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.\" This code is derived from software contributed to Berkeley by
.\" Chris Torek and the American National Standards Committee X3,
.\" on Information Processing Systems.
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.\"     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 .