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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | BUGS | SEE ALSO | COLOPHON |
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arc4random(3) Library Functions Manual arc4random(3)
arc4random, arc4random_uniform, arc4random_buf -
cryptographically-secure pseudorandom number generator
Standard C library (libc, -lc)
#include <stdlib.h>
uint32_t arc4random(void);
uint32_t arc4random_uniform(uint32_t upper_bound);
void arc4random_buf(size_t n;
void buf[n], size_t n);
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.
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.
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() │ │ │
└──────────────────────────────────────┴───────────────┴─────────┘
BSD.
OpenBSD 2.1, FreeBSD 3.0, NetBSD 1.6, DragonFly 1.0, libbsd, glibc
2.36.
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.
getrandom(3), rand(3), drand48(3), random(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-06-28 arc4random(3)
Pages that refer to this page: bash(1)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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