arc4random(3) — Linux manual page
arc4random(3) Library Functions Manual arc4random(3)
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).
┌─────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├─────────────────────────────────────┼───────────────┼─────────┤
│ arc4random(), arc4random_uniform(), │ Thread safety │ MT-Safe │
│ arc4random_buf() │ │ │
└─────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
BSD.
HISTORY
OpenBSD 2.1, FreeBSD 3.0, NetBSD 1.6, DragonFly 1.0, libbsd,
glibc 2.36.
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)
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