| RANDOM(3) | Library Functions Manual | RANDOM(3) |
initstate, random,
setstate, srandom,
srandomdev — better random
number generator; routines for changing generators
Standard C Library (libc, -lc)
#include
<stdlib.h>
char *
initstate(unsigned seed,
char *state, size_t size);
long
random(void);
char *
setstate(const char *state);
void
srandom(unsigned seed);
void
srandomdev(void);
The
random()
function uses a non-linear, additive feedback, random number generator,
employing a default table of size 31 long integers. It returns 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
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 of the bits generated by
random() are usable. For example,
‘random()&01’ will produce a
random binary value.
Like srand(3),
srandom()
sets the initial seed value for future calls to
random(). Like rand(3),
random() will by default produce a sequence of
numbers that can be duplicated by calling srandom()
with the same 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.
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.
#include
<stdlib.h>
char *
initstate(unsigned
long seed, char *state, long
size);;
char *
setstate(char
*state);;
void
srandom(unsigned
long seed);;
The type of each parameter is different in the legacy version.
These functions appeared in 4.2BSD.
Earl T. Cohen
About 2/3 the speed of rand(3).
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).
| June 4, 1993 | Mac OS X 12 |