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ARC4RANDOM(3) Library Functions Manual ARC4RANDOM(3) NAME arc4random, arc4random_buf, arc4random_uniform -- random number generator LIBRARY Standard C Library (libc, -lc) SYNOPSIS #include <stdlib.h> uint32_t arc4random(void); void arc4random_buf(void *buf, size_t nbytes); uint32_t arc4random_uniform(uint32_t upper_bound); DESCRIPTION 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 consump- tion because the other interfaces are deficient in either quality, porta- bility, 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 gener- ate 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 geten- tropy(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 distrib- uted 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)) SEE ALSO rand(3), rand48(3), random(3) Daniel J. Bernstein, ChaCha, a variant of Salsa20, http://cr.yp.to/pa- pers.html#chacha, 2008-01-28, Document ID: 4027b5256e17b9796842e6d0f68b0b5e. Daniel Lemire, "Fast Random Integer Generation in an Interval", Association for Computing Machinery, ACM Trans. Model. Comput. Simul., no. 1, vol. 29, pp. 1-12, New York, NY, USA, January 2019. HISTORY These functions first appeared in OpenBSD 2.1. arc4random() first appeared in FreeBSD 3.0. arc4random_buf() and arc4random_uniform() first appeared in FreeBSD 8.0. arc4random_stir() was removed in FreeBSD 12.0. 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 tech- niques advance. A good mnemonic is "A Replacement Call for Random". The arc4random() random number generator was first introduced in FreeBSD 2.2.6. The ChaCha20 based implementation was introduced in FreeBSD 12.0, with obsolete stir and addrandom interfaces removed at the same time. FreeBSD ports 15.quarterly November 18, 2024 ARC4RANDOM(3)
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | EXAMPLES | SEE ALSO | HISTORY
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