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RANDOM(3)              FreeBSD Library Functions Manual              RANDOM(3)

NAME
     random, srandom, srandomdev, initstate, setstate - better random number
     generator; routines for changing generators

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdlib.h>

     long
     random(void);

     void
     srandom(unsigned long seed);

     void
     srandomdev(void);

     char *
     initstate(unsigned long seed, char *state, long n);

     char *
     setstate(char *state);

DESCRIPTION
     The functions described in this manual page are not cryptographically
     secure.  Cryptographic applications should use arc4random(3) instead.

     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
     16*((2**31)-1).

     The random() and srandom() functions have (almost) the same calling
     sequence and initialization properties as the rand(3) and srand(3)
     functions.  The difference is that rand(3) produces a much less random
     sequence -- in fact, the low dozen bits generated by rand go through a
     cyclic pattern.  All the bits generated by random() are usable.  For
     example, `random()&01' will produce a random binary value.

     Like rand(3), random() will by default produce a sequence of numbers that
     can be duplicated by calling srandom() with `1' as the seed.

     The srandomdev() routine initializes a state array using pseudo-random
     numbers obtained from the kernel.  Note that this particular seeding
     procedure can generate states which are impossible to reproduce by
     calling srandom() with any value, since the succeeding terms in the state
     buffer are no longer derived from the LC algorithm applied to a fixed
     seed.

     The initstate() routine allows a state array, passed in as an argument,
     to be initialized for future use.  The size of the state array (in bytes)
     is used by initstate() to decide how sophisticated a random number
     generator it should use -- the more state, the better the random numbers
     will be.  (Current "optimal" values for the amount of state information
     are 8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to
     the nearest known amount.  Using less than 8 bytes will cause an error.)
     The seed for the initialization (which specifies a starting point for the
     random number sequence, and provides for restarting at the same point) is
     also an argument.  The initstate() function returns a pointer to the
     previous state information array.

     Once a state has been initialized, the setstate() routine provides for
     rapid switching between states.  The setstate() function returns a
     pointer to the previous state array; its argument state array is used for
     further random number generation until the next call to initstate() or
     setstate().

     Once a state array has been initialized, it may be restarted at a
     different point either by calling initstate() (with the desired seed, the
     state array, and its size) or by calling both setstate() (with the state
     array) and srandom() (with the desired seed).  The advantage of calling
     both setstate() and srandom() is that the size of the state array does
     not have to be remembered after it is initialized.

     With 256 bytes of state information, the period of the random number
     generator is greater than 2**69 which should be sufficient for most
     purposes.

DIAGNOSTICS
     If initstate() is called with less than 8 bytes of state information, or
     if setstate() detects that the state information has been garbled, error
     messages are printed on the standard error output.

SEE ALSO
     arc4random(3), lrand48(3), rand(3), random(4)

HISTORY
     These functions appeared in 4.2BSD.

AUTHORS
     Earl T. Cohen

BUGS
     About 2/3 the speed of rand(3).

     The historical implementation used to have a very weak seeding; the
     random sequence did not vary much with the seed.  The current
     implementation employs a better pseudo-random number generator for the
     initial state calculation.

FreeBSD 11.0-PRERELEASE          April 2, 2013         FreeBSD 11.0-PRERELEASE

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | DIAGNOSTICS | SEE ALSO | HISTORY | AUTHORS | BUGS

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