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18.6 Random number environment variables

The library allows you to choose a default generator and seed from the environment variables GSL_RNG_TYPE and GSL_RNG_SEED and the function gsl_rng_env_setup. This makes it easy try out different generators and seeds without having to recompile your program.

— Function: const gsl_rng_type * gsl_rng_env_setup (void)

This function reads the environment variables GSL_RNG_TYPE and GSL_RNG_SEED and uses their values to set the corresponding library variables gsl_rng_default and gsl_rng_default_seed. These global variables are defined as follows, 

          extern const gsl_rng_type *gsl_rng_default
          extern unsigned long int gsl_rng_default_seed

The environment variable GSL_RNG_TYPE should be the name of a generator, such as taus or mt19937. The environment variable GSL_RNG_SEED should contain the desired seed value. It is converted to an unsigned long int using the C library function strtoul.

If you don't specify a generator for GSL_RNG_TYPE then gsl_rng_mt19937 is used as the default. The initial value of gsl_rng_default_seed is zero.

Here is a short program which shows how to create a global generator using the environment variables GSL_RNG_TYPE and GSL_RNG_SEED, 

     #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;
     }

Running the program without any environment variables uses the initial defaults, an mt19937 generator with a seed of 0, 

     $ ./a.out

     generator type: mt19937
     seed = 0
     first value = 4293858116

By setting the two variables on the command line we can change the default generator and the seed, 

     $ 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