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arc4random(3) Library Functions Manual arc4random(3)
NAME
arc4random, arc4random_uniform, arc4random_buf - cryptographically-se-
cure 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 up-
per_bound for valid input, or 0 when upper_bound is invalid.
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(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_uni-
form(). 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)
Linux man-pages 6.04 2023-03-18 arc4random(3)
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