getrandom

TLDR

Get random bytes (shell)

$ head -c [16] /dev/urandom | xxd

Get random bytes (C function)

$ getrandom(buffer, length, flags)

SYNOPSIS

#include <sys/random.h>
ssize_t getrandom(void \*buf, size_t buflen, unsigned int flags);

getrandom() is a Linux system call that fills a buffer with random bytes from the kernel's random number generator. It's the recommended interface for obtaining random data in programs.
Unlike reading from /dev/urandom, getrandom() blocks during early boot until the entropy pool is initialized, ensuring strong randomness.

PARAMETERS

buf

Buffer to receive random bytes.

buflen

Number of bytes to read.

flags

GRNDRANDOM (use /dev/random) or GRNDNONBLOCK.

FLAGS

$ 0 - Default, blocks until entropy available
GRND_RANDOM - Use /dev/random pool (may block)
GRND_NONBLOCK - Don't block, return error instead

SHELL ALTERNATIVES

$ # Read random bytes
dd if=/dev/urandom bs=16 count=1

# Generate random hex
openssl rand -hex 16

# Generate random base64
openssl rand -base64 16

CAVEATS

System call, not a command. Available since Linux 3.17. On older systems, read from /dev/urandom. May block at early boot.

HISTORY

The getrandom() system call was added to Linux kernel 3.17 in 2014 by Theodore Ts'o to address issues with /dev/urandom returning weak random data before the entropy pool was initialized.