random(3)

NAME

random, srandom, srandomdev, initstate, setstate - better

random number
generator; routines for changing generators

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <stdlib.h>
long
random(void);
void
srandom(unsigned long seed);
void
srandomdev(void);
char *
initstate(unsigned long seed, char *state, long n);
char *
setstate(char *state);

DESCRIPTION

The random() function uses a non-linear additive feedback

random number
generator employing a default table of size 31 long integers

to return
successive pseudo-random numbers in the range from 0 to

(2**31)-1. The
period of this random number generator is very large, ap

proximately
16*((2**31)-1).

The random() and srandom() functions have (almost) the same

calling
sequence and initialization properties as the rand(3) and

srand(3) functions. The difference is that rand(3) produces a much less

random
sequence -- in fact, the low dozen bits generated by rand go

through a
cyclic pattern. All the bits generated by random() are us

able. For
example, `random()&01' will produce a random binary value.

Like rand(3), random() will by default produce a sequence of

numbers that
can be duplicated by calling srandom() with `1' as the seed.

The srandomdev() routine initializes a state array using the

random(4)
random number device which returns good random numbers,

suitable for
cryptographic use. Note that this particular seeding proce

dure can generate states which are impossible to reproduce by calling

srandom() with
any value, since the succeeding terms in the state buffer

are no longer
derived from the LC algorithm applied to a fixed seed.

The initstate() routine allows a state array, passed in as

an argument,
to be initialized for future use. The size of the state ar

ray (in bytes)
is used by initstate() to decide how sophisticated a random

number generator it should use -- the more state, the better the random

numbers will
be. (Current "optimal" values for the amount of state in

formation are 8,
32, 64, 128, and 256 bytes; other amounts will be rounded

down to the
nearest known amount. Using less than 8 bytes will cause an

error.) The
seed for the initialization (which specifies a starting

point for the
random number sequence, and provides for restarting at the

same point) is
also an argument. The initstate() function returns a point

er to the previous state information array.

Once a state has been initialized, the setstate() routine

provides for
rapid switching between states. The setstate() function re

turns a
pointer to the previous state array; its argument state ar

ray is used for
further random number generation until the next call to

initstate() or
setstate().

Once a state array has been initialized, it may be restarted

at a different point either by calling initstate() (with the desired

seed, the state
array, and its size) or by calling both setstate() (with the

state array)
and srandom() (with the desired seed). The advantage of

calling both
setstate() and srandom() is that the size of the state array

does not
have to be remembered after it is initialized.

With 256 bytes of state information, the period of the ran

dom number generator is greater than 2**69 which should be sufficient for

most purposes.

DIAGNOSTICS

If initstate() is called with less than 8 bytes of state in

formation, or
if setstate() detects that the state information has been

garbled, error
messages are printed on the standard error output.

SEE ALSO

arc4random(3), rand(3), srand(3), random(4)

HISTORY

These functions appeared in 4.2BSD.

AUTHORS

Earl T. Cohen

BUGS

About 2/3 the speed of rand(3).

The historical implementation used to have a very weak seed

ing; the random sequence did not vary much with the seed. The current

implementation
employs a better pseudo-random number generator for the ini

tial state
calculation.

Applications requiring cryptographic quality randomness

should use
arc4random(3).

BSD June 4, 1993

BSD