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Manual Pages  — RANDOM


arc4rand, arc4random, arc4random_buf, is_random_seeded, random, read_random, read_random_uio, srandom – supply pseudo-random numbers



#include <sys/libkern.h>


arc4random_buf(void *ptr, size_t len);

arc4rand(void *ptr, u_int length, int reseed);

#include <sys/random.h>


read_random(void *buffer, int count);

read_random_uio(struct uio *uio, bool nonblock);


#include <sys/libkern.h>

srandom(u_long seed);



The arc4random() and arc4random_buf() functions will return very good quality random numbers, suited for security-related purposes. Both are wrappers around the underlying arc4rand() interface. arc4random() returns a 32-bit random value, while arc4random_buf() fills ptr with len bytes of random data.

The arc4rand() CSPRNG is seeded from the random(4) kernel abstract entropy device. Automatic reseeding happens at unspecified time and bytes (of output) intervals. A reseed can be forced by passing a non-zero reseed value.

The read_random() function is used to read entropy directly from the kernel abstract entropy device. read_random() blocks if and until the entropy device is seeded. The provided buffer is filled with no more than count bytes. It is strongly advised that read_random() is not used directly; instead, use the arc4rand() family of functions.

The is_random_seeded() function can be used to check in advance if read_random() will block. (If random is seeded, it will not block.)

The read_random_uio() function behaves identically to read(2) on /dev/random. The uio argument points to a buffer where random data should be stored. If nonblock is true and the random device is not seeded, this function does not return any data. Otherwise, this function may block interruptibly until the random device is seeded. If the function is interrupted before the random device is seeded, no data is returned.

The legacy random() function will produce a sequence of numbers that can be duplicated by calling srandom() with some constant as the seed. The legacy srandom() function may be called with any seed value. It is strongly advised that the random() function not be used to generate random numbers. See SECURITY CONSIDERATIONS.


The arc4rand() function uses the Chacha20 algorithm to generate a pseudo-random sequence of bytes. The arc4random() function uses arc4rand() to generate pseudo-random numbers in the range from 0 to 232-1.

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

read_random_uio() returns zero when successful, otherwise an error code is returned.

The legacy random() function uses a non-linear additive feedback random number generator employing a default table of size 31 containing long integers to return successive pseudo-random numbers in the range from 0 to 231-1. The period of this random number generator is very large, approximately 16&#181;(231-1).


read_random_uio() may fail if:
  uio points to an invalid memory region.
  The random device is unseeded and nonblock is true.


Dan Moschuk wrote arc4random(). Mark R V Murray wrote read_random().


Do not use random() or srandom() in new code.

It is important to remember that the random() function is entirely predictable. It is easy for attackers to predict future output of random() by recording some generated values. We cannot emphasize strongly enough that random() must not be used to generate values that are intended to be unpredictable.

RANDOM (9) April 16, 2019

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