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

NAME
     mbuf -- memory management in the kernel IPC subsystem

SYNOPSIS
     #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);

     int
     MEXT_IS_REF(struct	mbuf *mbuf);

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

     M_ALIGN(struct mbuf *mbuf,	u_int len);

     MH_ALIGN(struct mbuf *mbuf, u_int len);

     int
     M_LEADINGSPACE(struct mbuf	*mbuf);

     int
     M_TRAILINGSPACE(struct mbuf *mbuf);

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

     MCHTYPE(struct mbuf *mbuf,	u_int type);

     int
     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);

     void
     m_freem(struct mbuf *mbuf);

   Mbuf	utility	functions
     void
     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);

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

     void
     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));

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

     u_int
     m_fixhdr(struct mbuf *mbuf);

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

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

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.

     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.

     An	mbuf consists of a variable-sized header and a small internal buffer
     for data.	The mbuf's total size, MSIZE, is a machine-dependent constant
     defined in	machine/param.h.  The mbuf header includes:

	   m_next     a	pointer	to the next buffer in the chain
	   m_nextpkt  a	pointer	to the next 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
     len).

     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.  MHLEN bytes of data
     can fit into an mbuf with the M_PKTHDR flag set, MLEN bytes can other-
     wise.

     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
     given mbuf's external storage 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	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 a clus-
     ter.  It's	equal to the sum of MLEN and MHLEN.  It	is typically prefer-
     able to store data	into an	mbuf's data region, if size permits, as	op-
     posed to allocating a separate mbuf cluster to hold the same data.

   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
	   the 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 mbuf-re-
	   lated functions and macros 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 an mbuf.  If the macro
	   fails, the M_EXT flag won't be set in the mbuf.

	   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
	   chain pointer or NULL is in mbuf after the call.

	   M_WRITABLE(mbuf)
	   This	macro will evaluate true if the	mbuf is	not marked 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	M_RDONLY flag can be set in the	mbuf's 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.

     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 chain to the orig	mbuf chain, if
	   it is non-NULL.  If the allocation fails at any point, free what-
	   ever	was allocated and return NULL.	If orig	is non-NULL, it	will
	   not be freed.  It is	possible to use	m_getm() to either append 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.

	   m_freem(mbuf)
	   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 chain, handle M_PKTHDR
	   properly.  Note: It doesn't allocate	any clusters, so len must be
	   less	than MLEN or MHLEN, depending on the M_PKTHDR flag setting.

	   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 chain on success, NULL on failure
	   (the	chain is freed in this case).  Note: It	doesn't	allocate any
	   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	clus-
	   ters	are not	copied,	only their reference counts are	incremented.

	   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 clusters are
	   not copied, only their reference counts are incremented.

	   m_dup(mbuf, how)
	   Copy	a packet header	mbuf chain into	a completely new chain,	in-
	   cluding 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 chain,
	   extending the mbuf chain if necessary.  Note: It doesn't allocate
	   any clusters, just adds mbufs to the	chain.	It's safe to set
	   offset beyond the current chain end:	zeroed mbufs will be allocated
	   to fill the space.

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

	   m_fixhdr(buf)
	   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 chains must be of the same	type.  N is
	   still valid after the function returned.  Note: It does not handle
	   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 chain to its original state.

STRESS TESTING
     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	mbuf-reliant
     parts of the kernel.

     net.inet.ip.mbuf_frag_size
	    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-
	    ers.

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

RETURN VALUES
     See above.

HISTORY
     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.

AUTHORS
     The original mbuf man page	was written by Yar Tikhiy.

BSD			       October 17, 2000				   BSD

NAME | SYNOPSIS | DESCRIPTION | STRESS TESTING | RETURN VALUES | HISTORY | AUTHORS

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