memcmp(3) — Linux manual page


memcmp(3)               Library Functions Manual               memcmp(3)

NAME         top

       memcmp - compare memory areas

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <string.h>

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

DESCRIPTION         top

       The memcmp() function compares the first n bytes (each
       interpreted as unsigned char) of the memory areas s1 and s2.

RETURN VALUE         top

       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         top

       For an explanation of the terms used in this section, see
       │ Interface                           Attribute     Value   │
       │ memcmp()                            │ Thread safety │ MT-Safe │

STANDARDS         top

       C11, POSIX.1-2008.

HISTORY         top

       POSIX.1-2001, C89, SVr4, 4.3BSD.

CAVEATS         top

       Do not use memcmp() to compare confidential data, such as
       cryptographic secrets, because the CPU time required for the
       comparison depends on the contents of the addresses compared,
       this function is subject to timing-based side-channel attacks.
       In such cases, a function that performs comparisons in
       deterministic time, depending only on n (the quantity of bytes
       compared) 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, you may need to
       implement such a function yourself.

SEE ALSO         top

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

Linux man-pages (unreleased)     (date)                        memcmp(3)

Pages that refer to this page: bcmp(3)bstring(3)size_t(3type)strcasecmp(3)strcmp(3)strcoll(3)strxfrm(3)void(3type)wmemcmp(3)signal-safety(7)