arc4random

NAME

arc4random, arc4random_uniform, arc4random_buf - cryptographically-secure pseudorandom number generator

LIBRARY

Standard C library (libc, -lc)

SYNOPSIS

#include <stdlib.h>

uint32_t arc4random(void);
uint32_t arc4random_uniform(uint32_t upper_bound);
void arc4random_buf(void buf[.n], size_t n);

DESCRIPTION

These functions give cryptographically-secure pseudorandom numbers.

arc4random() returns a uniformly-distributed value.

arc4random_uniform() returns a uniformly-distributed value less than upper_bound (see BUGS).

arc4random_buf() fills the memory pointed to by buf, with n bytes of pseudorandom data.

The rand(3) and drand48(3) families of functions should only be used where the quality of the pseudorandom numbers is not a concern and there’s a need for repeatability of the results. Unless you meet both of those conditions, use the arc4random() functions.

RETURN VALUE

arc4random() returns a pseudorandom number.

arc4random_uniform() returns a pseudorandom number less than upper_bound for valid input, or 0 when upper_bound is invalid.

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

STANDARDS

These nonstandard functions are present on several Unix systems.

BUGS

An upper_bound of 0 doesn’t make sense in a call to arc4random_uniform(). Such a call will fail, and return 0. Be careful, since that value is not less than upper_bound. In some cases, such as accessing an array, using that value could result in Undefined Behavior.

SEE ALSO

getrandom(3), rand(3), drand48(3), random(7)