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146 lines
5.2 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
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<title>Boost RNG Library - Non-Deterministic Random Number
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Generators</title>
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</head>
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<body bgcolor="#FFFFFF" text="#000000">
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<h1><img src="../../boost.png" alt="boost.png (6897 bytes)" align="middle"
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width="277" height="86">Header <a href=
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"../../boost/nondet_random.hpp"><boost/nondet_random.hpp></a></h1>
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<ul>
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<li><a href="#synopsis">Synopsis</a></li>
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<li><a href="#random_device">Class <code>random_device</code></a></li>
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<li><a href="#performance">Performance</a></li>
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</ul>
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<h2><a name="synopsis" id=
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"synopsis">Header</a><code><boost/nondet_random.hpp></code>
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Synopsis</h2>
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<pre>
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namespace boost {
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class random_device;
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} // namespace boost
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</pre>
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<h2><a name="random_device" id="random_device">Class
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<code>random_device</code></a></h2>
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<h3>Synopsis</h3>
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<pre>
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class random_device : noncopyable
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{
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public:
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typedef unsigned int result_type;
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static const bool has_fixed_range = true;
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static const result_type min_value = /* implementation defined */;
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static const result_type max_value = /* implementation defined */;
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result_type min() const;
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result_type max() const;
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explicit random_device(const std::string& token = default_token);
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~random_device();
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double entropy() const;
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unsigned int operator()();
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};
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</pre>
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<h3>Description</h3>
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<p>Class <code>random_device</code> models a <a href=
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"random-concepts.html#nondet-rng">non-deterministic random number
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generator</a>. It uses one or more implementation-defined stochastic
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processes to generate a sequence of uniformly distributed non-deterministic
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random numbers. For those environments where a non-deterministic random
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number generator is not available, class <code>random_device</code> must
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not be implemented. See</p>
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<blockquote>
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"Randomness Recommendations for Security", D. Eastlake, S. Crocker, J.
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Schiller, Network Working Group, RFC 1750, December 1994
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</blockquote>for further discussions.
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<p><em>Note:</em> Some operating systems abstract the computer hardware
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enough to make it difficult to non-intrusively monitor stochastic
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processes. However, several do provide a special device for exactly this
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purpose. It seems to be impossible to emulate the functionality using
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Standard C++ only, so users should be aware that this class may not be
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available on all platforms.</p>
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<h3>Members</h3>
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<pre>
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explicit random_device(const std::string& token = default_token)
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</pre><strong>Effects:</strong> Constructs a <code>random_device</code>,
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optionally using the given <code>token</code> as an access specification (for
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example, a URL) to some implementation-defined service for monitoring a
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stochastic process.
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<pre>
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double entropy() const
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</pre><strong>Returns:</strong> An entropy estimate for the random numbers
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returned by operator(), in the range <code>min()</code> to log<sub>2</sub>(
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<code>max()</code>+1). A deterministic random number generator (e.g. a
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pseudo-random number engine) has entropy 0.<br>
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<strong>Throws:</strong> Nothing.
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<h3>Implementation Note for Linux</h3>
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<p>On the Linux operating system, <code>token</code> is interpreted as a
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filesystem path. It is assumed that this path denotes an operating system
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pseudo-device which generates a stream of non-deterministic random numbers.
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The pseudo-device should never signal an error or end-of-file. Otherwise,
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<code>std::ios_base::failure</code> is thrown. By default,
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<code>random_device</code> uses the <code>/dev/urandom</code> pseudo-device
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to retrieve the random numbers. Another option would be to specify the
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<code>/dev/random</code> pseudo-device, which blocks on reads if the
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entropy pool has no more random bits available.</p>
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<h2><a name="performance" id="performance">Performance</a></h2>
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<p>The test program <a href=
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"nondet_random_speed.cpp">nondet_random_speed.cpp</a> measures the
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execution times of the <a href=
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"../../boost/nondet_random.hpp">nondet_random.hpp</a> implementation of the
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above algorithms in a tight loop. The performance has been evaluated on a
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Pentium Pro 200 MHz with gcc 2.95.2, Linux 2.2.13, glibc 2.1.2.</p>
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<table border="1" summary="">
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<tr>
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<th>class</th>
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<th>time per invocation [usec]</th>
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</tr>
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<tr>
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<td>random_device</td>
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<td>92.0</td>
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</tr>
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</table>
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<p>The measurement error is estimated at +/- 1 usec.</p>
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<hr>
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<p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src=
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"../../doc/images/valid-html401.png" alt="Valid HTML 4.01 Transitional"
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height="31" width="88"></a></p>
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<p>Revised
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<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->05
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December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38516" --></p>
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<p><i>Copyright © 2000-2003 <a href=
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"http://www.boost.org/people/jens_maurer.htm">Jens Maurer</a></i></p>
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<p><i>Distributed under the Boost Software License, Version 1.0. (See
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accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
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copy at <a href=
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"http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
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</body>
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</html>
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