fast random number generation

Hi,
I have made a program that generates random numbers. It takes arguments from the command line like this:
Usage: argv[0] [-n | -c [val]] [min-max] [num]

Default behaviour is to print [num] random numbers, ranging from [min] to [max], with a newline after each number. The option "-n" will cause no newlines to be printed between each value, OR, "-c val" will print an ascii value "val", between each number.

Here is the main processing function of the program (all the arguments have been checked):

Code:

---

/* Main function to output random numbers.
  It outputs "num" random numbers, between "min" and "max",
  with "ch" between each number. */
void OutputRanNums
(int ch, int min, int max, unsigned long int iterations)
{
    register long int rannum;
    register unsigned int diff;
    register unsigned long int num = iterations;

    /* Work out offset value, increment as rand()
        could return 0 as lowest value */
    diff = max - min;
    diff++;

    /* Any better ideas? */
    srand(time(NULL));
    for( ; num>0 ; num-- )
    {
        rannum = rand();
        rannum %= diff;
        rannum += min;
        printf("%ld",rannum);
        if( ch )
            printf("%c",ch);
    }
    fflush(stdout);
}

---

I wonder if any bit shifting could be done (or any other tricks) to speed this up...

Well, you're not generating the random numbers - rand() is - so actually, this is slower random number generation. Also, what's the use of the prog? I can understand a random(min,max) style function, but not one that just prints em out.

Someone told once in the forum the using % (mod) is a little bit slow, I think Quzah gave once another way to do that faster then using mod, so try searching the board.

I understand that rand() generates the psuedo-random numbers, but I want the random numbers in a certain range.
Eg. I want a print out of 100 random numbers ranging from 10-20.

In the past I have heard a few people asking how to produce N random numbers from range X to Y "in linux", in IRC channels (heh, don't ask me why=). I really just created the program because a good way to learn is to practice. Perhaps it may be useful in creating test data or something.
Who knows, one day, someone may even find it usefull. :)

PS
The source is also able to output the values in binary format, so "rand -n 65-90 15" could be used to produce a random password or something... just a thought.

"using % (mod) is a little bit slow"

Another way is to AND the returned value with the number. Of course, it only works for powers of two, but

rand() % 2 == rand() & 1
rand() % 4 == rand() & 3
rand() % 8 == rand() & 7

etc.