Партнёры:
Хостинг:
random srandom srandomdev initstate setstate - better random number generator; routines for changing generators
Lb libc
The
random ();
and
srandom ();
functions have (almost) the same calling sequence and initialization properties as the
rand(3)
and
srand(3)
functions.
The difference is that
rand(3)
produces a much less random sequence --- in fact, the low dozen bits
generated by rand 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
srandomdev ();
routine initializes a state array using the
random(4)
random number device which returns good random numbers,
suitable for cryptographic use.
Note that this particular seeding
procedure can generate states which are impossible to reproduce by
calling
srandom ();
with any value, since the succeeding terms in the
state buffer are no longer derived from the LC algorithm applied to
a fixed 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
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.
The historical implementation used to have a very weak seeding; the random sequence did not vary much with the seed. The current implementation employs a better pseudo-random number generator for the initial state calculation.
Applications requiring cryptographic quality randomness should use arc4random(3).
|
Закладки на сайте Проследить за страницей |
Created 1996-2024 by Maxim Chirkov Добавить, Поддержать, Вебмастеру |