---
name: "arc4random"
section: "3"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/arc4random"
---

# arc4random(3)

cryptographically-secure pseudorandom number generator

## NAME

arc4random, arc4random\_uniform, arc4random\_buf - cryptographically-secure pseudorandom number generator

## LIBRARY

Standard C library (*libc*, *-lc*)

## SYNOPSIS

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

**uint32\_t arc4random(void);** **uint32\_t arc4random\_uniform(uint32\_t** *upper\_bound***);** **void arc4random\_buf(**size\_t n;                     **void** *buf***\[***n***\], size\_t** *n***);**

## DESCRIPTION

These functions give cryptographically-secure pseudorandom numbers.

**arc4random**() returns a uniformly-distributed value.

**arc4random\_uniform**() returns a uniformly-distributed value less than *upper\_bound* (see BUGS).

**arc4random\_buf**() fills the memory pointed to by *buf*, with *n* bytes of pseudorandom data.

The [**rand**(3)](/rand/3.md) and [**drand48**(3)](/drand48/3.md) families of functions should only be used where the quality of the pseudorandom numbers is not a concern *and* there's a need for repeatability of the results. Unless you meet both of those conditions, use the **arc4random**() functions.

## RETURN VALUE

**arc4random**() returns a pseudorandom number.

**arc4random\_uniform**() returns a pseudorandom number less than *upper\_bound* for valid input, or **0** when *upper\_bound* is invalid.

## ATTRIBUTES

For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **arc4random**(), **arc4random\_uniform**(), **arc4random\_buf**() | Thread safety | MT-Safe |

## STANDARDS

BSD.

## HISTORY

OpenBSD 2.1, FreeBSD 3.0, NetBSD 1.6, DragonFly 1.0, libbsd, glibc 2.36.

## BUGS

An *upper\_bound* of **0** doesn't make sense in a call to **arc4random\_uniform**(). Such a call will fail, and return **0**. Be careful, since that value is *not* less than *upper\_bound*. In some cases, such as accessing an array, using that value could result in undefined behavior.

## SEE ALSO

**getrandom**(3), [**rand**(3)](/rand/3.md), [**drand48**(3)](/drand48/3.md), [**random**(7)](/random/7.md)