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RANDOM(3) FreeBSD Library Functions Manual RANDOM(3)
NAME
random, srandom, srandomdev, initstate, setstate - non-cryptographic
pseudorandom number generator; routines for changing generators
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <stdlib.h>
long
random(void);
void
srandom(unsigned int seed);
void
srandomdev(void);
char *
initstate(unsigned int seed, char *state, size_t n);
char *
setstate(char *state);
DESCRIPTION
The functions described in this manual page are not secure. Applications
which require unpredictable random numbers should use arc4random(3)
instead.
Unless initialized with less than 32 bytes of 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 16*((2**31)-1).
If initialized with less than 32 bytes of state, random() uses the poor-
quality 32-bit Park-Miller LCG.
The random() and srandom() functions are analagous to rand(3) and
srand(3).
Like rand(3), random() is implicitly initialized as if srandom(1) had
been invoked explicitly.
The srandomdev() routine initializes the state array using random numbers
obtained from the kernel. This can generate states which are impossible
to reproduce by calling srandom(), because the succeeding terms in the
state buffer are no longer derived from the Park-Miller LCG algorithm
applied to a fixed seed.
The initstate() routine initializes the provided state array of uint32_t
values and uses it in future random() invocations. (Despite the char *
type of state, the underlying object must be a naturally aligned array of
32-bit values.) 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.
returns a pointer to the previous state.
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.
DIAGNOSTICS
If initstate() is called with less than 8 bytes of state information, or
if setstate() detects that the state information has been garbled, NULL
is returned.
SEE ALSO
arc4random(3), lrand48(3), rand(3), random(4)
HISTORY
These functions appeared in 4.2BSD.
AUTHORS
Earl T. Cohen
FreeBSD 14.0-RELEASE-p6 February 1, 2020 FreeBSD 14.0-RELEASE-p6