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diff --git a/doc/html/boost_asio/overview/core/buffers.html b/doc/html/boost_asio/overview/core/buffers.html new file mode 100755 index 0000000000..64e4ff712a --- /dev/null +++ b/doc/html/boost_asio/overview/core/buffers.html @@ -0,0 +1,244 @@ +<html> +<head> +<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> +<title>Buffers</title> +<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css"> +<meta name="generator" content="DocBook XSL Stylesheets V1.76.1"> +<link rel="home" href="../../../boost_asio.html" title="Boost.Asio"> +<link rel="up" href="../core.html" title="Core Concepts and Functionality"> +<link rel="prev" href="strands.html" title="Strands: Use Threads Without Explicit Locking"> +<link rel="next" href="streams.html" title="Streams, Short Reads and Short Writes"> +</head> +<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> +<table cellpadding="2" width="100%"><tr> +<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td> +<td align="center"><a href="../../../../../index.html">Home</a></td> +<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td> +<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> +<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> +<td align="center"><a href="../../../../../more/index.htm">More</a></td> +</tr></table> +<hr> +<div class="spirit-nav"> +<a accesskey="p" href="strands.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../core.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../boost_asio.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="streams.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> +</div> +<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> +<h6> +<a name="boost_asio.overview.core.buffers.h0"></a> + <span><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" 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><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><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><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.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 © 2003-2012 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> +<hr> +<div class="spirit-nav"> +<a accesskey="p" href="strands.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../core.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../boost_asio.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="streams.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> +</div> +</body> +</html> |