xref: /freebsd/lib/libc/stdlib/random.3 (revision dc36d6f9bb1753f3808552f3afd30eda9a7b206a)

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.Dd February 1, 2020
.Dt RANDOM 3
.Os
.Sh NAME
.Nm random ,
.Nm srandom ,
.Nm srandomdev ,
.Nm initstate ,
.Nm setstate
.Nd non-cryptographic pseudorandom number generator; routines for changing generators
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdlib.h
.Ft long
.Fn random void
.Ft void
.Fn srandom "unsigned int seed"
.Ft void
.Fn srandomdev void
.Ft char *
.Fn initstate "unsigned int seed" "char *state" "size_t n"
.Ft char *
.Fn setstate "char *state"
.Sh DESCRIPTION
.Bf -symbolic
The functions described in this manual page are not secure.
Applications which require unpredictable random numbers should use
.Xr arc4random 3
instead.
.Ef
.Pp
Unless initialized with less than 32 bytes of state, the
.Fn 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
.if t 2\u\s731\s10\d\(mi1.
.if n (2**31)\(mi1.
The period of this random number generator is very large, approximately
.if t 16\(mu(2\u\s731\s10\d\(mi1).
.if n 16*((2**31)\(mi1).
.Pp
If initialized with less than 32 bytes of state,
.Fn random
uses the poor-quality 32-bit Park-Miller LCG.
.Pp
The
.Fn random
and
.Fn srandom
functions are analagous to
.Xr rand 3
and
.Xr srand 3 .
.Pp
Like
.Xr rand 3 ,
.Fn random
is implicitly initialized as if
.Fn srandom "1"
had been invoked explicitly.
.Pp
The
.Fn srandomdev
routine initializes the state array using random numbers obtained from the
kernel.
This can generate states which are impossible to reproduce by calling
.Fn srandom ,
because the succeeding terms in the state buffer are no longer derived from the
Park-Miller LCG algorithm applied to a fixed seed.
.Pp
The
.Fn initstate
routine initializes the provided state array of
.Vt uint32_t
values and uses it in future
.Fn random
invocations.
(Despite the
.Vt char *
type of
.Fa 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
.Fn initstate
to decide how sophisticated a random number generator it should use \(em 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
.Fa seed
is used as in
.Fn srandom .
The
.Fn initstate
function
returns a pointer to the previous state information array.
.Pp
The
.Fn setstate
routine switches
.Fn random
to using the provided state.
It returns a pointer to the previous state.
.Pp
Once a state array has been initialized, it may be restarted at a
different point either by calling
.Fn initstate
(with the desired seed, the state array, and its size) or by calling
both
.Fn setstate
(with the state array) and
.Fn srandom
(with the desired seed).
The advantage of calling both
.Fn setstate
and
.Fn srandom
is that the size of the state array does not have to be remembered after
it is initialized.
.Pp
With 256 bytes of state information, the period of the random number
generator is greater than
.if t 2\u\s769\s10\d,
.if n 2**69
which should be sufficient for most purposes.
.Sh DIAGNOSTICS
If
.Fn initstate
is called with less than 8 bytes of state information, or if
.Fn setstate
detects that the state information has been garbled,
NULL is returned.
.Sh SEE ALSO
.Xr arc4random 3 ,
.Xr lrand48 3 ,
.Xr rand 3 ,
.Xr random 4
.Sh HISTORY
These
functions appeared in
.Bx 4.2 .
.Sh AUTHORS
.An Earl T. Cohen