mbuf

最新推荐文章于 2025-07-03 21:08:42 发布

转载最新推荐文章于 2025-07-03 21:08:42 发布 · 2.1k 阅读

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#struct #ext #buffer #freebsd #header #interface

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mbuf结构体

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; };

/* 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。