Random number in a range. Assumes you have GNU GSL in /usr/local/*

Makefile

GSL_BUILDFLAGS=-I/usr/local/include -L/usr/local/lib -lgsl

all: rnd+gsl

rnd+gsl: rnd+gsl.c
	g++ $(GSL_BUILDFLAGS) -o rnd+gsl rnd+gsl.c

rnd+gsl.c

#include <stdio.h>
#include <math.h>
#include <gsl/gsl_rng.h>
#include <sys/time.h>

/* A didactic example of how to generate uniform integers between 1-6. 
   Please don't ever consider this cryptographically strong; it is not. */

unsigned long cur_usec()
{
  struct timeval c;
  gettimeofday(&c,0);
  return (unsigned long)(c.tv_usec);
}

int rndi_in_range(int lowerb, int upperb, gsl_rng *r)
{
  double upperbd = (double)upperb;
  return lowerb + floor(upperbd * gsl_rng_uniform(r));
}

int main (void)
{
  const gsl_rng_type *T;
  gsl_rng *r;  
  int i, n = 10;
  
  gsl_rng_env_setup();
  T = gsl_rng_default;
  r = gsl_rng_alloc(T);
  gsl_rng_set(r, cur_usec());
  
  for (i = 0; i < n; i++) 
  {
    int u = rndi_in_range(1, 6, r);
    printf ("%d\n", u);
  }
  
  gsl_rng_free(r);
  return 0;
}