memcmp

NAME

memcmp - compare memory areas

LIBRARY

Standard C library (libc, -lc)

SYNOPSIS

#include <string.h>

int memcmp(const void s1[.n], const void s2[.n], 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

STANDARDS

POSIX.1-2001, POSIX.1-2008, 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

bstring(3), strcasecmp(3), strcmp(3), strcoll(3), strncasecmp(3), strncmp(3), wmemcmp(3)