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.Dd October 24, 2024 33.Dt MEMSET 3 34.Os 35.Sh NAME 36.Nm memset 37.Nd write a byte to byte string 38.Sh LIBRARY 39.Lb libc 40.Sh SYNOPSIS 41.In string.h 42.Ft void * 43.Fn memset "void *dest" "int c" "size_t len" 44.Ft void * 45.Fn memset_explicit "void *dest" "int c" "size_t len" 46.Fd #define __STDC_WANT_LIB_EXT1__ 1 47.Ft errno_t 48.Fn memset_s "void *dest" "rsize_t destsz" "int c" "rsize_t len" 49.Sh DESCRIPTION 50The 51.Fn memset 52function 53writes 54.Fa len 55bytes of value 56.Fa c 57(converted to an 58.Vt "unsigned char" ) 59to the string 60.Fa dest . 61Undefined behaviour from 62.Fn memset , 63resulting from storage overflow, will occur if 64.Fa len 65is greater than the length of the 66.Fa dest 67buffer. 68The behaviour is also undefined if 69.Fa dest 70is an invalid pointer. 71.Pp 72The 73.Fn memset_explicit 74function behaves the same as 75.Fn memset, but will not be removed by a compiler's dead store 76optimization pass, making it useful for clearing sensitive memory 77such as a password. 78.Pp 79The 80.Fn memset_s 81function behaves the same as 82.Fn memset 83except that an error is returned and the currently registered 84runtime-constraint handler is called if 85.Fa dest 86is a null pointer, 87.Fa destsz 88or 89.Fa len 90is greater than 91.Dv RSIZE_MAX , 92or 93.Fa len 94is greater than 95.Fa destsz 96(buffer overflow would occur). 97The runtime-constraint handler is called first and may not return. 98If it does return, an error is returned to the caller. 99Like 100.Xr explicit_bzero 3 , 101.Fn memset_s 102is not removed through Dead Store Elimination (DSE), making it useful for 103clearing sensitive data. 104In contrast 105.Fn memset 106function 107may be optimized away if the object modified by the function is not accessed 108again. 109To clear memory that will not subsequently be accessed it is advised to use 110.Fn memset_s 111instead of 112.Fn memset . 113For instance, a buffer containing a password should be cleared with 114.Fn memset_s 115before 116.Xr free 3 . 117.Sh RETURN VALUES 118The 119.Fn memset 120and 121.Fn memset_explicit 122functions return their first argument. 123The 124.Fn memset_s 125function returns zero on success, non-zero on error. 126.Sh SEE ALSO 127.Xr bzero 3 , 128.Xr explicit_bzero 3 , 129.Xr set_constraint_handler_s 3 , 130.Xr swab 3 , 131.Xr wmemset 3 132.Sh STANDARDS 133The 134.Fn memset 135function 136conforms to 137.St -isoC . 138.Fn memset_s 139conforms to 140.St -isoC-2011 141K.3.7.4.1. 142.Fn memset_explicit 143conforms to 144.St -isoC-2024 . 145