From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Teuben <teuben@astro.umd.edu>
To: Brian Gough <bjg@network-theory.co.uk>
Cc: gsl-discuss@sourceware.cygnus.com
Subject: Re: fprintf(stderr,...) output in GSL routines (gsl_rng_env_setup)
Date: Wed, 19 Dec 2001 13:20:00 -0000
Message-id: <Pine.GSO.4.10.10109091030000.4459-100000@apus.astro.umd.edu>
References: <15259.15112.921941.197191@debian>
X-SW-Source: 2001/msg00485.html

> The environment variables are completely optional -- they are only
> used if you explicitly call gsl_rng_env_setup().  You can set the
> parameters by direct assignment in C.  e.g.
> 
> #include <gsl/gsl_rng.h>
> 
> int main () {
>        ....
>         gsl_rng_default_seed = 123;
>         gsl_rng_default_type = gsl_rng_ran3 ; 
> 

yes, I understood that also from the example, but i actually like the
idea of the user giving control of the random number generator. So in
the code the user can supply
	seed=0,ran3
where they are both optional. The method of assigning to gsl_rng_default_type
would require me to manually build a large list of (to me officially unknown)
names, whereas the putenv() method has the nice side-effect that if you
say
	seed=0,help
you get
...
Valid generator types are:
               cmrg              gfsr4             minstd                mrg
            mt19937               r250               ran0               ran1
               ran2               ran3             rand48               rand
      random128-bsd   random128-glibc2    random128-libc5      random256-bsd
   random256-glibc2    random256-libc5       random32-bsd    random32-glibc2
     random32-libc5       random64-bsd    random64-glibc2     random64-libc5
        random8-bsd     random8-glibc2      random8-libc5         random-bsd
      random-glibc2       random-libc5              randu               ranf
          ranlux389             ranlux            ranlxd1            ranlxd2
            ranlxs0            ranlxs1            ranlxs2             ranmar
             slatec               taus         transputer              tt800
              uni32                uni                vax                zuf


so, I got the following code from that idea:

    is = burststring(init,", ");                      /* parse init as "[seed[,name]]"  */
    nis = xstrlen(is,sizeof(string))-1;
    if (nis > 0) {                                          /* seed is first, but optional */
        iseed = natoi(is[0]);
	if (iseed > 0 || streq(is[0],"+0")) {
	  sprintf(my_gsl_seed,"%s=%s",env_seed,is[0]);
	} else {
	  iseed = set_xrandom(iseed);
	  sprintf(my_gsl_seed,"%s=%u",env_seed,iseed);
	}
	putenv(my_gsl_seed);
        if (nis > 1) {                                      /* name is second, also optional */
            sprintf(my_gsl_type,"%s=%s",env_type,is[1]);
            putenv(my_gsl_type);
        }
    }

    gsl_rng_env_setup();                          /* initialize the rng (name/seed) setup */
    my_T = gsl_rng_default;
    my_r = gsl_rng_alloc(my_T);


    dprintf(1,"GSL generator type: %s\n",gsl_rng_name(my_r));
    dprintf(1,"GSL seed = %u\n",gsl_rng_default_seed);
    dprintf(1,"GSL first value = %u\n",gsl_rng_get(my_r));



You can see a call to set_xrandom(), that replaces a seed of 0,-1,-2 by
seconds since 1970, clock-cycles or PID as seed. We found that to be useful
for simulations.  In linux there is also /dev/random, but I haven't 
been able to use that.

peter