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RANDOM(3)                BSD Library Functions Manual                RANDOM(3)

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

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdlib.h>

     char *
     initstate(unsigned seed, char *state, size_t size);

     long
     random(void);

     char *
     setstate(const char *state);

     void
     srandom(unsigned seed);

     void
     srandomdev(void);

DESCRIPTION
     The random() function uses a non-linear, additive feedback, random number generator, employing a
     default table of size 31 long integers.  It returns 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 proper-ties properties
     ties 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 of 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 the random(4) random number device which
     returns good random numbers, suitable for cryptographic use.  Note that this particular seeding proce-dure procedure
     dure 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 ran-dom random
     dom 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 pro-vides provides
     vides 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.

AUTHORS
     Earl T. Cohen

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.

LEGACY SYNOPSIS
     #include <stdlib.h>

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

     char *
     setstate(char *state);

     void
     srandom(unsigned long seed);

     The type of each parameter is different in the legacy version.

SEE ALSO
     arc4random(3), rand(3), srand(3), random(4), compat(5)

HISTORY
     These functions appeared in 4.2BSD.

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 ini-tial initial
     tial state calculation.

     Applications requiring cryptographic quality randomness should use arc4random(3).

BSD                              June 4, 1993                              BSD