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<title>Random number environment variables - GNU Scientific Library -- Reference Manual</title>
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<h3 class="section">17.6 Random number environment variables</h3>
<p>The library allows you to choose a default generator and seed from the
environment variables <code>GSL_RNG_TYPE</code> and <code>GSL_RNG_SEED</code> and
the function <code>gsl_rng_env_setup</code>. This makes it easy try out
different generators and seeds without having to recompile your program.
<div class="defun">
— Function: const gsl_rng_type * <b>gsl_rng_env_setup</b> (<var>void</var>)<var><a name="index-gsl_005frng_005fenv_005fsetup-1491"></a></var><br>
<blockquote><p>This function reads the environment variables <code>GSL_RNG_TYPE</code> and
<code>GSL_RNG_SEED</code> and uses their values to set the corresponding
library variables <code>gsl_rng_default</code> and
<code>gsl_rng_default_seed</code>. These global variables are defined as
follows,
<pre class="example"> extern const gsl_rng_type *gsl_rng_default
extern unsigned long int gsl_rng_default_seed
</pre>
<p>The environment variable <code>GSL_RNG_TYPE</code> should be the name of a
generator, such as <code>taus</code> or <code>mt19937</code>. The environment
variable <code>GSL_RNG_SEED</code> should contain the desired seed value. It
is converted to an <code>unsigned long int</code> using the C library function
<code>strtoul</code>.
<p>If you don't specify a generator for <code>GSL_RNG_TYPE</code> then
<code>gsl_rng_mt19937</code> is used as the default. The initial value of
<code>gsl_rng_default_seed</code> is zero.
</blockquote></div>
<p class="noindent">Here is a short program which shows how to create a global
generator using the environment variables <code>GSL_RNG_TYPE</code> and
<code>GSL_RNG_SEED</code>,
<pre class="example"><pre class="verbatim"> #include <stdio.h>
#include <gsl/gsl_rng.h>
gsl_rng * r; /* global generator */
int
main (void)
{
const gsl_rng_type * T;
gsl_rng_env_setup();
T = gsl_rng_default;
r = gsl_rng_alloc (T);
printf ("generator type: %s\n", gsl_rng_name (r));
printf ("seed = %lu\n", gsl_rng_default_seed);
printf ("first value = %lu\n", gsl_rng_get (r));
gsl_rng_free (r);
return 0;
}
</pre></pre>
<p class="noindent">Running the program without any environment variables uses the initial
defaults, an <code>mt19937</code> generator with a seed of 0,
<pre class="example"> $ ./a.out
<pre class="verbatim"> generator type: mt19937
seed = 0
first value = 4293858116
</pre></pre>
<p class="noindent">By setting the two variables on the command line we can
change the default generator and the seed,
<pre class="example"> $ GSL_RNG_TYPE="taus" GSL_RNG_SEED=123 ./a.out
GSL_RNG_TYPE=taus
GSL_RNG_SEED=123
generator type: taus
seed = 123
first value = 2720986350
</pre>
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