NAME¶

memcmp - memory compare

LIBRARY¶

Standard C library (libc, -lc)

SYNOPSIS¶

#include <string.h>  // see memory.h(3head)

int memcmp(size_t n;
           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).

VERSIONS¶

POSIX.1 specifies only that:

In glibc, as in most other implementations, the return value is the arithmetic result of subtracting the last compared byte in s2 from the last compared byte in s1. (If the two characters are equal, this difference is 0.)

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¶

memory.h(3head), memeq(3), wmemcmp(3)