132 lines
3.8 KiB
Groff
132 lines
3.8 KiB
Groff
.\" Copyright (c) 1983 Regents of the University of California.
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.\" All rights reserved. The Berkeley software License Agreement
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.\" specifies the terms and conditions for redistribution.
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.\"
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.\" @(#)random.3 6.2 (Berkeley) 9/29/85
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.\"
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.TH RANDOM 3 "September 29, 1985"
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.UC 5
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.SH NAME
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random, srandom, initstate, setstate \- better random number generator; routines for changing generators
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.SH SYNOPSIS
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.nf
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.ft B
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#include <stdlib.h>
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long random(void)
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void srandom(unsigned \fIseed\fP)
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char *initstate(unsigned \fIseed\fP, char *\fIstate\fP, int \fIn\fP)
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char *setstate(char *\fIstate\fP)
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.ft R
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.fi
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.SH DESCRIPTION
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.PP
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.B Random
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uses a non-linear additive feedback random number generator employing a
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default table of size 31 long integers to return successive pseudo-random
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numbers in the range from 0 to
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.if t 2\u\s731\s10\d\(mi1.
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.if n (2**31)\(mi1.
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The period of this random number generator is very large, approximately
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.if t 16\(mu(2\u\s731\s10\d\(mi1).
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.if n 16*((2**31)\(mi1).
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.PP
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.B Random/srandom
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have (almost) the same calling sequence and initialization properties as
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.B rand/srand.
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The difference is that
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.BR rand (3)
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produces a much less random sequence \(em in fact, the low dozen bits
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generated by rand go through a cyclic pattern. All the bits generated by
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.B random
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are usable. For example, ``random()&01'' will produce a random binary
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value.
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.PP
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Unlike
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.BR srand ,
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.B srandom
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does not return the old seed; the reason for this is that the amount of
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state information used is much more than a single word. (Two other
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routines are provided to deal with restarting/changing random
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number generators). Like
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.BR rand (3),
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however,
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.B random
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will by default produce a sequence of numbers that can be duplicated
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by calling
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.B srandom
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with
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.B 1
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as the seed.
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.PP
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The
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.B initstate
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routine allows a state array, passed in as an argument, to be initialized
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for future use. The size of the state array (in bytes) is used by
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.B initstate
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to decide how sophisticated a random number generator it should use -- the
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more state, the better the random numbers will be.
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(Current "optimal" values for the amount of state information are
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8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to
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the nearest known amount. Using less than 8 bytes will cause an error).
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The seed for the initialization (which specifies a starting point for
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the random number sequence, and provides for restarting at the same
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point) is also an argument.
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.B Initstate
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returns a pointer to the previous state information array.
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.PP
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Once a state has been initialized, the
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.B setstate
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routine provides for rapid switching between states.
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.B Setstate
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returns a pointer to the previous state array; its
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argument state array is used for further random number generation
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until the next call to
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.B initstate
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or
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.BR setstate .
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.PP
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Once a state array has been initialized, it may be restarted at a
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different point either by calling
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.B initstate
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(with the desired seed, the state array, and its size) or by calling
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both
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.B setstate
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(with the state array) and
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.B srandom
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(with the desired seed).
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The advantage of calling both
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.B setstate
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and
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.B srandom
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is that the size of the state array does not have to be remembered after
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it is initialized.
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.PP
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With 256 bytes of state information, the period of the random number
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generator is greater than
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.if t 2\u\s769\s10\d,
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.if n 2**69
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which should be sufficient for most purposes.
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.SH AUTHOR
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Earl T. Cohen
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.SH DIAGNOSTICS
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.PP
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If
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.B initstate
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is called with less than 8 bytes of state information, or if
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.B setstate
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detects that the state information has been garbled, error
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messages are printed on the standard error output.
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.SH "SEE ALSO"
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.BR rand (3).
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.SH NOTES
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.B initstate
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and
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.B setstate
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are not declared in
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.IR <stdlib.h> ,
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programmers must provide their own declarations.
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.SH BUGS
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About 2/3 the speed of
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.BR rand (3).
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