This is xnu-4903.221.2. See this file in:
/*
 * Copyright (c) 2017 Apple Inc. All rights reserved.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
 *
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. The rights granted to you under the License
 * may not be used to create, or enable the creation or redistribution of,
 * unlawful or unlicensed copies of an Apple operating system, or to
 * circumvent, violate, or enable the circumvention or violation of, any
 * terms of an Apple operating system software license agreement.
 *
 * Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this file.
 *
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
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 */

/*
 *  extern uint32_t os_cpu_in_cksum(const void *data, uint32_t len,
 *      uint32_t initial_sum);
 *
 *  input :
 *      data : starting address
 *      len : byte stream length
 *      initial_sum : 32-bit sum
 *
 *  output :
 *	This function returns the partial 16-bit checksum accumulated in
 *	a 32-bit variable (withouth 1's complement); caller is responsible
 *	for folding the 32-bit sum into 16-bit and performinng the 1's
 *	complement if applicable
 */