---
name: "getentropy"
section: "2"
platform: "macos"
package: "xnu"
version: "12377.121.6"
release: "macOS 26.5"
platforms: ["macos"]
url: "https://new.carta.tech/getentropy"
---

# getentropy(2)

get entropy

## NAME

**getentropy** – get entropy

## SYNOPSIS

**#include \<sys/random.h\>**

*int*\

**getentropy**(*void \*buf*, *size\_t buflen*);

## DESCRIPTION

**getentropy**() fills a buffer with random data, which can be used as input for process-context pseudorandom generators like [arc4random(3)](/arc4random/3.md).

The maximum buffer size permitted is 256 bytes. If *buflen* exceeds this, an error of EIO will be indicated.

**getentropy**() should be used as a replacement for [random(4)](/random/4.md) when random data derived directly from the kernel random byte generator is required. Unlike the [random(4)](/random/4.md) pseudo-devices, it is not vulnerable to file descriptor exhaustion attacks and is available when sandboxed or in a chroot, making it more reliable for security-critical applications.

However, it should be noted that **getentropy**() is primarily intended for use in the construction and seeding of userspace PRNGs like [arc4random(3)](/arc4random/3.md) or CC\_crypto(3). Clients who simply require random data should use [arc4random(3)](/arc4random/3.md), **CCRandomGenerateBytes**() from CC\_crypto(3), or **SecRandomCopyBytes**() from the Security framework instead of **getentropy**() or [random(4)](/random/4.md)

## RETURN VALUES

The **getentropy()** function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

## ERRORS

**getentropy**() will succeed unless:

- \[EINVAL\]

  The *buf* parameter points to an invalid address.

- \[EIO\]

  Too many bytes requested, or some other fatal error occurred.

## SEE ALSO

[arc4random(3)](/arc4random/3.md) CC\_crypto(3) [random(4)](/random/4.md)

## HISTORY

The **getentropy**() function appeared in OSX 10.12