arc4random(9) supply pseudo-random numbers

Other Alias

arc4rand, random, read_random, srandom


In sys/libkern.h Ft void Fn srandom u_long seed Ft u_long Fn random void Ft void Fn arc4rand void *ptr u_int length int reseed Ft uint32_t Fn arc4random void

In sys/random.h Ft int Fn read_random void *buffer int count


The Fn random function will by default produce a sequence of numbers that can be duplicated by calling Fn srandom with `1' as the Fa seed . The Fn srandom function may be called with any arbitrary Fa seed value to get slightly more unpredictable numbers. It is important to remember that the Fn random function is entirely predictable, and is therefore not of use where knowledge of the sequence of numbers may be of benefit to an attacker.

The Fn arc4rand function will return very good quality random numbers, slightly better suited for security-related purposes. The random numbers from Fn arc4rand are seeded from the entropy device if it is available. Automatic reseeds happen after a certain timeinterval and after a certain number of bytes have been delivered. A forced reseed can be forced by passing a non-zero value in the Fa reseed argument.

The Fn read_random function is used to return entropy directly from the entropy device if it has been loaded. If the entropy device is not loaded, then the Fa buffer is filled with output generated by Fn random . The Fa buffer is filled with no more than Fa count bytes. It is advised that Fn read_random is not used; instead use Fn arc4rand

All the bits generated by Fn random , Fn arc4rand and Fn read_random are usable. For example, `random()&01 ' will produce a random binary value.

The Fn arc4random is a convenience function which calls Fn arc4rand to return a 32 bit pseudo-random integer.


The Fn random function uses a non-linear additive feedback random number generator employing a default table of size 31 long integers to return successive pseudo-random numbers in the range from 0 to (2**31)-1. The period of this random number generator is very large, approximately 16*((2**31)-1).

The Fn arc4rand function uses the RC4 algorithm to generate successive pseudo-random bytes. The Fn arc4random function uses Fn arc4rand to generate pseudo-random numbers in the range from 0 to (2**32)-1.

The Fn read_random function returns the number of bytes placed in Fa buffer .


An Dan Moschuk wrote Fn arc4random . An Mark R V Murray wrote Fn read_random .