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| <div class="section"> |
| <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" 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" 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> |
| <a name="boost_asio.overview.core.buffers.streambuf_for_integration_with_iostreams"></a><h6> |
| <a name="id763050"></a> |
| <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" 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> |
| <a name="boost_asio.overview.core.buffers.bytewise_traversal_of_buffer_sequences"></a><h6> |
| <a name="id763236"></a> |
| <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> |
| <a name="boost_asio.overview.core.buffers.buffer_debugging"></a><h6> |
| <a name="id763588"></a> |
| <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> |
| <a name="boost_asio.overview.core.buffers.see_also"></a><h6> |
| <a name="id763988"></a> |
| <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.html#boost_asio.examples.buffers">buffers example</a>. |
| </p> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2003 - 2010 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> |
| </div></td> |
| </tr></table> |
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