man/man9/mbuf.9

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.\" $FreeBSD: src/share/man/man9/mbuf.9,v 1.27.2.1 2003/05/28 13:53:18 yar Exp $
.\"
.Dd December 27, 2023
.Dt MBUF 9
.Os
.\"
.Sh NAME
.Nm mbuf
.Nd "memory management in the kernel IPC subsystem"
.\"
.Sh SYNOPSIS
.In sys/param.h
.In sys/systm.h
.In 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"
.\"
.Ss Mbuf utility macros
.Ft void *
.Fn mtod "struct mbuf *mbuf" "type"
.Fn M_ALIGN "struct mbuf *mbuf" "u_int len"
.Fn MH_ALIGN "struct mbuf *mbuf" "u_int len"
.Ft int
.Fn M_LEADINGSPACE "struct mbuf *mbuf"
.Ft int
.Fn M_TRAILINGSPACE "struct mbuf *mbuf"
.Fn M_PREPEND "struct mbuf *mbuf" "int len" "int how"
.\"
.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 "const 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 "const struct mbuf *mbuf" "int offset" "int len" "void *buf"
.Ft void
.Fn m_copyback "struct mbuf *mbuf" "int offset" "int len" "const void *buf"
.Ft int
.Fn m_copyback2 "struct mbuf *mbuf" "int offset" "int len" "const void *buf" "int how"
.Ft struct mbuf *
.Fo m_devget
.Fa "void *buf"
.Fa "int len"
.Fa "int offset"
.Fa "struct ifnet *ifp"
.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"
.Ft struct mbuf *
.Fn m_unshare "struct mbuf *mbuf" "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 constant defined in
.In sys/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_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 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
.Dv 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_WAITOK
or
.Dv M_NOWAIT .
If
.Fa how
is
.Dv M_WAITOK ,
this macro will await resources if necessary;
if
.Fa how
is
.Dv M_NOWAIT
and resources are not available, this macro fails.
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.
.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
Similar to the
.Fn m_copyback2
function below, but
.Em forbid mbuf expansion .
So the caller must ensure that the mbuf (chain) has enough space;
otherwise, the copy would fail and it would be hard for the caller
to discriminate due to the lack of a return value.
In the failure case, the input mbuf (chain) may be modified but
will not be freed.
.\"
.It Fn m_copyback2 mbuf offset len buf how
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.
Mbuf clusters will be allocated if appropriate.
.Sy Note :
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 data from a device local memory pointed to by
.Fa buf
to an mbuf chain, using
.Fn bcopy .
.\"
.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.
.It Fn m_unshare mbuf how
Create a version of the specified mbuf chain whose
contents can be safely modified without affecting other users.
If allocation fails and this operation can not be completed,
.Dv NULL
will be returned.
The original mbuf chain is always reclaimed and the reference
count of any shared mbuf clusters is decremented.
As a side-effect of this process the returned
mbuf chain may be compacted.
.Pp
This function is especially useful in the transmit path of
network code, when data must be encrypted or otherwise
altered prior to transmission.
.
.El
.Sh STRESS TESTING
When running a kernel compiled with the option
.Dv MBUF_STRESS_TEST ,
the following
.Xr sysctl 8 Ns
-controlled options may be used to create
various failure/extreme cases for testing of network drivers
and other parts of the kernel that rely on
.Vt mbufs .
.Bl -tag -width ident
.It Va net.inet.ip.mbuf_frag_size
Causes
.Fn ip_output
to fragment outgoing
.Vt mbuf chains
into fragments of the specified size.
Setting this variable to 1 is an excellent way to
test the long
.Vt mbuf chain
handling ability of network drivers.
.It Va kern.ipc.m_defragrandomfailures
Causes the function
.Fn m_defrag
to randomly fail, returning
.Dv NULL .
Any piece of code which uses
.Fn m_defrag
should be tested with this feature.
.El
.Sh RETURN VALUES
See above.
.Sh HISTORY
.\" Please correct me if I'm wrong
Mbufs appeared in an early version of
.Bx .
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
.Nm
man page was written by
.An Yar Tikhiy .