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

NAME
     random, srandom, srandomdev, initstate, setstate - non-cryptographic
     pseudorandom number generator; routines for changing generators

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdlib.h>

     long
     random(void);

     void
     srandom(unsigned int seed);

     void
     srandomdev(void);

     char *
     initstate(unsigned int seed, char *state, size_t n);

     char *
     setstate(char *state);

DESCRIPTION
     The functions described in this manual page are not secure.  Applications
     which require unpredictable random numbers should use arc4random(3)
     instead.

     Unless initialized with less than 32 bytes of state, 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).

     If initialized with less than 32 bytes of state, random() uses the poor-
     quality 32-bit Park-Miller LCG.

     The random() and srandom() functions are analagous to rand(3) and
     srand(3).

     Like rand(3), random() is implicitly initialized as if srandom(1) had
     been invoked explicitly.

     The srandomdev() routine initializes the state array using random numbers
     obtained from the kernel.  This can generate states which are impossible
     to reproduce by calling srandom(), because the succeeding terms in the
     state buffer are no longer derived from the Park-Miller LCG algorithm
     applied to a fixed seed.

     The initstate() routine initializes the provided state array of uint32_t
     values and uses it in future random() invocations.  (Despite the char *
     type of state, the underlying object must be a naturally aligned array of
     32-bit values.)  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 is
     used as in srandom().  The initstate() function returns a pointer to the
     previous state information array.

     The setstate() routine switches random() to using the provided state.  It
     returns a pointer to the previous state.

     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, NULL
     is returned.

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

HISTORY
     These functions appeared in 4.2BSD.

AUTHORS
     Earl T. Cohen

FreeBSD 13.1-RELEASE-p6        February 1, 2020        FreeBSD 13.1-RELEASE-p6