RANDOM(3) | Library Functions Manual | RANDOM(3) |
random
, srandom
,
initstate
, setstate
—
#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);
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.
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.
random
(), srandom
(),
initstate
() and setstate
()
functions conform to IEEE Std 1003.1-2008
(“POSIX.1”).
June 12, 2014 | NetBSD 9.0 |