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


mbufmemory 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_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_getclr(int how, short type);

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(struct mbuf *mbuf, int how);

m_copydata(const struct mbuf *mbuf, int offset, int len, caddr_t buf);

m_copyback(struct mbuf *mbuf, int offset, int len, caddr_t buf);

struct mbuf *
m_devget(char *buf, int len, int offset, struct ifnet *ifp, void (*copy)(char *from, caddr_t to, u_int len));

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

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

m_fixhdr(struct mbuf *mbuf);

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

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

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

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

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_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:

(struct mbuf *) A pointer to the next mbuf in the mbuf chain.
(struct mbuf *) A pointer to the next mbuf chain in the queue.
(caddr_t) A pointer to data attached to this mbuf.
(int) The length of the data.
(short) The type of the data.
(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 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 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.

(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 () 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 mbufs.
(mbuf, how, type)
Allocate an mbuf and initialize it to contain a packet header and internal data. See MGET() for details.
(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.
(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 drivers.
(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().
(mbuf, len)
Serves the same purpose as M_ALIGN() does, but only for mbuf newly allocated with MGETHDR() or m_gethdr(), or initialized by () or ().
(mbuf, len)
Services the same purpose as M_ALIGN() but handles any type of mbuf.
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 mbuf.
(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.
(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.
(mbuf, type)
Change the type of mbuf to type. This is a relatively expensive operation and should be avoided.

The functions are:

(how, type)
A function version of () for non-critical paths.
(size, how, type, flags)
Allocate an mbuf with enough space to hold specified amount of data.
(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.
(how, type)
A function version of MGETHDR() for non-critical paths.
(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.
(how, type, flags, size)
This is like m_getcl() but it the size of the cluster allocated will be large enough for size bytes.
(how, type)
Allocate an mbuf and zero out the data region.
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.
(mbuf, len)
Trim len bytes from the head of an mbuf chain if len is positive, from the tail otherwise.
(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.
(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.
(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 MHLEN.
(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().
(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 incremented.
(mbuf, how)
Copy an entire packet including header, which must be present. This is an optimized version of the common case m_copym(mbuf, 0, M_COPYALL, how). Note: the copy is read-only, because the mbuf clusters are not copied, only their reference counts are incremented.
(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.
(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.
(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.
(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.
(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 () if copy is NULL.
(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, 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.
(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 () 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 () will return the error and stop iterating through the mbuf chain.

(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.

(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 (), how should be one of M_WAITOK or M_NOWAIT.
(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 compacted.

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


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:

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

The IP header checksum has been computed.
The IP header has a valid checksum. This flag can appear only in combination with CSUM_IP_CHECKED.
The checksum of the data portion of the IP packet has been computed and stored in the field csum_data in network byte order.
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 () to fragment outgoing mbuf chains into fragments of the specified size. Setting this variable to 1 is an excellent way to test the long mbuf chain handling ability of network drivers.
Causes the function () 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)


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.

February 29, 2016 Debian