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<div class="titlepage"><div><div><h4 class="title">
<a name="boost_asio.overview.core.basics"></a><a class="link" href="basics.html" title="Basic Boost.Asio Anatomy">Basic Boost.Asio Anatomy</a>
</h4></div></div></div>
<p>
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.
</p>
<p>
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.
</p>
<p>
<span class="inlinemediaobject"><img src="../../sync_op.png" alt="sync_op"></span>
</p>
<p>
<span class="bold"><strong>Your program</strong></span> will have at least one <span class="bold"><strong>io_service</strong></span> object. The <span class="bold"><strong>io_service</strong></span>
represents <span class="bold"><strong>your program</strong></span>'s link to the
<span class="bold"><strong>operating system</strong></span>'s I/O services.
</p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_service</span> <span class="identifier">io_service</span><span class="special">;</span>
</pre>
<p>
To perform I/O operations <span class="bold"><strong>your program</strong></span>
will need an <span class="bold"><strong>I/O object</strong></span> such as a TCP
socket:
</p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">ip</span><span class="special">::</span><span class="identifier">tcp</span><span class="special">::</span><span class="identifier">socket</span> <span class="identifier">socket</span><span class="special">(</span><span class="identifier">io_service</span><span class="special">);</span>
</pre>
<p>
When a synchronous connect operation is performed, the following sequence
of events occurs:
</p>
<p>
1. <span class="bold"><strong>Your program</strong></span> initiates the connect
operation by calling the <span class="bold"><strong>I/O object</strong></span>:
</p>
<pre class="programlisting"><span class="identifier">socket</span><span class="special">.</span><span class="identifier">connect</span><span class="special">(</span><span class="identifier">server_endpoint</span><span class="special">);</span>
</pre>
<p>
2. The <span class="bold"><strong>I/O object</strong></span> forwards the request
to the <span class="bold"><strong>io_service</strong></span>.
</p>
<p>
3. The <span class="bold"><strong>io_service</strong></span> calls on the <span class="bold"><strong>operating system</strong></span> to perform the connect operation.
</p>
<p>
4. The <span class="bold"><strong>operating system</strong></span> returns the result
of the operation to the <span class="bold"><strong>io_service</strong></span>.
</p>
<p>
5. The <span class="bold"><strong>io_service</strong></span> translates any error
resulting from the operation into an object of type <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span></code>.
An <code class="computeroutput"><span class="identifier">error_code</span></code> may be compared
with specific values, or tested as a boolean (where a <code class="computeroutput"><span class="keyword">false</span></code>
result means that no error occurred). The result is then forwarded back
up to the <span class="bold"><strong>I/O object</strong></span>.
</p>
<p>
6. The <span class="bold"><strong>I/O object</strong></span> throws an exception
of type <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">system_error</span></code> if the operation failed.
If the code to initiate the operation had instead been written as:
</p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">ec</span><span class="special">;</span>
<span class="identifier">socket</span><span class="special">.</span><span class="identifier">connect</span><span class="special">(</span><span class="identifier">server_endpoint</span><span class="special">,</span> <span class="identifier">ec</span><span class="special">);</span>
</pre>
<p>
then the <code class="computeroutput"><span class="identifier">error_code</span></code> variable
<code class="computeroutput"><span class="identifier">ec</span></code> would be set to the
result of the operation, and no exception would be thrown.
</p>
<p>
When an asynchronous operation is used, a different sequence of events
occurs.
</p>
<p>
<span class="inlinemediaobject"><img src="../../async_op1.png" alt="async_op1"></span>
</p>
<p>
1. <span class="bold"><strong>Your program</strong></span> initiates the connect
operation by calling the <span class="bold"><strong>I/O object</strong></span>:
</p>
<pre class="programlisting"><span class="identifier">socket</span><span class="special">.</span><span class="identifier">async_connect</span><span class="special">(</span><span class="identifier">server_endpoint</span><span class="special">,</span> <span class="identifier">your_completion_handler</span><span class="special">);</span>
</pre>
<p>
where <code class="computeroutput"><span class="identifier">your_completion_handler</span></code>
is a function or function object with the signature:
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">your_completion_handler</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&</span> <span class="identifier">ec</span><span class="special">);</span>
</pre>
<p>
The exact signature required depends on the asynchronous operation being
performed. The reference documentation indicates the appropriate form for
each operation.
</p>
<p>
2. The <span class="bold"><strong>I/O object</strong></span> forwards the request
to the <span class="bold"><strong>io_service</strong></span>.
</p>
<p>
3. The <span class="bold"><strong>io_service</strong></span> signals to the <span class="bold"><strong>operating system</strong></span> that it should start an asynchronous
connect.
</p>
<p>
Time passes. (In the synchronous case this wait would have been contained
entirely within the duration of the connect operation.)
</p>
<p>
<span class="inlinemediaobject"><img src="../../async_op2.png" alt="async_op2"></span>
</p>
<p>
4. The <span class="bold"><strong>operating system</strong></span> indicates that
the connect operation has completed by placing the result on a queue, ready
to be picked up by the <span class="bold"><strong>io_service</strong></span>.
</p>
<p>
5. <span class="bold"><strong>Your program</strong></span> must make a call to <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code>
(or to one of the similar <span class="bold"><strong>io_service</strong></span> member
functions) in order for the result to be retrieved. A call to <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code>
blocks while there are unfinished asynchronous operations, so you would
typically call it as soon as you have started your first asynchronous operation.
</p>
<p>
6. While inside the call to <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code>, the <span class="bold"><strong>io_service</strong></span>
dequeues the result of the operation, translates it into an <code class="computeroutput"><span class="identifier">error_code</span></code>, and then passes it to <span class="bold"><strong>your completion handler</strong></span>.
</p>
<p>
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.
</p>
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