xref: /freebsd/share/man/man9/mbuf.9 (revision 7b2fd1831feff3149edd09030e5fbafedce5fb51)

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.\" $FreeBSD$
.\"
.Dd October 17, 2000
.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"
.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
.Ft void *
.Fn mtod "struct mbuf *mbuf" "type"
.Ft int
.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"
.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"
.Fn MCHTYPE "struct mbuf *mbuf" "u_int type"
.Ft int
.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 "const 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 u_int
.Fn m_fixhdr "struct mbuf *mbuf"
.Ft u_int
.Fn m_length "struct mbuf *mbuf" "struct mbuf **last"
.Ft struct mbuf *
.Fn m_split "struct mbuf *mbuf" "int len" "int how"
.\"
.Sh DESCRIPTION
An
.Vt mbuf
is a basic unit of memory management in the kernel IPC subsystem.
Network packets and socket buffers are stored in
.Vt mbufs .
A network packet may span multiple
.Vt mbufs
arranged into a
.Vt mbuf chain
(linked list),
which allows adding or trimming
network headers with little overhead.
.Pp
While a developer should not bother with
.Vt mbuf
internals without serious
reason in order to avoid incompatibilities with future changes, it
is useful to understand the general structure of an
.Vt mbuf .
.Pp
An
.Vt mbuf
consists of a variable-sized header and a small internal
buffer for data.
The total size of an
.Vt mbuf ,
.Dv MSIZE ,
is a machine-dependent constant defined in
.Pa machine/param.h .
The
.Vt mbuf
header includes:
.Pp
.Bl -tag -width "m_nextpkt" -compact -offset indent
.It Va m_next
a pointer to the next
.Vt mbuf
in the
.Vt mbuf chain
.It Va m_nextpkt
a pointer to the next
.Vt mbuf chain
in the queue
.It Va m_data
a pointer to the data
.It Va m_len
the length of the data
.It Va m_type
the type of data
.It Va m_flags
the
.Vt mbuf
flags
.El
.Pp
The
.Vt 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
.Vt 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
.Vt struct pkthdr Va m_pkthdr
is added to the
.Vt mbuf
header.
It contains a pointer to the interface
the packet has been received from
.Pq Vt struct ifnet Va *rcvif ,
and the total packet length
.Pq Vt int Va len .
.Pp
If small enough, data is stored in the internal data buffer of an
.Vt mbuf .
If the data is sufficiently large, another
.Vt mbuf
may be added to the
.Vt mbuf chain ,
or external storage may be associated with the
.Vt mbuf .
.Dv MHLEN
bytes of data can fit into an
.Vt mbuf
with the
.Dv M_PKTHDR
flag set,
.Dv MLEN
bytes can otherwise.
.Pp
If external storage is being associated with an
.Vt mbuf ,
the
.Va 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
.Vt 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
external storage of a given
.Vt mbuf
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
.Vt mbuf cluster .
.Vt Mbuf clusters
can be allocated and configured with the use of the
.Dv MCLGET
macro.
Each
.Vt mbuf 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 an
.Vt mbuf cluster .
It's equal to the sum of
.Dv MLEN
and
.Dv MHLEN .
It is typically preferable to store data into the data region of an
.Vt mbuf ,
if size permits, as opposed to allocating a separate
.Vt 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
.Fa mbuf
pointer to a data pointer.
The macro expands to the data pointer cast to the pointer of the specified
.Fa type .
.Sy Note :
It is advisable to ensure that there is enough contiguous data in
.Fa mbuf .
See
.Fn m_pullup
for details.
.It Fn MGET mbuf how type
Allocate an
.Vt mbuf
and initialize it to contain internal data.
.Fa mbuf
will point to the allocated
.Vt 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 functions and macros related to
.Vt mbufs
have the same argument because they may
at some point need to allocate new
.Vt mbufs .
.Pp
Programmers should be careful not to confuse the
.Vt mbuf
allocation flag
.Dv M_DONTWAIT
with the
.Xr malloc 9
allocation flag,
.Dv M_NOWAIT .
They are not the same.
.It Fn MGETHDR mbuf how type
Allocate an
.Vt 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
.Vt mbuf cluster
to
.Fa mbuf .
If the macro fails, the
.Dv M_EXT
flag won't be set in
.Fa mbuf .
.It Fn M_PREPEND mbuf len how
This macro operates on an
.Vt 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
.Vt mbuf chain
pointer or
.Dv NULL
is in
.Fa mbuf
after the call.
.It Fn M_WRITABLE mbuf
This macro will evaluate true if
.Fa mbuf
is not marked
.Dv M_RDONLY
and if either
.Fa 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
.Fa mbuf->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
.Vt 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
.Vt mbufs
and
.Vt mbuf clusters
if necessary and append the resulting allocated
.Vt mbuf chain
to the
.Vt mbuf chain
.Fa orig ,
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
.Vt mbuf
or
.Vt mbuf chain
(for example, one which may be sitting in a pre-allocated ring)
or to simply perform an all-or-nothing
.Vt mbuf
and
.Vt 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
.Vt mbuf
and zero out the data region.
.El
.Pp
The functions below operate on
.Vt mbuf chains .
.Bl -ohang -offset indent
.It Fn m_freem mbuf
Free an entire
.Vt mbuf chain ,
including any external storage.
.\"
.It Fn m_adj mbuf len
Trim
.Fa len
bytes from the head of an
.Vt mbuf chain
if
.Fa len
is positive, from the tail otherwise.
.\"
.It Fn m_prepend mbuf len how
Allocate a new
.Vt mbuf
and prepend it to the
.Vt mbuf chain ,
handle
.Dv M_PKTHDR
properly.
.Sy Note :
It doesn't allocate any
.Vt mbuf 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
.Vt 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
.Vt mbuf chain
on success,
.Dv NULL
on failure
(the
.Vt mbuf chain
is freed in this case).
.Sy Note :
It doesn't allocate any
.Vt mbuf clusters ,
so
.Fa len
must be less than
.Dv MHLEN .
.\"
.It Fn m_copym mbuf offset len how
Make a copy of an
.Vt 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
.Vt mbuf chain .
.Sy Note :
The copy is read-only, because the
.Vt mbuf 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 the
.Vt mbuf clusters
are not copied, only their reference counts are incremented.
.\"
.It Fn m_dup mbuf how
Copy a packet header
.Vt mbuf chain
into a completely new
.Vt mbuf chain ,
including copying any
.Vt mbuf clusters .
Use this instead of
.Fn m_copypacket
when you need a writable copy of an
.Vt mbuf chain .
.\"
.It Fn m_copydata mbuf offset len buf
Copy data from an
.Vt 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
.Vt mbuf chain ,
starting at
.Fa offset
bytes from the beginning of the
.Vt mbuf chain ,
extending the
.Vt mbuf chain
if necessary.
.Sy Note :
It doesn't allocate any
.Vt mbuf clusters ,
just adds
.Vt mbufs
to the
.Vt mbuf chain .
It's safe to set
.Fa offset
beyond the current
.Vt mbuf chain
end: zeroed
.Vt mbufs
will be allocated to fill the space.
.\"
.It Fn m_length buf last
Return the length of the
.Vt mbuf chain ,
and optionally a pointer to the last
.Vt mbuf .
.\"
.It Fn m_fixhdr buf
Set the packet-header length to the length of the
.Vt mbuf chain .
.\"
.It Fn m_devget buf len offset ifp copy
Copy data from a device local memory pointed to by
.Fa buf
to an
.Vt 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
.Vt mbuf 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
.Vt 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
.Vt mbuf chain
to its original state.
.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
.Vt 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 Yar Tikhiy.