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<h1>srand</h1>
<div class="mod_az_list">
<a href="../index-functions.html">Perl functions A-Z</a> |
<a href="../index-functions-by-cat.html">Perl functions by category</a> |
<a href="../perlfunc.html">The 'perlfunc' manpage</a>
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<ul>
<li><a name="srand-EXPR"></a><b>srand EXPR</b>
<p> </p>
</li>
</ul>
<ul>
<li><a name="srand"></a><b>srand</b>
<p>Sets the random number seed for the <code class="inline"><a class="l_k" href="../functions/rand.html">rand</a></code> operator.</p>
<p>The point of the function is to "seed" the <code class="inline"><a class="l_k" href="../functions/rand.html">rand</a></code> function so that
<code class="inline"><a class="l_k" href="../functions/rand.html">rand</a></code> can produce a different sequence each time you run your
program.</p>
<p>If srand() is not called explicitly, it is called implicitly at the
first use of the <code class="inline"><a class="l_k" href="../functions/rand.html">rand</a></code> operator. However, this was not the case in
versions of Perl before 5.004, so if your script will run under older
Perl versions, it should call <code class="inline"><a class="l_k" href="../functions/srand.html">srand</a></code>.</p>
<p>Most programs won't even call srand() at all, except those that
need a cryptographically-strong starting point rather than the
generally acceptable default, which is based on time of day,
process ID, and memory allocation, or the <i>/dev/urandom</i> device,
if available.</p>
<p>You can call srand($seed) with the same $seed to reproduce the
<i>same</i> sequence from rand(), but this is usually reserved for
generating predictable results for testing or debugging.
Otherwise, don't call srand() more than once in your program.</p>
<p>Do <b>not</b> call srand() (i.e. without an argument) more than once in
a script. The internal state of the random number generator should
contain more entropy than can be provided by any seed, so calling
srand() again actually <i>loses</i> randomness.</p>
<p>Most implementations of <code class="inline"><a class="l_k" href="../functions/srand.html">srand</a></code> take an integer and will silently
truncate decimal numbers. This means <code class="inline"><a class="l_k" href="../functions/srand.html">srand(42)</a></code> will usually
produce the same results as <code class="inline"><a class="l_k" href="../functions/srand.html">srand(42.1)</a></code>. To be safe, always pass
<code class="inline"><a class="l_k" href="../functions/srand.html">srand</a></code> an integer.</p>
<p>In versions of Perl prior to 5.004 the default seed was just the
current <code class="inline"><a class="l_k" href="../functions/time.html">time</a></code>. This isn't a particularly good seed, so many old
programs supply their own seed value (often <code class="inline"><a class="l_k" href="../functions/time.html">time</a> ^ <span class="i">$$</span></code>
or <code class="inline"><a class="l_k" href="../functions/time.html">time</a> ^
<span class="s">(</span><span class="i">$$</span> + <span class="s">(</span><span class="i">$$</span> << <span class="n">15</span><span class="s">)</span><span class="s">)</span></code>
), but that isn't necessary any more.</p>
<p>For cryptographic purposes, however, you need something much more random
than the default seed. Checksumming the compressed output of one or more
rapidly changing operating system status programs is the usual method. For
example:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="../functions/srand.html">srand</a> <span class="s">(</span><a class="l_k" href="../functions/time.html">time</a> ^ <span class="i">$$</span> ^ <a class="l_k" href="../functions/unpack.html">unpack</a> <span class="q">"%L*"</span><span class="cm">,</span> <span class="q">`ps axww | gzip -f`</span><span class="s">)</span><span class="sc">;</span></li></ol></pre><p>If you're particularly concerned with this, see the <code class="inline"><span class="w">Math::TrulyRandom</span></code>
module in CPAN.</p>
<p>Frequently called programs (like CGI scripts) that simply use</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="../functions/time.html">time</a> ^ <span class="i">$$</span></li></ol></pre><p>for a seed can fall prey to the mathematical property that</p>
<pre class="verbatim"><ol><li> <span class="w">a</span>^<span class="w">b</span> == <span class="s">(</span><span class="w">a</span>+<span class="n">1</span><span class="s">)</span>^<span class="s">(</span><span class="w">b</span>+<span class="n">1</span><span class="s">)</span></li></ol></pre><p>one-third of the time. So don't do that.</p>
</li>
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