RANDOM(3) OpenBSD Programmer's Manual RANDOM(3)NAME
random, srandom, srandomdev, initstate, setstate - better random number
generator; routines for changing generators
srandom(unsigned int seed);
initstate(unsigned int seed, char *state, size_t n);
setstate(const char *state);
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
The random() and srandom() functions have (almost) the same calling
sequence and initialization properties as rand(3)/srand(3). The
difference is that rand 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.
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 srandomdev() routine initializes a state array using random numbers
obtained from the kernel, suitable for cryptographic use. Note that this
particular seeding procedure can generate states which are impossible to
reproduce by calling srandom() with any value, since the succeeding terms
in the state buffer are no longer derived from the LC algorithm applied
to a fixed 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 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.
SEE ALSOarc4random(3), drand48(3), rand(3), random(4)STANDARDS
The random(), srandom(), initstate(), and setstate() functions conform to
X/Open Portability Guide Issue 4.2 (``XPG4.2'').
The srandomdev() function is an extension.
These functions appeared in 4.2BSD.
Earl T. Cohen
About 2/3 the speed of rand(3).
OpenBSD 4.9 May 31, 2007 OpenBSD 4.9