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  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 5.05 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)