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MBUF(9)			 BSD Kernel Developer's	Manual		       MBUF(9)

     mbuf -- memory management in the kernel IPC subsystem

     #include <sys/param.h>
     #include <sys/systm.h>
     #include <sys/mbuf.h>

   Mbuf	allocation macros
     MGET(struct mbuf *mbuf, int how, short type);

     MGETHDR(struct mbuf *mbuf,	int how, short type);

     MCLGET(struct mbuf	*mbuf, int how);

     MEXTADD(struct mbuf *mbuf,	caddr_t	buf, u_int size,
	 void (*free)(void *opt_args), void *opt_args, short flags, int	type);

     MEXTFREE(struct mbuf *mbuf);

     MEXT_ADD_REF(struct mbuf *mbuf);

     MEXT_REM_REF(struct mbuf *mbuf);

     MFREE(struct mbuf *mbuf, struct mbuf *successor);

   Mbuf	utility	macros
     void *
     mtod(struct mbuf *mbuf, type);

     MEXT_IS_REF(struct	mbuf *mbuf);

     M_ALIGN(struct mbuf *mbuf,	u_int len);

     MH_ALIGN(struct mbuf *mbuf, u_int len);

     M_LEADINGSPACE(struct mbuf	*mbuf);

     M_TRAILINGSPACE(struct mbuf *mbuf);

     M_MOVE_PKTHDR(struct mbuf *to, struct mbuf	*from);

     M_PREPEND(struct mbuf *mbuf, int len, int how);

     MCHTYPE(struct mbuf *mbuf,	u_int type);

     M_WRITABLE(struct mbuf *mbuf);

   Mbuf	allocation functions
     struct mbuf *
     m_get(int how, int	type);

     struct mbuf *
     m_getm(struct mbuf	*orig, int len,	int how, int type);

     struct mbuf *
     m_getclr(int how, int type);

     struct mbuf *
     m_gethdr(int how, int type);

     struct mbuf *
     m_free(struct mbuf	*mbuf);

     m_freem(struct mbuf *mbuf);

   Mbuf	utility	functions
     m_adj(struct mbuf *mbuf, int len);

     struct mbuf *
     m_prepend(struct mbuf *mbuf, int len, int how);

     struct mbuf *
     m_pullup(struct mbuf *mbuf, int len);

     struct mbuf *
     m_copym(struct mbuf *mbuf,	int offset, int	len, int how);

     struct mbuf *
     m_copypacket(struct mbuf *mbuf, int how);

     struct mbuf *
     m_dup(struct mbuf *mbuf, int how);

     m_copydata(const struct mbuf *mbuf, int offset, int len, caddr_t buf);

     m_copyback(struct mbuf *mbuf, int offset, int len,	caddr_t	buf);

     struct mbuf *
     m_devget(char *buf, int len, int offset, struct ifnet *ifp,
	 void (*copy)(char *from, caddr_t to, u_int len));

     m_cat(struct mbuf *m, struct mbuf *n);

     m_fixhdr(struct mbuf *mbuf);

     m_dup_pkthdr(struct mbuf *to, struct mbuf *from);

     m_move_pkthdr(struct mbuf *to, struct mbuf	*from);

     m_length(struct mbuf *mbuf, struct	mbuf **last);

     struct mbuf *
     m_split(struct mbuf *mbuf,	int len, int how);

     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 mbuf chain	(linked	list), which
     allows adding or trimming network headers with little overhead.

     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 general structure	of an mbuf.

     An	mbuf consists of a variable-sized header and a small internal buffer
     for data.	The total size of an mbuf, MSIZE, is a machine-dependent con-
     stant defined in <machine/param.h>.  The mbuf header includes:

	   m_next     a	pointer	to the next mbuf in the	mbuf chain
	   m_nextpkt  a	pointer	to the next mbuf chain in the queue
	   m_data     a	pointer	to the data
	   m_len      the length of the	data
	   m_type     the type of data
	   m_flags    the mbuf flags

     The mbuf flag bits	are defined as follows:

     /*	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	*/

     The available mbuf	types are defined as follows:

     /*	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	*/

     If	the M_PKTHDR flag is set, a struct pkthdr m_pkthdr is added to the
     mbuf header.  It contains a pointer to the	interface the packet has been
     received from (struct ifnet *rcvif), and the total	packet length (int

     If	small enough, data is stored in	the internal data buffer of an mbuf.
     If	the data is sufficiently large,	another	mbuf may be added to the mbuf
     chain, or external	storage	may be associated with the mbuf.  MHLEN	bytes
     of	data can fit into an mbuf with the M_PKTHDR flag set, MLEN bytes can

     If	external storage is being associated with an mbuf, the 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 M_EXT flag set.

     The system	supplies a macro for allocating	the desired external storage
     buffer, MEXTADD.

     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 mbuf is greater than 1	with the MEXT_IS_REF
     macro.  Similarly,	the developer can directly add and remove references,
     if	absolutely necessary, with the use of the MEXT_ADD_REF and
     MEXT_REM_REF macros.

     The system	also supplies a	default	type of	external storage buffer	called
     an	mbuf cluster.  Mbuf clusters can be allocated and configured with the
     use of the	MCLGET macro.  Each mbuf cluster is MCLBYTES in	size, where
     MCLBYTES is a machine-dependent constant.	The system defines an advisory
     macro MINCLSIZE, which is the smallest amount of data to put into an mbuf
     cluster.  It's equal to the sum of	MLEN and MHLEN.	 It is typically
     preferable	to store data into the data region of an mbuf, if size per-
     mits, as opposed to allocating a separate mbuf cluster to hold the	same

   Macros and Functions
     There are numerous	predefined macros and functions	that provide the de-
     veloper with common utilities.

	   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.	Note:
	   It is advisable to ensure that there	is enough contiguous data in
	   mbuf.  See m_pullup() for details.

	   MGET(mbuf, how, type)
	   Allocate an mbuf and	initialize it to contain internal data.	 mbuf
	   will	point to the allocated mbuf on success,	or be set to NULL on
	   failure.  The how argument is to be set to M_TRYWAIT	or M_DONTWAIT.
	   It specifies	whether	the caller is willing to block if necessary.
	   If how is set to M_TRYWAIT, a failed	allocation will	result in the
	   caller being	put to sleep for a designated kern.ipc.mbuf_wait
	   (sysctl(8) tunable) number of ticks.	 A number of other functions
	   and macros related to mbufs have the	same argument because they may
	   at some point need to allocate new mbufs.

	   Programmers should be careful not to	confuse	the mbuf allocation
	   flag	M_DONTWAIT with	the malloc(9) allocation flag, M_NOWAIT.  They
	   are not the same.

	   MGETHDR(mbuf, how, type)
	   Allocate an mbuf and	initialize it to contain a packet header and
	   internal data.  See MGET() for details.

	   MCLGET(mbuf,	how)
	   Allocate and	attach an mbuf cluster to mbuf.	 If the	macro fails,
	   the M_EXT flag won't	be set in mbuf.

	   M_ALIGN(mbuf, len)
	   Set the pointer mbuf-_m_data	to place an object of the size len at
	   the end of the internal data	area of	mbuf, long word	aligned.  Ap-
	   plicable only if mbuf is newly allocated with MGET()	or m_get().

	   MH_ALIGN(mbuf, len)
	   Serves the same purpose as M_ALIGN()	does, but only for mbuf	newly
	   allocated with MGETHDR() or m_gethdr(), or initialized by
	   m_dup_pkthdr() or m_move_pkthdr().

	   Returns the number of bytes available before	the beginning of data
	   in mbuf.

	   Returns the number of bytes available after the end of data in

	   M_PREPEND(mbuf, len,	how)
	   This	macro operates on an mbuf chain.  It is	an optimized wrapper
	   for m_prepend() that	can make use of	possible empty space before
	   data	(e.g. left after trimming of a link-layer header).  The	new
	   mbuf	chain pointer or NULL is in mbuf after the call.

	   M_MOVE_PKTHDR(to, from)
	   Using this macro is equivalent to calling m_move_pkthdr(to, from).

	   This	macro will evaluate true if mbuf is not	marked M_RDONLY	and if
	   either mbuf does not	contain	external storage or, if	it does, then
	   if the reference count of the storage is not	greater	than 1.	 The
	   M_RDONLY flag can be	set in mbuf-_m_flags.  This can	be achieved
	   during setup	of the external	storage, by passing the	M_RDONLY bit
	   as a	flags argument to the MEXTADD()	macro, or can be directly set
	   in individual mbufs.

	   MCHTYPE(mbuf, type)
	   Change the type of mbuf to type.  This is a relatively expensive
	   operation and should	be avoided.

     The functions are:

	   m_get(how, type)
	   A function version of MGET()	for non-critical paths.

	   m_getm(orig,	len, how, type)
	   Allocate len	bytes worth of mbufs and mbuf clusters if necessary
	   and append the resulting allocated mbuf chain to the	mbuf chain
	   orig, if it is non-NULL.  If	the allocation fails at	any point,
	   free	whatever was allocated and return NULL.	 If orig is non-NULL,
	   it will not be freed.  It is	possible to use	m_getm() to either ap-
	   pend	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.

	   m_gethdr(how, type)
	   A function version of MGETHDR() for non-critical paths.

	   m_getclr(how, type)
	   Allocate an mbuf and	zero out the data region.

     The functions below operate on mbuf chains.

	   Free	an entire mbuf chain, including	any external storage.

	   m_adj(mbuf, len)
	   Trim	len bytes from the head	of an mbuf chain if len	is positive,
	   from	the tail otherwise.

	   m_prepend(mbuf, len,	how)
	   Allocate a new mbuf and prepend it to the mbuf chain, handle
	   M_PKTHDR properly.  Note: It	doesn't	allocate any mbuf clusters, so
	   len must be less than MLEN or MHLEN,	depending on the M_PKTHDR flag

	   m_pullup(mbuf, len)
	   Arrange that	the first len bytes of an mbuf chain are contiguous
	   and lay in the data area of mbuf, so	they are accessible with
	   mtod(mbuf, type).  Return the new mbuf chain	on success, NULL on
	   failure (the	mbuf chain is freed in this case).  Note: It doesn't
	   allocate any	mbuf clusters, so len must be less than	MHLEN.

	   m_copym(mbuf, offset, len, how)
	   Make	a copy of an mbuf chain	starting offset	bytes from the begin-
	   ning, continuing for	len bytes.  If len is M_COPYALL, copy to the
	   end of the mbuf chain.  Note: The copy is read-only,	because	the
	   mbuf	clusters are not copied, only their reference counts are in-

	   m_copypacket(mbuf, how)
	   Copy	an entire packet including header, which must be present.
	   This	is an optimized	version	of the common case m_copym(mbuf, 0,
	   M_COPYALL, how).  Note: the copy is read-only, because the mbuf
	   clusters are	not copied, only their reference counts	are incre-

	   m_dup(mbuf, how)
	   Copy	a packet header	mbuf chain into	a completely new mbuf chain,
	   including copying any mbuf clusters.	 Use this instead of
	   m_copypacket() when you need	a writable copy	of an mbuf chain.

	   m_copydata(mbuf, offset, len, buf)
	   Copy	data from an mbuf chain	starting off bytes from	the beginning,
	   continuing for len bytes, into the indicated	buffer buf.

	   m_copyback(mbuf, offset, len, buf)
	   Copy	len bytes from the buffer buf back into	the indicated mbuf
	   chain, starting at offset bytes from	the beginning of the mbuf
	   chain, extending the	mbuf chain if necessary.  Note:	It doesn't al-
	   locate any mbuf clusters, just adds mbufs to	the mbuf chain.	 It's
	   safe	to set offset beyond the current mbuf chain end: zeroed	mbufs
	   will	be allocated to	fill the space.

	   m_length(mbuf, last)
	   Return the length of	the mbuf chain,	and optionally a pointer to
	   the last mbuf.

	   m_dup_pkthdr(to, from, how)
	   Upon	the function's completion, the mbuf to will contain an identi-
	   cal copy of from-_m_pkthdr and the per-packet attributes found in
	   the mbuf chain from.	 The mbuf from must have the flag M_PKTHDR
	   initially set, and to must be empty on entry.

	   m_move_pkthdr(to, from)
	   Move	m_pkthdr and the per-packet attributes from the	mbuf chain
	   from	to the mbuf to.	 The mbuf from must have the flag M_PKTHDR
	   initially set, and to must be empty on entry.  Upon the function's
	   completion, from will have the flag M_PKTHDR	and the	per-packet at-
	   tributes cleared.

	   Set the packet-header length	to the length of the mbuf chain.

	   m_devget(buf, len, offset, ifp, copy)
	   Copy	data from a device local memory	pointed	to by buf to an	mbuf
	   chain.  The copy is done using a specified copy routine copy, or
	   bcopy() if copy is NULL.

	   m_cat(m, n)
	   Concatenate n to m.	Both mbuf chains must be of the	same type.  N
	   is still valid after	the function returned.	Note: It does not han-
	   dle M_PKTHDR	and friends.

	   m_split(mbuf, len, how)
	   Partition an	mbuf chain in two pieces, returning the	tail: all but
	   the first len bytes.	 In case of failure, it	returns	NULL and at-
	   tempts to restore the mbuf chain to its original state.

     When running a kernel compiled with the option MBUF_STRESS_TEST, the fol-
     lowing sysctl(8)-controlled options may be	used to	create various fail-
     ure/extreme cases for testing of network drivers and other	parts of the
     kernel that rely on mbufs.

	    Causes ip_output() to fragment outgoing mbuf chains	into fragments
	    of the specified size.  Setting this variable to 1 is an excellent
	    way	to test	the long mbuf chain handling ability of	network	driv-

	    Causes the function	m_defrag() to randomly fail, returning NULL.
	    Any	piece of code which uses m_defrag() should be tested with this

     See above.

     Mbufs appeared in an early	version	of BSD.	 Besides being used for	net-
     work packets, they	were used to store various dynamic structures, such as
     routing table entries, interface addresses, protocol control blocks, etc.

     The original mbuf man page	was written by Yar Tikhiy.

BSD			       October 17, 2000				   BSD


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