xref: /dragonfly/lib/libc/stdlib/random.3 (revision 8a7bdfea) 
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32.\"     @(#)random.3	8.1 (Berkeley) 6/4/93
33.\" $FreeBSD: src/lib/libc/stdlib/random.3,v 1.11.2.6 2003/06/03 19:13:16 schweikh Exp $
34.\" $DragonFly: src/lib/libc/stdlib/random.3,v 1.5 2007/06/29 19:34:41 swildner Exp $
35.\"
36.Dd June 4, 1993
37.Dt RANDOM 3
38.Os
39.Sh NAME
40.Nm random ,
41.Nm srandom ,
42.Nm srandomdev ,
43.Nm initstate ,
44.Nm setstate
45.Nd better random number generator; routines for changing generators
46.Sh LIBRARY
47.Lb libc
48.Sh SYNOPSIS
49.In stdlib.h
50.Ft long
51.Fn random void
52.Ft void
53.Fn srandom "unsigned long seed"
54.Ft void
55.Fn srandomdev void
56.Ft char *
57.Fn initstate "unsigned long seed" "char *state" "long n"
58.Ft char *
59.Fn setstate "char *state"
60.Sh DESCRIPTION
61The
62.Fn random
63function
64uses a non-linear additive feedback random number generator employing a
65default table of size 31 long integers to return successive pseudo-random
66numbers in the range from 0 to
67.if t 2\u\s731\s10\d\(mi1.
68.if n (2**31)\(mi1.
69The period of this random number generator is very large, approximately
70.if t 16\(mu(2\u\s731\s10\d\(mi1).
71.if n 16*((2**31)\(mi1).
72.Pp
73The
74.Fn random
75and
76.Fn srandom
77functions have (almost) the same calling sequence and initialization properties as the
78.Xr rand 3
79and
80.Xr srand 3
81functions.
82The difference is that
83.Xr rand 3
84produces a much less random sequence \(em in fact, the low dozen bits
85generated by rand go through a cyclic pattern.  All the bits generated by
86.Fn random
87are usable.  For example,
88.Sq Li random()&01
89will produce a random binary
90value.
91.Pp
92Like
93.Xr rand 3 ,
94.Fn random
95will by default produce a sequence of numbers that can be duplicated
96by calling
97.Fn srandom
98with
99.Ql 1
100as the seed.
101.Pp
102The
103.Fn srandomdev
104routine initializes a state array using the
105.Xr urandom 4
106random number device which returns good random numbers,
107suitable for cryptographic use.
108Note that this particular seeding
109procedure can generate states which are impossible to reproduce by
110calling
111.Fn srandom
112with any value, since the succeeding terms in the
113state buffer are no longer derived from the LC algorithm applied to
114a fixed seed.
115.Pp
116The
117.Fn initstate
118routine allows a state array, passed in as an argument, to be initialized
119for future use.  The size of the state array (in bytes) is used by
120.Fn initstate
121to decide how sophisticated a random number generator it should use \(em the
122more state, the better the random numbers will be.
123(Current "optimal" values for the amount of state information are
1248, 32, 64, 128, and 256 bytes; other amounts will be rounded down to
125the nearest known amount.  Using less than 8 bytes will cause an error.)
126The seed for the initialization (which specifies a starting point for
127the random number sequence, and provides for restarting at the same
128point) is also an argument.
129The
130.Fn initstate
131function
132returns a pointer to the previous state information array.
133.Pp
134Once a state has been initialized, the
135.Fn setstate
136routine provides for rapid switching between states.
137The
138.Fn setstate
139function
140returns a pointer to the previous state array; its
141argument state array is used for further random number generation
142until the next call to
143.Fn initstate
144or
145.Fn setstate .
146.Pp
147Once a state array has been initialized, it may be restarted at a
148different point either by calling
149.Fn initstate
150(with the desired seed, the state array, and its size) or by calling
151both
152.Fn setstate
153(with the state array) and
154.Fn srandom
155(with the desired seed).
156The advantage of calling both
157.Fn setstate
158and
159.Fn srandom
160is that the size of the state array does not have to be remembered after
161it is initialized.
162.Pp
163With 256 bytes of state information, the period of the random number
164generator is greater than
165.if t 2\u\s769\s10\d,
166.if n 2**69
167which should be sufficient for most purposes.
168.Sh DIAGNOSTICS
169If
170.Fn initstate
171is called with less than 8 bytes of state information, or if
172.Fn setstate
173detects that the state information has been garbled, error
174messages are printed on the standard error output.
175.Sh SEE ALSO
176.Xr arc4random 3 ,
177.Xr rand 3 ,
178.Xr srand 3 ,
179.Xr urandom 4
180.Sh HISTORY
181These
182functions appeared in
183.Bx 4.2 .
184.Sh AUTHORS
185.An Earl T. Cohen
186.Sh BUGS
187About 2/3 the speed of
188.Xr rand 3 .
189.Pp
190The historical implementation used to have a very weak seeding; the
191random sequence did not vary much with the seed.
192The current implementation employs a better pseudo-random number
193generator for the initial state calculation.
194