1MEMCMP(3)                  Linux Programmer's Manual                 MEMCMP(3)
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NAME

6       memcmp - compare memory areas
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SYNOPSIS

9       #include <string.h>
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11       int memcmp(const void *s1, const void *s2, size_t n);
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DESCRIPTION

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

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

29       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
30       tributes(7).
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32       ┌──────────┬───────────────┬─────────┐
33       │Interface │ Attribute     │ Value   │
34       ├──────────┼───────────────┼─────────┤
35       │memcmp()  │ Thread safety │ MT-Safe │
36       └──────────┴───────────────┴─────────┘

CONFORMING TO

38       POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
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NOTES

41       Do  not use memcmp() to compare security critical data, such as crypto‐
42       graphic secrets, because the required CPU time depends on the number of
43       equal bytes.  Instead, a function that performs comparisons in constant
44       time is required.  Some  operating  systems  provide  such  a  function
45       (e.g.,  NetBSD's  consttime_memequal()), but no such function is speci‐
46       fied in POSIX.  On Linux, it may be necessary to implement such a func‐
47       tion oneself.
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SEE ALSO

50       bcmp(3),   bstring(3),   strcasecmp(3),  strcmp(3),  strcoll(3),  strn‐
51       casecmp(3), strncmp(3), wmemcmp(3)
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COLOPHON

54       This page is part of release 5.10 of the Linux  man-pages  project.   A
55       description  of  the project, information about reporting bugs, and the
56       latest    version    of    this    page,    can     be     found     at
57       https://www.kernel.org/doc/man-pages/.
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61                                  2017-09-15                         MEMCMP(3)
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