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mbuf - "memory management in the kernel IPC subsystem"


Mbuf allocation macros
Mbuf utility macros
Mbuf allocation functions
Mbuf utility functions
Macros and Functions
Hardware-assisted Checksum Calculation
Stress Testing
Return Values
See Also


.In sys/param.h
.In sys/systm.h
.In 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"
.Fo MEXTADD "struct mbuf *mbuf" "caddr_t buf" "u_int size" "void (*free)(void *opt_args)" "void *opt_args" "short flags" "int type"
.Fc 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

mtod "struct mbuf *mbuf" "type" int MEXT_IS_REF "struct mbuf *mbuf" 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_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 *
.Fo m_devget "char *buf" "int len" "int offset" "struct ifnet *ifp" "void (*copy)(char *from, caddr_t to, u_int len)"
.Fc 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"


.Vt mbuf is a basic unit of memory management in the kernel IPC subsystem. Network packets and socket buffers are stored in
.Vt mbufs . A network packet may span multiple
.Vt mbufs arranged into a
.Vt mbuf chain (linked list), which allows adding or trimming network headers with little overhead.

While a developer should not bother with
.Vt mbuf internals without serious reason in order to avoid incompatibilities with future changes, it is useful to understand the general structure of an
.Vt mbuf .

.Vt mbuf consists of a variable-sized header and a small internal buffer for data. The total size of an
.Vt mbuf , MSIZE, is a constant defined in
.In sys/param.h . The
.Vt mbuf header includes:

m_next (Vt struct mbuf *) A pointer to the next
.Vt mbuf in the
.Vt mbuf chain .
m_nextpkt (Vt struct mbuf *) A pointer to the next
.Vt mbuf chain in the queue.
m_data (Vt caddr_t) A pointer to data attached to this
.Vt mbuf .
m_len (Vt int) The length of the data.
m_type (Vt short) The type of the data.
m_flags (Vt int) The
.Vt mbuf flags.

.Vt 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_FRAG0x0800 /* packet is fragment of larger packet */
#define M_FIRSTFRAG0x1000 /* packet is first fragment */
#define M_LASTFRAG 0x2000 /* packet is last fragment */

The available
.Vt mbuf types are defined as follows:
/* mbuf types */
#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
.Vt struct pkthdr Va m_pkthdr is added to the
.Vt mbuf header. It contains a pointer to the interface the packet has been received from (Vt struct ifnet *rcvif), and the total packet length (Vt int len). Optionally, it may also contain an attached list of packet tags (Vt "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
.Vt mbuf . If the data is sufficiently large, another
.Vt mbuf may be added to the
.Vt mbuf chain , or external storage may be associated with the
.Vt mbuf . MHLEN bytes of data can fit into an
.Vt mbuf with the M_PKTHDR flag set, MLEN bytes can otherwise.

If external storage is being associated with an
.Vt 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
.Vt 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
.Vt 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
.Vt mbuf cluster .
.Vt Mbuf clusters can be allocated and configured with the use of the MCLGET macro. Each
.Vt 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
.Vt 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
.Vt mbuf , if size permits, as opposed to allocating a separate
.Vt 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
.Vt mbuf and initialize it to contain internal data. mbuf will point to the allocated
.Vt 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
.Vt mbufs have the same argument because they may at some point need to allocate new
.Vt mbufs .

Programmers should be careful not to confuse the
.Vt mbuf allocation flag M_DONTWAIT with the malloc(9) allocation flag, M_NOWAIT. They are not the same.

MGETHDR mbuf how type Allocate an
.Vt mbuf and initialize it to contain a packet header and internal data. See MGET for details.
MCLGET mbuf how Allocate and attach an
.Vt 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
.Vt 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
.Vt 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
.Vt 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
.Vt mbufs and
.Vt mbuf clusters if necessary and append the resulting allocated
.Vt mbuf chain to the
.Vt 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
.Vt mbuf or
.Vt mbuf chain (for example, one which may be sitting in a pre-allocated ring) or to simply perform an all-or-nothing
.Vt mbuf and
.Vt mbuf cluster allocation.
m_gethdr how type A function version of MGETHDR for non-critical paths.
m_getcl how type flags Fetch an
.Vt mbuf with a
.Vt 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
.Vt mbuf and
.Vt mbuf cluster together, as it avoids having to unlock/relock between allocations. Returns NULL on failure.
m_getclr how type Allocate an
.Vt mbuf and zero out the data region.
m_free mbuf Frees
.Vt mbuf . Returns m_next of the freed
.Vt mbuf .

The functions below operate on
.Vt mbuf chains .

m_freem mbuf Free an entire
.Vt mbuf chain , including any external storage.
m_adj mbuf len Trim len bytes from the head of an
.Vt mbuf chain if len is positive, from the tail otherwise.
m_append mbuf len cp Append
.Vt len bytes of data
.Vt cp to the
.Vt mbuf chain . Extend the mbuf chain if the new data does not fit in existing space.
m_prepend mbuf len how Allocate a new
.Vt mbuf and prepend it to the
.Vt mbuf chain , handle M_PKTHDR properly. Note: It does not allocate any
.Vt 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
.Vt mbuf chain on success, and frees the
.Vt mbuf chain and returns NULL on failure. Note: The function does not allocate
.Vt mbuf clusters , so len + dstoff must be less than MHLEN.
m_pullup mbuf len Arrange that the first len bytes of an
.Vt mbuf chain are contiguous and lay in the data area of mbuf, so they are accessible with mtod mbuf type. Return the new
.Vt mbuf chain on success, NULL on failure (the
.Vt mbuf chain is freed in this case). Note: It does not allocate any
.Vt mbuf clusters , so len must be less than MHLEN.
m_copym mbuf offset len how Make a copy of an
.Vt mbuf chain starting offset bytes from the beginning, continuing for len bytes. If len is M_COPYALL, copy to the end of the
.Vt mbuf chain . Note: The copy is read-only, because the
.Vt 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
.Vt mbuf clusters are not copied, only their reference counts are incremented.
m_dup mbuf how Copy a packet header
.Vt mbuf chain into a completely new
.Vt mbuf chain , including copying any
.Vt mbuf clusters . Use this instead of m_copypacket when you need a writable copy of an
.Vt mbuf chain .
m_copydata mbuf offset len buf Copy data from an
.Vt 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
.Vt mbuf chain , starting at offset bytes from the beginning of the
.Vt mbuf chain , extending the
.Vt mbuf chain if necessary. Note: It does not allocate any
.Vt mbuf clusters , just adds
.Vt mbufs to the
.Vt mbuf chain . It is safe to set offset beyond the current
.Vt mbuf chain end: zeroed
.Vt mbufs will be allocated to fill the space.
m_length mbuf last Return the length of the
.Vt mbuf chain , and optionally a pointer to the last
.Vt mbuf .
m_dup_pkthdr to from how Upon the function’s completion, the
.Vt mbuf to will contain an identical copy of from->m_pkthdr and the per-packet attributes found in the
.Vt mbuf chain from. The
.Vt 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
.Vt mbuf chain from to the
.Vt mbuf to. The
.Vt 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
.Vt mbuf chain .
m_devget buf len offset ifp copy Copy data from a device local memory pointed to by buf to an
.Vt 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
.Vt 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
.Vt 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
.Vt mbuf chain to its original state.
m_apply mbuf off len f arg Apply a function to an
.Vt 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
.Vt 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
.Vt 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_TRYWAIT or M_DONTWAIT, 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.


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
.Vt mbuf of a packet contains two fields used for that purpose,
.Vt int Va csum_flags and
.Vt int Va 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
.Vt mbuf in the
.Vt 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 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 initially 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 following 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, correspondingly;
  • 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 location 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 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.

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.


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
.Vt mbufs .
Causes ip_output to fragment outgoing
.Vt mbuf chains into fragments of the specified size. Setting this variable to 1 is an excellent way to test the long
.Vt 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 feature.


See above.


ifnet(9), mbuf_tags(9)



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