arc4random_buf(3)

1arc4random(3)              Library Functions Manual              arc4random(3)
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NAME

6       arc4random,  arc4random_uniform, arc4random_buf - cryptographically-se‐
7       cure pseudorandom number generator
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LIBRARY

10       Standard C library (libc, -lc)
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SYNOPSIS

13       #include <stdlib.h>
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15       uint32_t arc4random(void);
16       uint32_t arc4random_uniform(uint32_t upper_bound);
17       void arc4random_buf(void buf[.n], size_t n);
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DESCRIPTION

20       These functions give cryptographically-secure pseudorandom numbers.
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22       arc4random() returns a uniformly-distributed value.
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24       arc4random_uniform() returns a uniformly-distributed  value  less  than
25       upper_bound (see BUGS).
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27       arc4random_buf()  fills  the  memory pointed to by buf, with n bytes of
28       pseudorandom data.
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30       The rand(3) and drand48(3) families of functions should  only  be  used
31       where  the  quality  of  the  pseudorandom numbers is not a concern and
32       there's a need for repeatability of the results.  Unless you meet  both
33       of those conditions, use the arc4random() functions.
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RETURN VALUE

36       arc4random() returns a pseudorandom number.
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38       arc4random_uniform()  returns  a  pseudorandom  number  less  than  up‐
39       per_bound for valid input, or 0 when upper_bound is invalid.
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ATTRIBUTES

42       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
43       tributes(7).
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45       ┌────────────────────────────────────────────┬───────────────┬─────────┐
46       │Interface                                   │ Attribute     │ Value   │
47       ├────────────────────────────────────────────┼───────────────┼─────────┤
48       │arc4random(), arc4random_uniform(),         │ Thread safety │ MT-Safe │
49       │arc4random_buf()                            │               │         │
50       └────────────────────────────────────────────┴───────────────┴─────────┘
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STANDARDS

53       BSD.
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HISTORY

56       OpenBSD 2.1, FreeBSD 3.0, NetBSD  1.6,  DragonFly  1.0,  libbsd,  glibc
57       2.36.
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BUGS

60       An   upper_bound   of   0   doesn't   make   sense   in   a   call   to
61       arc4random_uniform().  Such  a  call  will  fail,  and  return  0.   Be
62       careful, since that value is not less than upper_bound.  In some cases,
63       such as accessing an array, using that value could result in  Undefined
64       Behavior.
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