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     arc4rand, arc4random, random, read_random, read_random_uio, srandom — supply pseudo-random


     #include <sys/libkern.h>

     srandom(u_long seed);


     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);


     The random() function will by default produce a sequence of numbers that can be duplicated
     by calling srandom() with some constant 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, 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 ignored and zero
     is returned.  The buffer is filled with no more than count bytes.  It is strongly advised
     that read_random() is not used; instead use arc4rand() unless it is necessary to know that
     no entropy has been returned.

     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.  This function only returns
     data if the random device is seeded.  It blocks if unseeded, except when the nonblock
     argument is true.

     All the bits returned by random(), arc4rand(), read_random(), and read_random_uio() 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 containing 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 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 (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.


     read_random_uio() may fail if:

     [EFAULT]           uio points to an invalid memory region.

     [EWOULDBLOCK]      The random device is unseeded and nonblock is true.


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