memcmp - compare memory areas
The memcmp() function compares the first
bytes (each interpreted as
unsigned char) of the memory areas
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
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
n is zero, the return value is zero.
POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
Do not use memcmp() to compare security critical data, such as cryptographic 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 specified in POSIX. On Linux, it may be necessary to implement such a function oneself.
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