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
These nonstandard functions are present on several Unix systems.
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 6.03 2023-01-05 arc4random(3)
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