Linux下ARP头结构体的问题

在我的系统中

/usr/include/net/if_arp.h中arphdr的结构体定义如下

struct arphdr { unsigned short int ar_hrd;/* Format of hardware address. */ unsigned short int ar_pro;/* Format of protocol address. */ unsigned char ar_hln;/* Length of hardware address. */ unsigned char ar_pln;/* Length of protocol address. */ unsigned short int ar_op;/* ARP opcode (command). */#if 0 /* Ethernet looks like this : This bit is variable sized however... */ unsigned char __ar_sha[ETH_ALEN];/* Sender hardware address. */ unsigned char __ar_sip[4];/* Sender IP address. */ unsigned char __ar_tha[ETH_ALEN];/* Target hardware address. */ unsigned char __ar_tip[4];/* Target IP address. */#endif }; 

十分不解为什么要注释掉后面的部分

后来在/usr/include/netinet/if_ether.h找到

#include <net/if_arp.h>__BEGIN_DECLS/* * Ethernet Address Resolution Protocol. * * See RFC 826 for protocol description. Structure below is adapted * to resolving internet addresses. Field names used correspond to * RFC 826. */structether_arp {structarphdr ea_hdr;/* fixed-size header */u_int8_t arp_sha[ETH_ALEN];/* sender hardware address */u_int8_t arp_spa[4];/* sender protocol address */u_int8_t arp_tha[ETH_ALEN];/* target hardware address */u_int8_t arp_tpa[4];/* target protocol address */};#definearp_hrdea_hdr.ar_hrd#definearp_proea_hdr.ar_pro#definearp_hlnea_hdr.ar_hln#definearp_plnea_hdr.ar_pln#definearp_opea_hdr.ar_op