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