char *double2comma(double f, int places)
{
   static char tmp[40];
   char *s;
   int i=0;

   if ((places < 0) || (places > 38)) {
      tmp[0] = 0;
      return tmp;
   }
   sprintf(tmp, "%#.*f", places, f);
   s = strchr(tmp, '.');
   if (!places) {
      /* Get rid of trailing '.': */
      *s = '\0';
   }

   while(--s > tmp) {
      if (i++ == 2) {
         /* Are we just to the right of '-'? */
         if (s[-1] == '-') break;
         /* Insert a comma */
         i=0;
         /* Shift segment right to make room for ',': */
         memmove(s+1, s, strlen(s)+1);
         *s = ',';
      }
   }
   return tmp;
}

usage: printf("You have $%s in your account.", double2comma((double)123.45, 2));