RANDOM(3) Library Functions Manual RANDOM(3)
random, srandom, initstate, setstate - better random number generator;
routines for changing generators
char *initstate(seed, state, n)
Random 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 231-1. The period of this
random number generator is very large, approximately 16*(231-1).
Random/srandom have (almost) the same calling sequence and initializa-
tion properties as rand/srand. 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.
Unlike srand, srandom 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 restarting/chang-
ing random number generators). Like rand(3), however, random will by
default produce a sequence of numbers that can be duplicated by calling
srandom with 1 as the 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 num-
bers will be. (Current "optimal" values for the amount of state infor-
mation 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. Initstate returns a pointer to
the previous state information array.
Once a state has been initialized, the setstate routine provides for
rapid switching between states. Setstate returns a pointer to the pre-
vious 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 dif-
ferent 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 269, which should be sufficient for most pur-
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.
About 2/3 the speed of rand(3C).
4th Berkeley Distribution 19 January 1983 RANDOM(3)