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TCP/IP详解第二卷实现主要讲的是BSD的协议栈实现。看协议栈代码,首先看的就是经典的mbuf结构了。原来觉得mbuf结构比较复杂,硬着头皮看了几天后,感觉思路上清晰了一些。于是准备写点东西把理解到的一点点先记下来,省得以后忘记了。
书上是以4.4BSDlite源代码为基础讲的。可以说是目前各个bsd发行版的基础。在此基础上,FreeBSD、NetBSD、OpenBSD做了稍许的改动。下面是lite版的mbuf声明代码:
/*
* 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;
};
[color="#000000"]/* 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。
本文来自ChinaUnix博客,如果查看原文请点:http://blog.chinaunix.net/u/355/showart_291392.html |
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