-attempt to allocate an mbuf.
*/
struct mbuf *
m_retry(i, t)
int i, t;
{
register struct mbuf *m;
调用协议的注册函数释放内存
m_reclaim();
把m_retrydefine成NULL这样就直接返回NULL了，但这里怎么保证这个MGET中m_retry返回的是NULL，而上一个返回的是这个函数？？？？？？？＃define在预编译期间就做替换了。
这个的关键就是MGET是一个宏，而不是函数。
#define m_retry(i, t) (struct mbuf *)0
MGET(m, i, t);
#undef m_retry
return (m);
}
这个函数循环调用协议的drain函数分配内存
m_reclaim()
{
register struct domain *dp;
register struct protosw *pr;
提升处理器的优先级不被网络处理中断
int s = splimp();
for (dp = domains; dp; dp = dp->dom_next)
for (pr = dp->dom_protosw; pr < dp->dom_protoswNPROTOSW; pr++)
if (pr->pr_drain)
(*pr->pr_drain)();
恢复处理器的优先级
splx(s);
mbstat.m_drain++;
}
分配一个分组头部的mbuf，对m_data和m_flags进行初始化
#define MGETHDR(m, how, type) { \
MALLOC((m), struct mbuf *, MSIZE, mbtypes[type], (how)); \
if (m) { \
(m)->m_type = (type); \
MBUFLOCK(mbstat.m_mtypes[type]++;) \
(m)->m_next = (struct mbuf *)NULL; \
(m)->m_nextpkt = (struct mbuf *)NULL; \
(m)->m_data = (m)->m_pktdat; \
(m)->m_flags = M_PKTHDR; \
} else \
(m) = m_retryhdr((how), (type)); \
}
587 /*
588 * Routine to copy from device local memory into mbufs.
589 */
590 struct mbuf *
591 m_devget(buf, totlen, off0, ifp, copy)
592 char *buf;
593 int totlen, off0;
594 struct ifnet *ifp;
595 void (*copy)();
这个函数是对MGET和MGETHDR的封装，一般由设备驱动程序调用，分配mbuf空间。
1，如果数据长度《84，则在数据（IP数据包）的前面保留16个字节。为输出时添加14字节的MAC包头准备。（一个包含pak_hdr的mbuf最多放100字节的数据）
2，如果数据》85 ＆＆ 数据《100则不额外保留这16字节的数据
3，如果数据》100，则分配一个cluster进行数据的存放。
可见m_devget根据数据的长度，分配合适的mbuf
4.2mbuf到mbuf中data的转换
定义了两个宏
56 * mtod(m,t) - convert mbuf pointer to data pointer of correct type
57 * dtom(x) - convert data pointer within mbuf to mbuf pointer (XXX)
61 #define mtod(m,t) ((t)((m)->m_data))
MSIZE ＝＝ 128 这个基于mbuf是128字节对齐
62 #define dtom(x) ((struct mbuf *)((int)(x) & ~(MSIZE-1)))
dotm对cluster的数据有问题，不能正常转换到mbuf,所以需要下面的函数
/*
* Rearange an mbuf chain so that len bytes are contiguous
* and in the data area of an mbuf (so that mtod and dtom
* will work for a structure of size len). Returns the resulting
* mbuf chain on success, frees it and returns null on failure.
* If there is room, it will add up to max_protohdr-len extra bytes to the
* contiguous region in an attempt to avoid being called next time.
*/
这个函数从mbuf链表中取出len字节的数据放在第一个mbuf中，使dtom能正确运行
struct mbuf *
m_pullup(n, len)
register struct mbuf *n;
int len;
{
register struct mbuf *m;
register int count;
int space;
/*
* If first mbuf has no cluster, and has room for len bytes
* without shifting current data, pullup into it,
* otherwise allocate a new mbuf to prepend to the chain.
*/
if ((n->m_flags & M_EXT) == 0 &&
n->m_data + len < &n->m_dat[MLEN] && n->m_next) {
if (n->m_len >= len)
return (n);
m = n;
n = n->m_next;
len -= m->m_len;
} else {
if (len > MHLEN)
goto bad;
MGET(m, M_DONTWAIT, n->m_type);
if (m == 0)
goto bad;
m->m_len = 0;
if (n->m_flags & M_PKTHDR) {
M_COPY_PKTHDR(m, n);
n->m_flags &= ~M_PKTHDR;
}
}
space = &m->m_dat[MLEN] - (m->m_data + m->m_len);
do {
count = min(min(max(len, max_protohdr), space), n->m_len);
bcopy(mtod(n, caddr_t), mtod(m, caddr_t) + m->m_len,
(unsigned)count);
len -= count;
m->m_len += count;
n->m_len -= count;
space -= count;
if (n->m_len)
n->m_data += count;
else
n = m_free(n);
} while (len > 0 && n);
if (len > 0) {
(void) m_free(m);
g