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121 lines
5.2 KiB
HTML
121 lines
5.2 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<title>rand (The Red Hat newlib C Library)</title>
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<meta name="description" content="rand (The Red Hat newlib C Library)">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Document-Index.html#Document-Index" rel="index" title="Document Index">
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<link href="Document-Index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Stdlib.html#Stdlib" rel="up" title="Stdlib">
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<link href="random.html#random" rel="next" title="random">
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</head>
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<body lang="en">
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<a name="rand"></a>
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<div class="header">
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<p>
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Next: <a href="random.html#random" accesskey="n" rel="next">random</a>, Previous: <a href="qsort.html#qsort" accesskey="p" rel="prev">qsort</a>, Up: <a href="Stdlib.html#Stdlib" accesskey="u" rel="up">Stdlib</a> [<a href="Document-Index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Document-Index.html#Document-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="rand_002c-srand_002d_002d_002dpseudo_002drandom-numbers"></a>
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<h3 class="section">2.33 <code>rand</code>, <code>srand</code>—pseudo-random numbers</h3>
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<a name="index-rand"></a>
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<a name="index-srand"></a>
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<a name="index-rand_005fr"></a>
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<p><strong>Synopsis</strong>
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</p><div class="example">
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<pre class="example">#include <stdlib.h>
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int rand(void);
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void srand(unsigned int <var>seed</var>);
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int rand_r(unsigned int *<var>seed</var>);
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</pre></div>
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<p><strong>Description</strong><br>
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<code>rand</code> returns a different integer each time it is called; each
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integer is chosen by an algorithm designed to be unpredictable, so
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that you can use <code>rand</code> when you require a random number.
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The algorithm depends on a static variable called the “random seed”;
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starting with a given value of the random seed always produces the
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same sequence of numbers in successive calls to <code>rand</code>.
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</p>
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<p>You can set the random seed using <code>srand</code>; it does nothing beyond
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storing its argument in the static variable used by <code>rand</code>. You can
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exploit this to make the pseudo-random sequence less predictable, if
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you wish, by using some other unpredictable value (often the least
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significant parts of a time-varying value) as the random seed before
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beginning a sequence of calls to <code>rand</code>; or, if you wish to ensure
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(for example, while debugging) that successive runs of your program
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use the same “random” numbers, you can use <code>srand</code> to set the same
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random seed at the outset.
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</p>
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<br>
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<p><strong>Returns</strong><br>
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<code>rand</code> returns the next pseudo-random integer in sequence; it is a
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number between <code>0</code> and <code>RAND_MAX</code> (inclusive).
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</p>
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<p><code>srand</code> does not return a result.
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</p>
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<br>
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<p><strong>Notes</strong><br>
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<code>rand</code> and <code>srand</code> are unsafe for multi-threaded applications.
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<code>rand_r</code> is thread-safe and should be used instead.
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</p>
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<br>
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<p><strong>Portability</strong><br>
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<code>rand</code> is required by ANSI, but the algorithm for pseudo-random
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number generation is not specified; therefore, even if you use
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the same random seed, you cannot expect the same sequence of results
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on two different systems.
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</p>
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<p><code>rand</code> requires no supporting OS subroutines.
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</p>
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<br>
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<hr>
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<div class="header">
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<p>
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Next: <a href="random.html#random" accesskey="n" rel="next">random</a>, Previous: <a href="qsort.html#qsort" accesskey="p" rel="prev">qsort</a>, Up: <a href="Stdlib.html#Stdlib" accesskey="u" rel="up">Stdlib</a> [<a href="Document-Index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Document-Index.html#Document-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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