random(3) BSD Library Functions Manual random(3)
NAME
initstate, random, setstate, srandom, srandomdev -- better random number
generator; routines for changing generators
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <stdlib.h>
char *
initstate(unsigned seed, char *state, size_t size);
long
random(void);
char *
setstate(const char *state);
void
srandom(unsigned seed);
void
srandomdev(void);
DESCRIPTION
The random() function uses a non-linear, additive feedback, random number
generator, employing a default table of size 31 long integers. It
returns 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 random() and srandom() functions have (almost) the same calling
sequence and initialization properties as the rand(3) and srand(3) func-
tions. 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 of the bits generated by random() are usable. For
example, `random()&01' will produce a random binary value.
Like srand(3), srandom() sets the initial seed value for future calls to
random(). Like rand(3), random() will by default produce a sequence of
numbers that can be duplicated by calling srandom() with the same seed.
The srandomdev() routine initializes a state array, using the random(4)
random number device which returns good random numbers, suitable for
cryptographic use. Note that this particular seeding procedure can gen-
erate 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 gener-
ator 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 pre-
vious 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 differ-
ent 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 gen-
erator is greater than 2**69 , which should be sufficient for most pur-
poses.
DIAGNOSTICS
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.
LEGACY SYNOPSIS
#include <stdlib.h>
char *
initstate(unsigned long seed, char *state, long size);
char *
setstate(char *state);
void
srandom(unsigned long seed);
The type of each parameter is different in the legacy version.
SEE ALSO
arc4random(3), rand(3), srand(3), random(4), compat(5)
HISTORY
These functions appeared in 4.2BSD.
AUTHORS
Earl T. Cohen
BUGS
About 2/3 the speed of rand(3).
The historical implementation used to have a very weak seeding; the ran-
dom sequence did not vary much with the seed. The current implementation
employs a better pseudo-random number generator for the initial state
calculation.
Applications requiring cryptographic quality randomness should use
arc4random(3).
BSD June 4, 1993 BSD
Mac OS X 10.8 - Generated Thu Aug 30 10:59:48 CDT 2012