random, srandom, initstate, setstate - better random number generator; routines for chang-
char *initstate(seed, state, n)
Random uses a non-linear additive feedback random number generator employing a default ta-
ble 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
Random/srandom have (almost) the same calling sequence and initialization 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 gen-
erated by random are usable. For example, ``random()&01'' will produce a random binary
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 pro-
vided to deal with restarting/changing 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 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. 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 previous state array; its argument
state array is used for further random number generation until the next call to initstate
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
About 2/3 the speed of rand(3C).
4.2 Berkeley Distribution September 29, 1985 RANDOM(3)