NAME
random, srandom,
srandomdev, initstate,
setstate —
non-cryptographic pseudorandom number
generator; routines for changing generators
LIBRARY
library “libc”
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
Unless initialized with less than 32 bytes of state,
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).
If initialized with less than 32 bytes of state,
random()
uses the poor-quality 32-bit Park-Miller LCG.
The
random()
and
srandom()
functions are analagous to
rand(3) and
srand(3).
Like
rand(3),
random()
is implicitly initialized as if
srandom(1)
had been invoked explicitly.
The
srandomdev()
routine initializes the state array using random numbers obtained from the
kernel. This can generate states which are impossible to reproduce by
calling
srandom(),
because the succeeding terms in the state buffer are no longer derived from
the Park-Miller LCG algorithm applied to a fixed seed.
The
initstate()
routine initializes the provided state array of
uint32_t values and uses it in future
random() invocations. (Despite the
char * type of state, the
underlying object must be a naturally aligned array of 32-bit values.) 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 is used as in
srandom(). The initstate()
function returns a pointer to the previous state information array.
The
setstate()
routine switches random() to using the provided
state. It returns a pointer to the previous state.
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, NULL is returned.
SEE ALSO
HISTORY
These functions appeared in 4.2BSD.
AUTHORS
Earl T. Cohen