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 random numbers.
arc4random() returns a uniformly‐distributed value.
arc4random_uniform() returns a uniformly‐distributed value less than
upper_bound (see CAVEATS).
arc4random_buf() fills the memory pointed to by buf, with n bytes of
random data.
The rand(3) and rand48(3) families of functions should only be used
where the quality of the random 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 random number.
arc4random_uniform() returns a random 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 attrib‐
utes(7).
┌────────────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├────────────────────────────────────────────┼───────────────┼─────────┤
│arc4random(), arc4random_uniform(), │ Thread safety │ MT‐Safe │
│arc4random_buf() │ │ │
└────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
These nonstandard functions are present in several Unix systems.
CAVEATS
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), rand48(3), random(7)
Linux man‐pages (unreleased) (date) arc4random(3)
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