RANDOM(3) BSD Library Functions Manual RANDOM(3)
random, srandom, initstate, setstate -- better random number generator; routines for chang-
Standard C Library (libc, -lc)
srandom(unsigned long seed);
initstate(unsigned long seed, char *state, size_t n);
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 maximum value RANDOM_MAX is defined in <stdlib.h>.
The random() and srandom() have (almost) the same calling sequence and initialization prop-
erties as rand(3) and srand(3). The difference is that rand(3) produces a much less random
sequence -- in fact, the low dozen bits generated by rand(3) 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 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 ran-
dom 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 speci-
fies a starting point for the random number sequence, and provides for restarting at the
same point) is also an argument. The state array passed to initstate() must be aligned to a
32-bit boundary. This can be achieved by using a suitably-sized array of ints, and casting
the array to char * when passing it to initstate(). 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.
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 stan-
dard error output.
rand(3), srand(3), rnd(4), rnd(9)
The random(), srandom(), initstate() and setstate() functions conform to IEEE Std
These functions appeared in 4.2BSD.
Earl T. Cohen
About 2/3 the speed of rand(3).
BSD October 15, 2011 BSD