| RANDOM(3) | Library Functions Manual | RANDOM(3) |
random, srandom,
initstate, setstate —
better random number generator; routines for changing
generators
Standard C Library (libc, -lc)
#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);
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.
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.
The random(),
srandom(), initstate() and
setstate() functions conform to
IEEE Std 1003.1-2008 (“POSIX.1”).
These functions appeared in 4.2BSD.
Earl T. Cohen
About 2/3 the speed of rand(3).
| June 12, 2014 | NetBSD 11.0 |