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     random_harvest — gather entropy from the kernel for the entropy device


     #include <sys/types.h>
     #include <sys/random.h>

     random_harvest(void *entropy, u_int size, u_int bits, u_int frac,
         enum esource source);


     The random_harvest() function is used by device drivers and other kernel
     processes to pass data that is considered (at least partially) stochastic
     to the entropy device.

     The caller should pass a pointer (to no more than 16 bytes) of the
     “random” data in entropy.  The argument size contains the number of bytes
     pointed to.  The caller should very conservatively estimate the number of
     random bits in the sample, and pass this in bits or frac.  If the
     estimated number of bits per sample is an integer, then bits is used, and
     frac is 0.  Otherwise, for low-entropy samples, “fractional” entropy can
     be supplied in frac.  (This is considered to be frac / 1024 bits of
     entropy.)  The source is chosen from RANDOM_WRITE, RANDOM_KEYBOARD,
     RANDOM_MOUSE, RANDOM_NET and RANDOM_INTERRUPT, and is used to indicate
     the source of the entropy.

     Interrupt harvesting has been simplified for the kernel programmer.  If a
     device driver registers an interrupt handler with BUS_SETUP_INTR(9) or
     bus_setup_intr(9), then it is only necessary to include the INTR_ENTROPY
     bit in the flags argument to have that interrupt source be used for
     entropy harvesting.


     random(4), BUS_SETUP_INTR(9)


     The FreeBSD random(4) entropy device and supporting documentation was
     written by Mark R V Murray.