xref: /freebsd/lib/libc/stdlib/random.3 (revision 1e413cf93298b5b97441a21d9a50fdcd0ee9945e)
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28.\"     @(#)random.3	8.1 (Berkeley) 6/4/93
29.\" $FreeBSD$
30.\"
31.Dd June 4, 1993
32.Dt RANDOM 3
33.Os
34.Sh NAME
35.Nm random ,
36.Nm srandom ,
37.Nm srandomdev ,
38.Nm initstate ,
39.Nm setstate
40.Nd better random number generator; routines for changing generators
41.Sh LIBRARY
42.Lb libc
43.Sh SYNOPSIS
44.In stdlib.h
45.Ft long
46.Fn random void
47.Ft void
48.Fn srandom "unsigned long seed"
49.Ft void
50.Fn srandomdev void
51.Ft char *
52.Fn initstate "unsigned long seed" "char *state" "long n"
53.Ft char *
54.Fn setstate "char *state"
55.Sh DESCRIPTION
56The
57.Fn random
58function
59uses a non-linear additive feedback random number generator employing a
60default table of size 31 long integers to return successive pseudo-random
61numbers in the range from 0 to
62.if t 2\u\s731\s10\d\(mi1.
63.if n (2**31)\(mi1.
64The period of this random number generator is very large, approximately
65.if t 16\(mu(2\u\s731\s10\d\(mi1).
66.if n 16*((2**31)\(mi1).
67.Pp
68The
69.Fn random
70and
71.Fn srandom
72functions have (almost) the same calling sequence and initialization properties as the
73.Xr rand 3
74and
75.Xr srand 3
76functions.
77The difference is that
78.Xr rand 3
79produces a much less random sequence \(em in fact, the low dozen bits
80generated by rand go through a cyclic pattern.
81All the bits generated by
82.Fn random
83are usable.
84For example,
85.Sq Li random()&01
86will produce a random binary
87value.
88.Pp
89Like
90.Xr rand 3 ,
91.Fn random
92will by default produce a sequence of numbers that can be duplicated
93by calling
94.Fn srandom
95with
96.Ql 1
97as the seed.
98.Pp
99The
100.Fn srandomdev
101routine initializes a state array using the
102.Xr random 4
103random number device which returns good random numbers,
104suitable for cryptographic use.
105Note that this particular seeding
106procedure can generate states which are impossible to reproduce by
107calling
108.Fn srandom
109with any value, since the succeeding terms in the
110state buffer are no longer derived from the LC algorithm applied to
111a fixed seed.
112.Pp
113The
114.Fn initstate
115routine allows a state array, passed in as an argument, to be initialized
116for future use.
117The size of the state array (in bytes) is used by
118.Fn initstate
119to decide how sophisticated a random number generator it should use \(em the
120more state, the better the random numbers will be.
121(Current "optimal" values for the amount of state information are
1228, 32, 64, 128, and 256 bytes; other amounts will be rounded down to
123the nearest known amount.
124Using less than 8 bytes will cause an error.)
125The seed for the initialization (which specifies a starting point for
126the random number sequence, and provides for restarting at the same
127point) is also an argument.
128The
129.Fn initstate
130function
131returns a pointer to the previous state information array.
132.Pp
133Once a state has been initialized, the
134.Fn setstate
135routine provides for rapid switching between states.
136The
137.Fn setstate
138function
139returns a pointer to the previous state array; its
140argument state array is used for further random number generation
141until the next call to
142.Fn initstate
143or
144.Fn setstate .
145.Pp
146Once a state array has been initialized, it may be restarted at a
147different point either by calling
148.Fn initstate
149(with the desired seed, the state array, and its size) or by calling
150both
151.Fn setstate
152(with the state array) and
153.Fn srandom
154(with the desired seed).
155The advantage of calling both
156.Fn setstate
157and
158.Fn srandom
159is that the size of the state array does not have to be remembered after
160it is initialized.
161.Pp
162With 256 bytes of state information, the period of the random number
163generator is greater than
164.if t 2\u\s769\s10\d,
165.if n 2**69
166which should be sufficient for most purposes.
167.Sh DIAGNOSTICS
168If
169.Fn initstate
170is called with less than 8 bytes of state information, or if
171.Fn setstate
172detects that the state information has been garbled, error
173messages are printed on the standard error output.
174.Sh SEE ALSO
175.Xr arc4random 3 ,
176.Xr rand 3 ,
177.Xr srand 3 ,
178.Xr random 4
179.Sh HISTORY
180These
181functions appeared in
182.Bx 4.2 .
183.Sh AUTHORS
184.An Earl T. Cohen
185.Sh BUGS
186About 2/3 the speed of
187.Xr rand 3 .
188.Pp
189The historical implementation used to have a very weak seeding; the
190random sequence did not vary much with the seed.
191The current implementation employs a better pseudo-random number
192generator for the initial state calculation.
193.Pp
194Applications requiring cryptographic quality randomness should use
195.Xr arc4random 3 .
196