Maximum return value of arc4random()

Question

For some reason I'm not able to find any sort of official doc telling me what the range is for arc4random(). I've seen in some non-authoritative sources that it's 2^32 - 1, a little over 4 billion.

Can anyone confirm this? If so, can you link to an official doc showing this number somewhere? I'm trying to generate a random number on my iOS app and need to have a pretty official upper end for the algorithm to work correctly.

Answer 1

The man page for arc4random says:

The arc4random() function returns pseudo-random numbers in the range of 0 to 2³² – 1…

And, happily, this matches its return type of uint32_t. That is, arc4random may return any 32-bit unsigned integer. The inclusive upper bound is UINT32_MAX (or UInt32.max in Swift).

However, if you just need numbers uniformly distributed within a particular range, you should use arc4random_uniform(n) (the range is exclusive: arc4random_uniform(5) can return 0, 1, 2, 3, or 4). This is more correct — and more convenient! — than using % to truncate the range returned by regular arc4random().

Answer 2

You are right,

DESCRIPTION

 The arc4random() function uses the key stream generator employed by the arc4 cipher, which uses 8*8 8
 bit S-Boxes.  The S-Boxes can be in about (2**1700) states.  The arc4random() function returns pseudo-random pseudorandom
 random numbers in the range of 0 to (2**32)-1, and therefore has twice the range of rand(3) and
 random(3).

arc4random_buf() function fills the region buf of length nbytes with ARC4-derived random data.

arc4random_uniform() will return a uniformly distributed random number less than upper_bound. arc4random_uniform() is recommended over constructions like ``arc4random() % upper_bound'' as it avoids "modulo bias" when the upper bound is not a power of two.

The arc4random_stir() function reads data from /dev/urandom and uses it to permute the S-Boxes via arc4random_addrandom().

There is no need to call arc4random_stir() before using arc4random() functions family, since they auto-matically automatically matically initialize themselves.

Source https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man3/arc4random.3.html