random
, srandom
,
srandomdev
, initstate
,
setstate
—
non-cryptographic pseudorandom number generator; routines for
changing generators
Standard C Library (libc, -lc)
#include <stdlib.h>
long
random
(void);
void
srandom
(unsigned
int seed);
void
srandomdev
(void);
char *
initstate
(unsigned
int seed, char
*state, size_t
n);
char *
setstate
(char
*state);
The functions described in this manual page are not secure.
Applications which require unpredictable random numbers should use
arc4random(3)
instead.
Unless initialized with less than 32 bytes of 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).
If initialized with less than 32 bytes of state,
random
() uses the poor-quality 32-bit Park-Miller
LCG.
The random
() and
srandom
() functions are analagous to
rand(3)
and
srand(3).
Like
rand(3),
random
() is implicitly initialized as if
srandom
(1) had been invoked
explicitly.
The srandomdev
() routine initializes the
state array using random numbers obtained from the kernel. This can generate
states which are impossible to reproduce by calling
srandom
(), because the succeeding terms in the state
buffer are no longer derived from the Park-Miller LCG algorithm applied to a
fixed seed.
The initstate
() routine initializes the
provided state array of uint32_t values and uses it in
future random
() invocations. (Despite the
char * type of state, the
underlying object must be a naturally aligned array of 32-bit values.) 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 is used as in
srandom
(). The initstate
()
function returns a pointer to the previous state information array.
The setstate
() routine switches
random
() to using the provided state. It returns a
pointer to the previous state.
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, NULL is returned.
These functions appeared in 4.2BSD.