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<a name="rand_002c-srand_002d_002d_002dpseudo_002drandom-numbers"></a>
<h3 class="section">2.33 <code>rand</code>, <code>srand</code>&mdash;pseudo-random numbers</h3>
<a name="index-rand"></a>
<a name="index-srand"></a>
<a name="index-rand_005fr"></a>
<p><strong>Synopsis</strong>
</p><div class="example">
<pre class="example">#include &lt;stdlib.h&gt;
int rand(void);
void srand(unsigned int <var>seed</var>);
int rand_r(unsigned int *<var>seed</var>);

</pre></div>
<p><strong>Description</strong><br>
<code>rand</code> returns a different integer each time it is called; each
integer is chosen by an algorithm designed to be unpredictable, so
that you can use <code>rand</code> when you require a random number.
The algorithm depends on a static variable called the &ldquo;random seed&rdquo;;
starting with a given value of the random seed always produces the
same sequence of numbers in successive calls to <code>rand</code>.
</p>
<p>You can set the random seed using <code>srand</code>; it does nothing beyond
storing its argument in the static variable used by <code>rand</code>.  You can
exploit this to make the pseudo-random sequence less predictable, if
you wish, by using some other unpredictable value (often the least
significant parts of a time-varying value) as the random seed before
beginning a sequence of calls to <code>rand</code>; or, if you wish to ensure
(for example, while debugging) that successive runs of your program
use the same &ldquo;random&rdquo; numbers, you can use <code>srand</code> to set the same
random seed at the outset.
</p>
<br>
<p><strong>Returns</strong><br>
<code>rand</code> returns the next pseudo-random integer in sequence; it is a
number between <code>0</code> and <code>RAND_MAX</code> (inclusive).
</p>
<p><code>srand</code> does not return a result.
</p>
<br>
<p><strong>Notes</strong><br>
<code>rand</code> and <code>srand</code> are unsafe for multi-threaded applications.
<code>rand_r</code> is thread-safe and should be used instead.
</p>
<br>
<p><strong>Portability</strong><br>
<code>rand</code> is required by ANSI, but the algorithm for pseudo-random
number generation is not specified; therefore, even if you use
the same random seed, you cannot expect the same sequence of results
on two different systems.
</p>
<p><code>rand</code> requires no supporting OS subroutines.
</p>
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