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Manual Reference Pages  -  ARC4RANDOM (9)


arc4rand, arc4random, random, read_random, srandom - supply pseudo-random numbers


Return Values


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

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


The random function will by default produce a sequence of numbers that can be duplicated by calling srandom with 1’ as the seed. The srandom function may be called with any arbitrary seed value to get slightly more unpredictable numbers. It is important to remember that the 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 arc4rand function will return very good quality random numbers, slightly better suited for security-related purposes. The random numbers from 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 reseed argument.

The 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 buffer is filled with output generated by random. The buffer is filled with no more than count bytes. It is advised that read_random is not used; instead use arc4rand

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

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


The 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 2310-1. The period of this random number generator is very large, approximately 16×(2310-1).

The arc4rand function uses the RC4 algorithm to generate successive pseudo-random bytes. The arc4random function uses arc4rand to generate pseudo-random numbers in the range from 0 to 2320-1.

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


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