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1992-08-10
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RANDOM(3) UNIX Programmer's Manual RANDOM(3)
NNAAMMEE
rraannddoomm, ssrraannddoomm, iinniittssttaattee, sseettssttaattee - better random number generator;
routines for changing generators
SSYYNNOOPPSSIISS
##iinncclluuddee <<ssttddlliibb>>
_l_o_n_g
rraannddoomm(_v_o_i_d)
_v_o_i_d
ssrraannddoomm(_u_n_s_i_g_n_e_d _s_e_e_d)
_c_h_a_r _*
iinniittssttaattee(_u_n_s_i_g_n_e_d _s_e_e_d, _c_h_a_r _*_s_t_a_t_e, _i_n_t _n)
_c_h_a_r _*
sseettssttaattee(_c_h_a_r _*_s_t_a_t_e)
DDEESSCCRRIIPPTTIIOONN
The rraannddoomm() function uses a nonlinear additive feedback random number
generator employing a default table of size 31 long integers to return
successive pseudorandom 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 rraannddoomm()/ ssrraannddoomm() have (almost) the same calling sequence and ini
tialization properties as rand(3)/ srand(3). The difference is that
rand 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
rraannddoomm() are usable. For example, `random()&01' will produce a random
binary value.
Unlike srand, ssrraannddoomm() does not return the old seed; the reason for
this is that the amount of state information used is much more than a
single word. (Two other routines are provided to deal with restart
ing/changing random number generators). Like rand(3), however, rraannddoomm()
will by default produce a sequence of numbers that can be duplicated by
calling ssrraannddoomm() with `1' as the seed.
The iinniittssttaattee() 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 iinniittssttaattee() to decide how sophisticated a random number gener
ator 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 iinniittssttaattee() function returns a pointer to the pre
vious state information array.
Once a state has been initialized, the sseettssttaattee() routine provides for
rapid switching between states. The sseettssttaattee() function returns a point
er to the previous state array; its argument state array is used for fur
ther random number generation until the next call to iinniittssttaattee() or
sseettssttaattee().
Once a state array has been initialized, it may be restarted at a differ
ent point either by calling iinniittssttaattee() (with the desired seed, the state
array, and its size) or by calling both sseettssttaattee() (with the state array)
and ssrraannddoomm() (with the desired seed). The advantage of calling both
sseettssttaattee() and ssrraannddoomm() 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 gen
erator is greater than 2**69 which should be sufficient for most purpos
es.
AAUUTTHHOORR
Earl T. Cohen
DDIIAAGGNNOOSSTTIICCSS
If iinniittssttaattee() is called with less than 8 bytes of state information, or
if sseettssttaattee() detects that the state information has been garbled, error
messages are printed on the standard error output.
SSEEEE AALLSSOO
rand(3)
HHIISSTTOORRYY
These functions appeared in 4.2BSD.
BBUUGGSS
About 2/3 the speed of rand(3).
4.2 Berkeley Distribution April 19, 1991 2