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<div class="titlepage"><div><div><h4 class="title">
<a name="boost_asio.overview.core.buffers"></a><a class="link" href="buffers.html" title="Buffers">Buffers</a>
</h4></div></div></div>
<p>
Fundamentally, I/O involves the transfer of data to and from contiguous
regions of memory, called buffers. These buffers can be simply expressed
as a tuple consisting of a pointer and a size in bytes. However, to allow
the development of efficient network applications, Boost.Asio includes
support for scatter-gather operations. These operations involve one or
more buffers:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
A scatter-read receives data into multiple buffers.
</li>
<li class="listitem">
A gather-write transmits multiple buffers.
</li>
</ul></div>
<p>
Therefore we require an abstraction to represent a collection of buffers.
The approach used in Boost.Asio is to define a type (actually two types)
to represent a single buffer. These can be stored in a container, which
may be passed to the scatter-gather operations.
</p>
<p>
In addition to specifying buffers as a pointer and size in bytes, Boost.Asio
makes a distinction between modifiable memory (called mutable) and non-modifiable
memory (where the latter is created from the storage for a const-qualified
variable). These two types could therefore be defined as follows:
</p>
<pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">void</span><span class="special">*,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">></span> <span class="identifier">mutable_buffer</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">void</span><span class="special">*,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">></span> <span class="identifier">const_buffer</span><span class="special">;</span>
</pre>
<p>
Here, a mutable_buffer would be convertible to a const_buffer, but conversion
in the opposite direction is not valid.
</p>
<p>
However, Boost.Asio does not use the above definitions as-is, but instead
defines two classes: <code class="computeroutput"><span class="identifier">mutable_buffer</span></code>
and <code class="computeroutput"><span class="identifier">const_buffer</span></code>. The goal
of these is to provide an opaque representation of contiguous memory, where:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
Types behave as std::pair would in conversions. That is, a <code class="computeroutput"><span class="identifier">mutable_buffer</span></code> is convertible to
a <code class="computeroutput"><span class="identifier">const_buffer</span></code>, but
the opposite conversion is disallowed.
</li>
<li class="listitem">
There is protection against buffer overruns. Given a buffer instance,
a user can only create another buffer representing the same range of
memory or a sub-range of it. To provide further safety, the library
also includes mechanisms for automatically determining the size of
a buffer from an array, <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">array</span></code>
or <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span></code> of POD elements, or from a
<code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>.
</li>
<li class="listitem">
Type safety violations must be explicitly requested using the <code class="computeroutput"><span class="identifier">buffer_cast</span></code> function. In general
an application should never need to do this, but it is required by
the library implementation to pass the raw memory to the underlying
operating system functions.
</li>
</ul></div>
<p>
Finally, multiple buffers can be passed to scatter-gather operations (such
as <a class="link" href="../../reference/read.html" title="read">read()</a> or <a class="link" href="../../reference/write.html" title="write">write()</a>)
by putting the buffer objects into a container. The <code class="computeroutput"><span class="identifier">MutableBufferSequence</span></code>
and <code class="computeroutput"><span class="identifier">ConstBufferSequence</span></code>
concepts have been defined so that containers such as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span></code>,
<code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span></code>, <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span></code>
or <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">array</span></code> can be used.
</p>
<h6>
<a name="boost_asio.overview.core.buffers.h0"></a>
<span class="phrase"><a name="boost_asio.overview.core.buffers.streambuf_for_integration_with_iostreams"></a></span><a class="link" href="buffers.html#boost_asio.overview.core.buffers.streambuf_for_integration_with_iostreams">Streambuf
for Integration with Iostreams</a>
</h6>
<p>
The class <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">basic_streambuf</span></code> is derived from <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_streambuf</span></code> to associate the input
sequence and output sequence with one or more objects of some character
array type, whose elements store arbitrary values. These character array
objects are internal to the streambuf object, but direct access to the
array elements is provided to permit them to be used with I/O operations,
such as the send or receive operations of a socket:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
The input sequence of the streambuf is accessible via the <a class="link" href="../../reference/basic_streambuf/data.html" title="basic_streambuf::data">data()</a>
member function. The return type of this function meets the <code class="computeroutput"><span class="identifier">ConstBufferSequence</span></code> requirements.
</li>
<li class="listitem">
The output sequence of the streambuf is accessible via the <a class="link" href="../../reference/basic_streambuf/data.html" title="basic_streambuf::data">prepare()</a>
member function. The return type of this function meets the <code class="computeroutput"><span class="identifier">MutableBufferSequence</span></code> requirements.
</li>
<li class="listitem">
Data is transferred from the front of the output sequence to the back
of the input sequence by calling the <a class="link" href="../../reference/basic_streambuf/commit.html" title="basic_streambuf::commit">commit()</a>
member function.
</li>
<li class="listitem">
Data is removed from the front of the input sequence by calling the
<a class="link" href="../../reference/basic_streambuf/consume.html" title="basic_streambuf::consume">consume()</a>
member function.
</li>
</ul></div>
<p>
The streambuf constructor accepts a <code class="computeroutput"><span class="identifier">size_t</span></code>
argument specifying the maximum of the sum of the sizes of the input sequence
and output sequence. Any operation that would, if successful, grow the
internal data beyond this limit will throw a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">length_error</span></code>
exception.
</p>
<h6>
<a name="boost_asio.overview.core.buffers.h1"></a>
<span class="phrase"><a name="boost_asio.overview.core.buffers.bytewise_traversal_of_buffer_sequences"></a></span><a class="link" href="buffers.html#boost_asio.overview.core.buffers.bytewise_traversal_of_buffer_sequences">Bytewise
Traversal of Buffer Sequences</a>
</h6>
<p>
The <code class="computeroutput"><span class="identifier">buffers_iterator</span><span class="special"><></span></code>
class template allows buffer sequences (i.e. types meeting <code class="computeroutput"><span class="identifier">MutableBufferSequence</span></code> or <code class="computeroutput"><span class="identifier">ConstBufferSequence</span></code> requirements) to
be traversed as though they were a contiguous sequence of bytes. Helper
functions called buffers_begin() and buffers_end() are also provided, where
the buffers_iterator<> template parameter is automatically deduced.
</p>
<p>
As an example, to read a single line from a socket and into a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>, you may write:
</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">streambuf</span> <span class="identifier">sb</span><span class="special">;</span>
<span class="special">...</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">n</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">read_until</span><span class="special">(</span><span class="identifier">sock</span><span class="special">,</span> <span class="identifier">sb</span><span class="special">,</span> <span class="char">'\n'</span><span class="special">);</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">streambuf</span><span class="special">::</span><span class="identifier">const_buffers_type</span> <span class="identifier">bufs</span> <span class="special">=</span> <span class="identifier">sb</span><span class="special">.</span><span class="identifier">data</span><span class="special">();</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">line</span><span class="special">(</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffers_begin</span><span class="special">(</span><span class="identifier">bufs</span><span class="special">),</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffers_begin</span><span class="special">(</span><span class="identifier">bufs</span><span class="special">)</span> <span class="special">+</span> <span class="identifier">n</span><span class="special">);</span>
</pre>
<h6>
<a name="boost_asio.overview.core.buffers.h2"></a>
<span class="phrase"><a name="boost_asio.overview.core.buffers.buffer_debugging"></a></span><a class="link" href="buffers.html#boost_asio.overview.core.buffers.buffer_debugging">Buffer
Debugging</a>
</h6>
<p>
Some standard library implementations, such as the one that ships with
Microsoft Visual C++ 8.0 and later, provide a feature called iterator debugging.
What this means is that the validity of iterators is checked at runtime.
If a program tries to use an iterator that has been invalidated, an assertion
will be triggered. For example:
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">v</span><span class="special">(</span><span class="number">1</span><span class="special">)</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">iterator</span> <span class="identifier">i</span> <span class="special">=</span> <span class="identifier">v</span><span class="special">.</span><span class="identifier">begin</span><span class="special">();</span>
<span class="identifier">v</span><span class="special">.</span><span class="identifier">clear</span><span class="special">();</span> <span class="comment">// invalidates iterators</span>
<span class="special">*</span><span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="comment">// assertion!</span>
</pre>
<p>
Boost.Asio takes advantage of this feature to add buffer debugging. Consider
the following code:
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">dont_do_this</span><span class="special">()</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">msg</span> <span class="special">=</span> <span class="string">"Hello, world!"</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">async_write</span><span class="special">(</span><span class="identifier">sock</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffer</span><span class="special">(</span><span class="identifier">msg</span><span class="special">),</span> <span class="identifier">my_handler</span><span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
When you call an asynchronous read or write you need to ensure that the
buffers for the operation are valid until the completion handler is called.
In the above example, the buffer is the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>
variable <code class="computeroutput"><span class="identifier">msg</span></code>. This variable
is on the stack, and so it goes out of scope before the asynchronous operation
completes. If you're lucky then the application will crash, but random
failures are more likely.
</p>
<p>
When buffer debugging is enabled, Boost.Asio stores an iterator into the
string until the asynchronous operation completes, and then dereferences
it to check its validity. In the above example you would observe an assertion
failure just before Boost.Asio tries to call the completion handler.
</p>
<p>
This feature is automatically made available for Microsoft Visual Studio
8.0 or later and for GCC when <code class="computeroutput"><span class="identifier">_GLIBCXX_DEBUG</span></code>
is defined. There is a performance cost to this checking, so buffer debugging
is only enabled in debug builds. For other compilers it may be enabled
by defining <code class="computeroutput"><span class="identifier">BOOST_ASIO_ENABLE_BUFFER_DEBUGGING</span></code>.
It can also be explicitly disabled by defining <code class="computeroutput"><span class="identifier">BOOST_ASIO_DISABLE_BUFFER_DEBUGGING</span></code>.
</p>
<h6>
<a name="boost_asio.overview.core.buffers.h3"></a>
<span class="phrase"><a name="boost_asio.overview.core.buffers.see_also"></a></span><a class="link" href="buffers.html#boost_asio.overview.core.buffers.see_also">See
Also</a>
</h6>
<p>
<a class="link" href="../../reference/buffer.html" title="buffer">buffer</a>, <a class="link" href="../../reference/buffers_begin.html" title="buffers_begin">buffers_begin</a>,
<a class="link" href="../../reference/buffers_end.html" title="buffers_end">buffers_end</a>, <a class="link" href="../../reference/buffers_iterator.html" title="buffers_iterator">buffers_iterator</a>,
<a class="link" href="../../reference/const_buffer.html" title="const_buffer">const_buffer</a>,
<a class="link" href="../../reference/const_buffers_1.html" title="const_buffers_1">const_buffers_1</a>,
<a class="link" href="../../reference/mutable_buffer.html" title="mutable_buffer">mutable_buffer</a>,
<a class="link" href="../../reference/mutable_buffers_1.html" title="mutable_buffers_1">mutable_buffers_1</a>,
<a class="link" href="../../reference/streambuf.html" title="streambuf">streambuf</a>, <a class="link" href="../../reference/ConstBufferSequence.html" title="Constant buffer sequence requirements">ConstBufferSequence</a>,
<a class="link" href="../../reference/MutableBufferSequence.html" title="Mutable buffer sequence requirements">MutableBufferSequence</a>,
<a class="link" href="../../examples/cpp03_examples.html#boost_asio.examples.cpp03_examples.buffers">buffers example
(C++03)</a>, <a class="link" href="../../examples/cpp11_examples.html#boost_asio.examples.cpp11_examples.buffers">buffers
example (c++11)</a>.
</p>
</div>
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<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2003-2015 Christopher M.
Kohlhoff<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
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