/* 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 */
/*
* 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;
else
n = 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。
----------------------------------------------------------
-----------------------------------------------------------
| /*
* Mbufs are of a single size, MSIZE (machine/machparam.h), which
* includes overhead. An mbuf may add a single "mbuf cluster" of size
* MCLBYTES (also in machine/machparam.h), which has no additional overhead
* and is used instead of the internal data area; this is done when
* at least MINCLSIZE of data must be stored.
*/
#define MLEN (MSIZE - sizeof(struct m_hdr))/* normal data len */
#define MHLEN (MLEN - sizeof(struct pkthdr))/* data len w/pkthdr */
/* header at beginning of each mbuf: */
struct m_hdr {
struct mbuf *mh_next;/* next buffer in chain */
struct mbuf *mh_nextpkt;/* next chain in queue/record */
caddr_t mh_data; /* location of data */
int mh_len;/* amount of data in this mbuf */
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 */
struct pkthdr {
struct ifnet *rcvif;/* rcv interface */
int len;/* total packet length */
};
/* 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 */
};
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;
};
/* mbuf flags */
#define M_EXT 0x0001 /* has associated external storage */
#define M_PKTHDR 0x0002 /* start of record */
#define M_EOR 0x0004 /* end of record */
|
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。
转自： http://blog.chinaunix.net/uid-22832715-id-279147.html
