random, srandom, srandomdev, initstate, setstate -- better random number
generator; routines for changing generators
Standard C Library (libc, -lc)
#include <stdlib.h>
long
random(void);
void
srandom(unsigned long seed);
void
srandomdev(void);
char *
initstate(unsigned long seed, char *state, long n);
char *
setstate(char *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).
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.
Earl T. Cohen
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.
arc4random(3), rand(3), srand(3), random(4)
These functions appeared in 4.2BSD.
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).
FreeBSD 5.2.1 June 4, 1993 FreeBSD 5.2.1 [ Back ] |
