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<h3 class="section">19.8 Pseudo-Random Numbers</h3>

<p><a name="index-random-numbers-2282"></a><a name="index-pseudo_002drandom-numbers-2283"></a><a name="index-seed-_0028for-random-numbers_0029-2284"></a>
This section describes the GNU facilities for generating a series of
pseudo-random numbers.  The numbers generated are not truly random;
typically, they form a sequence that repeats periodically, with a period
so large that you can ignore it for ordinary purposes.  The random
number generator works by remembering a <dfn>seed</dfn> value which it uses
to compute the next random number and also to compute a new seed.

   <p>Although the generated numbers look unpredictable within one run of a
program, the sequence of numbers is <em>exactly the same</em> from one run
to the next.  This is because the initial seed is always the same.  This
is convenient when you are debugging a program, but it is unhelpful if
you want the program to behave unpredictably.  If you want a different
pseudo-random series each time your program runs, you must specify a
different seed each time.  For ordinary purposes, basing the seed on the
current time works well.

   <p>You can obtain repeatable sequences of numbers on a particular machine type
by specifying the same initial seed value for the random number
generator.  There is no standard meaning for a particular seed value;
the same seed, used in different C libraries or on different CPU types,
will give you different random numbers.

   <p>The GNU library supports the standard ISO&nbsp;C<!-- /@w --> random number functions
plus two other sets derived from BSD and SVID.  The BSD and ISO&nbsp;C<!-- /@w -->
functions provide identical, somewhat limited functionality.  If only a
small number of random bits are required, we recommend you use the
ISO&nbsp;C<!-- /@w --> interface, <code>rand</code> and <code>srand</code>.  The SVID functions
provide a more flexible interface, which allows better random number
generator algorithms, provides more random bits (up to 48) per call, and
can provide random floating-point numbers.  These functions are required
by the XPG standard and therefore will be present in all modern Unix
systems.

<ul class="menu">
<li><a accesskey="1" href="ISO-Random.html#ISO-Random">ISO Random</a>:                   <code>rand</code> and friends. 
<li><a accesskey="2" href="BSD-Random.html#BSD-Random">BSD Random</a>:                   <code>random</code> and friends. 
<li><a accesskey="3" href="SVID-Random.html#SVID-Random">SVID Random</a>:                  <code>drand48</code> and friends. 
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