### 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