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ARC4RANDOM(9) Kernel Developer's Manual ARC4RANDOM(9)

arc4random, arc4random_buf, arc4random_uniform, enqueue_randomness
kernel random subsystem

#include <sys/systm.h>
u_int32_t
arc4random(void);
void
arc4random_buf(void *buf, size_t nbytes);
u_int32_t
arc4random_uniform(u_int32_t upper_bound);
#include <dev/rndvar.h>
void
enqueue_randomness(int);

The add_random() functions below mix input into the system entropy pool, which is then used to create a key for the ChaCha stream cipher used in the arc4random() series of functions.
enqueue_randomness() causes the supplied data argument to be added to the entropy pool, increasing the entropy counter by 32 bits.
arc4random() and arc4random_buf() provide random numbers and are intended to be called in any circumstance where random numbers are required.
arc4random_uniform() will return a uniformly distributed random number less than upper_bound, avoiding "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.
arc4random(), arc4random_buf(), and arc4random_uniform() may be called in nearly any kernel context.

arc4random(3), pchb(4), random(4)
April 28, 2018 OpenBSD-current