---
name: "random"
section: "4"
platform: "macos"
package: "xnu"
version: "12377.121.6"
release: "macOS 26.5"
platforms: ["linux", "freebsd", "openbsd", "macos"]
url: "https://new.carta.tech/random/4"
---

# random(4)

random data source devices.

## NAME

**random** , **urandom** – random data source devices.

## SYNOPSIS

**pseudo-device random**

## DESCRIPTION

The **random** device produces uniformly distributed random byte values of potentially high quality.

To obtain random bytes, open **/dev/random** for reading and read from it.

The same random data is also available from [getentropy(2)](/getentropy/2.md). Using the [getentropy(2)](/getentropy/2.md) system call interface will provide resiliency to file descriptor exhaustion, chroot, or sandboxing which can make **/dev/random** unavailable.  Additionally, the [arc4random(3)](/arc4random/3.md) API provides a fast userspace random number generator built on the **random** data source and is preferred over directly accessing the system's random device.

**/dev/urandom** is a compatibility nod to Linux. On Linux, **/dev/urandom** will produce lower quality output if the entropy pool drains, while **/dev/random** will prefer to block and wait for additional entropy to be collected. With Fortuna, this choice and distinction is not necessary, and the two devices behave identically. You may use either.

The **random** device implements the **Fortuna** pseudo random number generator algorithm and maintains its entropy pool. The kernel automatically seeds the algorithm with additional entropy during normal execution.

## FILES

- */dev/random*

  - */dev/urandom*

  ## HISTORY

A **random** device appeared in the Linux operating system.