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     arc4rand, arc4random, random, read_random, 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);


     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

     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 (2**31)−1.  The
     period of this random number generator is very large, approximately

     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.


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