校验和计算方法

unsigned short checksum(unsigned short *buffer,int size) //校验和的求法
{
  unsigned long cksum=0;
  while(size>0) //各位求和
  {
    cksum+=*buffer++;
    size-=sizeof(unsigned short);
  }
  if(size)
  cksum+=*(unsigned char *)buffer;
  cksum=(cksum>>16)+(cksum & 0xffff); //移位，位与运算
  cksum+=(cksum>>16);
  return (unsigned short)(~cksum); //再取反
}