From: "John M. Aldrich" <fighteer AT cs DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: What does "srand" and "srandom" do?
Date: Sat, 07 Mar 1998 16:34:28 -0500
Organization: Two pounds of chaos and a pinch of salt.
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Ahn Ki-yung wrote:
> 
> I wonder what does these libc functions do ?
> In the libc reference info file there are 3 random functions.
> They are rand, random, srandom.
> The function "rand(unsigned int)" is not found but there is a prototype
> in "stdlib.h"

There are actually four related functions:  rand(), srand(), random(),
and srandom(), all declared in <stdlib.h>, but the documentation for
srand() was omitted from DJGPP v2.01 by mistake.

These functions all deal with pseudo-random numbers, an essential
feature of any program that depends on "chance," "randomness," "free
will," or any similar concept.  Games, simulations, and many other types
of programs absolutely depend on having access to random numbers.

DJGPP has two pseudo-random number generators:  rand() and random(). 
Both of these make use of a number called a "seed", which is used to
determine the next pseudo-random number.  If you don't initialize this
seed value, it starts at zero, and therefore the sequence of
pseudo-random numbers will be the same each time you run your program. 
To initalize the seed, you must call the srand() or srandom() functions,
respectively, each of which sets the seed of the corresponding
pseudo-random number generator.  This process is called "seeding."

The biggest problem is picking the value to use as the seed.  Obviously,
using the same number will result in the same sequence of pseudo-random
numbers, so most programmers use a value that is virtually guaranteed to
be unique each time the program is run:  the system clock.  The easiest
way to retrieve this value is with the time() library function, declared
in <time.h>.  Example code:

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

int main( void )
{
    srandom( (int) time( NULL ) );
    printf( "Random value from 1 to 100:  %d\n", random() % 100 + 1 );
    return 0;
}

srand() returns a value from 0 to RAND_MAX, defined in <limits.h>. 
random() returns a value from 0 to MAXINT.  You must clip these values
to the desired range before using them.

Finally, some notes.  The term "pseudo-random" is used because these are
not true random numbers.  The period of most RNGs (i.e., the number of
iterations before the sequence is repeated) is usually the same as their
maximum range (i.e., RAND_MAX or MAXINT).  Also, not all random number
generators are equal:  the DJGPP rand() tends to be non-random in its
low order bits.  random() is a better RNG, but takes more cycles to
run.  Many programmers recommend implementing your own custom random
number generators from books of algorithms such as Knuth, Volume 2.

Hope this helps!

-- 
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