Planet
navi homePPSaboutscreenshotsdownloaddevelopmentforum

source: downloads/boost_1_34_1/libs/random/nondet_random.html @ 33

Last change on this file since 33 was 29, checked in by landauf, 16 years ago

updated boost from 1_33_1 to 1_34_1
File size: 5.2 KB
Line 
1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
2
3<html>
4<head>
5  <meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
6
7  <title>Boost RNG Library - Non-Deterministic Random Number
8  Generators</title>
9</head>
10
11<body bgcolor="#FFFFFF" text="#000000">
12  <h1><img src="../../boost.png" alt="boost.png (6897 bytes)" align="middle"
13  width="277" height="86">Header <a href=
14  "../../boost/nondet_random.hpp">&lt;boost/nondet_random.hpp&gt;</a></h1>
15
16  <ul>
17    <li><a href="#synopsis">Synopsis</a></li>
18
19    <li><a href="#random_device">Class <code>random_device</code></a></li>
20
21    <li><a href="#performance">Performance</a></li>
22  </ul>
23
24  <h2><a name="synopsis" id=
25  "synopsis">Header</a><code>&lt;boost/nondet_random.hpp&gt;</code>
26  Synopsis</h2>
27  <pre>
28namespace boost {
29  class random_device;
30} // namespace boost
31</pre>
32
33  <h2><a name="random_device" id="random_device">Class
34  <code>random_device</code></a></h2>
35
36  <h3>Synopsis</h3>
37  <pre>
38class random_device : noncopyable
39{
40public:
41  typedef unsigned int result_type;
42  static const bool has_fixed_range = true;
43  static const result_type min_value = /* implementation defined */;
44  static const result_type max_value = /* implementation defined */;
45  result_type min() const;
46  result_type max() const;
47  explicit random_device(const std::string&amp; token = default_token);
48  ~random_device();
49  double entropy() const;
50  unsigned int operator()();
51};
52</pre>
53
54  <h3>Description</h3>
55
56  <p>Class <code>random_device</code> models a <a href=
57  "random-concepts.html#nondet-rng">non-deterministic random number
58  generator</a>. It uses one or more implementation-defined stochastic
59  processes to generate a sequence of uniformly distributed non-deterministic
60  random numbers. For those environments where a non-deterministic random
61  number generator is not available, class <code>random_device</code> must
62  not be implemented. See</p>
63
64  <blockquote>
65    "Randomness Recommendations for Security", D. Eastlake, S. Crocker, J.
66    Schiller, Network Working Group, RFC 1750, December 1994
67  </blockquote>for further discussions.
68
69  <p><em>Note:</em> Some operating systems abstract the computer hardware
70  enough to make it difficult to non-intrusively monitor stochastic
71  processes. However, several do provide a special device for exactly this
72  purpose. It seems to be impossible to emulate the functionality using
73  Standard C++ only, so users should be aware that this class may not be
74  available on all platforms.</p>
75
76  <h3>Members</h3>
77  <pre>
78explicit random_device(const std::string&amp; token = default_token)
79</pre><strong>Effects:</strong> Constructs a <code>random_device</code>,
80optionally using the given <code>token</code> as an access specification (for
81example, a URL) to some implementation-defined service for monitoring a
82stochastic process.
83  <pre>
84    double entropy() const
85</pre><strong>Returns:</strong> An entropy estimate for the random numbers
86returned by operator(), in the range <code>min()</code> to log<sub>2</sub>(
87<code>max()</code>+1). A deterministic random number generator (e.g. a
88pseudo-random number engine) has entropy 0.<br>
89  <strong>Throws:</strong> Nothing.
90
91  <h3>Implementation Note for Linux</h3>
92
93  <p>On the Linux operating system, <code>token</code> is interpreted as a
94  filesystem path. It is assumed that this path denotes an operating system
95  pseudo-device which generates a stream of non-deterministic random numbers.
96  The pseudo-device should never signal an error or end-of-file. Otherwise,
97  <code>std::ios_base::failure</code> is thrown. By default,
98  <code>random_device</code> uses the <code>/dev/urandom</code> pseudo-device
99  to retrieve the random numbers. Another option would be to specify the
100  <code>/dev/random</code> pseudo-device, which blocks on reads if the
101  entropy pool has no more random bits available.</p>
102
103  <h2><a name="performance" id="performance">Performance</a></h2>
104
105  <p>The test program <a href=
106  "nondet_random_speed.cpp">nondet_random_speed.cpp</a> measures the
107  execution times of the <a href=
108  "../../boost/nondet_random.hpp">nondet_random.hpp</a> implementation of the
109  above algorithms in a tight loop. The performance has been evaluated on a
110  Pentium Pro 200 MHz with gcc 2.95.2, Linux 2.2.13, glibc 2.1.2.</p>
111
112  <table border="1" summary="">
113    <tr>
114      <th>class</th>
115
116      <th>time per invocation [usec]</th>
117    </tr>
118
119    <tr>
120      <td>random_device</td>
121
122      <td>92.0</td>
123    </tr>
124  </table>
125
126  <p>The measurement error is estimated at +/- 1 usec.</p>
127  <hr>
128
129  <p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src=
130  "http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01 Transitional"
131  height="31" width="88"></a></p>
132
133  <p>Revised
134  <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->05
135  December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38516" --></p>
136
137  <p><i>Copyright &copy; 2000-2003 <a href=
138  "../../people/jens_maurer.htm">Jens Maurer</a></i></p>
139
140  <p><i>Distributed under the Boost Software License, Version 1.0. (See
141  accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
142  copy at <a href=
143  "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
144</body>
145</html>
Note: See TracBrowser for help on using the repository browser.