[unix-history] / usr / man / man3 / random.3

.TH RANDOM 3 "19 January 1983"
.UC 4
.SH NAME
random, srandom, initstate, setstate \- better random number generator; routines for changing generators
.SH SYNOPSIS
.nf
.B long  random()
.PP
.B srandom(seed)
.B int  seed;
.PP
.B char  *initstate(seed, state, n)
.B unsigned  seed;
.B char  *state;
.B int  n;
.PP
.B char  *setstate(state)
.B char  *state;
.fi
.SH DESCRIPTION
.PP
.I Random
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\u\s731\s10\d\-1.  The period of this
random number generator is very large, approximately 16*(2\u\s731\s10\d\-1).
.PP
.I Random/srandom
have (almost) the same calling sequence and initialization properties as
.I rand/srand.
The difference is that
.IR 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
.I random
are usable.  For example, \*(lqrandom()&01\*(rq will produce a random binary
value.
.PP
Unlike
.IR srand ,
.I srandom
does not return the old seed; the reason for this is that the amount of
state information used is much more than a single word.  (Two other
routines are provided to deal with restarting/changing random
number generators).  Like
.IR rand (3),
however,
.I random
will by default produce a sequence of numbers that can be duplicated
by calling
.I srandom
with 
.I 1
as the seed.
.PP
The
.I 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
.I 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 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.
.I Initstate
returns a pointer to the previous state information array.
.PP
Once a state has been initialized, the
.I setstate
routine provides for rapid switching between states.
.I Setstate returns a pointer to the previous state array; its
argument state array is used for further random number generation
until the next call to
.I initstate
or
.I setstate.
.PP
Once a state array has been initialized, it may be restarted at a
different point either by calling
.I initstate
(with the desired seed, the state array, and its size) or by calling
both
.I setstate
(with the state array) and
.I srandom
(with the desired seed).
The advantage of calling both
.I setstate
and
.I 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 2\u\s769\s10\d, which should be sufficient for
most purposes.
.SH AUTHOR
Earl T. Cohen
.SH DIAGNOSTICS
.PP
If
.I initstate
is called with less than 8 bytes of state information, or if
.I setstate
detects that the state information has been garbled, error
messages are printed on the standard error output.
.SH "SEE ALSO"
rand(3)
.SH BUGS
About 2/3 the speed of
.IR rand (3C).
Commit	Line	Data
5436e645 C	1	.TH RANDOM 3 "19 January 1983"
	2	.UC 4
	3	.SH NAME
	4	random, srandom, initstate, setstate \- better random number generator; routines for changing generators
	5	.SH SYNOPSIS
	6	.nf
	7	.B long random()
	8	.PP
	9	.B srandom(seed)
	10	.B int seed;
	11	.PP
	12	.B char *initstate(seed, state, n)
	13	.B unsigned seed;
	14	.B char *state;
	15	.B int n;
	16	.PP
	17	.B char *setstate(state)
	18	.B char *state;
	19	.fi
	20	.SH DESCRIPTION
	21	.PP
	22	.I Random
	23	uses a non-linear additive feedback random number generator employing a
	24	default table of size 31 long integers to return successive pseudo-random
	25	numbers in the range from 0 to 2\u\s731\s10\d\-1. The period of this
	26	random number generator is very large, approximately 16*(2\u\s731\s10\d\-1).
	27	.PP
	28	.I Random/srandom
	29	have (almost) the same calling sequence and initialization properties as
	30	.I rand/srand.
	31	The difference is that
	32	.IR rand (3)
	33	produces a much less random sequence -- in fact, the low dozen bits
	34	generated by rand go through a cyclic pattern. All the bits generated by
	35	.I random
	36	are usable. For example, \(lqrandom()&01\(rq will produce a random binary
	37	value.
	38	.PP
	39	Unlike
	40	.IR srand ,
	41	.I srandom
	42	does not return the old seed; the reason for this is that the amount of
	43	state information used is much more than a single word. (Two other
	44	routines are provided to deal with restarting/changing random
	45	number generators). Like
	46	.IR rand (3),
	47	however,
	48	.I random
	49	will by default produce a sequence of numbers that can be duplicated
	50	by calling
	51	.I srandom
	52	with
	53	.I 1
	54	as the seed.
	55	.PP
	56	The
	57	.I initstate
	58	routine allows a state array, passed in as an argument, to be initialized
	59	for future use. The size of the state array (in bytes) is used by
	60	.I initstate
	61	to decide how sophisticated a random number generator it should use -- the
	62	more state, the better the random numbers will be.
	63	(Current "optimal" values for the amount of state information are
	64	8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to
65	the nearest known amount. Using less than 8 bytes will cause an error).
66	The seed for the initialization (which specifies a starting point for
67	the random number sequence, and provides for restarting at the same
68	point) is also an argument.
69	.I Initstate
70	returns a pointer to the previous state information array.
71	.PP
72	Once a state has been initialized, the
73	.I setstate
74	routine provides for rapid switching between states.
75	.I Setstate returns a pointer to the previous state array; its
76	argument state array is used for further random number generation
77	until the next call to
78	.I initstate
79	or
80	.I setstate.
81	.PP
82	Once a state array has been initialized, it may be restarted at a
83	different point either by calling
84	.I initstate
85	(with the desired seed, the state array, and its size) or by calling
86	both
87	.I setstate
88	(with the state array) and
89	.I srandom
90	(with the desired seed).
91	The advantage of calling both
92	.I setstate
93	and
94	.I srandom
95	is that the size of the state array does not have to be remembered after
96	it is initialized.
97	.PP
98	With 256 bytes of state information, the period of the random number
99	generator is greater than 2\u\s769\s10\d, which should be sufficient for
100	most purposes.
101	.SH AUTHOR
102	Earl T. Cohen
103	.SH DIAGNOSTICS
104	.PP
105	If
106	.I initstate
107	is called with less than 8 bytes of state information, or if
108	.I setstate
109	detects that the state information has been garbled, error
110	messages are printed on the standard error output.
111	.SH "SEE ALSO"
112	rand(3)
113	.SH BUGS
114	About 2/3 the speed of
115	.IR rand (3C).