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

       For an explanation of the terms used in this section, see attributes(7).

       ┌───────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │Interface                                                      │ Attribute     │ Value   │
       ├───────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │memcmp()                                                       │ Thread safety │ MT-Safe │
       └───────────────────────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

       C11, POSIX.1-2008.

HISTORY

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

CAVEATS

       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

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