NAME
random,
srandom,
initstate,
setstate —
better random number generator; routines for changing
generators
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <stdlib.h>
long
random(
void);
void
srandom(
unsigned
int seed);
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 maximum value
RANDOM_MAX
is defined in
<stdlib.h>.
The
random() and
srandom() have (almost) the
same calling sequence and initialization properties as
rand(3) and
srand(3). The difference is that
rand(3) produces a much less
random sequence — in fact, the low dozen bits generated by
rand(3) 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
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 state array passed to
initstate()
must be aligned to a 32-bit boundary. This can be achieved by using a
suitably-sized array of ints, and casting the array to char * when passing it
to
initstate(). 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
rand(3),
srand(3),
rnd(4),
rnd(9)
STANDARDS
The
random(),
srandom(),
initstate() and
setstate() functions
conform to
IEEE Std 1003.1-2008
(“POSIX.1”).
HISTORY
These functions appeared in
4.2BSD.
AUTHORS
Earl T. Cohen
BUGS
About 2/3 the speed of
rand(3).