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

NAME
     mbuf, m_get, m_gethdr, m_devget, m_copym, m_copypacket, m_copydata,
     m_copyback, m_copyback_cow, m_cat, m_dup, m_makewritable, m_pulldown,
     m_pullup, m_copyup, m_split, m_adj, m_apply, m_free, m_freem, mtod, MGET,
     MGETHDR, MEXTMALLOC, MEXTADD, MCLGET, M_COPY_PKTHDR, M_MOVE_PKTHDR,
     m_remove_pkthdr, M_ALIGN, MH_ALIGN, M_LEADINGSPACE, M_TRAILINGSPACE,
     M_PREPEND, MCHTYPE -- functions and macros for managing memory used by
     networking code

SYNOPSIS
     #include <sys/mbuf.h>

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

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

     struct mbuf *
     m_devget(char *buf, int totlen, int off, struct ifnet *ifp);

     struct mbuf *
     m_copym(struct mbuf *m, int off, int len, int wait);

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

     void
     m_copydata(struct mbuf *m, int off, int len, void *cp);

     void
     m_copyback(struct mbuf *m0, int off, int len, void *cp);

     struct mbuf *
     m_copyback_cow(struct mbuf *m0, int off, int len, void *cp, int how);

     int
     m_makewritable(struct mbuf **mp, int off, int len, int how);

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

     struct mbuf *
     m_dup(struct mbuf *m, int off, int len, int wait);

     struct mbuf *
     m_pulldown(struct mbuf *m, int off, int len, int *offp);

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

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

     struct mbuf *
     m_split(struct mbuf *m0, int len, int wait);

     void
     m_adj(struct mbuf *mp, int req_len);

     int
     m_apply(struct mbuf *m, int off, int len,
         int *f(void *, void *, unsigned int), void *arg);

     struct mbuf *
     m_free(struct mbuf *m);

     void
     m_freem(struct mbuf *m);

     datatype
     mtod(struct mbuf *m, datatype);

     void
     MGET(struct mbuf *m, int how, int type);

     void
     MGETHDR(struct mbuf *m, int how, int type);

     void
     MEXTMALLOC(struct mbuf *m, int len, int how);

     void
     MEXTADD(struct mbuf *m, void *buf, int size, int type,
         void (*free)(struct mbuf *, void *, size_t, void *), void *arg);

     void
     MCLGET(struct mbuf *m, int how);

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

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

     void
     m_remove_pkthdr(struct mbuf *m);

     void
     M_ALIGN(struct mbuf *m, int len);

     void
     MH_ALIGN(struct mbuf *m, int len);

     int
     M_LEADINGSPACE(struct mbuf *m);

     int
     M_TRAILINGSPACE(struct mbuf *m);

     void
     M_PREPEND(struct mbuf *m, int plen, int how);

     void
     MCHTYPE(struct mbuf *m, int type);

DESCRIPTION
     The mbuf functions and macros provide an easy and consistent way to han-
     dle a networking stack's memory management needs.  An mbuf consists of a
     header and a data area.  It is of a fixed size, MSIZE (defined in
     <machine/param.h>), which includes the size of the header.  The header
     contains a pointer to the next mbuf in the `mbuf chain', a pointer to the
     next `mbuf chain', a pointer to the data area, the amount of data in this
     mbuf, its type and a flags field.

     The type variable can signify:
           MT_FREE      the mbuf should be on the ``free'' list
           MT_DATA      data was dynamically allocated
           MT_HEADER    data is a packet header
           MT_SONAME    data is a socket name
           MT_SOOPTS    data is socket options
           MT_FTABLE    data is the fragment reassembly header
           MT_CONTROL   mbuf contains ancillary (protocol control) data
           MT_OOBDATA   mbuf contains out-of-band data.

     The flags variable contains information describing the mbuf, notably:
           M_EXT        has external storage
           M_PKTHDR     is start of record
           M_EOR        is end of record

     If an mbuf designates the start of a record (M_PKTHDR), its flags field
     may contain additional information describing the content of the record:
           M_BCAST      sent/received as link-level broadcast
           M_MCAST      sent/received as link-level multicast

     An mbuf may add a single `mbuf cluster' of MCLBYTES bytes (defined in
     <machine/param.h>), which has no additional overhead and is used instead
     of the internal data area; this is done when at least MINCLSIZE bytes of
     data must be stored.

     When the M_EXT flag is set on an mbuf, the external storage area could be
     shared among multiple mbufs.  Therefore, care must be taken when over-
     writing the data content of an mbuf, because its external storage could
     be considered as read-only.

     m_get(int how, int type)
              Allocates an mbuf and initializes it to contain internal data.
              The how parameter is a choice of M_WAIT / M_DONTWAIT from
              caller.  M_WAIT means the call cannot fail, but may take for-
              ever.  The type parameter is an mbuf type.

     m_gethdr(int how, int type)
              Allocates an mbuf and initializes it to contain a packet header
              and internal data.  The how parameter is a choice of M_WAIT /
              M_DONTWAIT from caller.  The type parameter is an mbuf type.

     m_devget(char *buf, int totlen, int off, struct ifnet *ifp)
              Copies len bytes from device local memory into mbufs.  If param-
              eter off is non-zero, the packet is supposed to be trailer-
              encapsulated and off bytes plus the type and length fields will
              be skipped before copying.  Returns the top of the mbuf chain it
              created.

     m_copym(struct mbuf *m, int off, int len, int wait)
              Creates a copy of an mbuf chain starting off bytes from the
              beginning, continuing for len bytes.  If the len requested is
              M_COPYALL, the complete mbuf chain will be copied.  The wait
              parameter is a choice of M_WAIT / M_DONTWAIT from caller.

     m_copypacket(struct mbuf *m, int how)
              Copies an entire packet, including header.  This function is an
              optimization of the common case m_copym (m, 0, M_COPYALL, how).
              However, contrary to m_copym(), a header must be present.  It is
              incorrect to use m_copypacket() with an mbuf that does not have
              a header.

     m_copydata(struct mbuf *m, int off, int len, void *cp)
              Copies len bytes data from mbuf chain m into the buffer cp,
              starting off bytes from the beginning.

     m_copyback(struct mbuf *m0, int off, int len, void *cp)
              Copies len bytes data from buffer cp back into the mbuf chain
              m0, starting off bytes from the beginning of the chain, extend-
              ing the mbuf chain if necessary.  m_copyback() can only fail
              when extending the chain.  The caller should check for this kind
              of failure by checking the resulting length of the chain in that
              case.  It is an error to use m_copyback() on read-only mbufs.

     m_copyback_cow(struct mbuf *m0, int off, int len, void *cp, int how)
              Copies len bytes data from buffer cp back into the mbuf chain m0
              as m_copyback() does.  Unlike m_copyback(), it is safe to use
              m_copyback_cow() on read-only mbufs.  If needed,
              m_copyback_cow() automatically allocates new mbufs and adjusts
              the chain.  On success, it returns a pointer to the resulting
              mbuf chain, and frees the original mbuf m0.  Otherwise, it
              returns NULL.  The how parameter is a choice of M_WAIT /
              M_DONTWAIT from the caller.  Unlike m_copyback(), extending the
              mbuf chain isn't supported.  It is an error to attempt to extend
              the mbuf chain using m_copyback_cow().

     m_makewritable(struct mbuf **mp, int off, int len, int how)
              Rearranges an mbuf chain so that len bytes from offset off are
              writable.  When it meets read-only mbufs, it allocates new
              mbufs, adjusts the chain as m_copyback_cow() does, and copies
              the original content into them.  m_makewritable() does not guar-
              antee that all len bytes at off are consecutive.  The how param-
              eter is a choice of M_WAIT / M_DONTWAIT from the caller.
              m_makewritable() preserves the contents of the mbuf chain even
              in the case of failure.  It updates a pointer to the mbuf chain
              pointed to by mp.  It returns 0 on success.  Otherwise, it
              returns an error code, typically ENOBUFS.

     m_cat(struct mbuf *m, struct mbuf *n)
              Concatenates mbuf chain n to m.  Both chains must be of the same
              type; packet headers will not be updated if present.

     m_dup(struct mbuf *m, int off, int len, int wait)
              Similarly to m_copym(), the function creates a copy of an mbuf
              chain starting off bytes from the beginning, continuing for len
              bytes.  While m_copym() tries to share external storage for
              mbufs with M_EXT flag, m_dup() will deep-copy the whole data
              content into new mbuf chain and avoids shared external storage.

     m_pulldown(struct mbuf *m, int off, int len, int *offp)
              Rearranges an mbuf chain so that len bytes from offset off are
              contiguous and in the data area of an mbuf.  The return value
              points to an mbuf in the middle of the mbuf chain m.  If we call
              the return value n, the contiguous data region is available at
              mtod(n, void *) + *offp, or mtod(n, void *) if offp is NULL.
              The top of the mbuf chain m, and mbufs up to off, will not be
              modified.  On successful return, it is guaranteed that the mbuf
              pointed to by n does not have a shared external storage, there-
              fore it is safe to update the contiguous region.  Returns NULL
              and frees the mbuf chain on failure.  len must be smaller than
              or equal to MCLBYTES.

     m_pullup(struct mbuf *m, int len)
              Rearranges an mbuf chain so that len bytes are contiguous and in
              the data area of an mbuf (so that mtod() will work for a struc-
              ture of size len).  Returns the resulting mbuf chain on success,
              frees it and returns NULL on failure.  If there is room, it will
              add up to max_protohdr - len extra bytes to the contiguous
              region to possibly avoid being called again.  len must be
              smaller or equal than MHLEN.

     m_copyup(struct mbuf *m, int len, int 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 that the function does not allocate mbuf
              clusters, so len + dstoff must be less than MHLEN.

     m_split(struct mbuf *m0, int len, int wait)
              Partitions an mbuf chain in two pieces, returning the tail,
              which is all but the first len bytes.  In case of failure, it
              returns NULL and restores the chain to its original state.

     m_adj(struct mbuf *mp, int req_len)
              Shaves off req_len bytes from head or tail of the (valid) data
              area.  If req_len is greater than zero, front bytes are being
              shaved off, if it's smaller, from the back (and if it is zero,
              the mbuf will stay bearded).  This function does not move data
              in any way, but is used to manipulate the data area pointer and
              data length variable of the mbuf in a non-clobbering way.

     m_apply(struct mbuf *m, int off, int len, int (*f)(void *, void *,
              unsigned int), void *arg)
              Apply function f to the data in an mbuf chain starting off bytes
              from the beginning, continuing for len bytes.  Neither off nor
              len may be negative.  arg will be supplied as first argument for
              f, the second argument will be the pointer to the data buffer of
              a packet (starting after off bytes in the stream), and the third
              argument is the amount of data in bytes in this call.  If f
              returns something not equal to zero m_apply() will bail out,
              returning the return code of f.  Upon successful completion it
              will return zero.

     m_free(struct mbuf *m)
              Frees mbuf m.

     m_freem(struct mbuf *m)
              Frees the mbuf chain beginning with m.  This function contains
              the elementary sanity check for a NULL pointer.

     mtod(struct mbuf *m, datatype)
              Returns a pointer to the data contained in the specified mbuf m,
              type-casted to the specified data type datatype.

     MGET(struct mbuf *m, int how, int type)
              Allocates mbuf m and initializes it to contain internal data.
              See m_get().

     MGETHDR(struct mbuf *m, int how, int type)
              Allocates mbuf m and initializes it to contain a packet header.
              See m_gethdr().

     MEXTMALLOC(struct mbuf *m, int len, int how)
              Allocates external storage of size len for mbuf m.  The how
              parameter is a choice of M_WAIT / M_DONTWAIT from caller.  The
              flag M_EXT is set upon success.

     MEXTADD(struct mbuf *m, void *buf, int size, int type, void
              (*free)(struct mbuf *, void *, size_t, void *), void *arg)
              Adds pre-allocated external storage buf to a normal mbuf m; the
              parameters size, type, free and arg describe the external stor-
              age.  size is the size of the storage, type describes its
              malloc(9) type, free is a free routine (if not the usual one),
              and arg is a possible argument to the free routine.  The flag
              M_EXT is set upon success.  If a free routine is specified, it
              will be called when the mbuf is freed.  In the case of former,
              the first argument for a free routine is the mbuf m and the rou-
              tine is expected to free it in addition to the external storage
              pointed by second argument.  In the case of latter, the first
              argument for the routine is NULL.

     MCLGET(struct mbuf *m, int how)
              Allocates and adds an mbuf cluster to a normal mbuf m.  The how
              parameter is a choice of M_WAIT / M_DONTWAIT from caller.  The
              flag M_EXT is set upon success.

     M_COPY_PKTHDR(struct mbuf *to, struct mbuf *from)
              Copies the mbuf pkthdr from mbuf from to mbuf to.  from must
              have the type flag M_PKTHDR set, and to must be empty.

     M_MOVE_PKTHDR(struct mbuf *to, struct mbuf *from)
              Moves the mbuf pkthdr from mbuf from to mbuf to.  from must have
              the type flag M_PKTHDR set, and to must be empty.  The flag
              M_PKTHDR in mbuf from will be cleared.

     m_remove_pkthdr(struct mbuf *m)
              Removes the mbuf pkthdr from mbuf m.  m must have the flag
              M_PKTHDR set.  This flag will be cleared.

     M_ALIGN(struct mbuf *m, int len)
              Sets the data pointer of a newly allocated mbuf m to len bytes
              from the end of the mbuf data area, so that len bytes of data
              written to the mbuf m, starting at the data pointer, will be
              aligned to the end of the data area.

     MH_ALIGN(struct mbuf *m, int len)
              Sets the data pointer of a newly allocated packetheader mbuf m
              to len bytes from the end of the mbuf data area, so that len
              bytes of data written to the mbuf m, starting at the data
              pointer, will be aligned to the end of the data area.

     M_LEADINGSPACE(struct mbuf *m)
              Returns the amount of space available before the current start
              of valid data in mbuf m.  Returns 0 if the mbuf data part is
              shared across multiple mbufs (i.e. not writable).

     M_TRAILINGSPACE(struct mbuf *m)
              Returns the amount of space available after the current end of
              valid data in mbuf m.  Returns 0 if the mbuf data part is shared
              across multiple mbufs (i.e. not writable).

     M_PREPEND(struct mbuf *m, int plen, int how)
              Prepends space of size plen to mbuf m.  If a new mbuf must be
              allocated, how specifies whether to wait.  If how is M_DONTWAIT
              and allocation fails, the original mbuf chain is freed and m is
              set to NULL.  It is illegal for the plen parameter to be greater
              than MHLEN.

     MCHTYPE(struct mbuf *m, int type)
              Change mbuf m to new type type.

CODE REFERENCES
     The mbuf management functions are implemented within the file
     sys/kern/uipc_mbuf.c.  Function prototypes, and the functions implemented
     as macros are located in sys/sys/mbuf.h.

SEE ALSO
     /usr/share/doc/smm/18.net, netstat(1), m_tag(9), malloc(9)

     Jun-ichiro Hagino, "Mbuf issues in 4.4BSD IPv6/IPsec support (experiences
     from KAME IPv6/IPsec implementation)", Proceedings of the freenix track:
     2000 USENIX annual technical conference, June 2000.

AUTHORS
     The original mbuf data structures were designed by Rob Gurwitz when he
     did the initial TCP/IP implementation at BBN.

     Further extensions and enhancements were made by Bill Joy, Sam Leffler,
     and Mike Karels at CSRG.

     Current implementation of external storage by Matt Thomas
     <matt@3am-software.com> and Jason R. Thorpe <thorpej@NetBSD.org>.

NetBSD 8.0                        May 7, 2018                       NetBSD 8.0

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