long
random(
void
)
void
srandom(
unsigned long seed
)
char
*
initstate(
unsigned long seed
, char *state
, size_t n
)
char
*
setstate(
char *state
)
)
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
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.
)
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.