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.Dd February 1, 2020 33.Dt RAND 3 34.Os 35.Sh NAME 36.Nm rand , 37.Nm srand , 38.Nm rand_r 39.Nd bad random number generator 40.Sh LIBRARY 41.Lb libc 42.Sh SYNOPSIS 43.In stdlib.h 44.Ft void 45.Fn srand "unsigned seed" 46.Ft int 47.Fn rand void 48.Ft int 49.Fn rand_r "unsigned *ctx" 50.Sh DESCRIPTION 51.Bf -symbolic 52The functions described in this manual page are not cryptographically 53secure. 54Applications which require unpredictable random numbers should use 55.Xr arc4random 3 56instead. 57.Ef 58.Pp 59The 60.Fn rand 61function computes a sequence of pseudo-random integers in the range 62of 0 to 63.Dv RAND_MAX , 64inclusive. 65.Pp 66The 67.Fn srand 68function seeds the algorithm with the 69.Fa seed 70parameter. 71Repeatable sequences of 72.Fn rand 73output may be obtained by calling 74.Fn srand 75with the same 76.Fa seed . 77.Fn rand 78is implicitly initialized as if 79.Fn srand "1" 80had been invoked explicitly. 81.Pp 82In 83.Fx 13 , 84.Fn rand 85is implemented using the same 128-byte state LFSR generator algorithm as 86.Xr random 3 . 87However, the legacy 88.Fn rand_r 89function is not (and can not be, because of its limited 90.Fa *ctx 91size). 92.Fn rand_r 93implements the historical, poor-quality Park-Miller 32-bit LCG and should not 94be used in new designs. 95.Sh IMPLEMENTATION NOTES 96Since 97.Fx 13 , 98.Fn rand 99is implemented with the same generator as 100.Xr random 3 , 101so the low-order bits should no longer be significantly worse than the 102high-order bits. 103.Sh SEE ALSO 104.Xr arc4random 3 , 105.Xr random 3 , 106.Xr random 4 107.Sh STANDARDS 108The 109.Fn rand 110and 111.Fn srand 112functions 113conform to 114.St -isoC . 115.Pp 116The 117.Fn rand_r 118function is not part of 119.St -isoC 120and is marked obsolescent in 121.St -p1003.1-2008 . 122It may be removed in a future revision of POSIX. 123.Sh CAVEATS 124Prior to 125.Fx 13 , 126.Fn rand 127used the historical Park-Miller generator with 32 bits of state and produced 128poor quality output, especially in the lower bits. 129.Fn rand 130in earlier versions of 131.Fx , 132as well as other standards-conforming implementations, may continue to produce 133poor quality output. 134.Pp 135.Em These functions should not be used in portable applications that want a 136.Em high quality or high performance pseudorandom number generator . 137One possible replacement, 138.Xr random 3 , 139is portable to Linux — but it is not especially fast, nor standardized. 140.Pp 141If broader portability or better performance is desired, any of the widely 142available and permissively licensed SFC64/32, JSF64/32, PCG64/32, or SplitMix64 143algorithm implementations may be embedded in your application. 144These algorithms have the benefit of requiring less space than 145.Xr random 3 146and being quite fast (in header inline implementations). 147