arc4random_uniform3
Random number generator
Part of arc4random(3).
Usage
#include <stdlib.h>
uint32_t
arc4random(void);
void
arc4random_buf(void *buf, size_t nbytes);
uint32_t
arc4random_uniform(uint32_t upper_bound);
Library
Standard C Library (libc, -lc)
Overview
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 pthread(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(4) subsystem using getentropy(3) on a regular basis, and also upon fork(2).
The arc4random() function returns a single 32-bit value. The arc4random() function returns pseudo-random numbers in the range of 0 to (2**32)−1, and therefore has twice the range of rand(3) and random(3).
arc4random_buf() 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.
Return values
These functions are always successful, and no return value is reserved to indicate an error.
Examples
The following produces a drop-in replacement for the traditional rand() and random() functions using arc4random():
#define foo4random() (arc4random_uniform(RAND_MAX + 1))
Releases
39 releases since FreeBSD 8.0-RELEASE; the text changed 26 times.