
