memcmp

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
       attributes(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
       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 5.06 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)