NAME
random, srandom,
srandom_deterministic,
srandomdev, initstate,
setstate —
pseudo-random number generator;
routines for changing generators
SYNOPSIS
#include
<stdlib.h>
long
random(void);
void
srandom(unsigned
int seed);
void
srandom_deterministic(unsigned
int seed);
void
srandomdev(void);
char *
initstate(unsigned
int seed, char
*state, size_t
n);
char *
setstate(char
*state);
DESCRIPTION
To satisfy portable code,
srandom()
or
srandomdev()
may be called to initialize the subsystem. In
OpenBSD the seed variable is
ignored, and strong random number results will be provided from
arc4random(3). In other systems, the seed
variable primes a simplistic deterministic algorithm.
If the standardized behavior is
required
srandom_deterministic()
can be substituted for srandom(), then subsequent
random() calls will return results using the
deterministic algorithm.
In non-deterministic (default) mode, the
random()
function returns results from
arc4random(3) in the range from 0 to (2**31)-1.
In deterministic mode, 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), but the
results are a deterministic sequence from the seed. The deterministic
sequence algorithm changed a number of times since original development, is
underspecified, and should not be relied upon to remain consistent between
platforms and over time.
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.
Use of
srandom_deterministic(),
initstate(), or setstate()
forces the subsystem into deterministic mode.
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(),
initstate(), and setstate()
functions conform to X/Open Portability Guide
Issue 4, Version 2 (“XPG4.2”).
The srandom() function does not conform to
X/Open Portability Guide Issue 4, Version 2
(“XPG4.2”), intentionally.
The srandomdev() function is an
extension.
The srandom_deterministic() function is an
OpenBSD extension.
HISTORY
These functions appeared in 4.2BSD.
AUTHORS
Earl T. Cohen