Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
RANDOM(3)	       FreeBSD Library Functions Manual		     RANDOM(3)

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

     Standard C	Library	(libc, -lc)

     #include <stdlib.h>


     srandom(unsigned int seed);


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

     char *
     setstate(char *state);

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

     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 se-
     quence and	initialization properties as the rand(3) and srand(3) func-
     tions.  The difference is that rand(3) produces a much less random	se-
     quence -- in fact,	the low	dozen bits generated by	rand go	through	a
     cyclic pattern.  All the bits generated by	random() are usable.  For ex-
     ample, `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 pro-
     cedure 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	gener-
     ator 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 pre-
     vious 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 differ-
     ent 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 gen-
     erator is greater than 2**69 which	should be sufficient for most pur-

     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.

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

     These functions appeared in 4.2BSD.

     Earl T. Cohen

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

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

FreeBSD	13.0			 July 26, 2016			  FreeBSD 13.0


Want to link to this manual page? Use this URL:

home | help