#include <stdio.h>
#include <time.h>
#include <stdlib.h>

/* To include mixed case, parenthesize this: */
#define MULT 36 + 26
#define LEN  20

#define DBL_RAND    (rand() / ((double) RAND_MAX + 1.0))
#define RAND_NUM(n) (DBL_RAND * n)
#define INT_RNUM(n) ((int) RAND_NUM(n))

int main (void)
{
  int ctr;
  const char *c = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";

  srand (time(NULL));

  for (ctr = 0; ctr < LEN; ctr++)
    putchar (c[INT_RNUM (MULT)]);

  putchar ('\n');
  return 0;
}