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    <p>See this article: <a href="http://en.wikipedia.org/wiki/List_of_random_number_generators" rel="noreferrer">http://en.wikipedia.org/wiki/List_of_random_number_generators</a></p> <p>This is the source code of glibc's <code>rand()</code>:</p> <pre><code>/* Reentrant random function from POSIX.1c. Copyright (C) 1996, 1999, 2009 Free Software Foundation, Inc. This file is part of the GNU C Library. Contributed by Ulrich Drepper &lt;drepper@cygnus.com&gt;, 1996. The GNU C Library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version. The GNU C Library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details. You should have received a copy of the GNU Lesser General Public License along with the GNU C Library; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA. */ #include &lt;stdlib.h&gt; /* This algorithm is mentioned in the ISO C standard, here extended for 32 bits. */ int rand_r (unsigned int *seed) { unsigned int next = *seed; int result; next *= 1103515245; next += 12345; result = (unsigned int) (next / 65536) % 2048; next *= 1103515245; next += 12345; result &lt;&lt;= 10; result ^= (unsigned int) (next / 65536) % 1024; next *= 1103515245; next += 12345; result &lt;&lt;= 10; result ^= (unsigned int) (next / 65536) % 1024; *seed = next; return result; } </code></pre> <p>Source: <a href="https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=stdlib/rand_r.c;hb=HEAD" rel="noreferrer">https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=stdlib/rand_r.c;hb=HEAD</a></p> <p>As you can see, it's simply multiply with an addition and a shift. The values are carefully chosen to make sure that you get no repeat of the output for RAND_MAX iterations.</p> <p>Note that this is an old implementation which has been replaced by a more complex algorithm: <a href="https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=stdlib/random_r.c;hb=HEAD" rel="noreferrer">https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=stdlib/random_r.c;hb=HEAD</a></p> <p>If the link if broken, Google for "glibc rand_r"</p>
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    1. POWhat common algorithms are used for C's rand()?
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    1. USAaron Digulla
    1. USAaron Digulla
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    1. POWhat common algorithms are used for C's rand()?
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    1. COThat's it? Surprisingly simple. Thanks muchly.
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    2. COYes, that's it, most general purpose PNRGs are surprisingly simple to implement. If you're looking for something more complex try the Mersenne Twister.
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      1. USJasper Bekkers
    3. COEven MT19937 isn't exactly hard to implement.
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