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Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)random.3 8.1 (Berkeley) 6/4/93 .\" $FreeBSD: src/lib/libc/stdlib/random.3,v 1.22 2007/01/09 00:28:10 imp Exp $ .\" .Dd June 4, 1993 .Dt RANDOM 3 .Os .Sh NAME .Nm initstate , .Nm random , .Nm setstate , .Nm srandom , .Nm srandomdev .Nd better random number generator; routines for changing generators .Sh LIBRARY .Lb libc .Sh SYNOPSIS .In stdlib.h .Ft char * .Fo initstate .Fa "unsigned seed" .Fa "char *state" .Fa "size_t size" .Fc .Ft long .Fo random .Fa void .Fc .Ft char * .Fo setstate .Fa "const char *state" .Fc .Ft void .Fo srandom .Fa "unsigned seed" .Fc .Ft void .Fo srandomdev .Fa void .Fc .Sh DESCRIPTION The .Fn 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 .if t 2\u\s731\s10\d\(mi1. .if n (2**31)\(mi1. The period of this random number generator is very large, approximately .if t 16\(mu(2\u\s731\s10\d\(mi1). .if n 16*((2**31)\(mi1). .Pp The .Fn random and .Fn srandom functions have (almost) the same calling sequence and initialization properties as the .Xr rand 3 and .Xr srand 3 functions. The difference is that .Xr rand 3 produces a much less random sequence \(em in fact, the low dozen bits generated by rand go through a cyclic pattern. All of the bits generated by .Fn random are usable. For example, .Sq Li random()&01 will produce a random binary value. .Pp Like .Xr srand 3 , .Fn srandom sets the initial seed value for future calls to .Fn random . Like .Xr rand 3 , .Fn random will by default produce a sequence of numbers that can be duplicated by calling .Fn srandom with the same seed. .Pp The .Fn srandomdev routine initializes a state array, using the .Xr random 4 random number device which returns good random numbers, suitable for cryptographic use. Note that this particular seeding procedure can generate states which are impossible to reproduce by calling .Fn 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. .Pp The .Fn 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 .Fn initstate to decide how sophisticated a random number generator it should use \(em 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 .Fn initstate function returns a pointer to the previous state information array. .Pp Once a state has been initialized, the .Fn setstate routine provides for rapid switching between states. The .Fn 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 .Fn initstate or .Fn setstate . .Pp Once a state array has been initialized, it may be restarted at a different point either by calling .Fn initstate (with the desired seed, the state array, and its size) or by calling both .Fn setstate (with the state array) and .Fn srandom (with the desired seed). The advantage of calling both .Fn setstate and .Fn srandom is that the size of the state array does not have to be remembered after it is initialized. .Pp With 256 bytes of state information, the period of the random number generator is greater than .if t 2\u\s769\s10\d, .if n 2**69 , which should be sufficient for most purposes. .Sh DIAGNOSTICS If .Fn initstate is called with less than 8 bytes of state information, or if .Fn setstate detects that the state information has been garbled, error messages are printed on the standard error output. .Sh LEGACY SYNOPSIS .Fd #include <stdlib.h> .Pp .Ft char * .br .Fo initstate .Fa "unsigned long seed" .Fa "char *state" .Fa "long size" .Fc ; .Pp .Ft char * .br .Fo setstate .Fa "char *state" .Fc ; .Pp .Ft void .br .Fo srandom .Fa "unsigned long seed" .Fc ; .Pp The type of each parameter is different in the legacy version. .Sh SEE ALSO .Xr arc4random 3 , .Xr rand 3 , .Xr srand 3 , .Xr random 4 , .Xr compat 5 .Sh HISTORY These functions appeared in .Bx 4.2 . .Sh AUTHORS .An Earl T. Cohen .Sh BUGS About 2/3 the speed of .Xr rand 3 . .Pp 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 initial state calculation. .Pp Applications requiring cryptographic quality randomness should use .Xr arc4random 3 .