520 lines
14 KiB
Groff
520 lines
14 KiB
Groff
.\" Copyright (c) 2000 FreeBSD Inc.
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL [your name] OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd October 17, 2000
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.Dt MBUF 9
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.Os
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.\"
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.Sh NAME
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.Nm mbuf
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.Nd "memory management in the kernel IPC subsystem"
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.\"
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.Sh SYNOPSIS
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.In sys/param.h
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.In sys/systm.h
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.In sys/mbuf.h
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.\"
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.Ss Mbuf allocation macros
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.Fn MGET "struct mbuf *mbuf" "int how" "short type"
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.Fn MGETHDR "struct mbuf *mbuf" "int how" "short type"
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.Fn MCLGET "struct mbuf *mbuf" "int how"
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.Fo MEXTADD
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.Fa "struct mbuf *mbuf"
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.Fa "caddr_t buf"
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.Fa "u_int size"
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.Fa "void (*free)(void *opt_args)"
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.Fa "void *opt_args"
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.Fa "short flags"
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.Fa "int type"
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.Fc
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.Fn MEXTFREE "struct mbuf *mbuf"
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.Fn MEXT_ADD_REF "struct mbuf *mbuf"
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.Fn MEXT_REM_REF "struct mbuf *mbuf"
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.Fn MFREE "struct mbuf *mbuf" "struct mbuf *successor"
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.\"
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.Ss Mbuf utility macros
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.Ft void *
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.Fn mtod "struct mbuf *mbuf" "type"
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.Ft int
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.Fn MEXT_IS_REF "struct mbuf *mbuf"
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.Fn M_COPY_PKTHDR "struct mbuf *to" "struct mbuf *from"
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.Fn M_ALIGN "struct mbuf *mbuf" "u_int len"
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.Fn MH_ALIGN "struct mbuf *mbuf" "u_int len"
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.Ft int
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.Fn M_LEADINGSPACE "struct mbuf *mbuf"
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.Ft int
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.Fn M_TRAILINGSPACE "struct mbuf *mbuf"
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.Fn M_PREPEND "struct mbuf *mbuf" "int len" "int how"
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.Fn MCHTYPE "struct mbuf *mbuf" "u_int type"
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.Ft int
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.Fn M_WRITABLE "struct mbuf *mbuf"
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.\"
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.Ss Mbuf allocation functions
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.Ft struct mbuf *
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.Fn m_get "int how" "int type"
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.Ft struct mbuf *
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.Fn m_getm "struct mbuf *orig" "int len" "int how" "int type"
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.Ft struct mbuf *
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.Fn m_getclr "int how" "int type"
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.Ft struct mbuf *
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.Fn m_gethdr "int how" "int type"
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.Ft struct mbuf *
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.Fn m_free "struct mbuf *mbuf"
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.Ft void
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.Fn m_freem "struct mbuf *mbuf"
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.\"
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.Ss Mbuf utility functions
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.Ft void
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.Fn m_adj "struct mbuf *mbuf" "int len"
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.Ft struct mbuf *
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.Fn m_prepend "struct mbuf *mbuf" "int len" "int how"
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.Ft struct mbuf *
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.Fn m_pullup "struct mbuf *mbuf" "int len"
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.Ft struct mbuf *
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.Fn m_copym "struct mbuf *mbuf" "int offset" "int len" "int how"
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.Ft struct mbuf *
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.Fn m_copypacket "struct mbuf *mbuf" "int how"
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.Ft struct mbuf *
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.Fn m_dup "struct mbuf *mbuf" "int how"
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.Ft void
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.Fn m_copydata "const struct mbuf *mbuf" "int offset" "int len" "caddr_t buf"
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.Ft void
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.Fn m_copyback "struct mbuf *mbuf" "int offset" "int len" "caddr_t buf"
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.Ft struct mbuf *
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.Fo m_devget
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.Fa "char *buf"
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.Fa "int len"
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.Fa "int offset"
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.Fa "struct ifnet *ifp"
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.Fa "void (*copy)(char *from, caddr_t to, u_int len)"
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.Fc
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.Ft void
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.Fn m_cat "struct mbuf *m" "struct mbuf *n"
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.Ft struct mbuf *
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.Fn m_split "struct mbuf *mbuf" "int len" "int how"
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.\"
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.Sh DESCRIPTION
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An mbuf is a basic unit of memory management in the kernel IPC subsystem.
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Network packets and socket buffers are stored in mbufs.
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A network packet may span multiple mbufs arranged into a chain
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(linked list),
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which allows adding or trimming
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network headers with little overhead.
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.Pp
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While a developer should not bother with mbuf internals without serious
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reason in order to avoid incompatibilities with future changes, it
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is useful to understand the mbuf's general structure.
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.Pp
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An mbuf consists of a variable-sized header and a small internal
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buffer for data.
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The mbuf's total size,
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.Dv MSIZE ,
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is a machine-dependent constant defined in
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.Pa machine/param.h .
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The mbuf header includes:
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.Pp
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.Bl -tag -width "m_nextpkt" -compact -offset indent
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.It Fa m_next
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a pointer to the next buffer in the chain
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.It Fa m_nextpkt
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a pointer to the next chain in the queue
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.It Fa m_data
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a pointer to the data
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.It Fa m_len
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the length of the data
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.It Fa m_type
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the type of data
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.It Fa m_flags
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the mbuf flags
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.El
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.Pp
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The mbuf flag bits are defined as follows:
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.Bd -literal
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/* mbuf flags */
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#define M_EXT 0x0001 /* has associated external storage */
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#define M_PKTHDR 0x0002 /* start of record */
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#define M_EOR 0x0004 /* end of record */
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#define M_RDONLY 0x0008 /* associated data marked read-only */
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#define M_PROTO1 0x0010 /* protocol-specific */
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#define M_PROTO2 0x0020 /* protocol-specific */
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#define M_PROTO3 0x0040 /* protocol-specific */
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#define M_PROTO4 0x0080 /* protocol-specific */
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#define M_PROTO5 0x0100 /* protocol-specific */
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/* mbuf pkthdr flags, also in m_flags */
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#define M_BCAST 0x0200 /* send/received as link-level broadcast */
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#define M_MCAST 0x0400 /* send/received as link-level multicast */
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#define M_FRAG 0x0800 /* packet is fragment of larger packet */
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#define M_FIRSTFRAG 0x1000 /* packet is first fragment */
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#define M_LASTFRAG 0x2000 /* packet is last fragment */
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.Ed
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.Pp
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The available mbuf types are defined as follows:
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.Bd -literal
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/* mbuf types */
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#define MT_FREE 0 /* should be on free list */
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#define MT_DATA 1 /* dynamic (data) allocation */
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#define MT_HEADER 2 /* packet header */
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#define MT_SONAME 8 /* socket name */
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#define MT_FTABLE 11 /* fragment reassembly header */
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#define MT_CONTROL 14 /* extra-data protocol message */
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#define MT_OOBDATA 15 /* expedited data */
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.Ed
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.Pp
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If the
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.Dv M_PKTHDR
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flag is set, a
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.Li struct pkthdr m_pkthdr
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is added to the mbuf header.
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It contains a pointer to the interface
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the packet has been received from
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.Pq Fa struct ifnet *rcvif ,
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and the total packet length
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.Pq Fa int len .
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.Pp
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If small enough, data is stored in the mbuf's internal data buffer.
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If the data is sufficiently large, another mbuf may be added to the chain,
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or external storage may be associated with the mbuf.
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.Dv MHLEN
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bytes of data can fit into an mbuf with the
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.Dv M_PKTHDR
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flag set,
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.Dv MLEN
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bytes can otherwise.
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.Pp
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If external storage is being associated with an mbuf, the
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.Dv m_ext
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header is added at the cost of losing the internal data buffer.
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It includes a pointer to external storage, the size of the storage,
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a pointer to a function used for freeing the storage,
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a pointer to an optional argument that can be passed to the function,
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and a pointer to a reference counter.
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An mbuf using external storage has the
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.Dv M_EXT
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flag set.
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.Pp
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The system supplies a macro for allocating the desired external storage
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buffer,
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.Dv MEXTADD .
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.Pp
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The allocation and management of the reference counter is handled by the
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subsystem.
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The developer can check whether the reference count for the
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given mbuf's external storage is greater than 1 with the
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.Dv MEXT_IS_REF
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macro.
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Similarly, the developer can directly add and remove references,
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if absolutely necessary, with the use of the
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.Dv MEXT_ADD_REF
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and
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.Dv MEXT_REM_REF
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macros.
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.Pp
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The system also supplies a default type of external storage buffer called an
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.Dq mbuf cluster .
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Mbuf clusters can be allocated and configured with the use of the
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.Dv MCLGET
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macro.
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Each cluster is
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.Dv MCLBYTES
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in size, where MCLBYTES is a machine-dependent constant.
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The system defines an advisory macro
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.Dv MINCLSIZE ,
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which is the smallest amount of data to put into a cluster.
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It's equal to the sum of
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.Dv MLEN
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and
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.Dv MHLEN .
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It is typically preferable to store data into an mbuf's data region, if size
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permits, as opposed to allocating a separate mbuf cluster to hold the same
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data.
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.\"
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.Ss Macros and Functions
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There are numerous predefined macros and functions that provide the
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developer with common utilities.
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.\"
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.Bl -ohang -offset indent
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.It Fn mtod mbuf type
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Convert an mbuf pointer to a data pointer.
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The macro expands to the data pointer cast to the pointer of the specified type.
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.Sy Note :
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It is advisable to ensure that there is enough contiguous data in the mbuf.
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See
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.Fn m_pullup
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for details.
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.It Fn MGET mbuf how type
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Allocate an mbuf and initialize it to contain internal data.
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.Fa mbuf
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will point to the allocated mbuf on success, or be set to
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.Dv NULL
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on failure.
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The
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.Fa how
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argument is to be set to
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.Dv M_TRYWAIT
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or
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.Dv M_DONTWAIT .
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It specifies whether the caller is willing to block if necessary.
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If
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.Fa how
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is set to
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.Dv M_TRYWAIT ,
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a failed allocation will result in the caller being put
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to sleep for a designated
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kern.ipc.mbuf_wait
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.Xr ( sysctl 8
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tunable)
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number of ticks.
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A number of other mbuf-related
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functions and macros have the same argument because they may
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at some point need to allocate new mbufs.
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.It Fn MGETHDR mbuf how type
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Allocate an mbuf and initialize it to contain a packet header
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and internal data.
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See
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.Fn MGET
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for details.
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.It Fn MCLGET mbuf how
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Allocate and attach an mbuf cluster to an mbuf.
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If the macro fails, the
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.Dv M_EXT
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flag won't be set in the mbuf.
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.It Fn M_PREPEND mbuf len how
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This macro operates on an mbuf chain.
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It is an optimized wrapper for
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.Fn m_prepend
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that can make use of possible empty space before data
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(e.g. left after trimming of a link-layer header).
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The new chain pointer or
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.Dv NULL
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is in
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.Fa mbuf
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after the call.
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.It Fn M_WRITABLE mbuf
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This macro will evaluate true if the mbuf is not marked
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.Dv M_RDONLY
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and if either the mbuf does not contain external storage or,
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if it does,
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then if the reference count of the storage is not greater than 1.
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The
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.Dv M_RDONLY
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flag can be set in the mbuf's
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.Dv m_flags .
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This can be achieved during setup of the external storage,
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by passing the
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.Dv M_RDONLY
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bit as a
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.Fa flags
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argument to the
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.Fn MEXTADD
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macro, or can be directly set in individual mbufs.
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.El
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.Pp
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The functions are:
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.Bl -ohang -offset indent
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.It Fn m_get how type
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A function version of
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.Fn MGET
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for non-critical paths.
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.It Fn m_getm orig len how type
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Allocate
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.Fa len
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bytes worth of mbufs and mbuf clusters if necessary and append the resulting
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allocated chain to the
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.Fa orig
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mbuf chain, if it is
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.No non- Ns Dv NULL .
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If the allocation fails at any point,
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free whatever was allocated and return
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.Dv NULL .
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If
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.Fa orig
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is
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.No non- Ns Dv NULL ,
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it will not be freed.
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It is possible to use
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.Fn m_getm
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to either append
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.Fa len
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bytes to an existing mbuf or mbuf chain
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(for example, one which may be sitting in a pre-allocated ring)
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or to simply perform an all-or-nothing mbuf and mbuf cluster allocation.
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.It Fn m_gethdr how type
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A function version of
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.Fn MGETHDR
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for non-critical paths.
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.It Fn m_getclr how type
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Allocate an mbuf and zero out the data region.
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.El
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.Pp
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The functions below operate on mbuf chains.
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.Bl -ohang -offset indent
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.It Fn m_freem mbuf
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Free an entire mbuf chain, including any external
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storage.
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.\"
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.It Fn m_adj mbuf len
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Trim
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.Fa len
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bytes from the head of an mbuf chain if
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.Fa len
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is positive, from the tail otherwise.
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.\"
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.It Fn m_prepend mbuf len how
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Allocate a new mbuf and prepend it to the chain, handle
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.Dv M_PKTHDR
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properly.
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.Sy Note :
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It doesn't allocate any clusters, so
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.Fa len
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must be less than
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.Dv MLEN
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or
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.Dv MHLEN ,
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depending on the
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.Dv M_PKTHDR
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flag setting.
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.\"
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.It Fn m_pullup mbuf len
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Arrange that the first
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.Fa len
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bytes of an mbuf chain are contiguous and lay in the data area of
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.Fa mbuf ,
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so they are accessible with
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.Fn mtod mbuf type .
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Return the new chain on success,
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.Dv NULL
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on failure
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(the chain is freed in this case).
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.Sy Note :
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It doesn't allocate any clusters, so
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.Fa len
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must be less than
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.Dv MHLEN .
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.\"
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.It Fn m_copym mbuf offset len how
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Make a copy of an mbuf chain starting
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.Fa offset
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bytes from the beginning, continuing for
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.Fa len
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bytes.
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If
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.Fa len
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is
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.Dv M_COPYALL ,
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copy to the end of the mbuf chain.
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.Sy Note :
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The copy is read-only, because clusters are not
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copied, only their reference counts are incremented.
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.\"
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.It Fn m_copypacket mbuf how
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Copy an entire packet including header, which must be present.
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This is an optimized version of the common case
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.Fn m_copym mbuf 0 M_COPYALL how .
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.Sy Note :
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the copy is read-only, because clusters are not
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copied, only their reference counts are incremented.
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.\"
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.It Fn m_dup mbuf how
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Copy a packet header mbuf chain into a completely new chain, including
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copying any mbuf clusters.
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Use this instead of
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.Fn m_copypacket
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when you need a writable copy of an mbuf chain.
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.\"
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.It Fn m_copydata mbuf offset len buf
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Copy data from an mbuf chain starting
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.Fa off
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bytes from the beginning, continuing for
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.Fa len
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bytes, into the indicated buffer
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.Fa buf .
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.\"
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.It Fn m_copyback mbuf offset len buf
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Copy
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.Fa len
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bytes from the buffer
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.Fa buf
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back into the indicated mbuf chain,
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starting at
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.Fa offset
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bytes from the beginning of the chain, extending the mbuf chain if necessary.
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.Sy Note :
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It doesn't allocate any clusters, just adds mbufs to the chain.
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It's safe to set
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.Fa offset
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beyond the current chain end: zeroed mbufs will be allocated to fill the
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space.
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.\"
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.It Fn m_devget buf len offset ifp copy
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Copy data from a device local memory pointed to by
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.Fa buf
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to an mbuf chain.
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The copy is done using a specified copy routine
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.Fa copy ,
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or
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.Fn bcopy
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if
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.Fa copy
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is
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.Dv NULL .
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.\"
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.It Fn m_cat m n
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Concatenate
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.Fa n
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to
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.Fa m .
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Both chains must be of the same type.
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.Fa N
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is still valid after the function returned.
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.Sy Note :
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It does not handle
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.Dv M_PKTHDR
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and friends.
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.\"
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.It Fn m_split mbuf len how
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Partition an mbuf chain in two pieces, returning the tail:
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all but the first
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.Fa len
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bytes.
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In case of failure, it returns
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.Dv NULL
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and attempts to restore the chain to its original state.
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.El
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.Sh RETURN VALUES
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See above.
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.Sh HISTORY
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.\" Please correct me if I'm wrong
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Mbufs appeared in an early version of
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.Bx .
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Besides for being used for network packets, they were used
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to store various dynamic structures, such as routing table
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entries, interface addresses, protocol control blocks, etc.
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.Sh AUTHORS
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The original
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.Nm
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man page was written by Yar Tikhiy.
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