/* header at beginning of each mbuf: */这个结构用来描述mbuf跟具体的内容无关struct m_hdr {struct mbuf *mh_next; /* next buffer in chain */struct mbuf *mh_nextpkt; /* next chain in queue/record */int mh_len; /* amount of data in this mbuf */caddr_t mh_data; /* location of data */short mh_type; /* type of data in this mbuf */short mh_flags; /* flags; see below */};
/* record/packet header in first mbuf of chain; valid if M_PKTHDR set */对mbuf中数据的描述，len和接收接口struct pkthdr {int len; /* total packet length */struct ifnet *rcvif; /* rcv interface */};
/* description of external storage mapped into mbuf, valid if M_EXT set */struct m_ext {caddr_t ext_buf; /* start of buffer */void (*ext_free)(); /* free routine if not the usual */u_int ext_size; /* size of buffer, for ext_free */};这个就是mbuf的描述，设计的比较巧妙struct mbuf {struct m_hdr m_hdr;union {struct {struct pkthdr MH_pkthdr; /* M_PKTHDR set */union {struct m_ext MH_ext; /* M_EXT set */char MH_databuf[MHLEN];} MH_dat;} MH;char M_databuf[MLEN]; /* !M_PKTHDR, !M_EXT */} M_dat;};
2，mbuf中mhdr.md_flags:/* mbuf flags */一个mbuf的大小是128字节，猜测是一个cacheline的大小。如果数据比较多，就需要多个mbuf连起来或者用一个叫cluster的东西来存储数据。M_EXT就是这个标志#define M_EXT0x0001/* has associated external storage */表明分组的第一个mbuf，在数据区中有pkthdr#define M_PKTHDR0x0002/* start of record */表明记录的尾部，TCP是一个字节流，不设置这个标志#define M_EOR0x0004/* end of record */
/* mbuf pkthdr flags, also in m_flags */#define M_BCAST0x0100/* send/received as link-levelbroadcast */#define M_MCAST0x0200/* send/received as link-levelmulticast */
/* flags copied when copying m_pkthdr */这个具体干嘛用的不懂。。。#define M_COPYFLAGS(M_PKTHDR|M_EOR|M_BCAST|M_MCAST)
3，mbuf的种类
/* mbuf types */#define MT_FREE0/* should be on free list */#define MT_DATA1/* dynamic (data) allocation */ 数据就是这个类型#define MT_HEADER2/* packet header */#define MT_SOCKET3/* socket structure */#define MT_PCB4/* protocol control block */#define MT_RTABLE5/* routing tables */#define MT_HTABLE6/* IMP host tables */#define MT_ATABLE7/* address resolution tables */#define MT_SONAME8/* socket name */#define MT_SOOPTS10/* socket options */#define MT_FTABLE11/* fragment reassembly header */#define MT_RIGHTS12/* access rights */#define MT_IFADDR13/* interface address */#define MT_CONTROL 14/* extra-data protocol message */#define MT_OOBDATA 15/* expedited data */
4，mbuf相关函数
4.1mbuf的分配/* * mbuf allocation/deallocation macros: * * MGET(struct mbuf *m, int how, int type) * allocates an mbuf and initializes it to contain internal data. * * MGETHDR(struct mbuf *m, int how, int type) * allocates an mbuf and initializes it to contain a packet header * and internal data. */#define MGET(m, how, type) { mbtypes[type]把mbuf的type转换成MALLOC需要的type,如M_MBUF,M_SOCKET等MALLOC((m), struct mbuf *, MSIZE, mbtypes[type], (how));if (m) { (m)->m_type = (type); MBUFLOCK改变处理器优先级，防止被网络处理器中断，共享资源的保护MBUFLOCK(mbstat.m_mtypes[type]++;) (m)->m_next = (struct mbuf *)NULL; (m)->m_nextpkt = (struct mbuf *)NULL; #define m_dat M_dat.M_databuf 为pkthdr和m_ext预留了空间 (m)->m_data = (m)->m_dat; (m)->m_flags = 0; } else 尝试重新分配，一个主要的问题，分配的内存从哪里来？详见后面(m) = m_retry((how), (type)); }
/* * When MGET failes, ask protocols to free space when short of memory, * then re-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;elsen = m_free(n);} while (len > 0 && n);if (len > 0) {(void) m_free(m);goto bad;}m->m_next = n;return (m);bad:m_freem(n);MPFail++;return (0);}
有些细节的东西还不明确，还需要进一步整理。 ---------------------------------------------------------------------------------------------------------------------
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可以看到mbuf大体由两部分组成，m_hdr和M_dat。
M_dat是个联合体，使用其中哪个成员是由m_hdr.mh_flags来决定的。
- 如果mh_flags里没有M_EXT与M_PKTHDR，则表面此mbuf完全是用来存放数据的。总共可以存放MLEN字节的数据；
- 如果mh_flags里有M_EXT则表面此mbuf使用外部簇来存放数据；
- 如果mh_flags里有M_PKTHDR，则表面此mbuf是一个记录的起始mbuf，由于MH_pkthdr占用了部分空间，总共只能存放MHLEN字节的数据；
- 如果mh_flags里既有M_PKTHDR又有M_EXT则表面此mbuf是起始mbuf，但是使用外部簇来存放数据。