# NAME

**arc4random**,

**arc4random_buf**,

**arc4random_uniform** —

random number
generator
# SYNOPSIS

**#include <stdlib.h>**
`uint32_t`
**arc4random**(

`void`);

`void`
**arc4random_buf**(

`void
*buf`,

`size_t nbytes`);

`uint32_t`
**arc4random_uniform**(

`uint32_t
upper_bound`);

# DESCRIPTION

This family of functions provides higher quality data than those described in

rand(3),

random(3), and

rand48(3).

Use of these functions is encouraged for almost all random number consumption
because the other interfaces are deficient in either quality, portability,
standardization, or availability. These functions can be called in almost all
coding environments, including

pthreads(3) and

chroot(2).

High quality 32-bit pseudo-random numbers are generated very quickly. On each
call, a cryptographic pseudo-random number generator is used to generate a new
result. One data pool is used for all consumers in a process, so that
consumption under program flow can act as additional stirring. The subsystem
is re-seeded from the kernel random number subsystem using

getentropy(2) on a regular basis, and
also upon

fork(2).

The

**arc4random**() function returns a single 32-bit value.

**arc4random_buf**() fills the region

`buf` of
length

`nbytes` with random data.

**arc4random_uniform**() will return a single 32-bit value,
uniformly distributed but less than

`upper_bound`. This is
recommended over constructions like “

```
arc4random() %
upper_bound
```

” as it avoids "modulo bias" when the upper
bound is not a power of two. In the worst case, this function may consume
multiple iterations to ensure uniformity; see the source code to understand
the problem and solution.

# RETURN VALUES

These functions are always successful, and no return value is reserved to
indicate an error.

# SEE ALSO

rand(3),

rand48(3),

random(3)
# HISTORY

These functions first appeared in

OpenBSD 2.1.

The original version of this random number generator used the RC4 (also known as
ARC4) algorithm. In

OpenBSD 5.5 it was replaced with
the ChaCha20 cipher, and it may be replaced again in the future as
cryptographic techniques advance. A good mnemonic is “A Replacement Call
for Random”.