RANDOM(3) | Library Functions Manual | RANDOM(3) |

`random`

, `srandom`

,
`srandomdev`

, `initstate`

,
`setstate`

— better random
number generator; routines for changing generators

```
#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`);

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).

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 `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, error messages are printed on
the standard error output.

The `random`

(),
`srandom`

(), `initstate`

(), and
`setstate`

() functions conform to
X/Open Portability Guide Issue 4, Version 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).

June 5, 2013 | OpenBSD-5.4 |