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NAME    [Toc]    [Back]

     mbuf -- memory management in the kernel IPC subsystem

SYNOPSIS    [Toc]    [Back]

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

   Mbuf allocation macros    [Toc]    [Back]
     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    [Toc]    [Back]
     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    [Toc]    [Back]
     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    [Toc]    [Back]
     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);

DESCRIPTION    [Toc]    [Back]

     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 constant
 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 permits,
 as opposed to allocating a separate mbuf cluster to hold the same

   Macros and Functions    [Toc]    [Back]
     There are numerous predefined macros and functions that provide the
     developer 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.
	   Applicable 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
	   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.

	   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 beginning,
 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

	   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 incremented.

	   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
	   allocate 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 identical
 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
	   attributes 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 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
	   attempts to restore the mbuf chain to its original state.

STRESS TESTING    [Toc]    [Back]

     When running a kernel compiled with the option MBUF_STRESS_TEST, the following
 sysctl(8)-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 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 drivers.

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

RETURN VALUES    [Toc]    [Back]

     See above.

HISTORY    [Toc]    [Back]

     Mbufs appeared in an early version of BSD.  Besides 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    [Toc]    [Back]

     The original mbuf man page was written by Yar Tikhiy.

FreeBSD 5.2.1		       October 17, 2000 		 FreeBSD 5.2.1
[ Back ]
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