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

`random`

,
`srandom`

,
`srandomdev`

,
`initstate`

,
`setstate`

—
better random number generator; routines for changing
generators

```
#include
<stdlib.h>
```

`random`

(`srandom`

(`srandomdev`

(`initstate`

(`setstate`

(This interface is not
cryptographically secure, so consider using
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 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 switches to an
algorithm using state derived from 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.
`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.
`random`

(),
`srandom`

(),
`initstate`

(), and
`setstate`

() functions conform to
X/Open Portability Guide Issue 4,
Version 2 (“XPG4.2”).
The `srandomdev`

() function is an extension.
July 18, 2014 | OpenBSD-5.6 |