1memcmp(3)                  Library Functions Manual                  memcmp(3)
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NAME

6       memcmp - compare memory areas
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LIBRARY

9       Standard C library (libc, -lc)
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SYNOPSIS

12       #include <string.h>
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14       int memcmp(const void s1[.n], const void s2[.n], size_t n);
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DESCRIPTION

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

21       The memcmp() function returns  an  integer  less  than,  equal  to,  or
22       greater than zero if the first n bytes of s1 is found, respectively, to
23       be less than, to match, or be greater than the first n bytes of s2.
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25       For a nonzero return value, the sign is determined by the sign  of  the
26       difference  between  the  first  pair of bytes (interpreted as unsigned
27       char) that differ in s1 and s2.
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29       If n is zero, the return value is zero.
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ATTRIBUTES

32       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
33       tributes(7).
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35       ┌────────────────────────────────────────────┬───────────────┬─────────┐
36       │Interface                                   │ Attribute     │ Value   │
37       ├────────────────────────────────────────────┼───────────────┼─────────┤
38       │memcmp()                                    │ Thread safety │ MT-Safe │
39       └────────────────────────────────────────────┴───────────────┴─────────┘
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STANDARDS

42       C11, POSIX.1-2008.
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HISTORY

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

48       Do not use memcmp() to compare confidential data, such as cryptographic
49       secrets, because the CPU time required for the  comparison  depends  on
50       the  contents  of  the  addresses compared, this function is subject to
51       timing-based side-channel attacks.  In  such  cases,  a  function  that
52       performs  comparisons  in  deterministic time, depending only on n (the
53       quantity of  bytes  compared)  is  required.   Some  operating  systems
54       provide  such  a function (e.g., NetBSD's consttime_memequal()), but no
55       such function is specified  in  POSIX.   On  Linux,  you  may  need  to
56       implement such a function yourself.
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SEE ALSO

59       bstring(3),   strcasecmp(3),   strcmp(3),  strcoll(3),  strncasecmp(3),
60       strncmp(3), wmemcmp(3)
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64Linux man-pages 6.05              2023-07-30                         memcmp(3)
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