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.\" 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 $ 27.\" 28.Dd August 18, 2007 29.Dt MBUF 9 30.Os 31.\" 32.Sh NAME 33.Nm mbuf 34.Nd "memory management in the kernel IPC subsystem" 35.\" 36.Sh SYNOPSIS 37.In sys/param.h 38.In sys/systm.h 39.In sys/mbuf.h 40.\" 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" 46.\" 47.Ss Mbuf utility macros 48.Ft void * 49.Fn mtod "struct mbuf *mbuf" "type" 50.Ft int 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" 54.Ft int 55.Fn M_LEADINGSPACE "struct mbuf *mbuf" 56.Ft int 57.Fn M_TRAILINGSPACE "struct mbuf *mbuf" 58.Fn M_PREPEND "struct mbuf *mbuf" "int len" "int how" 59.\" 60.Ss Mbuf allocation functions 61.Ft struct mbuf * 62.Fn m_get "int how" "int type" 63.Ft struct mbuf * 64.Fn m_getm "struct mbuf *orig" "int len" "int how" "int type" 65.Ft struct mbuf * 66.Fn m_getclr "int how" "int type" 67.Ft struct mbuf * 68.Fn m_gethdr "int how" "int type" 69.Ft struct mbuf * 70.Fn m_free "struct mbuf *mbuf" 71.Ft void 72.Fn m_freem "struct mbuf *mbuf" 73.\" 74.Ss Mbuf utility functions 75.Ft void 76.Fn m_adj "struct mbuf *mbuf" "int len" 77.Ft struct mbuf * 78.Fn m_prepend "struct mbuf *mbuf" "int len" "int how" 79.Ft struct mbuf * 80.Fn m_pullup "struct mbuf *mbuf" "int len" 81.Ft struct mbuf * 82.Fn m_copym "struct mbuf *mbuf" "int offset" "int len" "int how" 83.Ft struct mbuf * 84.Fn m_copypacket "struct mbuf *mbuf" "int how" 85.Ft struct mbuf * 86.Fn m_dup "struct mbuf *mbuf" "int how" 87.Ft void 88.Fn m_copydata "const struct mbuf *mbuf" "int offset" "int len" "caddr_t buf" 89.Ft void 90.Fn m_copyback "struct mbuf *mbuf" "int offset" "int len" "caddr_t buf" 91.Ft struct mbuf * 92.Fo m_devget 93.Fa "char *buf" 94.Fa "int len" 95.Fa "int offset" 96.Fa "struct ifnet *ifp" 97.Fa "void (*copy)(char *from, caddr_t to, u_int len)" 98.Fc 99.Ft void 100.Fn m_cat "struct mbuf *m" "struct mbuf *n" 101.Ft struct mbuf * 102.Fn m_split "struct mbuf *mbuf" "int len" "int how" 103.\" 104.Sh DESCRIPTION 105An mbuf is a basic unit of memory management in the kernel IPC subsystem. 106Network packets and socket buffers are stored in mbufs. 107A network packet may span multiple mbufs arranged into a chain 108(linked list), 109which allows adding or trimming 110network headers with little overhead. 111.Pp 112While a developer should not bother with mbuf internals without serious 113reason in order to avoid incompatibilities with future changes, it 114is useful to understand the mbuf's general structure. 115.Pp 116An mbuf consists of a variable-sized header and a small internal 117buffer for data. 118The mbuf's total size, 119.Dv MSIZE , 120is a machine-dependent constant defined in 121.In machine/param.h . 122The mbuf header includes: 123.Pp 124.Bl -tag -width "m_nextpkt" -compact -offset indent 125.It Fa m_next 126a pointer to the next buffer in the chain 127.It Fa m_nextpkt 128a pointer to the next chain in the queue 129.It Fa m_data 130a pointer to the data 131.It Fa m_len 132the length of the data 133.It Fa m_type 134the type of data 135.It Fa m_flags 136the mbuf flags 137.El 138.Pp 139The mbuf flag bits are defined as follows: 140.Bd -literal 141/* mbuf flags */ 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 */ 150 151/* 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 */ 157.Ed 158.Pp 159The available mbuf types are defined as follows: 160.Bd -literal 161/* mbuf types */ 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 */ 169.Ed 170.Pp 171If the 172.Dv M_PKTHDR 173flag is set, a 174.Li struct pkthdr m_pkthdr 175is added to the mbuf header. 176It contains a pointer to the interface 177the packet has been received from 178.Pq Fa struct ifnet *rcvif , 179and the total packet length 180.Pq Fa int len . 181.Pp 182If small enough, data is stored in the mbuf's internal data buffer. 183If the data is sufficiently large, another mbuf may be added to the chain, 184or external storage may be associated with the mbuf. 185.Dv MHLEN 186bytes of data can fit into an mbuf with the 187.Dv M_PKTHDR 188flag set, 189.Dv MLEN 190bytes can otherwise. 191.Pp 192If external storage is being associated with an mbuf, the 193.Dv m_ext 194header is added at the cost of losing the internal data buffer. 195It includes a pointer to external storage, the size of the storage, 196a pointer to a function used for freeing the storage, 197a pointer to an optional argument that can be passed to the function, 198and a pointer to a reference counter. 199An mbuf using external storage has the 200.Dv M_EXT 201flag set. 202.Pp 203The system supplies a default type of external storage buffer called an 204.Dq mbuf cluster . 205Mbuf clusters can be allocated and configured with the use of the 206.Dv MCLGET 207macro. 208Each cluster is 209.Dv MCLBYTES 210in size, where 211.Dv MCLBYTES 212is a machine-dependent constant. 213The system defines an advisory macro 214.Dv MINCLSIZE , 215which is the smallest amount of data to put into a cluster. 216It's equal to the sum of 217.Dv MLEN 218and 219.Dv MHLEN . 220It is typically preferable to store data into an mbuf's data region, if size 221permits, as opposed to allocating a separate mbuf cluster to hold the same 222data. 223.\" 224.Ss Macros and Functions 225There are numerous predefined macros and functions that provide the 226developer with common utilities. 227.\" 228.Bl -ohang -offset indent 229.It Fn mtod mbuf type 230Convert an mbuf pointer to a data pointer. 231The macro expands to the data pointer cast to the pointer of the specified type. 232.Sy Note : 233It is advisable to ensure that there is enough contiguous data in the mbuf. 234See 235.Fn m_pullup 236for details. 237.It Fn MGET mbuf how type 238Allocate an mbuf and initialize it to contain internal data. 239.Fa mbuf 240will point to the allocated mbuf on success, or be set to 241.Dv NULL 242on failure. 243The 244.Fa how 245argument is to be set to 246.Dv MB_WAIT 247or 248.Dv MB_DONTWAIT . 249It specifies whether the caller is willing to block if necessary. 250If 251.Fa how 252is set to 253.Dv MB_WAIT , 254a failed allocation will result in the caller being put 255to sleep for a designated 256.Va kern.ipc.mbuf_wait 257.Xr ( sysctl 8 258tunable) 259number of ticks. 260A number of other mbuf-related 261functions and macros have the same argument because they may 262at some point need to allocate new mbufs. 263.Pp 264Programmers should be careful not to confuse the mbuf allocation flag 265.Dv MB_DONTWAIT 266with the 267.Xr kmalloc 9 268allocation flag, 269.Dv M_NOWAIT . 270They are not the same. 271.It Fn MGETHDR mbuf how type 272Allocate an mbuf and initialize it to contain a packet header 273and internal data. 274See 275.Fn MGET 276for details. 277.It Fn MCLGET mbuf how 278Allocate and attach an mbuf cluster to an mbuf. 279If the macro fails, the 280.Dv M_EXT 281flag won't be set in the mbuf. 282.It Fn M_PREPEND mbuf len how 283This macro operates on an mbuf chain. 284It is an optimized wrapper for 285.Fn m_prepend 286that can make use of possible empty space before data 287(e.g. left after trimming of a link-layer header). 288The new chain pointer or 289.Dv NULL 290is in 291.Fa mbuf 292after the call. 293.El 294.Pp 295The functions are: 296.Bl -ohang -offset indent 297.It Fn m_get how type 298A function version of 299.Fn MGET 300for non-critical paths. 301.It Fn m_getm orig len how type 302Allocate 303.Fa len 304bytes worth of mbufs and mbuf clusters if necessary and append the resulting 305allocated chain to the 306.Fa orig 307mbuf chain, if it is 308.No non- Ns Dv NULL . 309If the allocation fails at any point, 310free whatever was allocated and return 311.Dv NULL . 312If 313.Fa orig 314is 315.No non- Ns Dv NULL , 316it will not be freed. 317It is possible to use 318.Fn m_getm 319to either append 320.Fa len 321bytes to an existing mbuf or mbuf chain 322(for example, one which may be sitting in a pre-allocated ring) 323or to simply perform an all-or-nothing mbuf and mbuf cluster allocation. 324.It Fn m_gethdr how type 325A function version of 326.Fn MGETHDR 327for non-critical paths. 328.It Fn m_getclr how type 329Allocate an mbuf and zero out the data region. 330.El 331.Pp 332The functions below operate on mbuf chains. 333.Bl -ohang -offset indent 334.It Fn m_freem mbuf 335Free an entire mbuf chain, including any external 336storage. 337.\" 338.It Fn m_adj mbuf len 339Trim 340.Fa len 341bytes from the head of an mbuf chain if 342.Fa len 343is positive, from the tail otherwise. 344.\" 345.It Fn m_prepend mbuf len how 346Allocate a new mbuf and prepend it to the chain, handle 347.Dv M_PKTHDR 348properly. 349.Sy Note : 350It doesn't allocate any clusters, so 351.Fa len 352must be less than 353.Dv MLEN 354or 355.Dv MHLEN , 356depending on the 357.Dv M_PKTHDR 358flag setting. 359.\" 360.It Fn m_pullup mbuf len 361Arrange that the first 362.Fa len 363bytes of an mbuf chain are contiguous and lay in the data area of 364.Fa mbuf , 365so they are accessible with 366.Fn mtod mbuf type . 367Return the new chain on success, 368.Dv NULL 369on failure 370(the chain is freed in this case). 371.Sy Note : 372It doesn't allocate any clusters, so 373.Fa len 374must be less than 375.Dv MHLEN . 376.\" 377.It Fn m_copym mbuf offset len how 378Make a copy of an mbuf chain starting 379.Fa offset 380bytes from the beginning, continuing for 381.Fa len 382bytes. 383If 384.Fa len 385is 386.Dv M_COPYALL , 387copy to the end of the mbuf chain. 388.Sy Note : 389The copy is read-only, because clusters are not 390copied, only their reference counts are incremented. 391.\" 392.It Fn m_copypacket mbuf how 393Copy an entire packet including header, which must be present. 394This is an optimized version of the common case 395.Fn m_copym mbuf 0 M_COPYALL how . 396.Sy Note : 397the copy is read-only, because clusters are not 398copied, only their reference counts are incremented. 399.\" 400.It Fn m_dup mbuf how 401Copy a packet header mbuf chain into a completely new chain, including 402copying any mbuf clusters. 403Use this instead of 404.Fn m_copypacket 405when you need a writable copy of an mbuf chain. 406.\" 407.It Fn m_copydata mbuf offset len buf 408Copy data from an mbuf chain starting 409.Fa off 410bytes from the beginning, continuing for 411.Fa len 412bytes, into the indicated buffer 413.Fa buf . 414.\" 415.It Fn m_copyback mbuf offset len buf 416Copy 417.Fa len 418bytes from the buffer 419.Fa buf 420back into the indicated mbuf chain, 421starting at 422.Fa offset 423bytes from the beginning of the chain, extending the mbuf chain if necessary. 424.Sy Note : 425It doesn't allocate any clusters, just adds mbufs to the chain. 426It's safe to set 427.Fa offset 428beyond the current chain end: zeroed mbufs will be allocated to fill the 429space. 430.\" 431.It Fn m_devget buf len offset ifp copy 432Copy data from a device local memory pointed to by 433.Fa buf 434to an mbuf chain. 435The copy is done using a specified copy routine 436.Fa copy , 437or 438.Fn bcopy 439if 440.Fa copy 441is 442.Dv NULL . 443.\" 444.It Fn m_cat m n 445Concatenate 446.Fa n 447to 448.Fa m . 449Both chains must be of the same type. 450.Fa N 451is still valid after the function returned. 452.Sy Note : 453It does not handle 454.Dv M_PKTHDR 455and friends. 456.\" 457.It Fn m_split mbuf len how 458Partition an mbuf chain in two pieces, returning the tail: 459all but the first 460.Fa len 461bytes. 462In case of failure, it returns 463.Dv NULL 464and attempts to restore the chain to its original state. 465.El 466.Sh STRESS TESTING 467When running a kernel compiled with the option 468.Dv MBUF_STRESS_TEST , 469the following 470.Xr sysctl 8 Ns 471-controlled options may be used to create 472various failure/extreme cases for testing of network drivers 473and other parts of the kernel that rely on 474.Vt mbufs . 475.Bl -tag -width ident 476.It Va net.inet.ip.mbuf_frag_size 477Causes 478.Fn ip_output 479to fragment outgoing 480.Vt mbuf chains 481into fragments of the specified size. 482Setting this variable to 1 is an excellent way to 483test the long 484.Vt mbuf chain 485handling ability of network drivers. 486.It Va kern.ipc.m_defragrandomfailures 487Causes the function 488.Fn m_defrag 489to randomly fail, returning 490.Dv NULL . 491Any piece of code which uses 492.Fn m_defrag 493should be tested with this feature. 494.El 495.Sh RETURN VALUES 496See above. 497.Sh HISTORY 498.\" Please correct me if I'm wrong 499Mbufs appeared in an early version of 500.Bx . 501Besides for being used for network packets, they were used 502to store various dynamic structures, such as routing table 503entries, interface addresses, protocol control blocks, etc. 504.Sh AUTHORS 505The original 506.Nm 507man page was written by Yar Tikhiy. 508