RANDOM(3) Library Functions Manual RANDOM(3)
random, srandom, initstate, setstate -- better random number generator;
routines for changing generators
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 properties 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 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 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
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
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.
rand(3), srand(3), rnd(4), rnd(9)
The random(), srandom(), initstate() and setstate() functions conform to
IEEE Std 1003.1-2008 (``POSIX.1'').
These functions appeared in 4.2BSD.
Earl T. Cohen
About 2/3 the speed of rand(3).
NetBSD 6.1.5 October 15, 2011 NetBSD 6.1.5