1.\" $NetBSD: explicit_memset.3,v 1.3 2024/11/02 02:43:48 riastradh Exp $ 2.\" 3.\" Copyright (c) 2013 The NetBSD Foundation, Inc. 4.\" All rights reserved. 5.\" 6.\" This documentation is derived from text contributed to The NetBSD 7.\" Foundation by Taylor R. Campbell. 8.\" 9.\" Redistribution and use in source and binary forms, with or without 10.\" modification, are permitted provided that the following conditions 11.\" are met: 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in the 16.\" documentation and/or other materials provided with the distribution. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 19.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 20.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 21.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 22.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 23.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 24.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 25.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 26.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 27.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 28.\" POSSIBILITY OF SUCH DAMAGE. 29.\" 30.Dd November 1, 2024 31.Dt EXPLICIT_MEMSET 3 32.Os 33.Sh NAME 34.Nm explicit_memset 35.Nm memset_explicit 36.Nd guarantee writing a byte to a byte string 37.Sh LIBRARY 38.Lb libc 39.Sh SYNOPSIS 40.In string.h 41.Ft void * 42.Fn explicit_memset "void *b" "int c" "size_t len" 43.Ft void * 44.Fn memset_explicit "void *b" "int c" "size_t len" 45.Sh DESCRIPTION 46The 47.Fn explicit_memset 48and 49.Fn memset_explicit 50functions write 51.Fa len 52bytes of value 53.Fa c 54(converted to an unsigned char) to the string 55.Fa b . 56They are guaranteed not to be optimized away by the compiler even if 57.Fa b 58is no longer used and is about to be freed or go out of scope. 59.Sh RETURN VALUES 60The 61.Fn explicit_memset 62function returns the original value of 63.Fa b . 64.Sh EXAMPLES 65Create a buffer on the stack for a secret key, use it, and then zero it 66in memory before throwing it away. 67.Bd -literal -offset indent 68void 69f(void) 70{ 71 uint8_t key[32]; 72 73 crypto_random(key, sizeof(key)); 74 do_crypto_stuff(key, sizeof(key)); 75 \&... 76 77 explicit_memset(key, 0, sizeof(key)); 78} 79.Ed 80.Sh SEE ALSO 81.Xr consttime_memequal 3 , 82.Xr memset 3 83.Sh STANDARDS 84The 85.Fn memset_explicit 86function conforms to 87.St -isoC-2023 . 88.Sh HISTORY 89The 90.Fn explicit_memset 91function appeared in 92.Nx 7.0 . 93The 94.Fn memset_explicit 95alias was added in 96.Nx 11.0 . 97