random, srandom, initstate, setstate - better random number
generator; routines for changing generators
unsigned int seed;
char *initstate(seed, state, n)
unsigned int seed;
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 (2**31)-1.
The period of this random number generator is very large,
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 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/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
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.
About 2/3 the speed of rand(3).