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.\" @(#)memset.3 8.1 (Berkeley) 6/4/93 33.\" $FreeBSD$ 34.\" 35.Dd August 19, 2018 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 *dest" "int c" "size_t len" 47.Fd #define __STDC_WANT_LIB_EXT1__ 1 48.Ft errno_t 49.Fn memset_s "void *dest" "rsize_t destsz" "int c" "rsize_t len" 50.Sh DESCRIPTION 51The 52.Fn memset 53function 54writes 55.Fa len 56bytes of value 57.Fa c 58(converted to an 59.Vt "unsigned char" ) 60to the string 61.Fa dest . 62Undefined behaviour from 63.Fn memset , 64resulting from storage overflow, will occur if 65.Fa len 66is greater than the length of the 67.Fa dest 68buffer. 69The behaviour is also undefined if 70.Fa dest 71is an invalid pointer. 72.Pp 73The 74.Fn memset_s 75function behaves the same as 76.Fn memset 77except that an error is returned and the currently registered 78runtime-constraint handler is called if 79.Fa dest 80is a null pointer, 81.Fa destsz 82or 83.Fa len 84is greater than 85.Dv RSIZE_MAX , 86or 87.Fa len 88is greater than 89.Fa destsz 90(buffer overflow would occur). 91The runtime-constraint handler is called first and may not return. 92If it does return, an error is returned to the caller. 93Like 94.Xr explicit_bzero 3 , 95.Fn memset_s 96is not removed through Dead Store Elimination (DSE), making it useful for 97clearing sensitive data. 98In contrast 99.Fn memset 100function 101may be optimized away if the object modified by the function is not accessed 102again. 103To clear memory that will not subsequently be accessed it is advised to use 104.Fn memset_s 105instead of 106.Fn memset . 107For instance, a buffer containing a password should be cleared with 108.Fn memset_s 109before 110.Xr free 3 . 111.Sh RETURN VALUES 112The 113.Fn memset 114function returns its first argument. 115The 116.Fn memset_s 117function returns zero on success, non-zero on error. 118.Sh SEE ALSO 119.Xr bzero 3 , 120.Xr explicit_bzero 3 , 121.Xr set_constraint_handler_s 3 , 122.Xr swab 3 , 123.Xr wmemset 3 124.Sh STANDARDS 125The 126.Fn memset 127function 128conforms to 129.St -isoC . 130.Fn memset_s 131conforms to 132.St -isoC-2011 133K.3.7.4.1. 134