xref: /minix3/lib/libc/string/memcmp.3 (revision 84d9c625bfea59e274550651111ae9edfdc40fbd)

.\" Copyright (c) 1990, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" This code is derived from software contributed to Berkeley by
.\" Chris Torek and the American National Standards Committee X3,
.\" on Information Processing Systems.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"     from: @(#)memcmp.3	8.1 (Berkeley) 6/4/93
.\"	$NetBSD: memcmp.3,v 1.11 2013/06/24 04:21:20 riastradh Exp $
.\"
.Dd June 23, 2013
.Dt MEMCMP 3
.Os
.Sh NAME
.Nm memcmp
.Nd compare byte string
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In string.h
.Ft int
.Fn memcmp "const void *b1" "const void *b2" "size_t len"
.Sh DESCRIPTION
The
.Fn memcmp
function
compares byte string
.Fa b1
against byte string
.Fa b2 .
Both strings are assumed to be
.Fa len
bytes long.
.Sh RETURN VALUES
The
.Fn memcmp
function
returns zero if the two strings are identical,
otherwise returns the difference between the first two differing bytes
(treated as unsigned char values, so that
.Sq Li \e200
is greater than
.Sq Li \&\e0 ,
for example).
Zero-length strings are always identical.
.Pp
Do not use
.Fn memcmp
to compare cryptographic secrets, because the time it takes varies
depending on how many bytes are the same, and thus leaks information
about the two strings by a timing side channel.
To compare secrets, hashes, message authentication codes, etc., use
.Xr consttime_memequal 3
instead.
.Sh SEE ALSO
.Xr bcmp 3 ,
.Xr consttime_memequal 3 ,
.Xr strcasecmp 3 ,
.Xr strcmp 3 ,
.Xr strcoll 3 ,
.Xr strxfrm 3
.Sh STANDARDS
The
.Fn memcmp
function
conforms to
.St -ansiC .