c4d9468ea0
Avoid using parenthesis enclosure macros (.Pq and .Po/.Pc) with plain text. Not only this slows down the mdoc(7) processing significantly, but it also has an undesired (in this case) effect of disabling hyphenation within the entire enclosed block.
511 lines
14 KiB
Groff
511 lines
14 KiB
Groff
.\" Copyright (c) 2000 FreeBSD Inc.
|
|
.\" All rights reserved.
|
|
.\"
|
|
.\" Redistribution and use in source and binary forms, with or without
|
|
.\" modification, are permitted provided that the following conditions
|
|
.\" are met:
|
|
.\" 1. Redistributions of source code must retain the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer.
|
|
.\" 2. Redistributions in binary form must reproduce the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer in the
|
|
.\" documentation and/or other materials provided with the distribution.
|
|
.\"
|
|
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
|
|
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
|
|
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
|
|
.\" ARE DISCLAIMED. IN NO EVENT SHALL [your name] OR CONTRIBUTORS BE LIABLE
|
|
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
|
|
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
|
|
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
|
|
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
|
|
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
|
|
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
|
|
.\" SUCH DAMAGE.
|
|
.\"
|
|
.\" $FreeBSD$
|
|
.\"
|
|
.Dd October 17, 2000
|
|
.Dt MBUF 9
|
|
.Os
|
|
.\"
|
|
.Sh NAME
|
|
.Nm mbuf
|
|
.Nd "memory management in the kernel IPC subsystem"
|
|
.\"
|
|
.Sh SYNOPSIS
|
|
.Fd #include <sys/param.h>
|
|
.Fd #include <sys/mbuf.h>
|
|
.\"
|
|
.Ss Mbuf allocation macros
|
|
.Fn MGET "struct mbuf *mbuf" "int how" "short type"
|
|
.Fn MGETHDR "struct mbuf *mbuf" "int how" "short type"
|
|
.Fn MCLGET "struct mbuf *mbuf" "int how"
|
|
.Fo MEXTADD
|
|
.Fa "struct mbuf *mbuf"
|
|
.Fa "caddr_t buf"
|
|
.Fa "u_int size"
|
|
.Fa "void (*free)(void *opt_args)"
|
|
.Fa "void *opt_args"
|
|
.Fa "short flags"
|
|
.Fa "int type"
|
|
.Fc
|
|
.Fn MEXTFREE "struct mbuf *mbuf"
|
|
.Fn MEXT_ADD_REF "struct mbuf *mbuf"
|
|
.Fn MEXT_REM_REF "struct mbuf *mbuf"
|
|
.Fn MFREE "struct mbuf *mbuf" "struct mbuf *successor"
|
|
.\"
|
|
.Ss Mbuf utility macros
|
|
.Fn mtod "struct mbuf *mbuf" "any type"
|
|
.Fn MEXT_IS_REF "struct mbuf *mbuf"
|
|
.Fn M_COPY_PKTHDR "struct mbuf *to" "struct mbuf *from"
|
|
.Fn M_ALIGN "struct mbuf *mbuf" "u_int len"
|
|
.Fn MH_ALIGN "struct mbuf *mbuf" "u_int len"
|
|
.Fn M_LEADINGSPACE "struct mbuf *mbuf"
|
|
.Fn M_TRAILINGSPACE "struct mbuf *mbuf"
|
|
.Fn M_PREPEND "struct mbuf *mbuf" "int len" "int how"
|
|
.Fn MCHTYPE "struct mbuf *mbuf" "u_int type"
|
|
.Fn M_WRITABLE "struct mbuf *mbuf"
|
|
.\"
|
|
.Ss Mbuf allocation functions
|
|
.Ft struct mbuf *
|
|
.Fn m_get "int how" "int type"
|
|
.Ft struct mbuf *
|
|
.Fn m_getm "struct mbuf *orig" "int len" "int how" "int type"
|
|
.Ft struct mbuf *
|
|
.Fn m_getclr "int how" "int type"
|
|
.Ft struct mbuf *
|
|
.Fn m_gethdr "int how" "int type"
|
|
.Ft struct mbuf *
|
|
.Fn m_free "struct mbuf *mbuf"
|
|
.Ft void
|
|
.Fn m_freem "struct mbuf *mbuf"
|
|
.\"
|
|
.Ss Mbuf utility functions
|
|
.Ft void
|
|
.Fn m_adj "struct mbuf *mbuf" "int len"
|
|
.Ft struct mbuf *
|
|
.Fn m_prepend "struct mbuf *mbuf" "int len" "int how"
|
|
.Ft struct mbuf *
|
|
.Fn m_pullup "struct mbuf *mbuf" "int len"
|
|
.Ft struct mbuf *
|
|
.Fn m_copym "struct mbuf *mbuf" "int offset" "int len" "int how"
|
|
.Ft struct mbuf *
|
|
.Fn m_copypacket "struct mbuf *mbuf" "int how"
|
|
.Ft struct mbuf *
|
|
.Fn m_dup "struct mbuf *mbuf" "int how"
|
|
.Ft void
|
|
.Fn m_copydata "struct mbuf *mbuf" "int offset" "int len" "caddr_t buf"
|
|
.Ft void
|
|
.Fn m_copyback "struct mbuf *mbuf" "int offset" "int len" "caddr_t buf"
|
|
.Ft struct mbuf *
|
|
.Fo m_devget
|
|
.Fa "char *buf"
|
|
.Fa "int len"
|
|
.Fa "int offset"
|
|
.Fa "struct ifnet *ifp"
|
|
.Fa "void (*copy)(char *from, caddr_t to, u_int len)"
|
|
.Fc
|
|
.Ft void
|
|
.Fn m_cat "struct mbuf *m" "struct mbuf *n"
|
|
.Ft struct mbuf *
|
|
.Fn m_split "struct mbuf *mbuf" "int len" "int how"
|
|
.\"
|
|
.Sh DESCRIPTION
|
|
An mbuf is a basic unit of memory management in the kernel IPC subsystem.
|
|
Network packets and socket buffers are stored in mbufs.
|
|
A network packet may span multiple mbufs arranged into a chain
|
|
(linked list),
|
|
which allows adding or trimming
|
|
network headers with little overhead.
|
|
.Pp
|
|
While a developer should not bother with mbuf internals without serious
|
|
reason in order to avoid incompatibilities with future changes, it
|
|
is useful to understand the mbuf's general structure.
|
|
.Pp
|
|
An mbuf consists of a variable-sized header and a small internal
|
|
buffer for data.
|
|
The mbuf's total size,
|
|
.Dv MSIZE ,
|
|
is a machine-dependent constant defined in
|
|
.Pa machine/param.h .
|
|
The mbuf header includes:
|
|
.Pp
|
|
.Bl -tag -width "m_nextpkt" -compact -offset indent
|
|
.It Fa m_next
|
|
a pointer to the next buffer in the chain
|
|
.It Fa m_nextpkt
|
|
a pointer to the next chain in the queue
|
|
.It Fa m_data
|
|
a pointer to the data
|
|
.It Fa m_len
|
|
the length of the data
|
|
.It Fa m_type
|
|
the type of data
|
|
.It Fa m_flags
|
|
the mbuf flags
|
|
.El
|
|
.Pp
|
|
The mbuf flag bits are defined as follows:
|
|
.Bd -literal
|
|
/* mbuf flags */
|
|
#define M_EXT 0x0001 /* has associated external storage */
|
|
#define M_PKTHDR 0x0002 /* start of record */
|
|
#define M_EOR 0x0004 /* end of record */
|
|
#define M_RDONLY 0x0008 /* associated data marked read-only */
|
|
#define M_PROTO1 0x0010 /* protocol-specific */
|
|
#define M_PROTO2 0x0020 /* protocol-specific */
|
|
#define M_PROTO3 0x0040 /* protocol-specific */
|
|
#define M_PROTO4 0x0080 /* protocol-specific */
|
|
#define M_PROTO5 0x0100 /* protocol-specific */
|
|
|
|
/* mbuf pkthdr flags, also in m_flags */
|
|
#define M_BCAST 0x0200 /* send/received as link-level broadcast */
|
|
#define M_MCAST 0x0400 /* send/received as link-level multicast */
|
|
#define M_FRAG 0x0800 /* packet is fragment of larger packet */
|
|
#define M_FIRSTFRAG 0x1000 /* packet is first fragment */
|
|
#define M_LASTFRAG 0x2000 /* packet is last fragment */
|
|
.Ed
|
|
.Pp
|
|
The available mbuf types are defined as follows:
|
|
.Bd -literal
|
|
/* mbuf types */
|
|
#define MT_FREE 0 /* should be on free list */
|
|
#define MT_DATA 1 /* dynamic (data) allocation */
|
|
#define MT_HEADER 2 /* packet header */
|
|
#define MT_SONAME 8 /* socket name */
|
|
#define MT_FTABLE 11 /* fragment reassembly header */
|
|
#define MT_CONTROL 14 /* extra-data protocol message */
|
|
#define MT_OOBDATA 15 /* expedited data */
|
|
.Ed
|
|
.Pp
|
|
If the
|
|
.Dv M_PKTHDR
|
|
flag is set, a
|
|
.Li struct pkthdr m_pkthdr
|
|
is added to the mbuf header.
|
|
It contains a pointer to the interface
|
|
the packet has been received from
|
|
.Pq Fa struct ifnet *rcvif ,
|
|
and the total packet length
|
|
.Pq Fa int len .
|
|
.Pp
|
|
If small enough, data is stored in the mbuf's internal data buffer.
|
|
If the data is sufficiently large, another mbuf may be added to the chain,
|
|
or external storage may be associated with the mbuf.
|
|
.Dv MHLEN
|
|
bytes of data can fit into an mbuf with the
|
|
.Dv M_PKTHDR
|
|
flag set,
|
|
.Dv MLEN
|
|
bytes can otherwise.
|
|
.Pp
|
|
If external storage is being associated with an mbuf, the
|
|
.Dv m_ext
|
|
header is added at the cost of losing the internal data buffer.
|
|
It includes a pointer to external storage, the size of the storage,
|
|
a pointer to a function used for freeing the storage,
|
|
a pointer to an optional argument that can be passed to the function,
|
|
and a pointer to a reference counter.
|
|
An mbuf using external storage has the
|
|
.Dv M_EXT
|
|
flag set.
|
|
.Pp
|
|
The system supplies a macro for allocating the desired external storage
|
|
buffer,
|
|
.Dv MEXTADD .
|
|
.Pp
|
|
The allocation and management of the reference counter is handled by the
|
|
subsystem.
|
|
The developer can check whether the reference count for the
|
|
given mbuf's external storage is greater than 1 with the
|
|
.Dv MEXT_IS_REF
|
|
macro.
|
|
Similarly, the developer can directly add and remove references,
|
|
if absolutely necessary, with the use of the
|
|
.Dv MEXT_ADD_REF
|
|
and
|
|
.Dv MEXT_REM_REF
|
|
macros.
|
|
.Pp
|
|
The system also supplies a default type of external storage buffer called an
|
|
.Dq mbuf cluster .
|
|
Mbuf clusters can be allocated and configured with the use of the
|
|
.Dv MCLGET
|
|
macro.
|
|
Each cluster is
|
|
.Dv MCLBYTES
|
|
in size, where MCLBYTES is a machine-dependent constant.
|
|
The system defines an advisory macro
|
|
.Dv MINCLSIZE ,
|
|
which is the smallest amount of data to put into a cluster.
|
|
It's equal to the sum of
|
|
.Dv MLEN
|
|
and
|
|
.Dv MHLEN .
|
|
It is typically preferable to store data into an mbuf's data region, if size
|
|
permits, as opposed to allocating a separate mbuf cluster to hold the same
|
|
data.
|
|
.\"
|
|
.Ss Macros and Functions
|
|
There are numerous predefined macros and functions that provide the
|
|
developer with common utilities.
|
|
.\"
|
|
.Bl -ohang -offset indent
|
|
.It Fn mtod mbuf type
|
|
Convert an mbuf pointer to a data pointer.
|
|
The macro expands to the data pointer cast to the pointer of the specified type.
|
|
.Sy Note :
|
|
It is advisable to ensure that there is enough contiguous data in the mbuf.
|
|
See
|
|
.Fn m_pullup
|
|
for details.
|
|
.It Fn MGET mbuf how type
|
|
Allocate an mbuf and initialize it to contain internal data.
|
|
.Fa mbuf
|
|
will point to the allocated mbuf on success, or be set to
|
|
.Dv NULL
|
|
on failure.
|
|
The
|
|
.Fa how
|
|
argument is to be set to
|
|
.Dv M_TRYWAIT
|
|
or
|
|
.Dv M_DONTWAIT .
|
|
It specifies whether the caller is willing to block if necessary.
|
|
If
|
|
.Fa how
|
|
is set to
|
|
.Dv M_TRYWAIT ,
|
|
a failed allocation will result in the caller being put
|
|
to sleep for a designated
|
|
kern.ipc.mbuf_wait
|
|
.Xr ( sysctl 8
|
|
tunable)
|
|
number of ticks.
|
|
A number of other mbuf-related
|
|
functions and macros have the same argument because they may
|
|
at some point need to allocate new mbufs.
|
|
.It Fn MGETHDR mbuf how type
|
|
Allocate an mbuf and initialize it to contain a packet header
|
|
and internal data.
|
|
See
|
|
.Fn MGET
|
|
for details.
|
|
.It Fn MCLGET mbuf how
|
|
Allocate and attach an mbuf cluster to an mbuf.
|
|
If the macro fails, the
|
|
.Dv M_EXT
|
|
flag won't be set in the mbuf.
|
|
.It Fn M_PREPEND mbuf len how
|
|
This macro operates on an mbuf chain.
|
|
It is an optimized wrapper for
|
|
.Fn m_prepend
|
|
that can make use of possible empty space before data
|
|
(e.g. left after trimming of a link-layer header).
|
|
The new chain pointer or
|
|
.Dv NULL
|
|
is in
|
|
.Fa mbuf
|
|
after the call.
|
|
.It Fn M_WRITABLE mbuf
|
|
This macro will evaluate true if the mbuf is not marked
|
|
.Dv M_RDONLY
|
|
and if either the mbuf does not contain external storage or,
|
|
if it does,
|
|
then if the reference count of the storage is not greater than 1.
|
|
The
|
|
.Dv M_RDONLY
|
|
flag can be set in the mbuf's
|
|
.Dv m_flags .
|
|
This can be achieved during setup of the external storage,
|
|
by passing the
|
|
.Dv M_RDONLY
|
|
bit as a
|
|
.Fa flags
|
|
argument to the
|
|
.Fn MEXTADD
|
|
macro, or can be directly set in individual mbufs.
|
|
.El
|
|
.Pp
|
|
The functions are:
|
|
.Bl -ohang -offset indent
|
|
.It Fn m_get how type
|
|
A function version of
|
|
.Fn MGET
|
|
for non-critical paths.
|
|
.It Fn m_getm orig len how type
|
|
Allocate
|
|
.Fa len
|
|
bytes worth of mbufs and mbuf clusters if necessary and append the resulting
|
|
allocated chain to the
|
|
.Fa orig
|
|
mbuf chain, if it is
|
|
.No non- Ns Dv NULL .
|
|
If the allocation fails at any point,
|
|
free whatever was allocated and return
|
|
.Dv NULL .
|
|
If
|
|
.Fa orig
|
|
is
|
|
.No non- Ns Dv NULL ,
|
|
it will not be freed.
|
|
It is possible to use
|
|
.Fn m_getm
|
|
to either append
|
|
.Fa len
|
|
bytes to an existing mbuf or mbuf chain
|
|
(for example, one which may be sitting in a pre-allocated ring)
|
|
or to simply perform an all-or-nothing mbuf and mbuf cluster allocation.
|
|
.It Fn m_gethdr how type
|
|
A function version of
|
|
.Fn MGETHDR
|
|
for non-critical paths.
|
|
.It Fn m_getclr how type
|
|
Allocate an mbuf and zero out the data region.
|
|
.El
|
|
.Pp
|
|
The functions below operate on mbuf chains.
|
|
.Bl -ohang -offset indent
|
|
.It Fn m_freem mbuf
|
|
Free an entire mbuf chain, including any external
|
|
storage.
|
|
.\"
|
|
.It Fn m_adj mbuf len
|
|
Trim
|
|
.Fa len
|
|
bytes from the head of an mbuf chain if
|
|
.Fa len
|
|
is positive, from the tail otherwise.
|
|
.\"
|
|
.It Fn m_prepend mbuf len how
|
|
Allocate a new mbuf and prepend it to the chain, handle
|
|
.Dv M_PKTHDR
|
|
properly.
|
|
.Sy Note :
|
|
It doesn't allocate any clusters, so
|
|
.Fa len
|
|
must be less than
|
|
.Dv MLEN
|
|
or
|
|
.Dv MHLEN ,
|
|
depending on the
|
|
.Dv M_PKTHDR
|
|
flag setting.
|
|
.\"
|
|
.It Fn m_pullup mbuf len
|
|
Arrange that the first
|
|
.Fa len
|
|
bytes of an mbuf chain are contiguous and lay in the data area of
|
|
.Fa mbuf ,
|
|
so they are accessible with
|
|
.Fn mtod mbuf type .
|
|
Return the new chain on success,
|
|
.Dv NULL
|
|
on failure
|
|
(the chain is freed in this case).
|
|
.Sy Note :
|
|
It doesn't allocate any clusters, so
|
|
.Fa len
|
|
must be less than
|
|
.Dv MHLEN .
|
|
.\"
|
|
.It Fn m_copym mbuf offset len how
|
|
Make a copy of an mbuf chain starting
|
|
.Fa offset
|
|
bytes from the beginning, continuing for
|
|
.Fa len
|
|
bytes.
|
|
If
|
|
.Fa len
|
|
is
|
|
.Dv M_COPYALL ,
|
|
copy to the end of the mbuf chain.
|
|
.Sy Note :
|
|
The copy is read-only, because clusters are not
|
|
copied, only their reference counts are incremented.
|
|
.\"
|
|
.It Fn m_copypacket mbuf how
|
|
Copy an entire packet including header, which must be present.
|
|
This is an optimized version of the common case
|
|
.Fn m_copym mbuf 0 M_COPYALL how .
|
|
.Sy Note :
|
|
the copy is read-only, because clusters are not
|
|
copied, only their reference counts are incremented.
|
|
.\"
|
|
.It Fn m_dup mbuf how
|
|
Copy a packet header mbuf chain into a completely new chain, including
|
|
copying any mbuf clusters.
|
|
Use this instead of
|
|
.Fn m_copypacket
|
|
when you need a writable copy of an mbuf chain.
|
|
.\"
|
|
.It Fn m_copydata mbuf offset len buf
|
|
Copy data from an mbuf chain starting
|
|
.Fa off
|
|
bytes from the beginning, continuing for
|
|
.Fa len
|
|
bytes, into the indicated buffer
|
|
.Fa buf .
|
|
.\"
|
|
.It Fn m_copyback mbuf offset len buf
|
|
Copy
|
|
.Fa len
|
|
bytes from the buffer
|
|
.Fa buf
|
|
back into the indicated mbuf chain,
|
|
starting at
|
|
.Fa offset
|
|
bytes from the beginning of the chain, extending the mbuf chain if necessary.
|
|
.Sy Note :
|
|
It doesn't allocate any clusters, just adds mbufs to the chain.
|
|
It's safe to set
|
|
.Fa offset
|
|
beyond the current chain end: zeroed mbufs will be allocated to fill the
|
|
space.
|
|
.\"
|
|
.It Fn m_devget buf len offset ifp copy
|
|
Copy data from a device local memory pointed to by
|
|
.Fa buf
|
|
to an mbuf chain.
|
|
The copy is done using a specified copy routine
|
|
.Fa copy ,
|
|
or
|
|
.Fn bcopy
|
|
if
|
|
.Fa copy
|
|
is
|
|
.Dv NULL .
|
|
.\"
|
|
.It Fn m_cat m n
|
|
Concatenate
|
|
.Fa n
|
|
to
|
|
.Fa m .
|
|
Both chains must be of the same type.
|
|
.Fa N
|
|
is still valid after the function returned.
|
|
.Sy Note :
|
|
It does not handle
|
|
.Dv M_PKTHDR
|
|
and friends.
|
|
.\"
|
|
.It Fn m_split mbuf len how
|
|
Partition an mbuf chain in two pieces, returning the tail:
|
|
all but the first
|
|
.Fa len
|
|
bytes.
|
|
In case of failure, it returns
|
|
.Dv NULL
|
|
and attempts to restore the chain to its original state.
|
|
.El
|
|
.Sh RETURN VALUES
|
|
See above.
|
|
.Sh HISTORY
|
|
.\" Please correct me if I'm wrong
|
|
Mbufs appeared in an early version of BSD.
|
|
Besides for being used for network packets, they were used
|
|
to store various dynamic structures, such as routing table
|
|
entries, interface addresses, protocol control blocks, etc.
|
|
.Sh AUTHORS
|
|
The original mbuf(9) man page was written by Yar Tikhiy.
|