关于一段代码的注释


以下是一段代码，摘录自http://www.blogjava.net/yczz/articles/100172.html  
希望懂的可以帮忙注释一下，我实在是看不懂。谢谢了!//GetData()用于解析由PacketReceivePacket()函数收到的报文，关于分析ARP_REPLY报文以便获得局域网主机列表
void getdata(LPPACKET lp,int op) 
{
    ULONG  ulbytesreceived,off,tlen,ulen,ulLines;
    ULONG  j,k;
    ETHDR  *eth;
    ARPHDR *arp;
    PIPHDR ip;
    char   *buf,*pChar,*pLine,*base;
    struct bpf_hdr      *hdr;
    struct sockaddr_in  sin;
    ulbytesreceived=lp->ulBytesReceived;
    buf=(char *)lp->Buffer;    off=0;
    while(off<ulbytesreceived)
    {
        if(kbhit())
        {
            return ;
        }
        hdr=(struct bpf_hdr *)(buf+off);
        off+=hdr->bh_hdrlen;        pChar=(char *)(buf+off);
        base=pChar;
        off=Packet_WORDALIGN(off+hdr->bh_caplen);        eth=(PETHDR)pChar;                
        arp=(PARPHDR)(pChar+sizeof(ETHDR));         if(eth->eh_type==htons(ETH_IP)) 
        {
                    ip=(PIPHDR)(pChar+sizeof(ETHDR));            if(fm && sm && (op==3))  
            {  
                if((((ip->sourceip!=htonl(myip)) && (ip->destip!=htonl(myip)) 
                                && !strcmp((char *)eth->eh_dst,(char *)mmac)) 
                && ((ip->sourceip==htonl(firstip)) || (ip->destip==htonl(firstip)) 
                || (ip->sourceip==htonl(secondip)) || (ip->destip==htonl(secondip))))
                                || ((firstip==myip) && (secondip==myip)))
                {
                    memset(msg,0,sizeof(msg));                    sin.sin_addr.s_addr=ip->sourceip;                
                    printf("[IP:]%16s ---> [IP:]",inet_ntoa(sin.sin_addr));                                        strcpy(msg,inet_ntoa(sin.sin_addr));
                    strcat(msg+15," ---> ");                    sin.sin_addr.s_addr=ip->destip;
                    printf("%16s\n",inet_ntoa(sin.sin_addr));
                   
                    strcat(msg+23,inet_ntoa(sin.sin_addr));
                    fseek(fp,-2,1);
                    fwrite("\r\n\r\n\r\n",6,1,fp);
                    fwrite(msg,38,1,fp);
                    fwrite("\r\n",2,1,fp);                    ulLines=(hdr->bh_caplen+15)/16;
                    for(k=0;k<ulLines;k++)
                    {
                        pLine=pChar;
                        printf("%08lx : ",pChar-base);                        ulen=tlen;
                        ulen=(ulen>16) ? 16 : ulen;
                        tlen-=ulen;                        for(j=0;j<ulen;j++)
                            printf("%02x ",*(BYTE *)pChar++);                        if(ulen<16)
                            printf("%*s",(16-ulen)*3," ");                        pChar=pLine;                        for(j=0;j<ulen;j++,pChar++)
                        {
                            printf("%c",isprint(*pChar)? *pChar : '.');
                            fputc(isprint(*pChar) ? *pChar : '.',fp); 
                        }
                        printf("\n");
                    }
                    printf("\n");
                                        fwrite("\r\n",2,1,fp);  
                }            }
            continue;
        }
                else if((eth->eh_type==htons(ETH_ARP)) && (arp->arp_opt==htons(ARP_REPLY)))  
        {
            sin.sin_addr.s_addr=arp->arp_spa;             if(sin.sin_addr.s_addr==htonl(myip)) 
        {
            memcpy(mmac,eth->eh_src,6);
        if(!mm)
             {
            printf("\t");    
                     for(k=0;k<5;k++)
                        printf("%.2x-",eth->eh_src[k]);
                  printf("%.2x\n",eth->eh_src[5]);                    switch(op)
            {
                    case 1:
                         printf("\n[MAC LIST:]");
                        break;
                     case 2:
                        printf("\n[Sniffing Host:]");     
                         break;
                      default:                    
                         break;
            }
        }
            mm=TRUE;
        }             if((op==1) || (op==2))
            {
                printf("\n[IP:] %.16s  [MAC:] ",inet_ntoa(sin.sin_addr));
                for(k=0;k<5;k++)
                    printf("%.2x-",eth->eh_src[k]);
                printf("%.2x",eth->eh_src[5]);
        }
               else if(((op==3) || (op==4)) && (!fm || !sm))
        {
                if(arp->arp_spa==htonl(firstip))
            {
                    memcpy(fmac,eth->eh_src,6);
                    fm=TRUE;
            }
                
            if(arp->arp_spa==htonl(secondip))
            {
                    memcpy(smac,eth->eh_src,6);
                    sm=TRUE;
            }
        }
    }
    }
    return ;
}