memcmp(3) Library Functions Manual memcmp(3)
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 attrib-
utes(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 per-
forms comparisons in deterministic time, depending only on n (the quan-
tity 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)
Linux man-pages 6.7 2023-10-31 memcmp(3)
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