MEMCMP(3) | Library Functions Manual | MEMCMP(3)

# NAME

`memcmp` —
compare byte string

# LIBRARY

Standard C Library (libc, -lc)

# SYNOPSIS

`#include <string.h>`

int
`memcmp`(const void *b1, const void *b2, size_t len);

# DESCRIPTION

The `memcmp`() function compares byte string b1 against byte string b2. Both strings are assumed to be len bytes long.

# RETURN VALUES

The `memcmp`() function returns zero if the two strings are identical, otherwise returns the difference between the first two differing bytes (treated as unsigned char values, so that ‘`\200`’ is greater than ‘`\0`’, for example). Zero-length strings are always identical.

Do not use `memcmp`() to compare cryptographic secrets, because the time it takes varies depending on how many bytes are the same, and thus leaks information about the two strings by a timing side channel. To compare secrets, hashes, message authentication codes, etc., use consttime_memequal(3) instead.

# SEE ALSO

bcmp(3), consttime_memequal(3), strcasecmp(3), strcmp(3), strcoll(3), strxfrm(3)

# STANDARDS

The `memcmp`() function conforms to ANSI X3.159-1989 (“ANSI C89”).

June 23, 2013 | NetBSD 9.2