checksum.c

ReactOS是一些高手根据Windows XP的内核编写出的类XP。内核实现机理和API函数调用几乎相同。甚至可以兼容XP的程序。喜欢研究系统内核的人可以看一看。

/*
 * COPYRIGHT:   See COPYING in the top level directory
 * PROJECT:     ReactOS TCP/IP protocol driver
 * FILE:        tcpip/checksum.c
 * PURPOSE:     Checksum routines
 * NOTES:       The checksum routine is from RFC 1071
 * PROGRAMMERS: Casper S. Hornstrup ([e-mail])
 * REVISIONS:
 *   CSH 01/08-2000 Created
 */

#include "precomp.h"


ULONG ChecksumFold(
  ULONG Sum)
{
  /* Fold 32-bit sum to 16 bits */
  while (Sum >> 16)
    {
      Sum = (Sum & 0xFFFF) + (Sum >> 16);
    }

  return Sum;
}

ULONG ChecksumCompute(
  PVOID Data,
  UINT Count,
  ULONG Seed)
/*
 * FUNCTION: Calculate checksum of a buffer
 * ARGUMENTS:
 *     Data  = Pointer to buffer with data
 *     Count = Number of bytes in buffer
 *     Seed  = Previously calculated checksum (if any)
 * RETURNS:
 *     Checksum of buffer
 */
{
  register ULONG Sum = Seed;

  while (Count > 1)
    {
      Sum += *(PUSHORT)Data;
      Count -= 2;
      Data = (PVOID)((ULONG_PTR) Data + 2);
    }

  /* Add left-over byte, if any */
  if (Count > 0)
    {
      Sum += *(PUCHAR)Data;
    }

  return Sum;
}