Command Section
MBUF(9)                FreeBSD 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_arg1, void *opt_arg2), void *opt_arg1,)(void *opt_arg1, void *opt_arg2), void *opt_arg1,
         void *opt_arg2, short flags, int type);

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

     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, short type);

     M_WRITABLE(struct mbuf *mbuf);

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

     struct mbuf *
     m_get2(int size, int how, short type, int flags);

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

     struct mbuf *
     m_getjcl(int how, short type, int flags, int size);

     struct mbuf *
     m_getcl(int how, short type, int flags);

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

     struct mbuf *
     m_free(struct mbuf *mbuf);

     m_freem(struct mbuf *mbuf);

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

     m_align(struct mbuf *mbuf, int len);

     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(const 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));)(char *from, caddr_t to, u_int len));

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

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

     m_fixhdr(struct mbuf *mbuf);

     m_dup_pkthdr(struct mbuf *to, const struct mbuf *from, int how);

     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);

     m_apply(struct mbuf *mbuf, int off, int len,
         int (*f)(void *arg, void *data, u_int len), void *arg);)(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_collapse(struct mbuf *m0, int how, int maxfrags);

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

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

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

           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           0x00000001 /* has associated external storage */
     #define M_PKTHDR        0x00000002 /* start of record */
     #define M_EOR           0x00000004 /* end of record */
     #define M_RDONLY        0x00000008 /* associated data marked read-only */
     #define M_PROTO1        0x00001000 /* protocol-specific */
     #define M_PROTO2        0x00002000 /* protocol-specific */
     #define M_PROTO3        0x00004000 /* protocol-specific */
     #define M_PROTO4        0x00008000 /* protocol-specific */
     #define M_PROTO5        0x00010000 /* protocol-specific */
     #define M_PROTO6        0x00020000 /* protocol-specific */
     #define M_PROTO7        0x00040000 /* protocol-specific */
     #define M_PROTO8        0x00080000 /* protocol-specific */
     #define M_PROTO9        0x00100000 /* protocol-specific */
     #define M_PROTO10       0x00200000 /* protocol-specific */
     #define M_PROTO11       0x00400000 /* protocol-specific */
     #define M_PROTO12       0x00800000 /* protocol-specific */

     /* mbuf pkthdr flags (also stored in m_flags) */
     #define M_BCAST         0x00000010 /* send/received as link-level broadcast */
     #define M_MCAST         0x00000020 /* send/received as link-level multicast */

     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 */

     The available external buffer types are defined as follows:

     /* external buffer types */
     #define EXT_CLUSTER     1       /* mbuf cluster */
     #define EXT_SFBUF       2       /* sendfile(2)'s sf_bufs */
     #define EXT_JUMBOP      3       /* jumbo cluster 4096 bytes */
     #define EXT_JUMBO9      4       /* jumbo cluster 9216 bytes */
     #define EXT_JUMBO16     5       /* jumbo cluster 16184 bytes */
     #define EXT_PACKET      6       /* mbuf+cluster from packet zone */
     #define EXT_MBUF        7       /* external mbuf reference (M_IOVEC) */
     #define EXT_NET_DRV     252     /* custom ext_buf provided by net driver(s) */
     #define EXT_MOD_TYPE    253     /* custom module's ext_buf type */
     #define EXT_DISPOSABLE  254     /* can throw this buffer away w/page flipping */
     #define EXT_EXTREF      255     /* has externally maintained ref_cnt ptr */

     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

     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

     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 MHLEN plus one.  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 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 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_WAITOK or M_NOWAIT.
           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

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

           MEXTADD(mbuf, buf, size, free, opt_arg1, opt_arg2, flags, type)
           Associate externally managed data with mbuf.  Any internal data
           contained in the mbuf will be discarded, and the M_EXT flag will be
           set.  The buf and size arguments are the address and length,
           respectively, of the data.  The free argument points to a function
           which will be called to free the data when the mbuf is freed; it is
           only used if type is EXT_EXTREF.  The opt_arg1 and opt_arg2
           arguments will be passed unmodified to free.  The flags argument
           specifies additional mbuf flags; it is not necessary to specify
           M_EXT.  Finally, the type argument specifies the type of external
           data, which controls how it will be disposed of when the mbuf is
           freed.  In most cases, the correct value is EXT_EXTREF.

           MCLGET(mbuf, how)
           Allocate and attach an mbuf cluster to mbuf.  On success, a non-
           zero value returned; otherwise, 0.  Historically, consumers would
           check for success by testing the M_EXT flag on the mbuf, but this
           is now discouraged to avoid unnecessary awareness of the
           implementation of external storage in protocol stacks and device

           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

           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_get2(size, how, type, flags)
           Allocate an mbuf with enough space to hold specified amount of

           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_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_getjcl(how, type, flags, size)
           This is like m_getcl() but it the size of the cluster allocated
           will be large enough for size bytes.

           Frees mbuf.  Returns m_next of the freed mbuf.

     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_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
           failure (the mbuf chain is freed in this case).  Note: It does not
           allocate any mbuf clusters, so len must be less than or equal to

           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 offsetp is NULL,
           the region may be accessed using mtod(mbuf, type).  If offsetp is
           non-NULL, the region may be accessed using mtod(mbuf, uint8_t) +
           *offsetp.  The region of the mbuf chain between its beginning and
           offset 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
           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

           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
           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 not guaranteed to be valid after m_cat() returns.  m_cat() does
           not update any packet header fields or free mbuf tags.

           m_catpkt(m, n)
           A variant of m_cat() that operates on packets.  Both m and n must
           contain packet headers.  n is not guaranteed to be valid after
           m_catpkt() returns.

           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
           offset 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
           completed, 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_WAITOK or
           M_NOWAIT, depending on the caller's preference.

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

           m_collapse(m0, how, maxfrags)
           Defragment an mbuf chain, returning a chain of at most maxfrags
           mbufs and clusters.  If allocation fails or the chain cannot be
           collapsed as requested, NULL will be returned, with the original
           chain possibly modified.  As with m_defrag(), how should be one of
           M_WAITOK or M_NOWAIT.

           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_WAITOK or M_NOWAIT, depending on the caller's preference.  As a
           side-effect of this process the returned mbuf chain may be

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

     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
     capabilities 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

           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
     initially set by the TCP/IP module to the checksum of the pseudo header
     defined by the TCP and UDP specifications.

     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_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 validation 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.

     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

            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.

     ifnet(9), mbuf_tags(9)

     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.  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
     allocator 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.

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

FreeBSD 11.1-RELEASE-p4        October 10, 2016        FreeBSD 11.1-RELEASE-p4
Command Section