1.\" Copyright (c) 1990, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" This code is derived from software contributed to Berkeley by 5.\" the American National Standards Committee X3, on Information 6.\" Processing Systems. 7.\" 8.\" Redistribution and use in source and binary forms, with or without 9.\" modification, are permitted provided that the following conditions 10.\" are met: 11.\" 1. Redistributions of source code must retain the above copyright 12.\" notice, this list of conditions and the following disclaimer. 13.\" 2. Redistributions in binary form must reproduce the above copyright 14.\" notice, this list of conditions and the following disclaimer in the 15.\" documentation and/or other materials provided with the distribution. 16.\" 3. Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)rand.3 8.1 (Berkeley) 6/4/93 33.\" $FreeBSD$ 34.\" 35.Dd February 1, 2020 36.Dt RAND 3 37.Os 38.Sh NAME 39.Nm rand , 40.Nm srand , 41.Nm rand_r 42.Nd bad random number generator 43.Sh LIBRARY 44.Lb libc 45.Sh SYNOPSIS 46.In stdlib.h 47.Ft void 48.Fn srand "unsigned seed" 49.Ft int 50.Fn rand void 51.Ft int 52.Fn rand_r "unsigned *ctx" 53.Sh DESCRIPTION 54.Bf -symbolic 55The functions described in this manual page are not cryptographically 56secure. 57Applications which require unpredictable random numbers should use 58.Xr arc4random 3 59instead. 60.Ef 61.Pp 62The 63.Fn rand 64function computes a sequence of pseudo-random integers in the range 65of 0 to 66.Dv RAND_MAX , 67inclusive. 68.Pp 69The 70.Fn srand 71function seeds the algorithm with the 72.Fa seed 73parameter. 74Repeatable sequences of 75.Fn rand 76output may be obtained by calling 77.Fn srand 78with the same 79.Fa seed . 80.Fn rand 81is implicitly initialized as if 82.Fn srand "1" 83had been invoked explicitly. 84.Pp 85In 86.Fx 13 , 87.Fn rand 88is implemented using the same 128-byte state LFSR generator algorithm as 89.Xr random 3 . 90However, the legacy 91.Fn rand_r 92function is not (and can not be, because of its limited 93.Fa *ctx 94size). 95.Fn rand_r 96implements the historical, poor-quality Park-Miller 32-bit LCG and should not 97be used in new designs. 98.Sh IMPLEMENTATION NOTES 99Since 100.Fx 13 , 101.Fn rand 102is implemented with the same generator as 103.Xr random 3 , 104so the low-order bits should no longer be significantly worse than the 105high-order bits. 106.Sh SEE ALSO 107.Xr arc4random 3 , 108.Xr random 3 , 109.Xr random 4 110.Sh STANDARDS 111The 112.Fn rand 113and 114.Fn srand 115functions 116conform to 117.St -isoC . 118.Pp 119The 120.Fn rand_r 121function is not part of 122.St -isoC 123and is marked obsolescent in 124.St -p1003.1-2008 . 125It may be removed in a future revision of POSIX. 126.Sh CAVEATS 127Prior to 128.Fx 13 , 129.Fn rand 130used the historical Park-Miller generator with 32 bits of state and produced 131poor quality output, especially in the lower bits. 132.Fn rand 133in earlier versions of 134.Fx , 135as well as other standards-conforming implementations, may continue to produce 136poor quality output. 137.Pp 138.Em These functions should not be used in portable applications that want a 139.Em high quality or high performance pseudorandom number generator . 140One possible replacement, 141.Xr random 3 , 142is portable to Linux — but it is not especially fast, nor standardized. 143.Pp 144If broader portability or better performance is desired, any of the widely 145available and permissively licensed SFC64/32, JSF64/32, PCG64/32, or SplitMix64 146algorithm implementations may be embedded in your application. 147These algorithms have the benefit of requiring less space than 148.Xr random 3 149and being quite fast (in header inline implementations). 150