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