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RANDOM(3)		  OpenBSD Programmer's Manual		     RANDOM(3)

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

     #include <stdlib.h>


     srandom(unsigned int seed);


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

     char *
     setstate(const char *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

     The random() and srandom() functions have (almost) the same calling
     sequence and initialization properties as rand(3)/srand(3).  The
     difference is that rand 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 random numbers
     obtained from the kernel, suitable for cryptographic use.	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

     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

     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.

     arc4random(3), drand48(3), rand(3), random(4)

     The random(), srandom(), initstate(), and setstate() functions conform to
     X/Open Portability Guide Issue 4.2 (``XPG4.2'').

     The srandomdev() function is an extension.

     These functions appeared in 4.2BSD.

     Earl T. Cohen

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

OpenBSD 4.9			 May 31, 2007			   OpenBSD 4.9

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