這里說一下find_entry這個函數,這個函數主要是查找ARP表和在ARP表中申請一個empty的表項
這里有必要先翻譯一下開頭的注釋:
函數有兩個參數,一個ip地址,一個flag
如果ip地址有效,就返回一個pending或者stable狀態的表項索引。
如果找不到,就創建一個新的,並把這個ip地址設置上去,狀態變為empty。狀態有上層調用函數去改變。
如果ip地址是空的。也返回一個empty的表項索引
上面的都不符合並且flasg標志為ETHARP_TRY_HARD,那么就嘗試從ARP表中刪除一個最早的表項,並返回之索引。
刪除會有一個優先級別。
在代碼搜索了一遍ARP表的時候,已經記錄了一下表項的狀態和生存時間
1.返回第一個empty狀態的表項,代碼中用empty表示
2.返回生存時間最長的stable表項。代碼中用old_stable表示
3.返回生存時間最長且沒有數據緩沖項的索引,代碼中用old_pending表示
4.返回生存時間最長而且有數據緩沖項的索引,代碼中用old_queue表示
下面給代碼,照例刪除一下調試用的代碼,減少篇幅
static s8_t find_entry ( struct ip_addr *ipaddr, u8_t flags )
{
s8_t old_pending = ARP_TABLE_SIZE, old_stable = ARP_TABLE_SIZE;
s8_t empty = ARP_TABLE_SIZE;
u8_t i = 0, age_pending = 0, age_stable = 0;
#if ARP_QUEUEING
/* oldest entry with packets on queue */
s8_t old_queue = ARP_TABLE_SIZE;
/* its age */
u8_t age_queue = 0;
#endif
/*上面注意empty ,old_queue ,old_pending ,old_stable 這幾個變量*/
if ( ipaddr )
{
/*etharp_cached_entry保存了上次返回的索引*/
if ( arp_table[etharp_cached_entry].state == ETHARP_STATE_STABLE )
{
/*如果上次的還有效,並且是這個要找的ip,直接返回,這個技巧很快*/
/* the cached entry is stable */
if ( ip_addr_cmp ( ipaddr, &arp_table[etharp_cached_entry].ipaddr ) )
{
/* cached entry was the right one! */
ETHARP_STATS_INC ( etharp.cachehit );
return etharp_cached_entry;
}
}
}
/*下面的for做搜索,並記錄一下表項的狀態和生存時間*/
for ( i = 0; i < ARP_TABLE_SIZE; ++i )
{
/* no empty entry found yet and now we do find one? */
if ( ( empty == ARP_TABLE_SIZE ) && ( arp_table[i].state == ETHARP_STATE_EMPTY ) )
{
/*empty == ARP_TABLE_SIZE
這個第一次生效會生效,這里只進入一次,就是記錄第一個出現empty表項的位置*/
/* remember first empty entry */
empty = i;
}
/* pending entry? */
else if ( arp_table[i].state == ETHARP_STATE_PENDING )
{
/**看ip是否匹配,匹配的話就直接返回索引更新etharp_cached_entry,*/
/* if given, does IP address match IP address in ARP entry? */
if ( ipaddr && ip_addr_cmp ( ipaddr, &arp_table[i].ipaddr ) )
{
/* found exact IP address match, simply bail out */
etharp_cached_entry = i;
return i;
#if ARP_QUEUEING
/* pending with queued packets? */
}
else if ( arp_table[i].q != NULL )
{
/*找生存時間最長的*/
if ( arp_table[i].ctime >= age_queue )
{
old_queue = i;
age_queue = arp_table[i].ctime;
}
#endif
/* pending without queued packets? */
}
else
{
/*找生存時間最長的*/
if ( arp_table[i].ctime >= age_pending )
{
old_pending = i;
age_pending = arp_table[i].ctime;
}
}
}
/* stable entry? */
else if ( arp_table[i].state == ETHARP_STATE_STABLE )
{
/* if given, does IP address match IP address in ARP entry? */
if ( ipaddr && ip_addr_cmp ( ipaddr, &arp_table[i].ipaddr ) )
{
/*找到匹配的就直接返回了,就是它了!!*/
etharp_cached_entry = i;
return i;
/* remember entry with oldest stable entry in oldest, its age in maxtime */
}
else if ( arp_table[i].ctime >= age_stable )
{/*沒有就記錄一下它活了多久,一會會有判斷,如果活得太久了,就干掉它,返回一個empty的給上層*/
old_stable = i;
age_stable = arp_table[i].ctime;
}
}
}
/* { we have no match } => try to create a new entry */
/* no empty entry found and not allowed to recycle? */
if ( ( ( empty == ARP_TABLE_SIZE ) && ( ( flags & ETHARP_TRY_HARD ) == 0 ) )
/* or don't create new entry, only search? */
|| ( ( flags & ETHARP_FIND_ONLY ) != 0 ) )
{
/*找不到空位了,又不給殺人,只能返回錯誤了*/
return ( s8_t ) ERR_MEM;
}
/* b) choose the least destructive entry to recycle:
* 1) empty entry
* 2) oldest stable entry
* 3) oldest pending entry without queued packets
* 4) oldest pending entry with queued packets
*
* { ETHARP_TRY_HARD is set at this point }
*/
下面找到最合適殺掉的那一個。上面是解釋,就不翻譯了
/* 1) empty entry available? */
if ( empty < ARP_TABLE_SIZE )
{
i = empty;
}
/* 2) found recyclable stable entry? */
else if ( old_stable < ARP_TABLE_SIZE )
{
/* recycle oldest stable*/
i = old_stable;
#if ARP_QUEUEING
/* no queued packets should exist on stable entries */
LWIP_ASSERT ( "arp_table[i].q == NULL", arp_table[i].q == NULL );
#endif
/* 3) found recyclable pending entry without queued packets? */
}
else if ( old_pending < ARP_TABLE_SIZE )
{
/* recycle oldest pending */
i = old_pending;
#if ARP_QUEUEING
/* 4) found recyclable pending entry with queued packets? */
}
else if ( old_queue < ARP_TABLE_SIZE )
{
/* recycle oldest pending */
i = old_queue;
free_etharp_q ( arp_table[i].q );
arp_table[i].q = NULL;
#endif
/* no empty or recyclable entries found */
}
else
{
return ( s8_t ) ERR_MEM;
}
/* { empty or recyclable entry found } */
LWIP_ASSERT ( "i < ARP_TABLE_SIZE", i < ARP_TABLE_SIZE );
/*殺人開始*/
/*從索引中刪除*/
if ( arp_table[i].state != ETHARP_STATE_EMPTY )
{
snmp_delete_arpidx_tree ( arp_table[i].netif, &arp_table[i].ipaddr );
}
/* recycle entry (no-op for an already empty entry) */
arp_table[i].state = ETHARP_STATE_EMPTY;
/* IP address given? */
/*如果上面傳了個ip下來,就把這個ip寫進去*/
if ( ipaddr != NULL )
{
/* set IP address */
ip_addr_set ( &arp_table[i].ipaddr, ipaddr );
}
arp_table[i].ctime = 0;/*出生時間0*/
etharp_cached_entry = i;
return ( err_t ) i;
}