1，mbuf就是存储要发送数据的memery buf，类似于skb_buf.不过结构比较简单。
/* 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_EXT 0x0001 /* has associated external storage */
表明分组的第一个mbuf，在数据区中有pkthdr
#define M_PKTHDR 0x0002 /* 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_EOR 0x0004 /* end of record */
/* mbuf pkthdr flags, also in m_flags */
#define M_BCAST 0x0100 /* send/received as link-level broadcast */
#define M_MCAST 0x0200 /* send/received as link-level multicast */
/* 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_FREE 0 /* should be on free list */
#define MT_DATA 1 /* dynamic (data) allocation */ 数据就是这个类型
#define MT_HEADER 2 /* packet header */
#define MT_SOCKET 3 /* socket structure */
#define MT_PCB 4 /* protocol control block */
#define MT_RTABLE 5 /* routing tables */
#define MT_HTABLE 6 /* IMP host tables */
#define MT_ATABLE 7 /* address resolution tables */
#define MT_SONAME 8 /* socket name */
#define MT_SOOPTS 10 /* socket options */
#define MT_FTABLE 11 /* fragment reassembly header */
#define MT_RIGHTS 12 /* access rights */
#define MT_IFADDR 13 /* 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