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25 .\" $FreeBSD: src/share/man/man9/mbuf.9,v 1.27.2.1 2003/05/28 13:53:18 yar Exp $
26 .\" $DragonFly: src/share/man/man9/mbuf.9,v 1.8 2007/08/18 18:58:20 swildner Exp $
34 .Nd "memory management in the kernel IPC subsystem"
41 .Ss Mbuf allocation macros
42 .Fn MGET "struct mbuf *mbuf" "int how" "short type"
43 .Fn MGETHDR "struct mbuf *mbuf" "int how" "short type"
44 .Fn MCLGET "struct mbuf *mbuf" "int how"
45 .Fn MFREE "struct mbuf *mbuf" "struct mbuf *successor"
47 .Ss Mbuf utility macros
49 .Fn mtod "struct mbuf *mbuf" "type"
51 .Fn M_COPY_PKTHDR "struct mbuf *to" "struct mbuf *from"
52 .Fn M_ALIGN "struct mbuf *mbuf" "u_int len"
53 .Fn MH_ALIGN "struct mbuf *mbuf" "u_int len"
55 .Fn M_LEADINGSPACE "struct mbuf *mbuf"
57 .Fn M_TRAILINGSPACE "struct mbuf *mbuf"
58 .Fn M_PREPEND "struct mbuf *mbuf" "int len" "int how"
60 .Ss Mbuf allocation functions
62 .Fn m_get "int how" "int type"
64 .Fn m_getm "struct mbuf *orig" "int len" "int how" "int type"
66 .Fn m_getclr "int how" "int type"
68 .Fn m_gethdr "int how" "int type"
70 .Fn m_free "struct mbuf *mbuf"
72 .Fn m_freem "struct mbuf *mbuf"
74 .Ss Mbuf utility functions
76 .Fn m_adj "struct mbuf *mbuf" "int len"
78 .Fn m_prepend "struct mbuf *mbuf" "int len" "int how"
80 .Fn m_pullup "struct mbuf *mbuf" "int len"
82 .Fn m_copym "struct mbuf *mbuf" "int offset" "int len" "int how"
84 .Fn m_copypacket "struct mbuf *mbuf" "int how"
86 .Fn m_dup "struct mbuf *mbuf" "int how"
88 .Fn m_copydata "const struct mbuf *mbuf" "int offset" "int len" "caddr_t buf"
90 .Fn m_copyback "struct mbuf *mbuf" "int offset" "int len" "caddr_t buf"
96 .Fa "struct ifnet *ifp"
97 .Fa "void (*copy)(char *from, caddr_t to, u_int len)"
100 .Fn m_cat "struct mbuf *m" "struct mbuf *n"
102 .Fn m_split "struct mbuf *mbuf" "int len" "int how"
105 An mbuf is a basic unit of memory management in the kernel IPC subsystem.
106 Network packets and socket buffers are stored in mbufs.
107 A network packet may span multiple mbufs arranged into a chain
109 which allows adding or trimming
110 network headers with little overhead.
112 While a developer should not bother with mbuf internals without serious
113 reason in order to avoid incompatibilities with future changes, it
114 is useful to understand the mbuf's general structure.
116 An mbuf consists of a variable-sized header and a small internal
118 The mbuf's total size,
120 is a machine-dependent constant defined in
121 .In machine/param.h .
122 The mbuf header includes:
124 .Bl -tag -width "m_nextpkt" -compact -offset indent
126 a pointer to the next buffer in the chain
128 a pointer to the next chain in the queue
130 a pointer to the data
132 the length of the data
139 The mbuf flag bits are defined as follows:
142 #define M_EXT 0x0001 /* has associated external storage */
143 #define M_PKTHDR 0x0002 /* start of record */
144 #define M_EOR 0x0004 /* end of record */
145 #define M_PROTO1 0x0010 /* protocol-specific */
146 #define M_PROTO2 0x0020 /* protocol-specific */
147 #define M_PROTO3 0x0040 /* protocol-specific */
148 #define M_PROTO4 0x0080 /* protocol-specific */
149 #define M_PROTO5 0x0100 /* protocol-specific */
151 /* mbuf pkthdr flags, also in m_flags */
152 #define M_BCAST 0x0200 /* send/received as link-level broadcast */
153 #define M_MCAST 0x0400 /* send/received as link-level multicast */
154 #define M_FRAG 0x0800 /* packet is fragment of larger packet */
155 #define M_FIRSTFRAG 0x1000 /* packet is first fragment */
156 #define M_LASTFRAG 0x2000 /* packet is last fragment */
159 The available mbuf types are defined as follows:
162 #define MT_FREE 0 /* should be on free list */
163 #define MT_DATA 1 /* dynamic (data) allocation */
164 #define MT_HEADER 2 /* packet header */
165 #define MT_SONAME 8 /* socket name */
166 #define MT_FTABLE 11 /* fragment reassembly header */
167 #define MT_CONTROL 14 /* extra-data protocol message */
168 #define MT_OOBDATA 15 /* expedited data */
174 .Li struct pkthdr m_pkthdr
175 is added to the mbuf header.
176 It contains a pointer to the interface
177 the packet has been received from
178 .Pq Fa struct ifnet *rcvif ,
179 and the total packet length
182 If small enough, data is stored in the mbuf's internal data buffer.
183 If the data is sufficiently large, another mbuf may be added to the chain,
184 or external storage may be associated with the mbuf.
186 bytes of data can fit into an mbuf with the
192 If external storage is being associated with an mbuf, the
194 header is added at the cost of losing the internal data buffer.
195 It includes a pointer to external storage, the size of the storage,
196 a pointer to a function used for freeing the storage,
197 a pointer to an optional argument that can be passed to the function,
198 and a pointer to a reference counter.
199 An mbuf using external storage has the
203 The system supplies a default type of external storage buffer called an
205 Mbuf clusters can be allocated and configured with the use of the
212 is a machine-dependent constant.
213 The system defines an advisory macro
215 which is the smallest amount of data to put into a cluster.
216 It's equal to the sum of
220 It is typically preferable to store data into an mbuf's data region, if size
221 permits, as opposed to allocating a separate mbuf cluster to hold the same
224 .Ss Macros and Functions
225 There are numerous predefined macros and functions that provide the
226 developer with common utilities.
228 .Bl -ohang -offset indent
229 .It Fn mtod mbuf type
230 Convert an mbuf pointer to a data pointer.
231 The macro expands to the data pointer cast to the pointer of the specified type.
233 It is advisable to ensure that there is enough contiguous data in the mbuf.
237 .It Fn MGET mbuf how type
238 Allocate an mbuf and initialize it to contain internal data.
240 will point to the allocated mbuf on success, or be set to
245 argument is to be set to
249 It specifies whether the caller is willing to block if necessary.
254 a failed allocation will result in the caller being put
255 to sleep for a designated
256 .Va kern.ipc.mbuf_wait
260 A number of other mbuf-related
261 functions and macros have the same argument because they may
262 at some point need to allocate new mbufs.
264 Programmers should be careful not to confuse the mbuf allocation flag
270 They are not the same.
271 .It Fn MGETHDR mbuf how type
272 Allocate an mbuf and initialize it to contain a packet header
277 .It Fn MCLGET mbuf how
278 Allocate and attach an mbuf cluster to an mbuf.
279 If the macro fails, the
281 flag won't be set in the mbuf.
282 .It Fn M_PREPEND mbuf len how
283 This macro operates on an mbuf chain.
284 It is an optimized wrapper for
286 that can make use of possible empty space before data
287 (e.g. left after trimming of a link-layer header).
288 The new chain pointer or
296 .Bl -ohang -offset indent
297 .It Fn m_get how type
298 A function version of
300 for non-critical paths.
301 .It Fn m_getm orig len how type
304 bytes worth of mbufs and mbuf clusters if necessary and append the resulting
305 allocated chain to the
308 .No non- Ns Dv NULL .
309 If the allocation fails at any point,
310 free whatever was allocated and return
315 .No non- Ns Dv NULL ,
316 it will not be freed.
317 It is possible to use
321 bytes to an existing mbuf or mbuf chain
322 (for example, one which may be sitting in a pre-allocated ring)
323 or to simply perform an all-or-nothing mbuf and mbuf cluster allocation.
324 .It Fn m_gethdr how type
325 A function version of
327 for non-critical paths.
328 .It Fn m_getclr how type
329 Allocate an mbuf and zero out the data region.
332 The functions below operate on mbuf chains.
333 .Bl -ohang -offset indent
335 Free an entire mbuf chain, including any external
338 .It Fn m_adj mbuf len
341 bytes from the head of an mbuf chain if
343 is positive, from the tail otherwise.
345 .It Fn m_prepend mbuf len how
346 Allocate a new mbuf and prepend it to the chain, handle
350 It doesn't allocate any clusters, so
360 .It Fn m_pullup mbuf len
361 Arrange that the first
363 bytes of an mbuf chain are contiguous and lay in the data area of
365 so they are accessible with
367 Return the new chain on success,
370 (the chain is freed in this case).
372 It doesn't allocate any clusters, so
377 .It Fn m_copym mbuf offset len how
378 Make a copy of an mbuf chain starting
380 bytes from the beginning, continuing for
387 copy to the end of the mbuf chain.
389 The copy is read-only, because clusters are not
390 copied, only their reference counts are incremented.
392 .It Fn m_copypacket mbuf how
393 Copy an entire packet including header, which must be present.
394 This is an optimized version of the common case
395 .Fn m_copym mbuf 0 M_COPYALL how .
397 the copy is read-only, because clusters are not
398 copied, only their reference counts are incremented.
400 .It Fn m_dup mbuf how
401 Copy a packet header mbuf chain into a completely new chain, including
402 copying any mbuf clusters.
405 when you need a writable copy of an mbuf chain.
407 .It Fn m_copydata mbuf offset len buf
408 Copy data from an mbuf chain starting
410 bytes from the beginning, continuing for
412 bytes, into the indicated buffer
415 .It Fn m_copyback mbuf offset len buf
418 bytes from the buffer
420 back into the indicated mbuf chain,
423 bytes from the beginning of the chain, extending the mbuf chain if necessary.
425 It doesn't allocate any clusters, just adds mbufs to the chain.
428 beyond the current chain end: zeroed mbufs will be allocated to fill the
431 .It Fn m_devget buf len offset ifp copy
432 Copy data from a device local memory pointed to by
435 The copy is done using a specified copy routine
449 Both chains must be of the same type.
451 is still valid after the function returned.
457 .It Fn m_split mbuf len how
458 Partition an mbuf chain in two pieces, returning the tail:
462 In case of failure, it returns
464 and attempts to restore the chain to its original state.
467 When running a kernel compiled with the option
468 .Dv MBUF_STRESS_TEST ,
471 -controlled options may be used to create
472 various failure/extreme cases for testing of network drivers
473 and other parts of the kernel that rely on
475 .Bl -tag -width ident
476 .It Va net.inet.ip.mbuf_frag_size
481 into fragments of the specified size.
482 Setting this variable to 1 is an excellent way to
485 handling ability of network drivers.
486 .It Va kern.ipc.m_defragrandomfailures
489 to randomly fail, returning
491 Any piece of code which uses
493 should be tested with this feature.
498 .\" Please correct me if I'm wrong
499 Mbufs appeared in an early version of
501 Besides for being used for network packets, they were used
502 to store various dynamic structures, such as routing table
503 entries, interface addresses, protocol control blocks, etc.
507 man page was written by Yar Tikhiy.