memcmp - compare memory areas
#include <string.h>
int memcmp(const void *s1, const void *s2, size_t n);
The
memcmp() function compares the first
n bytes (each interpreted
as
unsigned char) of the memory areas
s1 and
s2.
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.
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.
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.
bcmp(3),
bstring(3),
strcasecmp(3),
strcmp(3),
strcoll(3),
strncasecmp(3),
strncmp(3),
wmemcmp(3)