1.\" Copyright (c) 2000 FreeBSD Inc.
2.\" All rights reserved.
3.\"
4.\" Redistribution and use in source and binary forms, with or without
5.\" modification, are permitted provided that the following conditions
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7.\" 1. Redistributions of source code must retain the above copyright
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| 1.\" Copyright (c) 2000 FreeBSD Inc.
2.\" All rights reserved.
3.\"
4.\" Redistribution and use in source and binary forms, with or without
5.\" modification, are permitted provided that the following conditions
6.\" are met:
7.\" 1. Redistributions of source code must retain the above copyright
8.\" notice, this list of conditions and the following disclaimer.
9.\" 2. Redistributions in binary form must reproduce the above copyright
10.\" notice, this list of conditions and the following disclaimer in the
11.\" documentation and/or other materials provided with the distribution.
12.\"
13.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
14.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
15.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
16.\" ARE DISCLAIMED. IN NO EVENT SHALL [your name] OR CONTRIBUTORS BE LIABLE
17.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
18.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
19.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
20.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
21.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
22.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
23.\" SUCH DAMAGE.
24.\"
|
26.\"
27.Dd October 17, 2000
28.Dt MBUF 9
29.Os
30.\"
31.Sh NAME
32.Nm mbuf
33.Nd "memory management in the kernel IPC subsystem"
34.\"
35.Sh SYNOPSIS
36.Fd #include <sys/param.h>
37.Fd #include <sys/mbuf.h>
38.\"
39.Ss Mbuf allocation macros
40.Fn MGET "struct mbuf *mbuf" "int how" "short type"
41.Fn MGETHDR "struct mbuf *mbuf" "int how" "short type"
42.Fn MCLGET "struct mbuf *mbuf" "int how"
43.Fo MEXTADD
44.Fa "struct mbuf *mbuf"
45.Fa "caddr_t buf"
46.Fa "u_int size"
47.Fa "void (*free)(void *opt_args)"
48.Fa "void *opt_args"
49.Fa "short flags"
50.Fa "int type"
51.Fc
52.Fn MEXTFREE "struct mbuf *mbuf"
53.Fn MEXT_ADD_REF "struct mbuf *mbuf"
54.Fn MEXT_REM_REF "struct mbuf *mbuf"
55.Fn MFREE "struct mbuf *mbuf" "struct mbuf *successor"
56.\"
57.Ss Mbuf utility macros
58.Fn mtod "struct mbuf *mbuf" "any type"
59.Fn MEXT_IS_REF "struct mbuf *mbuf"
60.Fn M_COPY_PKTHDR "struct mbuf *to" "struct mbuf *from"
61.Fn M_ALIGN "struct mbuf *mbuf" "u_int len"
62.Fn MH_ALIGN "struct mbuf *mbuf" "u_int len"
63.Fn M_LEADINGSPACE "struct mbuf *mbuf"
64.Fn M_TRAILINGSPACE "struct mbuf *mbuf"
65.Fn M_PREPEND "struct mbuf *mbuf" "int len" "int how"
66.Fn MCHTYPE "struct mbuf *mbuf" "u_int type"
67.Fn M_WRITABLE "struct mbuf *mbuf"
68.\"
69.Ss Mbuf allocation functions
70.Ft struct mbuf *
71.Fn m_get "int how" "int type"
72.Ft struct mbuf *
73.Fn m_getm "struct mbuf *orig" "int len" "int how" "int type"
74.Ft struct mbuf *
75.Fn m_getclr "int how" "int type"
76.Ft struct mbuf *
77.Fn m_gethdr "int how" "int type"
78.Ft struct mbuf *
79.Fn m_free "struct mbuf *mbuf"
80.Ft void
81.Fn m_freem "struct mbuf *mbuf"
82.\"
83.Ss Mbuf utility functions
84.Ft void
85.Fn m_adj "struct mbuf *mbuf" "int len"
86.Ft struct mbuf *
87.Fn m_prepend "struct mbuf *mbuf" "int len" "int how"
88.Ft struct mbuf *
89.Fn m_pullup "struct mbuf *mbuf" "int len"
90.Ft struct mbuf *
91.Fn m_copym "struct mbuf *mbuf" "int offset" "int len" "int how"
92.Ft struct mbuf *
93.Fn m_copypacket "struct mbuf *mbuf" "int how"
94.Ft struct mbuf *
95.Fn m_dup "struct mbuf *mbuf" "int how"
96.Ft void
97.Fn m_copydata "struct mbuf *mbuf" "int offset" "int len" "caddr_t buf"
98.Ft void
99.Fn m_copyback "struct mbuf *mbuf" "int offset" "int len" "caddr_t buf"
100.Ft struct mbuf *
101.Fo m_devget
102.Fa "char *buf"
103.Fa "int len"
104.Fa "int offset"
105.Fa "struct ifnet *ifp"
106.Fa "void (*copy)(char *from, caddr_t to, u_int len)"
107.Fc
108.Ft void
109.Fn m_cat "struct mbuf *m" "struct mbuf *n"
110.Ft struct mbuf *
111.Fn m_split "struct mbuf *mbuf" "int len" "int how"
112.\"
113.Sh DESCRIPTION
114An mbuf is a basic unit of memory management in the kernel IPC subsystem.
115Network packets and socket buffers are stored in mbufs.
116A network packet may span multiple mbufs arranged into a chain
117(linked list),
118which allows adding or trimming
119network headers with little overhead.
120.Pp
121While a developer should not bother with mbuf internals without serious
122reason in order to avoid incompatibilities with future changes, it
123is useful to understand the mbuf's general structure.
124.Pp
125An mbuf consists of a variable-sized header and a small internal
126buffer for data.
127The mbuf's total size,
128.Dv MSIZE ,
129is a machine-dependent constant defined in
130.Pa machine/param.h .
131The mbuf header includes:
132.Pp
133.Bl -tag -width "m_nextpkt" -compact -offset indent
134.It Fa m_next
135a pointer to the next buffer in the chain
136.It Fa m_nextpkt
137a pointer to the next chain in the queue
138.It Fa m_data
139a pointer to the data
140.It Fa m_len
141the length of the data
142.It Fa m_type
143the type of data
144.It Fa m_flags
145the mbuf flags
146.El
147.Pp
148The mbuf flag bits are defined as follows:
149.Bd -literal
150/* mbuf flags */
151#define M_EXT 0x0001 /* has associated external storage */
152#define M_PKTHDR 0x0002 /* start of record */
153#define M_EOR 0x0004 /* end of record */
154#define M_RDONLY 0x0008 /* associated data marked read-only */
155#define M_PROTO1 0x0010 /* protocol-specific */
156#define M_PROTO2 0x0020 /* protocol-specific */
157#define M_PROTO3 0x0040 /* protocol-specific */
158#define M_PROTO4 0x0080 /* protocol-specific */
159#define M_PROTO5 0x0100 /* protocol-specific */
160
161/* mbuf pkthdr flags, also in m_flags */
162#define M_BCAST 0x0200 /* send/received as link-level broadcast */
163#define M_MCAST 0x0400 /* send/received as link-level multicast */
164#define M_FRAG 0x0800 /* packet is fragment of larger packet */
165#define M_FIRSTFRAG 0x1000 /* packet is first fragment */
166#define M_LASTFRAG 0x2000 /* packet is last fragment */
167.Ed
168.Pp
169The available mbuf types are defined as follows:
170.Bd -literal
171/* mbuf types */
172#define MT_FREE 0 /* should be on free list */
173#define MT_DATA 1 /* dynamic (data) allocation */
174#define MT_HEADER 2 /* packet header */
175#define MT_SONAME 8 /* socket name */
176#define MT_FTABLE 11 /* fragment reassembly header */
177#define MT_CONTROL 14 /* extra-data protocol message */
178#define MT_OOBDATA 15 /* expedited data */
179.Ed
180.Pp
181If the
182.Dv M_PKTHDR
183flag is set, a
184.Li struct pkthdr m_pkthdr
185is added to the mbuf header.
186It contains a pointer to the interface
187the packet has been received from
188.Pq Fa struct ifnet *rcvif ,
189and the total packet length
190.Pq Fa int len .
191.Pp
192If small enough, data is stored in the mbuf's internal data buffer.
193If the data is sufficiently large, another mbuf may be added to the chain,
194or external storage may be associated with the mbuf.
195.Dv MHLEN
196bytes of data can fit into an mbuf with the
197.Dv M_PKTHDR
198flag set,
199.Dv MLEN
200bytes can otherwise.
201.Pp
202If external storage is being associated with an mbuf, the
203.Dv m_ext
204header is added at the cost of losing the internal data buffer.
205It includes a pointer to external storage, the size of the storage,
206a pointer to a function used for freeing the storage,
207a pointer to an optional argument that can be passed to the function,
208and a pointer to a reference counter.
209An mbuf using external storage has the
210.Dv M_EXT
211flag set.
212.Pp
213The system supplies a macro for allocating the desired external storage
214buffer,
215.Dv MEXTADD .
216.Pp
217The allocation and management of the reference counter is handled by the
218subsystem.
219The developer can check whether the reference count for the
220given mbuf's external storage is greater than 1 with the
221.Dv MEXT_IS_REF
222macro.
223Similarly, the developer can directly add and remove references,
224if absolutely necessary, with the use of the
225.Dv MEXT_ADD_REF
226and
227.Dv MEXT_REM_REF
228macros.
229.Pp
230The system also supplies a default type of external storage buffer called an
231.Dq mbuf cluster .
232Mbuf clusters can be allocated and configured with the use of the
233.Dv MCLGET
234macro.
235Each cluster is
236.Dv MCLBYTES
237in size, where MCLBYTES is a machine-dependent constant.
238The system defines an advisory macro
239.Dv MINCLSIZE ,
240which is the smallest amount of data to put into a cluster.
241It's equal to the sum of
242.Dv MLEN
243and
244.Dv MHLEN .
245It is typically preferable to store data into an mbuf's data region, if size
246permits, as opposed to allocating a separate mbuf cluster to hold the same
247data.
248.\"
249.Ss Macros and Functions
250There are numerous predefined macros and functions that provide the
251developer with common utilities.
252.\"
253.Bl -ohang -offset indent
254.It Fn mtod mbuf type
255Convert an mbuf pointer to a data pointer.
256The macro expands to the data pointer cast to the pointer of the specified type.
257.Sy Note :
258It is advisable to ensure that there is enough contiguous data in the mbuf.
259See
260.Fn m_pullup
261for details.
262.It Fn MGET mbuf how type
263Allocate an mbuf and initialize it to contain internal data.
264.Fa mbuf
265will point to the allocated mbuf on success, or be set to
266.Dv NULL
267on failure.
268The
269.Fa how
270argument is to be set to
271.Dv M_TRYWAIT
272or
273.Dv M_DONTWAIT .
274It specifies whether the caller is willing to block if necessary.
275If
276.Fa how
277is set to
278.Dv M_TRYWAIT ,
279a failed allocation will result in the caller being put
280to sleep for a designated
281kern.ipc.mbuf_wait
282.Xr ( sysctl 8
283tunable)
284number of ticks.
285A number of other mbuf-related
286functions and macros have the same argument because they may
287at some point need to allocate new mbufs.
288.It Fn MGETHDR mbuf how type
289Allocate an mbuf and initialize it to contain a packet header
290and internal data.
291See
292.Fn MGET
293for details.
294.It Fn MCLGET mbuf how
295Allocate and attach an mbuf cluster to an mbuf.
296If the macro fails, the
297.Dv M_EXT
298flag won't be set in the mbuf.
299.It Fn M_PREPEND mbuf len how
300This macro operates on an mbuf chain.
301It is an optimized wrapper for
302.Fn m_prepend
303that can make use of possible empty space before data
304(e.g. left after trimming of a link-layer header).
305The new chain pointer or
306.Dv NULL
307is in
308.Fa mbuf
309after the call.
310.It Fn M_WRITABLE mbuf
311This macro will evaluate true if the mbuf is not marked
312.Dv M_RDONLY
313and if either the mbuf does not contain external storage or,
314if it does,
315then if the reference count of the storage is not greater than 1.
316The
317.Dv M_RDONLY
318flag can be set in the mbuf's
319.Dv m_flags .
320This can be achieved during setup of the external storage,
321by passing the
322.Dv M_RDONLY
323bit as a
324.Fa flags
325argument to the
326.Fn MEXTADD
327macro, or can be directly set in individual mbufs.
328.El
329.Pp
330The functions are:
331.Bl -ohang -offset indent
332.It Fn m_get how type
333A function version of
334.Fn MGET
335for non-critical paths.
336.It Fn m_getm orig len how type
337Allocate
338.Fa len
339bytes worth of mbufs and mbuf clusters if necessary and append the resulting
340allocated chain to the
341.Fa orig
342mbuf chain, if it is
343.No non- Ns Dv NULL .
344If the allocation fails at any point,
345free whatever was allocated and return
346.Dv NULL .
347If
348.Fa orig
349is
350.No non- Ns Dv NULL ,
351it will not be freed.
352It is possible to use
353.Fn m_getm
354to either append
355.Fa len
356bytes to an existing mbuf or mbuf chain
357(for example, one which may be sitting in a pre-allocated ring)
358or to simply perform an all-or-nothing mbuf and mbuf cluster allocation.
359.It Fn m_gethdr how type
360A function version of
361.Fn MGETHDR
362for non-critical paths.
363.It Fn m_getclr how type
364Allocate an mbuf and zero out the data region.
365.El
366.Pp
367The functions below operate on mbuf chains.
368.Bl -ohang -offset indent
369.It Fn m_freem mbuf
370Free an entire mbuf chain, including any external
371storage.
372.\"
373.It Fn m_adj mbuf len
374Trim
375.Fa len
376bytes from the head of an mbuf chain if
377.Fa len
378is positive, from the tail otherwise.
379.\"
380.It Fn m_prepend mbuf len how
381Allocate a new mbuf and prepend it to the chain, handle
382.Dv M_PKTHDR
383properly.
384.Sy Note :
385It doesn't allocate any clusters, so
386.Fa len
387must be less than
388.Dv MLEN
389or
390.Dv MHLEN ,
391depending on the
392.Dv M_PKTHDR
393flag setting.
394.\"
395.It Fn m_pullup mbuf len
396Arrange that the first
397.Fa len
398bytes of an mbuf chain are contiguous and lay in the data area of
399.Fa mbuf ,
400so they are accessible with
401.Fn mtod mbuf type .
402Return the new chain on success,
403.Dv NULL
404on failure
405(the chain is freed in this case).
406.Sy Note :
407It doesn't allocate any clusters, so
408.Fa len
409must be less than
410.Dv MHLEN .
411.\"
412.It Fn m_copym mbuf offset len how
413Make a copy of an mbuf chain starting
414.Fa offset
415bytes from the beginning, continuing for
416.Fa len
417bytes.
418If
419.Fa len
420is
421.Dv M_COPYALL ,
422copy to the end of the mbuf chain.
423.Sy Note :
424The copy is read-only, because clusters are not
425copied, only their reference counts are incremented.
426.\"
427.It Fn m_copypacket mbuf how
428Copy an entire packet including header, which must be present.
429This is an optimized version of the common case
430.Fn m_copym mbuf 0 M_COPYALL how .
431.Sy Note :
432the copy is read-only, because clusters are not
433copied, only their reference counts are incremented.
434.\"
435.It Fn m_dup mbuf how
436Copy a packet header mbuf chain into a completely new chain, including
437copying any mbuf clusters.
438Use this instead of
439.Fn m_copypacket
440when you need a writable copy of an mbuf chain.
441.\"
442.It Fn m_copydata mbuf offset len buf
443Copy data from an mbuf chain starting
444.Fa off
445bytes from the beginning, continuing for
446.Fa len
447bytes, into the indicated buffer
448.Fa buf .
449.\"
450.It Fn m_copyback mbuf offset len buf
451Copy
452.Fa len
453bytes from the buffer
454.Fa buf
455back into the indicated mbuf chain,
456starting at
457.Fa offset
458bytes from the beginning of the chain, extending the mbuf chain if necessary.
459.Sy Note :
460It doesn't allocate any clusters, just adds mbufs to the chain.
461It's safe to set
462.Fa offset
463beyond the current chain end: zeroed mbufs will be allocated to fill the
464space.
465.\"
466.It Fn m_devget buf len offset ifp copy
467Copy data from a device local memory pointed to by
468.Fa buf
469to an mbuf chain.
470The copy is done using a specified copy routine
471.Fa copy ,
472or
473.Fn bcopy
474if
475.Fa copy
476is
477.Dv NULL .
478.\"
479.It Fn m_cat m n
480Concatenate
481.Fa n
482to
483.Fa m .
484Both chains must be of the same type.
485.Fa N
486is still valid after the function returned.
487.Sy Note :
488It does not handle
489.Dv M_PKTHDR
490and friends.
491.\"
492.It Fn m_split mbuf len how
493Partition an mbuf chain in two pieces, returning the tail:
494all but the first
495.Fa len
496bytes.
497In case of failure, it returns
498.Dv NULL
499and attempts to restore the chain to its original state.
500.El
501.Sh RETURN VALUES
502See above.
503.Sh HISTORY
504.\" Please correct me if I'm wrong
| 26.\"
27.Dd October 17, 2000
28.Dt MBUF 9
29.Os
30.\"
31.Sh NAME
32.Nm mbuf
33.Nd "memory management in the kernel IPC subsystem"
34.\"
35.Sh SYNOPSIS
36.Fd #include <sys/param.h>
37.Fd #include <sys/mbuf.h>
38.\"
39.Ss Mbuf allocation macros
40.Fn MGET "struct mbuf *mbuf" "int how" "short type"
41.Fn MGETHDR "struct mbuf *mbuf" "int how" "short type"
42.Fn MCLGET "struct mbuf *mbuf" "int how"
43.Fo MEXTADD
44.Fa "struct mbuf *mbuf"
45.Fa "caddr_t buf"
46.Fa "u_int size"
47.Fa "void (*free)(void *opt_args)"
48.Fa "void *opt_args"
49.Fa "short flags"
50.Fa "int type"
51.Fc
52.Fn MEXTFREE "struct mbuf *mbuf"
53.Fn MEXT_ADD_REF "struct mbuf *mbuf"
54.Fn MEXT_REM_REF "struct mbuf *mbuf"
55.Fn MFREE "struct mbuf *mbuf" "struct mbuf *successor"
56.\"
57.Ss Mbuf utility macros
58.Fn mtod "struct mbuf *mbuf" "any type"
59.Fn MEXT_IS_REF "struct mbuf *mbuf"
60.Fn M_COPY_PKTHDR "struct mbuf *to" "struct mbuf *from"
61.Fn M_ALIGN "struct mbuf *mbuf" "u_int len"
62.Fn MH_ALIGN "struct mbuf *mbuf" "u_int len"
63.Fn M_LEADINGSPACE "struct mbuf *mbuf"
64.Fn M_TRAILINGSPACE "struct mbuf *mbuf"
65.Fn M_PREPEND "struct mbuf *mbuf" "int len" "int how"
66.Fn MCHTYPE "struct mbuf *mbuf" "u_int type"
67.Fn M_WRITABLE "struct mbuf *mbuf"
68.\"
69.Ss Mbuf allocation functions
70.Ft struct mbuf *
71.Fn m_get "int how" "int type"
72.Ft struct mbuf *
73.Fn m_getm "struct mbuf *orig" "int len" "int how" "int type"
74.Ft struct mbuf *
75.Fn m_getclr "int how" "int type"
76.Ft struct mbuf *
77.Fn m_gethdr "int how" "int type"
78.Ft struct mbuf *
79.Fn m_free "struct mbuf *mbuf"
80.Ft void
81.Fn m_freem "struct mbuf *mbuf"
82.\"
83.Ss Mbuf utility functions
84.Ft void
85.Fn m_adj "struct mbuf *mbuf" "int len"
86.Ft struct mbuf *
87.Fn m_prepend "struct mbuf *mbuf" "int len" "int how"
88.Ft struct mbuf *
89.Fn m_pullup "struct mbuf *mbuf" "int len"
90.Ft struct mbuf *
91.Fn m_copym "struct mbuf *mbuf" "int offset" "int len" "int how"
92.Ft struct mbuf *
93.Fn m_copypacket "struct mbuf *mbuf" "int how"
94.Ft struct mbuf *
95.Fn m_dup "struct mbuf *mbuf" "int how"
96.Ft void
97.Fn m_copydata "struct mbuf *mbuf" "int offset" "int len" "caddr_t buf"
98.Ft void
99.Fn m_copyback "struct mbuf *mbuf" "int offset" "int len" "caddr_t buf"
100.Ft struct mbuf *
101.Fo m_devget
102.Fa "char *buf"
103.Fa "int len"
104.Fa "int offset"
105.Fa "struct ifnet *ifp"
106.Fa "void (*copy)(char *from, caddr_t to, u_int len)"
107.Fc
108.Ft void
109.Fn m_cat "struct mbuf *m" "struct mbuf *n"
110.Ft struct mbuf *
111.Fn m_split "struct mbuf *mbuf" "int len" "int how"
112.\"
113.Sh DESCRIPTION
114An mbuf is a basic unit of memory management in the kernel IPC subsystem.
115Network packets and socket buffers are stored in mbufs.
116A network packet may span multiple mbufs arranged into a chain
117(linked list),
118which allows adding or trimming
119network headers with little overhead.
120.Pp
121While a developer should not bother with mbuf internals without serious
122reason in order to avoid incompatibilities with future changes, it
123is useful to understand the mbuf's general structure.
124.Pp
125An mbuf consists of a variable-sized header and a small internal
126buffer for data.
127The mbuf's total size,
128.Dv MSIZE ,
129is a machine-dependent constant defined in
130.Pa machine/param.h .
131The mbuf header includes:
132.Pp
133.Bl -tag -width "m_nextpkt" -compact -offset indent
134.It Fa m_next
135a pointer to the next buffer in the chain
136.It Fa m_nextpkt
137a pointer to the next chain in the queue
138.It Fa m_data
139a pointer to the data
140.It Fa m_len
141the length of the data
142.It Fa m_type
143the type of data
144.It Fa m_flags
145the mbuf flags
146.El
147.Pp
148The mbuf flag bits are defined as follows:
149.Bd -literal
150/* mbuf flags */
151#define M_EXT 0x0001 /* has associated external storage */
152#define M_PKTHDR 0x0002 /* start of record */
153#define M_EOR 0x0004 /* end of record */
154#define M_RDONLY 0x0008 /* associated data marked read-only */
155#define M_PROTO1 0x0010 /* protocol-specific */
156#define M_PROTO2 0x0020 /* protocol-specific */
157#define M_PROTO3 0x0040 /* protocol-specific */
158#define M_PROTO4 0x0080 /* protocol-specific */
159#define M_PROTO5 0x0100 /* protocol-specific */
160
161/* mbuf pkthdr flags, also in m_flags */
162#define M_BCAST 0x0200 /* send/received as link-level broadcast */
163#define M_MCAST 0x0400 /* send/received as link-level multicast */
164#define M_FRAG 0x0800 /* packet is fragment of larger packet */
165#define M_FIRSTFRAG 0x1000 /* packet is first fragment */
166#define M_LASTFRAG 0x2000 /* packet is last fragment */
167.Ed
168.Pp
169The available mbuf types are defined as follows:
170.Bd -literal
171/* mbuf types */
172#define MT_FREE 0 /* should be on free list */
173#define MT_DATA 1 /* dynamic (data) allocation */
174#define MT_HEADER 2 /* packet header */
175#define MT_SONAME 8 /* socket name */
176#define MT_FTABLE 11 /* fragment reassembly header */
177#define MT_CONTROL 14 /* extra-data protocol message */
178#define MT_OOBDATA 15 /* expedited data */
179.Ed
180.Pp
181If the
182.Dv M_PKTHDR
183flag is set, a
184.Li struct pkthdr m_pkthdr
185is added to the mbuf header.
186It contains a pointer to the interface
187the packet has been received from
188.Pq Fa struct ifnet *rcvif ,
189and the total packet length
190.Pq Fa int len .
191.Pp
192If small enough, data is stored in the mbuf's internal data buffer.
193If the data is sufficiently large, another mbuf may be added to the chain,
194or external storage may be associated with the mbuf.
195.Dv MHLEN
196bytes of data can fit into an mbuf with the
197.Dv M_PKTHDR
198flag set,
199.Dv MLEN
200bytes can otherwise.
201.Pp
202If external storage is being associated with an mbuf, the
203.Dv m_ext
204header is added at the cost of losing the internal data buffer.
205It includes a pointer to external storage, the size of the storage,
206a pointer to a function used for freeing the storage,
207a pointer to an optional argument that can be passed to the function,
208and a pointer to a reference counter.
209An mbuf using external storage has the
210.Dv M_EXT
211flag set.
212.Pp
213The system supplies a macro for allocating the desired external storage
214buffer,
215.Dv MEXTADD .
216.Pp
217The allocation and management of the reference counter is handled by the
218subsystem.
219The developer can check whether the reference count for the
220given mbuf's external storage is greater than 1 with the
221.Dv MEXT_IS_REF
222macro.
223Similarly, the developer can directly add and remove references,
224if absolutely necessary, with the use of the
225.Dv MEXT_ADD_REF
226and
227.Dv MEXT_REM_REF
228macros.
229.Pp
230The system also supplies a default type of external storage buffer called an
231.Dq mbuf cluster .
232Mbuf clusters can be allocated and configured with the use of the
233.Dv MCLGET
234macro.
235Each cluster is
236.Dv MCLBYTES
237in size, where MCLBYTES is a machine-dependent constant.
238The system defines an advisory macro
239.Dv MINCLSIZE ,
240which is the smallest amount of data to put into a cluster.
241It's equal to the sum of
242.Dv MLEN
243and
244.Dv MHLEN .
245It is typically preferable to store data into an mbuf's data region, if size
246permits, as opposed to allocating a separate mbuf cluster to hold the same
247data.
248.\"
249.Ss Macros and Functions
250There are numerous predefined macros and functions that provide the
251developer with common utilities.
252.\"
253.Bl -ohang -offset indent
254.It Fn mtod mbuf type
255Convert an mbuf pointer to a data pointer.
256The macro expands to the data pointer cast to the pointer of the specified type.
257.Sy Note :
258It is advisable to ensure that there is enough contiguous data in the mbuf.
259See
260.Fn m_pullup
261for details.
262.It Fn MGET mbuf how type
263Allocate an mbuf and initialize it to contain internal data.
264.Fa mbuf
265will point to the allocated mbuf on success, or be set to
266.Dv NULL
267on failure.
268The
269.Fa how
270argument is to be set to
271.Dv M_TRYWAIT
272or
273.Dv M_DONTWAIT .
274It specifies whether the caller is willing to block if necessary.
275If
276.Fa how
277is set to
278.Dv M_TRYWAIT ,
279a failed allocation will result in the caller being put
280to sleep for a designated
281kern.ipc.mbuf_wait
282.Xr ( sysctl 8
283tunable)
284number of ticks.
285A number of other mbuf-related
286functions and macros have the same argument because they may
287at some point need to allocate new mbufs.
288.It Fn MGETHDR mbuf how type
289Allocate an mbuf and initialize it to contain a packet header
290and internal data.
291See
292.Fn MGET
293for details.
294.It Fn MCLGET mbuf how
295Allocate and attach an mbuf cluster to an mbuf.
296If the macro fails, the
297.Dv M_EXT
298flag won't be set in the mbuf.
299.It Fn M_PREPEND mbuf len how
300This macro operates on an mbuf chain.
301It is an optimized wrapper for
302.Fn m_prepend
303that can make use of possible empty space before data
304(e.g. left after trimming of a link-layer header).
305The new chain pointer or
306.Dv NULL
307is in
308.Fa mbuf
309after the call.
310.It Fn M_WRITABLE mbuf
311This macro will evaluate true if the mbuf is not marked
312.Dv M_RDONLY
313and if either the mbuf does not contain external storage or,
314if it does,
315then if the reference count of the storage is not greater than 1.
316The
317.Dv M_RDONLY
318flag can be set in the mbuf's
319.Dv m_flags .
320This can be achieved during setup of the external storage,
321by passing the
322.Dv M_RDONLY
323bit as a
324.Fa flags
325argument to the
326.Fn MEXTADD
327macro, or can be directly set in individual mbufs.
328.El
329.Pp
330The functions are:
331.Bl -ohang -offset indent
332.It Fn m_get how type
333A function version of
334.Fn MGET
335for non-critical paths.
336.It Fn m_getm orig len how type
337Allocate
338.Fa len
339bytes worth of mbufs and mbuf clusters if necessary and append the resulting
340allocated chain to the
341.Fa orig
342mbuf chain, if it is
343.No non- Ns Dv NULL .
344If the allocation fails at any point,
345free whatever was allocated and return
346.Dv NULL .
347If
348.Fa orig
349is
350.No non- Ns Dv NULL ,
351it will not be freed.
352It is possible to use
353.Fn m_getm
354to either append
355.Fa len
356bytes to an existing mbuf or mbuf chain
357(for example, one which may be sitting in a pre-allocated ring)
358or to simply perform an all-or-nothing mbuf and mbuf cluster allocation.
359.It Fn m_gethdr how type
360A function version of
361.Fn MGETHDR
362for non-critical paths.
363.It Fn m_getclr how type
364Allocate an mbuf and zero out the data region.
365.El
366.Pp
367The functions below operate on mbuf chains.
368.Bl -ohang -offset indent
369.It Fn m_freem mbuf
370Free an entire mbuf chain, including any external
371storage.
372.\"
373.It Fn m_adj mbuf len
374Trim
375.Fa len
376bytes from the head of an mbuf chain if
377.Fa len
378is positive, from the tail otherwise.
379.\"
380.It Fn m_prepend mbuf len how
381Allocate a new mbuf and prepend it to the chain, handle
382.Dv M_PKTHDR
383properly.
384.Sy Note :
385It doesn't allocate any clusters, so
386.Fa len
387must be less than
388.Dv MLEN
389or
390.Dv MHLEN ,
391depending on the
392.Dv M_PKTHDR
393flag setting.
394.\"
395.It Fn m_pullup mbuf len
396Arrange that the first
397.Fa len
398bytes of an mbuf chain are contiguous and lay in the data area of
399.Fa mbuf ,
400so they are accessible with
401.Fn mtod mbuf type .
402Return the new chain on success,
403.Dv NULL
404on failure
405(the chain is freed in this case).
406.Sy Note :
407It doesn't allocate any clusters, so
408.Fa len
409must be less than
410.Dv MHLEN .
411.\"
412.It Fn m_copym mbuf offset len how
413Make a copy of an mbuf chain starting
414.Fa offset
415bytes from the beginning, continuing for
416.Fa len
417bytes.
418If
419.Fa len
420is
421.Dv M_COPYALL ,
422copy to the end of the mbuf chain.
423.Sy Note :
424The copy is read-only, because clusters are not
425copied, only their reference counts are incremented.
426.\"
427.It Fn m_copypacket mbuf how
428Copy an entire packet including header, which must be present.
429This is an optimized version of the common case
430.Fn m_copym mbuf 0 M_COPYALL how .
431.Sy Note :
432the copy is read-only, because clusters are not
433copied, only their reference counts are incremented.
434.\"
435.It Fn m_dup mbuf how
436Copy a packet header mbuf chain into a completely new chain, including
437copying any mbuf clusters.
438Use this instead of
439.Fn m_copypacket
440when you need a writable copy of an mbuf chain.
441.\"
442.It Fn m_copydata mbuf offset len buf
443Copy data from an mbuf chain starting
444.Fa off
445bytes from the beginning, continuing for
446.Fa len
447bytes, into the indicated buffer
448.Fa buf .
449.\"
450.It Fn m_copyback mbuf offset len buf
451Copy
452.Fa len
453bytes from the buffer
454.Fa buf
455back into the indicated mbuf chain,
456starting at
457.Fa offset
458bytes from the beginning of the chain, extending the mbuf chain if necessary.
459.Sy Note :
460It doesn't allocate any clusters, just adds mbufs to the chain.
461It's safe to set
462.Fa offset
463beyond the current chain end: zeroed mbufs will be allocated to fill the
464space.
465.\"
466.It Fn m_devget buf len offset ifp copy
467Copy data from a device local memory pointed to by
468.Fa buf
469to an mbuf chain.
470The copy is done using a specified copy routine
471.Fa copy ,
472or
473.Fn bcopy
474if
475.Fa copy
476is
477.Dv NULL .
478.\"
479.It Fn m_cat m n
480Concatenate
481.Fa n
482to
483.Fa m .
484Both chains must be of the same type.
485.Fa N
486is still valid after the function returned.
487.Sy Note :
488It does not handle
489.Dv M_PKTHDR
490and friends.
491.\"
492.It Fn m_split mbuf len how
493Partition an mbuf chain in two pieces, returning the tail:
494all but the first
495.Fa len
496bytes.
497In case of failure, it returns
498.Dv NULL
499and attempts to restore the chain to its original state.
500.El
501.Sh RETURN VALUES
502See above.
503.Sh HISTORY
504.\" Please correct me if I'm wrong
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