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'\" t
.\" Copyright (C) 2023 Alejandro Colomar <alx@kernel.org>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH arc4random 3 2024-05-02 "Linux man-pages 6.8"
.SH NAME
arc4random, arc4random_uniform, arc4random_buf
\- cryptographically-secure pseudorandom number generator
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.P
.B uint32_t arc4random(void);
.BI "uint32_t arc4random_uniform(uint32_t " upper_bound );
.BI "void arc4random_buf(void " buf [. n "], size_t " n );
.fi
.SH DESCRIPTION
These functions give cryptographically-secure pseudorandom numbers.
.P
.BR arc4random ()
returns a uniformly-distributed value.
.P
.BR arc4random_uniform ()
returns a uniformly-distributed value less than
.I upper_bound
(see BUGS).
.P
.BR arc4random_buf ()
fills the memory pointed to by
.IR buf ,
with
.I n
bytes of pseudorandom data.
.P
The
.BR rand (3)
and
.BR drand48 (3)
families of functions should only be used where
the quality of the pseudorandom numbers is not a concern
.I and
there's a need for repeatability of the results.
Unless you meet both of those conditions,
use the
.BR arc4random ()
functions.
.SH RETURN VALUE
.BR arc4random ()
returns a pseudorandom number.
.P
.BR arc4random_uniform ()
returns a pseudorandom number less than
.I upper_bound
for valid input, or
.B 0
when
.I upper_bound
is invalid.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface Attribute Value
T{
.na
.nh
.BR arc4random (),
.BR arc4random_uniform (),
.BR arc4random_buf ()
T} Thread safety MT-Safe
.TE
.SH STANDARDS
BSD.
.SH HISTORY
OpenBSD 2.1,
FreeBSD 3.0,
NetBSD 1.6,
DragonFly 1.0,
libbsd,
glibc 2.36.
.SH BUGS
An
.I upper_bound
of
.B 0
doesn't make sense in a call to
.BR arc4random_uniform ().
Such a call will fail, and return
.BR 0 .
Be careful,
since that value is
.I not
less than
.IR upper_bound .
In some cases,
such as accessing an array,
using that value could result in Undefined Behavior.
.SH SEE ALSO
.BR getrandom (3),
.BR rand (3),
.BR drand48 (3),
.BR random (7)
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