NAME¶
memcmp - compare memory areas
SYNOPSIS¶
#include <string.h>
int memcmp(const void *s1, const void *s2, size_t n);
DESCRIPTION¶
The
memcmp() function compares the first
n bytes (each interpreted
as
unsigned char) of the memory areas
s1 and
s2.
RETURN VALUE¶
The
memcmp() function returns an integer less than, equal to, or greater
than zero if the first
n bytes of
s1 is found, respectively, to
be less than, to match, or be greater than the first
n bytes of
s2.
For a nonzero return value, the sign is determined by the sign of the difference
between the first pair of bytes (interpreted as
unsigned char) that
differ in
s1 and
s2.
If
n is zero, the return value is zero.
ATTRIBUTES¶
For an explanation of the terms used in this section, see
attributes(7).
Interface |
Attribute |
Value |
memcmp () |
Thread safety |
MT-Safe |
POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
NOTES¶
Do not use
memcmp() to compare security critical data, such as
cryptographic secrets, because the required CPU time depends on the number of
equal bytes. Instead, a function that performs comparisons in constant time is
required. Some operating systems provide such a function (e.g., NetBSD's
consttime_memequal()), but no such function is specified in POSIX. On
Linux, it may be necessary to implement such a function oneself.
SEE ALSO¶
bcmp(3),
bstring(3),
strcasecmp(3),
strcmp(3),
strcoll(3),
strncasecmp(3),
strncmp(3),
wmemcmp(3)
COLOPHON¶
This page is part of release 4.10 of the Linux
man-pages project. A
description of the project, information about reporting bugs, and the latest
version of this page, can be found at
https://www.kernel.org/doc/man-pages/.