MEMCMP(3) Linux Programmer's Manual MEMCMP(3)
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 at-
tributes(7).
+----------+---------------+---------+
|Interface | Attribute | Value |
+----------+---------------+---------+
|memcmp() | Thread safety | MT-Safe |
+----------+---------------+---------+
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
NOTES
Do not use memcmp() to compare security critical data, such as crypto-
graphic 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 speci-
fied in POSIX. On Linux, it may be necessary to implement such a func-
tion oneself.
SEE ALSO
bcmp(3), bstring(3), strcasecmp(3), strcmp(3), strcoll(3), strn-
casecmp(3), strncmp(3), wmemcmp(3)
COLOPHON
This page is part of release 5.07 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/.
2017-09-15 MEMCMP(3)