Copyright 1997 Niels Provos <provos@physnet.uni-hamburg.de>
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Manual page, using -mandoc macros
.Dd "Dec 31, 2014" .Dt ARC4RANDOM 3C .Os .Sh NAME .Nm arc4random , .Nm arc4random_buf , .Nm arc4random_uniform .Nd random number generator .Sh SYNOPSIS n stdlib.h .Ft uint32_t .Fn arc4random "void" .Ft void .Fn arc4random_buf "void *buf" "size_t nbytes" .Ft uint32_t .Fn arc4random_uniform "uint32_t upper_bound" .Sh DESCRIPTION This family of functions provides higher quality data than those described in .Xr rand 3C , .Xr random 3C , and .Xr rand48 3C .
p 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 .Xr pthreads 5 and .Xr chroot 2 .
p 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 .Xr getentropy 2 on a regular basis, and also upon .Xr fork 2 .
p The .Fn arc4random function returns a single 32-bit value.
p .Fn arc4random_buf fills the region .Fa buf of length .Fa nbytes with random data.
p .Fn arc4random_uniform will return a single 32-bit value, uniformly distributed but less than .Fa upper_bound . This is recommended over constructions like .Dq Li 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. .Sh RETURN VALUES These functions are always successful, and no return value is reserved to indicate an error. .Sh INTERFACE STABILITY .Sy Committed .Sh MT-LEVEL .Sy MT-Safe .Sh SEE ALSO .Xr rand 3C , .Xr rand48 3C , .Xr random 3C