malloc(n, M_IFADDR, M_WAITOK);

if (ifnet_addrs) {

bcopy((caddr_t)ifnet_addrs, (caddr_t)q, n/2);

free((caddr_t)ifnet_addrs, M_IFADDR);

}

ifnet_addrs = q;

}

/*

* create a Link Level name for this device

*/

这个函数也很有意思，很有意思

把ifp->if_unit转换成字符串，相当于itoa()....

unitname = sprint_d((u_int)ifp->if_unit, workbuf, sizeof(workbuf));

namelen = strlen(ifp->if_name);

unitlen = strlen(unitname);

#define _offsetof(t, m) ((int)((caddr_t)&((t *)0)->m))

masklen = _offsetof(struct sockaddr_dl, sdl_data[0]) +

unitlen + namelen;

比较抽象，看看书上那个图就明白内存布局了。

从前往后是name,unit,mask,分别由namelen,unitlen,masklen标识内存的长度

socksize = masklen + ifp->if_addrlen;

#define ROUNDUP(a) (1 + (((a) - 1) | (sizeof(long) - 1)))

socksize = ROUNDUP(socksize);

if (socksize < sizeof(*sdl))

socksize = sizeof(*sdl);

ifasize = sizeof(*ifa) + 2 * socksize;

这代码我是看的很晕，没细看，看71页的图就理解了

if (ifa = (struct ifaddr *)malloc(ifasize, M_IFADDR, M_WAITOK)) {

bzero((caddr_t)ifa, ifasize);

第一个sockaddr_dl 表示name和uinit信息，如eth0

物理地址是在后面的ether_attach中附给第一个sdl的

sdl = (struct sockaddr_dl *)(ifa + 1);

sdl->sdl_len = socksize;

sdl->sdl_family = AF_LINK;

bcopy(ifp->if_name, sdl->sdl_data, namelen);

bcopy(unitname, namelen + (caddr_t)sdl->sdl_data, unitlen);

sdl->sdl_nlen = (namelen += unitlen);

sdl->sdl_index = ifp->if_index;

sdl->sdl_type = ifp->if_type;

ifnet_addrs[if_index - 1] = ifa;

ifa->ifa_ifp = ifp;

ifa->ifa_next = ifp->if_addrlist;

ifa->ifa_rtrequest = link_rtrequest;

ifp->if_addrlist = ifa;

ifa->ifa_addr = (struct sockaddr *)sdl;

第2个sdl,表示netmask信息

sdl = (struct sockaddr_dl *)(socksize + (caddr_t)sdl);

ifa->ifa_netmask = (struct sockaddr *)sdl;

sdl->sdl_len = masklen;

while (namelen != 0)

sdl->sdl_data[--namelen] = 0xff;

}

/* XXX -- Temporary fix before changing 10 ethernet drivers */

if (ifp->if_output == ether_output)

ether_ifattach(ifp);

}

至此，over