NAME
random, srandom,
srandomdev, initstate,
setstate —
better random number generator;
routines for changing generators
SYNOPSIS
#include
<stdlib.h>
long
random(void);
void
srandom(unsigned
int seed);
void
srandomdev(void);
char *
initstate(unsigned
int seed, char
*state, size_t
n);
char *
setstate(char
*state);
DESCRIPTION
The
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 (2**31)-1. The period of this
random number generator is very large, approximately 16*((2**31)-1).
The
random()
and
srandom()
functions have (almost) the same calling sequence and initialization
properties as rand(3)/srand(3). The difference is that
rand 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
random() are usable. For example,
‘random()&01’ will produce a
random binary value.
Like
rand(3),
random()
will by default produce a sequence of numbers that can be duplicated by
calling
srandom()
with ‘1’ as the seed.
The
srandomdev()
routine initializes a state array using random numbers obtained from the
kernel, suitable for cryptographic use. Note that this particular seeding
procedure can generate states which are impossible to reproduce by calling
srandom()
with any value, since the succeeding terms in the state buffer are no longer
derived from the LC algorithm applied to a fixed seed.
The
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
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. The initstate()
function returns a pointer to the previous state information array.
Once a state has been initialized, the
setstate()
routine provides for rapid switching between states. The
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 initstate()
or setstate().
Once a state array has been initialized, it may
be restarted at a different point either by calling
initstate()
(with the desired seed, the state array, and its size) or by calling both
setstate() (with the state array) and
srandom() (with the desired seed). The advantage of
calling both setstate() and
srandom() is that the size of the state array does
not have to be remembered after it is initialized.
With 256 bytes of state information, the period of the random number generator is greater than 2**69 which should be sufficient for most purposes.
DIAGNOSTICS
If initstate() is called with less than 8
bytes of state information, or if setstate() detects
that the state information has been garbled, error messages are printed on
the standard error output.
SEE ALSO
STANDARDS
The random(),
srandom(), initstate(), and
setstate() functions conform to
X/Open Portability Guide Issue 4, Version 2
(“XPG4.2”).
The srandomdev() function is an
extension.
HISTORY
These functions appeared in 4.2BSD.
AUTHORS
Earl T. Cohen
BUGS
About 2/3 the speed of rand(3).