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

FreeBSD Manual Pages


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

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

     #include <stdlib.h>


     srandom(unsigned int seed);

     srandom_deterministic(unsigned int	seed);


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

     char *
     setstate(char *state);

     Standards insist that this	interface return deterministic results.	 Un-
     safe usage	is very	common,	so OpenBSD changed the subsystem to return
     non-deterministic results by default.

     To	satisfy	portable code, srandom() or srandomdev() may be	called to ini-
     tialize the subsystem.  In	OpenBSD	the seed variable is ignored, and
     strong random number results will be provided from	arc4random(3).	In
     other systems, the	seed variable primes a simplistic deterministic	algo-

     If	the standardized behavior is required srandom_deterministic() can be
     substituted for srandom(),	then subsequent	random() calls will return re-
     sults using the deterministic algorithm.

     In	non-deterministic (default) mode, the random() function	returns	re-
     sults from	arc4random(3) in the range from	0 to (2**31)-1.

     In	deterministic mode, 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), but the results are a	deterministic
     sequence from the seed.  The deterministic	sequence algorithm changed a
     number of times since original development, is underspecified, and	should
     not be relied upon	to remain consistent between platforms and over	time.

     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.

     Use of srandom_deterministic(), initstate(), or setstate()	forces the
     subsystem into deterministic mode.

     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(), initstate(),	and setstate() functions conform to X/Open
     Portability Guide Issue 4,	Version	2 ("XPG4.2").

     The srandom() function does not conform to	X/Open Portability Guide
     Issue 4, Version 2	("XPG4.2"), intentionally.

     The srandomdev() function is an extension.

     The srandom_deterministic() function is an	OpenBSD	extension.

     These functions appeared in 4.2BSD.

     Earl T. Cohen

FreeBSD	13.0		       February	12, 2021		  FreeBSD 13.0


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

home | help