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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_arg1, void *opt_arg2), void *opt_arg1,
	 void *opt_arg2, short flags, int type);

     MEXTFREE(struct mbuf *mbuf);

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

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

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

     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_getcl(int how, short type, int flags);

     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);

     void
     m_align(struct mbuf *mbuf, int len);

     int
     m_append(struct mbuf *mbuf, int len, c_caddr_t cp);

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

     struct mbuf *
     m_copyup(struct mbuf *mbuf, int len, int dstoff);

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

     struct mbuf *
     m_pulldown(struct mbuf *mbuf, int offset, int len, int *offsetp);

     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);

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

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

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

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

     int
     m_apply(struct mbuf *mbuf, int off, int len,
	 int (*f)(void *arg, void *data, u_int len), void *arg);

     struct mbuf *
     m_getptr(struct mbuf *mbuf, int loc, int *off);

     struct mbuf *
     m_defrag(struct mbuf *m0, int how);

     struct mbuf *
     m_unshare(struct mbuf *m0, 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 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 constant defined in
     <sys/param.h>.  The mbuf header includes:

	   m_next     (struct mbuf *) A pointer to the next mbuf in the mbuf
		      chain.

	   m_nextpkt  (struct mbuf *) A pointer to the next mbuf chain in the
		      queue.

	   m_data     (caddr_t) A pointer to data attached to this mbuf.

	   m_len      (int) The length of the data.

	   m_type     (short) The type of the data.

	   m_flags    (int) 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 */
     #define M_PROTO6	     0x4000  /* protocol-specific (avoid M_BCAST conflict) */
     #define M_FREELIST	     0x8000  /* mbuf is on the free list */

     /* mbuf pkthdr flags (also stored 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_DATA	     1	     /* dynamic (data) allocation */
     #define MT_HEADER	     MT_DATA /* packet header */
     #define MT_SONAME	     8	     /* socket name */
     #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).  Optionally, it may also contain an attached list of packet tags
     (struct m_tag).  See mbuf_tags(9) for details.  Fields used in offloading
     checksum calculation to the hardware are kept in m_pkthdr as well.	 See
     HARDWARE-ASSISTED CHECKSUM CALCULATION for details.

     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
     otherwise.

     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 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 is 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
     data.

   Macros and Functions
     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_WAIT or M_DONTWAIT.
	   It specifies whether the caller is willing to block if necessary.
	   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 will not 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().

	   m_align(mbuf, len)
	   Services the same purpose as M_ALIGN() but handles any type of
	   mbuf.

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

	   M_TRAILINGSPACE(mbuf)
	   Returns the number of bytes available after the end of data in
	   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
	   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).

	   M_WRITABLE(mbuf)
	   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 per‐
	   form an all-or-nothing mbuf and mbuf cluster allocation.

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

	   m_getcl(how, type, flags)
	   Fetch an mbuf with a mbuf cluster attached to it.  If one of the
	   allocations fails, the entire allocation fails.  This routine is
	   the preferred way of fetching both the mbuf and mbuf cluster
	   together, as it avoids having to unlock/relock between allocations.
	   Returns NULL on failure.

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

	   m_free(mbuf)
	   Frees mbuf.	Returns m_next of the freed mbuf.

     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_append(mbuf, len, cp)
	   Append len bytes of data cp to the mbuf chain.  Extend the mbuf
	   chain if the new data does not fit in existing space.

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

	   m_copyup(mbuf, len, dstoff)
	   Similar to m_pullup() but copies len bytes of data into a new mbuf
	   at dstoff bytes into the mbuf.  The dstoff argument aligns the data
	   and leaves room for a link layer header.  Returns the new mbuf
	   chain on success, and frees the mbuf chain and returns NULL on
	   failure.  Note: The function does not allocate mbuf clusters, so
	   len + dstoff must be less than MHLEN.

	   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).  It is important to remember that this may
	   involve reallocating some mbufs and moving data so all pointers
	   referencing data within the old mbuf chain must be recalculated or
	   made invalid.  Return the new mbuf chain on success, NULL on fail‐
	   ure (the mbuf chain is freed in this case).	Note: It does not
	   allocate any mbuf clusters, so len must be less than MHLEN.

	   m_pulldown(mbuf, offset, len, offsetp)
	   Arrange that len bytes between offset and offset + len in the mbuf
	   chain are contiguous and lay in the data area of mbuf, so they are
	   accessible with mtod(mbuf, type).  len must be smaller than, or
	   equal to, the size of an mbuf cluster.  Return a pointer to an
	   intermediate mbuf in the chain containing the requested region; the
	   offset in the data region of the mbuf chain to the data contained
	   in the returned mbuf is stored in *offsetp.	If offp is NULL, the
	   region may be accessed using mtod(mbuf, type).  If offp is non-
	   NULL, the region may be accessed using mtod(mbuf, uint8_t, +,
	   *offsetp).  The region of the mbuf chain between its beginning and
	   off is not modified, therefore it is safe to hold pointers to data
	   within this region before calling m_pulldown().

	   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
	   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 the mbuf
	   clusters are not copied, only their reference counts are incre‐
	   mented.

	   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 does not
	   allocate any mbuf clusters, just adds mbufs to the mbuf chain.  It
	   is 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
	   attributes cleared.

	   m_fixhdr(mbuf)
	   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
	   attempts to restore the mbuf chain to its original state.

	   m_apply(mbuf, off, len, f, arg)
	   Apply a function to an mbuf chain, at offset off, for length len
	   bytes.  Typically used to avoid calls to m_pullup() which would
	   otherwise be unnecessary or undesirable.  arg is a convenience
	   argument which is passed to the callback function f.

	   Each time f() is called, it will be passed arg, a pointer to the
	   data in the current mbuf, and the length len of the data in this
	   mbuf to which the function should be applied.

	   The function should return zero to indicate success; otherwise, if
	   an error is indicated, then m_apply() will return the error and
	   stop iterating through the mbuf chain.

	   m_getptr(mbuf, loc, off)
	   Return a pointer to the mbuf containing the data located at loc
	   bytes from the beginning of the mbuf chain.	The corresponding off‐
	   set into the mbuf will be stored in *off.

	   m_defrag(m0, how)
	   Defragment an mbuf chain, returning the shortest possible chain of
	   mbufs and clusters.	If allocation fails and this can not be com‐
	   pleted, NULL will be returned and the original chain will be
	   unchanged.  Upon success, the original chain will be freed and the
	   new chain will be returned.	how should be either M_WAIT or
	   M_DONTWAIT, depending on the caller's preference.

	   This function is especially useful in network drivers, where cer‐
	   tain long mbuf chains must be shortened before being added to TX
	   descriptor lists.

	   m_unshare(m0, 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, NULL will be returned.
	   The original mbuf chain is always reclaimed and the reference count
	   of any shared mbuf clusters is decremented.	how should be either
	   M_WAIT or M_DONTWAIT, depending on the caller's preference.	As a
	   side-effect of this process the returned mbuf chain may be com‐
	   pacted.

	   This function is especially useful in the transmit path of network
	   code, when data must be encrypted or otherwise altered prior to
	   transmission.

HARDWARE-ASSISTED CHECKSUM CALCULATION
     This section currently applies to TCP/IP only.  In order to save the host
     CPU resources, computing checksums is offloaded to the network interface
     hardware if possible.  The m_pkthdr member of the leading mbuf of a
     packet contains two fields used for that purpose, int csum_flags and int
     csum_data.	 The meaning of those fields depends on the direction a packet
     flows in, and on whether the packet is fragmented.	 Henceforth,
     csum_flags or csum_data of a packet will denote the corresponding field
     of the m_pkthdr member of the leading mbuf in the mbuf chain containing
     the packet.

     On output, checksum offloading is attempted after the outgoing interface
     has been determined for a packet.	The interface-specific field
     ifnet.if_data.ifi_hwassist (see ifnet(9)) is consulted for the capabili‐
     ties of the interface to assist in computing checksums.  The csum_flags
     field of the packet header is set to indicate which actions the interface
     is supposed to perform on it.  The actions unsupported by the network
     interface are done in the software prior to passing the packet down to
     the interface driver; such actions will never be requested through
     csum_flags.

     The flags demanding a particular action from an interface are as follows:

	   CSUM_IP   The IP header checksum is to be computed and stored in
		     the corresponding field of the packet.  The hardware is
		     expected to know the format of an IP header to determine
		     the offset of the IP checksum field.

	   CSUM_TCP  The TCP checksum is to be computed.  (See below.)

	   CSUM_UDP  The UDP checksum is to be computed.  (See below.)

     Should a TCP or UDP checksum be offloaded to the hardware, the field
     csum_data will contain the byte offset of the checksum field relative to
     the end of the IP header.	In this case, the checksum field will be ini‐
     tially set by the TCP/IP module to the checksum of the pseudo header
     defined by the TCP and UDP specifications.

     For outbound packets which have been fragmented by the host CPU, the fol‐
     lowing will also be true, regardless of the checksum flag settings:

	   ·   all fragments will have the flag M_FRAG set in their m_flags
	       field;

	   ·   the first and the last fragments in the chain will have
	       M_FIRSTFRAG or M_LASTFRAG set in their m_flags, correspond‐
	       ingly;

	   ·   the first fragment in the chain will have the total number of
	       fragments contained in its csum_data field.

     The last rule for fragmented packets takes precedence over the one for a
     TCP or UDP checksum.  Nevertheless, offloading a TCP or UDP checksum is
     possible for a fragmented packet if the flag CSUM_IP_FRAGS is set in the
     field ifnet.if_data.ifi_hwassist associated with the network interface.
     However, in this case the interface is expected to figure out the loca‐
     tion of the checksum field within the sequence of fragments by itself
     because csum_data contains a fragment count instead of a checksum offset
     value.

     On input, an interface indicates the actions it has performed on a packet
     by setting one or more of the following flags in csum_flags associated
     with the packet:

	   CSUM_IP_CHECKED  The IP header checksum has been computed.

	   CSUM_IP_VALID    The IP header has a valid checksum.	 This flag can
			    appear only in combination with CSUM_IP_CHECKED.

	   CSUM_DATA_VALID  The checksum of the data portion of the IP packet
			    has been computed and stored in the field
			    csum_data in network byte order.

	   CSUM_PSEUDO_HDR  Can be set only along with CSUM_DATA_VALID to
			    indicate that the IP data checksum found in
			    csum_data allows for the pseudo header defined by
			    the TCP and UDP specifications.  Otherwise the
			    checksum of the pseudo header must be calculated
			    by the host CPU and added to csum_data to obtain
			    the final checksum to be used for TCP or UDP vali‐
			    dation purposes.

     If a particular network interface just indicates success or failure of
     TCP or UDP checksum validation without returning the exact value of the
     checksum to the host CPU, its driver can mark CSUM_DATA_VALID and
     CSUM_PSEUDO_HDR in csum_flags, and set csum_data to 0xFFFF hexadecimal to
     indicate a valid checksum.	 It is a peculiarity of the algorithm used
     that the Internet checksum calculated over any valid packet will be
     0xFFFF as long as the original checksum field is included.

     For inbound packets which are IP fragments, all csum_data fields will be
     summed during reassembly to obtain the final checksum value passed to an
     upper layer in the csum_data field of the reassembled packet.  The
     csum_flags fields of all fragments will be consolidated using logical AND
     to obtain the final value for csum_flags.	Thus, in order to successfully
     offload checksum computation for fragmented data, all fragments should
     have the same value of csum_flags.

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 parts of the
     kernel that rely on mbufs.

     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.

SEE ALSO
     ifnet(9), mbuf_tags(9)

HISTORY
     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.
     In more recent FreeBSD use of mbufs is almost entirely limited to packet
     storage, with uma(9) zones being used directly to store other network-
     related memory.

     Historically, the mbuf allocator has been a special-purpose memory allo‐
     cator able to run in interrupt contexts and allocating from a special
     kernel address space map.	As of FreeBSD 5.3, the mbuf allocator is a
     wrapper around uma(9), allowing caching of mbufs, clusters, and mbuf +
     cluster pairs in per-CPU caches, as well as bringing other benefits of
     slab allocation.

AUTHORS
     The original mbuf manual page was written by Yar Tikhiy.  The uma(9) mbuf
     allocator was written by Bosko Milekic.

BSD				March 25, 2008				   BSD
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