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: head/lib/libc/string/memset.3 338051 2018-08-19 14:39:57Z kib $ 34.\" 35.Dd September 14, 2019 36.Dt MEMSET 3 37.Os 38.Sh NAME 39.Nm memset , 40.Nm memset_s 41.Nd write a byte to byte string 42.Sh LIBRARY 43.Lb libc 44.Sh SYNOPSIS 45.In string.h 46.Ft void * 47.Fn memset "void *dest" "int c" "size_t len" 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 the length of buffer 67.Fa dest . 68The behaviour is also undefined if 69.Fa dest 70is an invalid pointer. 71.Pp 72The 73.Fn memset_s 74function behaves the same as 75.Fn memset 76except that an error is returned and the currently registered 77runtime-constraint handler is called if 78.Fa dest 79is a null pointer, 80.Fa destsz 81or 82.Fa len 83is greater than 84.Dv RSIZE_MAX , 85or 86.Fa len 87is greater than 88.Fa destsz 89(buffer overflow would occur). 90The runtime-constraint handler is called first and may not return. 91If it does return, an error is returned to the caller. 92Like 93.Xr explicit_bzero 3 , 94.Fn memset_s 95is not removed through Dead Store Elimination (DSE), making it useful for 96clearing sensitive data. 97In contrast 98.Fn memset 99function 100may be optimized away if the object modified by the function is not accessed 101again. 102To clear memory that will not subsequently be accessed it is advised to use 103.Fn memset_s 104instead of 105.Fn memset . 106For instance, a buffer containing a password should be cleared with 107.Fn memset_s 108before 109.Xr free 3 . 110.Sh RETURN VALUES 111The 112.Fn memset 113function returns its first argument. 114The 115.Fn memset_s 116function returns zero on success, non-zero on error. 117.Sh SEE ALSO 118.Xr bzero 3 , 119.Xr explicit_bzero 3 , 120.Xr set_constraint_handler_s 3 , 121.Xr swab 3 , 122.Xr wmemset 3 123.Sh STANDARDS 124The 125.Fn memset 126function 127conforms to 128.St -isoC . 129.Fn memset_s 130conforms to 131.St -isoC-2011 132K.3.7.4.1. 133