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RANDOM(9) FreeBSD Kernel Developer's Manual RANDOM(9)
NAME arc4rand, arc4random, arc4random_buf, is_random_seeded, random, read_random, read_random_uio - supply pseudo-random numbers
SYNOPSIS #include <sys/libkern.h>
uint32_t arc4random(void);
void arc4random_buf(void *ptr, size_t len);
void arc4rand(void *ptr, u_int length, int reseed);
#include <sys/random.h>
bool is_random_seeded(void);
void read_random(void *buffer, int count);
int read_random_uio(struct uio *uio, bool nonblock);
LEGACY ROUTINES #include <sys/libkern.h>
u_long random(void);
DESCRIPTION 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 instead and see SECURITY CONSIDERATIONS.
RETURN VALUES 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 (2**32)-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.
random() returns numbers in the range from 0 to (2**31)-1.

ERRORS read_random_uio() may fail if:
[EFAULT] uio points to an invalid memory region.
[EWOULDBLOCK] The random device is unseeded and nonblock is true.
AUTHORS Dan Moschuk wrote arc4random(). Mark R V Murray wrote read_random().
SECURITY CONSIDERATIONS Do not use random() 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.
FreeBSD 14.0-RELEASE-p11 March 22, 2021 FreeBSD 14.0-RELEASE-p11