getrandom(2)

Fill a buffer with random bytes from the kernel CSPRNG, without opening /dev/urandom.

Since: Linux 3.17check your kernel →
glibc: glibc 2.25
System: Linux
Man page: getrandom(2)

Synopsis

#include <sys/random.h>

ssize_t getrandom(void buf[size], size_t size, unsigned int flags);

Rod's take

getrandom(2) showed up in Linux 3.17 and fixed the early-boot /dev/urandom mess, but every call still crossed into the kernel and back. The BSDs have done userspace ChaCha20 behind arc4random for years: seed once, generate in-process, reseed when it matters. Linux finally caught up in 6.11 with a vDSO getrandom that keeps per-thread state in userspace and notices fork and reseeds on its own. Fine. Better late than never. For your code nothing changes: ask for small chunks, check the return value every time, and let the default block until the pool is seeded.

Flags and arguments that matter

Name	What it does
GRND_NONBLOCK	Don't block if the pool isn't initialized (or random source is empty); fail with EAGAIN instead.
GRND_RANDOM	Draw from the random source (/dev/random) instead of urandom; may return fewer bytes than asked.
GRND_INSECURELinux 5.6	Return bytes even before the pool is seeded, with no cryptographic guarantee. Early-boot use only.

Errors you will actually hit

errno	Meaning
EAGAIN	Requested entropy wasn't available and GRND_NONBLOCK was set.
EINTR	Interrupted by a signal handler; small (<= 256 byte) urandom reads after init don't hit this.
EINVAL	Invalid flag in flags.
EFAULT	buf points outside the accessible address space.
ENOSYS	glibc wrapper found the kernel doesn't implement getrandom().

Example

unsigned char nonce[12];
if (getrandom(nonce, sizeof nonce, 0) != (ssize_t)sizeof nonce)
    err(1, "getrandom");

/* early-boot code that must not block */
unsigned char seed[32];
ssize_t n = getrandom(seed, sizeof seed, GRND_NONBLOCK);
if (n == -1 && errno == EAGAIN) {
    /* pool not initialized yet: retry later */
}

Trimmed from the full walkthrough in getrandom(2) without the syscall: what Linux 6.11 finally shipped.

BSD equivalent

FreeBSD: getrandom(2) since FreeBSD 12.0, plus arc4random(3). OpenBSD: getentropy(2) since OpenBSD 5.6; arc4random(3) for bulk use.

The long version, from the console

getrandom(2) without the syscall: what Linux 6.11 finally shipped

Linux 6.11 added getrandom(2) to the vDSO. The kernel shares RNG state with userspace, ChaCha20 runs in the process, and the syscall disappears from hot paths. BSD got here in the 1990s.

2026-09-14

Sources

Version numbers are read off the cited man pages, not memory. Spot one that's wrong? Tell Rod.