arc4random.3bsd - Man Page

arc4 random number generator


library “libbsd”


#include <stdlib.h> (See libbsd(7) for include usage.)

arc4random_buf(void *buf, size_t nbytes);

arc4random_uniform(uint32_t upper_bound);


arc4random_addrandom(unsigned char *dat, int datlen);


This family of functions provides higher quality data than those described in rand(3), random(3), and rand48(3).

Use of these functions is encouraged for almost all random number consumption because the other interfaces are deficient in either quality, portability, standardization, or availability. These functions can be called in almost all coding environments, including pthreads(3) and chroot(2).

High quality 32-bit pseudo-random numbers are generated very quickly. On each call, a cryptographic pseudo-random number generator is used to generate a new result. One data pool is used for all consumers in a process, so that consumption under program flow can act as additional stirring. The subsystem is re-seeded from the kernel random number subsystem using getentropy(2) on a regular basis, and also upon fork(2).

The arc4random() function returns a single 32-bit value.

The arc4random_buf() function fills the region buf of length nbytes with random data.

arc4random_uniform() will return a single 32-bit value, uniformly distributed but less than upper_bound. This is recommended over constructions like “arc4random() % upper_bound” as it avoids "modulo bias" when the upper bound is not a power of two. In the worst case, this function may consume multiple iterations to ensure uniformity; see the source code to understand the problem and solution.

The arc4random_stir() function reads data from getentropy(2) and uses it to re-seed the subsystem via arc4random_addrandom().

There is no need to call arc4random_stir() before using arc4random() functions family, since they automatically initialize themselves.

Return Values

These functions are always successful, and no return value is reserved to indicate an error.

See Also

rand(3), rand48(3), random(3)


These functions first appeared in OpenBSD 2.1, FreeBSD 3.0, NetBSD 1.6, and DragonFly 1.0.

The original version of this random number generator used the RC4 (also known as ARC4) algorithm. In OpenBSD 5.5 it was replaced with the ChaCha20 cipher, and it may be replaced again in the future as cryptographic techniques advance. A good mnemonic is “A Replacement Call for Random”.

Referenced By


The man pages arc4random_addrandom.3bsd(3), arc4random_buf.3bsd(3), arc4random_stir.3bsd(3) and arc4random_uniform.3bsd(3) are aliases of arc4random.3bsd(3).

July 19, 2014