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Standard C Library (libc, -lc)

#include <stdlib.h>

The functions described in this manual page are not secure.
Applications which require unpredictable random numbers should use
arc4random(3)
instead.

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
16µ(2^{31}-1).

The
`random()`
and
`srandom()`
functions have (almost) the same calling sequence and initialization properties as the
rand(3)
and
srand(3)
functions.
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 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
pseudo-random numbers obtained from the kernel.
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
`setstate()`.

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.

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.

These
functions appeared in
*BSD 4.2 .*

About 2/3 the speed of
rand(3).

The historical implementation used to have a very weak seeding; the random sequence did not vary much with the seed. The current implementation employs a better pseudo-random number generator for the initial state calculation.

RANDOM (3) | July 26, 2016 |

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