NetBSD/lib/libc/stdlib/random.3

.\"	$NetBSD: random.3,v 1.19 2008/02/28 16:16:35 christos Exp $
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.\"     from: @(#)random.3	8.1 (Berkeley) 6/4/93
.\"
.Dd February 28, 2008
.Dt RANDOM 3
.Os
.Sh NAME
.Nm random ,
.Nm srandom ,
.Nm initstate ,
.Nm setstate
.Nd better random number generator; routines for changing generators
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdlib.h
.Ft long
.Fn random void
.Ft void
.Fn srandom "unsigned long seed"
.Ft char *
.Fn initstate "unsigned long seed" "char *state" "size_t n"
.Ft char *
.Fn setstate "char *state"
.Sh DESCRIPTION
The
.Fn 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
.if t 2\u\s731\s10\d\(mi1.
.if n (2**31)\(mi1.
The period of this random number generator is very large, approximately
.if t 16\(mu(2\u\s731\s10\d\(mi1).
.if n 16*((2**31)\(mi1).
The maximum value
.Dv RANDOM_MAX
is defined in
.In stdlib.h .
.Pp
The
.Fn random
and
.Fn srandom
have (almost) the same calling sequence and initialization properties as
.Xr rand 3
and
.Xr srand 3 .
The difference is that
.Xr rand 3
produces a much less random sequence \(em in fact, the low dozen bits
generated by
.Xr rand 3
go through a cyclic pattern.
All the bits generated by
.Fn random
are usable.
For example,
.Sq Li random()\*[Am]01
will produce a random binary value.
.Pp
Like
.Xr rand 3 ,
.Fn random
will by default produce a sequence of numbers that can be duplicated
by calling
.Fn srandom
with
.Ql 1
as the seed.
.Pp
The
.Fn initstate
routine allows a state array, passed in as an argument, to be initialized
for future use.
The size of the state array (in bytes) is used by
.Fn initstate
to decide how sophisticated a random number generator it should use \(em the
more state, the better the random numbers will be.
(Current "optimal" values for the amount of state information 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 state array passed to
.Fn initstate
must be aligned to a 32-bit boundary.
This can be achieved by using
a suitably-sized array of ints, and casting the array to char * when
passing it to
.Fn initstate .
The
.Fn initstate
function
returns a pointer to the previous state information array.
.Pp
Once a state has been initialized, the
.Fn setstate
routine provides for rapid switching between states.
The
.Fn setstate
function
returns a pointer to the previous state array; its
argument state array is used for further random number generation
until the next call to
.Fn initstate
or
.Fn setstate .
.Pp
Once a state array has been initialized, it may be restarted at a
different point either by calling
.Fn initstate
(with the desired seed, the state array, and its size) or by calling
both
.Fn setstate
(with the state array) and
.Fn srandom
(with the desired seed).
The advantage of calling both
.Fn setstate
and
.Fn srandom
is that the size of the state array does not have to be remembered after
it is initialized.
.Pp
With 256 bytes of state information, the period of the random number
generator is greater than
.if t 2\u\s769\s10\d,
.if n 2**69
which should be sufficient for most purposes.
.Sh DIAGNOSTICS
If
.Fn initstate
is called with less than 8 bytes of state information, or if
.Fn setstate
detects that the state information has been garbled, error
messages are printed on the standard error output.
.Sh SEE ALSO
.Xr rand 3 ,
.Xr srand 3 ,
.Xr rnd 4 ,
.Xr rnd 9
.Sh HISTORY
These
functions appeared in
.Bx 4.2 .
.Sh AUTHORS
.An Earl T. Cohen
.Sh BUGS
About 2/3 the speed of
.Xr rand 3 .