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.\" Chris Torek and the American National Standards Committee X3, 6.\" on Information 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.\" from: @(#)memset.3 8.1 (Berkeley) 6/4/93 33.\" $NetBSD: memset.3,v 1.12 2013/06/24 07:56:49 wiz Exp $ 34.\" 35.Dd June 23, 2013 36.Dt MEMSET 3 37.Os 38.Sh NAME 39.Nm memset 40.Nd write a byte to byte string 41.Sh LIBRARY 42.Lb libc 43.Sh SYNOPSIS 44.In string.h 45.Ft void * 46.Fn memset "void *b" "int c" "size_t len" 47.Sh DESCRIPTION 48The 49.Fn memset 50function 51writes 52.Fa len 53bytes of value 54.Fa c 55(converted to an unsigned char) to the string 56.Fa b . 57.Sh RETURN VALUES 58The 59.Fn memset 60function 61returns the original value of 62.Fa b . 63.Pp 64Note that the compiler may optimize away a call to 65.Fn memset 66if it can prove that the string will not be used by the program again, 67for example if it is allocated on the stack and about to out of scope. 68If you want to guarantee that zeros are written to memory, for example 69to sanitize a buffer holding a cryptographic secret, use 70.Xr explicit_memset 3 . 71.Sh SEE ALSO 72.Xr bzero 3 , 73.Xr explicit_memset 3 , 74.Xr swab 3 75.Sh STANDARDS 76The 77.Fn memset 78function 79conforms to 80.St -ansiC . 81