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.\" $FreeBSD$ 26.\" 27.Dd June 5, 2014 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.In sys/param.h 37.In sys/systm.h 38.In sys/mbuf.h 39.\" 40.Ss Mbuf allocation macros 41.Fn MGET "struct mbuf *mbuf" "int how" "short type" 42.Fn MGETHDR "struct mbuf *mbuf" "int how" "short type" 43.Fn MCLGET "struct mbuf *mbuf" "int how" 44.Fo MEXTADD 45.Fa "struct mbuf *mbuf" 46.Fa "caddr_t buf" 47.Fa "u_int size" 48.Fa "void (*free)(void *opt_arg1, void *opt_arg2)" 49.Fa "void *opt_arg1" 50.Fa "void *opt_arg2" 51.Fa "short flags" 52.Fa "int type" 53.Fc 54.Fn MEXTFREE "struct mbuf *mbuf" 55.Fn MFREE "struct mbuf *mbuf" "struct mbuf *successor" 56.\" 57.Ss Mbuf utility macros 58.Fn mtod "struct mbuf *mbuf" "type" 59.Fn M_ALIGN "struct mbuf *mbuf" "u_int len" 60.Fn MH_ALIGN "struct mbuf *mbuf" "u_int len" 61.Ft int 62.Fn M_LEADINGSPACE "struct mbuf *mbuf" 63.Ft int 64.Fn M_TRAILINGSPACE "struct mbuf *mbuf" 65.Fn M_MOVE_PKTHDR "struct mbuf *to" "struct mbuf *from" 66.Fn M_PREPEND "struct mbuf *mbuf" "int len" "int how" 67.Fn MCHTYPE "struct mbuf *mbuf" "short type" 68.Ft int 69.Fn M_WRITABLE "struct mbuf *mbuf" 70.\" 71.Ss Mbuf allocation functions 72.Ft struct mbuf * 73.Fn m_get "int how" "short type" 74.Ft struct mbuf * 75.Fn m_get2 "int size" "int how" "short type" "int flags" 76.Ft struct mbuf * 77.Fn m_getm "struct mbuf *orig" "int len" "int how" "short type" 78.Ft struct mbuf * 79.Fn m_getjcl "int how" "short type" "int flags" "int size" 80.Ft struct mbuf * 81.Fn m_getcl "int how" "short type" "int flags" 82.Ft struct mbuf * 83.Fn m_getclr "int how" "short type" 84.Ft struct mbuf * 85.Fn m_gethdr "int how" "short type" 86.Ft struct mbuf * 87.Fn m_free "struct mbuf *mbuf" 88.Ft void 89.Fn m_freem "struct mbuf *mbuf" 90.\" 91.Ss Mbuf utility functions 92.Ft void 93.Fn m_adj "struct mbuf *mbuf" "int len" 94.Ft void 95.Fn m_align "struct mbuf *mbuf" "int len" 96.Ft int 97.Fn m_append "struct mbuf *mbuf" "int len" "c_caddr_t cp" 98.Ft struct mbuf * 99.Fn m_prepend "struct mbuf *mbuf" "int len" "int how" 100.Ft struct mbuf * 101.Fn m_copyup "struct mbuf *mbuf" "int len" "int dstoff" 102.Ft struct mbuf * 103.Fn m_pullup "struct mbuf *mbuf" "int len" 104.Ft struct mbuf * 105.Fn m_pulldown "struct mbuf *mbuf" "int offset" "int len" "int *offsetp" 106.Ft struct mbuf * 107.Fn m_copym "struct mbuf *mbuf" "int offset" "int len" "int how" 108.Ft struct mbuf * 109.Fn m_copypacket "struct mbuf *mbuf" "int how" 110.Ft struct mbuf * 111.Fn m_dup "struct mbuf *mbuf" "int how" 112.Ft void 113.Fn m_copydata "const struct mbuf *mbuf" "int offset" "int len" "caddr_t buf" 114.Ft void 115.Fn m_copyback "struct mbuf *mbuf" "int offset" "int len" "caddr_t buf" 116.Ft struct mbuf * 117.Fo m_devget 118.Fa "char *buf" 119.Fa "int len" 120.Fa "int offset" 121.Fa "struct ifnet *ifp" 122.Fa "void (*copy)(char *from, caddr_t to, u_int len)" 123.Fc 124.Ft void 125.Fn m_cat "struct mbuf *m" "struct mbuf *n" 126.Ft u_int 127.Fn m_fixhdr "struct mbuf *mbuf" 128.Ft void 129.Fn m_dup_pkthdr "struct mbuf *to" "struct mbuf *from" 130.Ft void 131.Fn m_move_pkthdr "struct mbuf *to" "struct mbuf *from" 132.Ft u_int 133.Fn m_length "struct mbuf *mbuf" "struct mbuf **last" 134.Ft struct mbuf * 135.Fn m_split "struct mbuf *mbuf" "int len" "int how" 136.Ft int 137.Fn m_apply "struct mbuf *mbuf" "int off" "int len" "int (*f)(void *arg, void *data, u_int len)" "void *arg" 138.Ft struct mbuf * 139.Fn m_getptr "struct mbuf *mbuf" "int loc" "int *off" 140.Ft struct mbuf * 141.Fn m_defrag "struct mbuf *m0" "int how" 142.Ft struct mbuf * 143.Fn m_unshare "struct mbuf *m0" "int how" 144.\" 145.Sh DESCRIPTION 146An 147.Vt mbuf 148is a basic unit of memory management in the kernel IPC subsystem. 149Network packets and socket buffers are stored in 150.Vt mbufs . 151A network packet may span multiple 152.Vt mbufs 153arranged into a 154.Vt mbuf chain 155(linked list), 156which allows adding or trimming 157network headers with little overhead. 158.Pp 159While a developer should not bother with 160.Vt mbuf 161internals without serious 162reason in order to avoid incompatibilities with future changes, it 163is useful to understand the general structure of an 164.Vt mbuf . 165.Pp 166An 167.Vt mbuf 168consists of a variable-sized header and a small internal 169buffer for data. 170The total size of an 171.Vt mbuf , 172.Dv MSIZE , 173is a constant defined in 174.In sys/param.h . 175The 176.Vt mbuf 177header includes: 178.Bl -tag -width "m_nextpkt" -offset indent 179.It Va m_next 180.Pq Vt struct mbuf * 181A pointer to the next 182.Vt mbuf 183in the 184.Vt mbuf chain . 185.It Va m_nextpkt 186.Pq Vt struct mbuf * 187A pointer to the next 188.Vt mbuf chain 189in the queue. 190.It Va m_data 191.Pq Vt caddr_t 192A pointer to data attached to this 193.Vt mbuf . 194.It Va m_len 195.Pq Vt int 196The length of the data. 197.It Va m_type 198.Pq Vt short 199The type of the data. 200.It Va m_flags 201.Pq Vt int 202The 203.Vt mbuf 204flags. 205.El 206.Pp 207The 208.Vt mbuf 209flag bits are defined as follows: 210.Bd -literal 211/* mbuf flags */ 212#define M_EXT 0x00000001 /* has associated external storage */ 213#define M_PKTHDR 0x00000002 /* start of record */ 214#define M_EOR 0x00000004 /* end of record */ 215#define M_RDONLY 0x00000008 /* associated data marked read-only */ 216#define M_PROTO1 0x00001000 /* protocol-specific */ 217#define M_PROTO2 0x00002000 /* protocol-specific */ 218#define M_PROTO3 0x00004000 /* protocol-specific */ 219#define M_PROTO4 0x00008000 /* protocol-specific */ 220#define M_PROTO5 0x00010000 /* protocol-specific */ 221#define M_PROTO6 0x00020000 /* protocol-specific */ 222#define M_PROTO7 0x00040000 /* protocol-specific */ 223#define M_PROTO8 0x00080000 /* protocol-specific */ 224#define M_PROTO9 0x00100000 /* protocol-specific */ 225#define M_PROTO10 0x00200000 /* protocol-specific */ 226#define M_PROTO11 0x00400000 /* protocol-specific */ 227#define M_PROTO12 0x00800000 /* protocol-specific */ 228 229/* mbuf pkthdr flags (also stored in m_flags) */ 230#define M_BCAST 0x00000010 /* send/received as link-level broadcast */ 231#define M_MCAST 0x00000020 /* send/received as link-level multicast */ 232.Ed 233.Pp 234The available 235.Vt mbuf 236types are defined as follows: 237.Bd -literal 238/* mbuf types */ 239#define MT_DATA 1 /* dynamic (data) allocation */ 240#define MT_HEADER MT_DATA /* packet header */ 241#define MT_SONAME 8 /* socket name */ 242#define MT_CONTROL 14 /* extra-data protocol message */ 243#define MT_OOBDATA 15 /* expedited data */ 244.Ed 245.Pp 246The available external buffer types are defined as follows: 247.Bd -literal 248/* external buffer types */ 249#define EXT_CLUSTER 1 /* mbuf cluster */ 250#define EXT_SFBUF 2 /* sendfile(2)'s sf_bufs */ 251#define EXT_JUMBOP 3 /* jumbo cluster 4096 bytes */ 252#define EXT_JUMBO9 4 /* jumbo cluster 9216 bytes */ 253#define EXT_JUMBO16 5 /* jumbo cluster 16184 bytes */ 254#define EXT_PACKET 6 /* mbuf+cluster from packet zone */ 255#define EXT_MBUF 7 /* external mbuf reference (M_IOVEC) */ 256#define EXT_NET_DRV 252 /* custom ext_buf provided by net driver(s) */ 257#define EXT_MOD_TYPE 253 /* custom module's ext_buf type */ 258#define EXT_DISPOSABLE 254 /* can throw this buffer away w/page flipping */ 259#define EXT_EXTREF 255 /* has externally maintained ref_cnt ptr */ 260.Ed 261.Pp 262If the 263.Dv M_PKTHDR 264flag is set, a 265.Vt struct pkthdr Va m_pkthdr 266is added to the 267.Vt mbuf 268header. 269It contains a pointer to the interface 270the packet has been received from 271.Pq Vt struct ifnet Va *rcvif , 272and the total packet length 273.Pq Vt int Va len . 274Optionally, it may also contain an attached list of packet tags 275.Pq Vt "struct m_tag" . 276See 277.Xr mbuf_tags 9 278for details. 279Fields used in offloading checksum calculation to the hardware are kept in 280.Va m_pkthdr 281as well. 282See 283.Sx HARDWARE-ASSISTED CHECKSUM CALCULATION 284for details. 285.Pp 286If small enough, data is stored in the internal data buffer of an 287.Vt mbuf . 288If the data is sufficiently large, another 289.Vt mbuf 290may be added to the 291.Vt mbuf chain , 292or external storage may be associated with the 293.Vt mbuf . 294.Dv MHLEN 295bytes of data can fit into an 296.Vt mbuf 297with the 298.Dv M_PKTHDR 299flag set, 300.Dv MLEN 301bytes can otherwise. 302.Pp 303If external storage is being associated with an 304.Vt mbuf , 305the 306.Va m_ext 307header is added at the cost of losing the internal data buffer. 308It includes a pointer to external storage, the size of the storage, 309a pointer to a function used for freeing the storage, 310a pointer to an optional argument that can be passed to the function, 311and a pointer to a reference counter. 312An 313.Vt mbuf 314using external storage has the 315.Dv M_EXT 316flag set. 317.Pp 318The system supplies a macro for allocating the desired external storage 319buffer, 320.Dv MEXTADD . 321.Pp 322The allocation and management of the reference counter is handled by the 323subsystem. 324.Pp 325The system also supplies a default type of external storage buffer called an 326.Vt mbuf cluster . 327.Vt Mbuf clusters 328can be allocated and configured with the use of the 329.Dv MCLGET 330macro. 331Each 332.Vt mbuf cluster 333is 334.Dv MCLBYTES 335in size, where MCLBYTES is a machine-dependent constant. 336The system defines an advisory macro 337.Dv MINCLSIZE , 338which is the smallest amount of data to put into an 339.Vt mbuf cluster . 340It is equal to 341.Dv MHLEN 342plus one. 343It is typically preferable to store data into the data region of an 344.Vt mbuf , 345if size permits, as opposed to allocating a separate 346.Vt mbuf cluster 347to hold the same data. 348.\" 349.Ss Macros and Functions 350There are numerous predefined macros and functions that provide the 351developer with common utilities. 352.\" 353.Bl -ohang -offset indent 354.It Fn mtod mbuf type 355Convert an 356.Fa mbuf 357pointer to a data pointer. 358The macro expands to the data pointer cast to the pointer of the specified 359.Fa type . 360.Sy Note : 361It is advisable to ensure that there is enough contiguous data in 362.Fa mbuf . 363See 364.Fn m_pullup 365for details. 366.It Fn MGET mbuf how type 367Allocate an 368.Vt mbuf 369and initialize it to contain internal data. 370.Fa mbuf 371will point to the allocated 372.Vt mbuf 373on success, or be set to 374.Dv NULL 375on failure. 376The 377.Fa how 378argument is to be set to 379.Dv M_WAITOK 380or 381.Dv M_NOWAIT . 382It specifies whether the caller is willing to block if necessary. 383A number of other functions and macros related to 384.Vt mbufs 385have the same argument because they may 386at some point need to allocate new 387.Vt mbufs . 388.It Fn MGETHDR mbuf how type 389Allocate an 390.Vt mbuf 391and initialize it to contain a packet header 392and internal data. 393See 394.Fn MGET 395for details. 396.It Fn MEXTADD mbuf buf size free opt_arg1 opt_arg2 flags type 397Associate externally managed data with 398.Fa mbuf . 399Any internal data contained in the mbuf will be discarded, and the 400.Dv M_EXT 401flag will be set. 402The 403.Fa buf 404and 405.Fa size 406arguments are the address and length, respectively, of the data. 407The 408.Fa free 409argument points to a function which will be called to free the data 410when the mbuf is freed; it is only used if 411.Fa type 412is 413.Dv EXT_EXTREF . 414The 415.Fa opt_arg1 416and 417.Fa opt_arg2 418arguments will be passed unmodified to 419.Fa free . 420The 421.Fa flags 422argument specifies additional 423.Vt mbuf 424flags; it is not necessary to specify 425.Dv M_EXT . 426Finally, the 427.Fa type 428argument specifies the type of external data, which controls how it 429will be disposed of when the 430.Vt mbuf 431is freed. 432In most cases, the correct value is 433.Dv EXT_EXTREF . 434.It Fn MCLGET mbuf how 435Allocate and attach an 436.Vt mbuf cluster 437to 438.Fa mbuf . 439If the macro fails, the 440.Dv M_EXT 441flag will not be set in 442.Fa mbuf . 443.It Fn M_ALIGN mbuf len 444Set the pointer 445.Fa mbuf->m_data 446to place an object of the size 447.Fa len 448at the end of the internal data area of 449.Fa mbuf , 450long word aligned. 451Applicable only if 452.Fa mbuf 453is newly allocated with 454.Fn MGET 455or 456.Fn m_get . 457.It Fn MH_ALIGN mbuf len 458Serves the same purpose as 459.Fn M_ALIGN 460does, but only for 461.Fa mbuf 462newly allocated with 463.Fn MGETHDR 464or 465.Fn m_gethdr , 466or initialized by 467.Fn m_dup_pkthdr 468or 469.Fn m_move_pkthdr . 470.It Fn m_align mbuf len 471Services the same purpose as 472.Fn M_ALIGN 473but handles any type of mbuf. 474.It Fn M_LEADINGSPACE mbuf 475Returns the number of bytes available before the beginning 476of data in 477.Fa mbuf . 478.It Fn M_TRAILINGSPACE mbuf 479Returns the number of bytes available after the end of data in 480.Fa mbuf . 481.It Fn M_PREPEND mbuf len how 482This macro operates on an 483.Vt mbuf chain . 484It is an optimized wrapper for 485.Fn m_prepend 486that can make use of possible empty space before data 487(e.g.\& left after trimming of a link-layer header). 488The new 489.Vt mbuf chain 490pointer or 491.Dv NULL 492is in 493.Fa mbuf 494after the call. 495.It Fn M_MOVE_PKTHDR to from 496Using this macro is equivalent to calling 497.Fn m_move_pkthdr to from . 498.It Fn M_WRITABLE mbuf 499This macro will evaluate true if 500.Fa mbuf 501is not marked 502.Dv M_RDONLY 503and if either 504.Fa mbuf 505does not contain external storage or, 506if it does, 507then if the reference count of the storage is not greater than 1. 508The 509.Dv M_RDONLY 510flag can be set in 511.Fa mbuf->m_flags . 512This can be achieved during setup of the external storage, 513by passing the 514.Dv M_RDONLY 515bit as a 516.Fa flags 517argument to the 518.Fn MEXTADD 519macro, or can be directly set in individual 520.Vt mbufs . 521.It Fn MCHTYPE mbuf type 522Change the type of 523.Fa mbuf 524to 525.Fa type . 526This is a relatively expensive operation and should be avoided. 527.El 528.Pp 529The functions are: 530.Bl -ohang -offset indent 531.It Fn m_get how type 532A function version of 533.Fn MGET 534for non-critical paths. 535.It Fn m_get2 size how type flags 536Allocate an 537.Vt mbuf 538with enough space to hold specified amount of data. 539.It Fn m_getm orig len how type 540Allocate 541.Fa len 542bytes worth of 543.Vt mbufs 544and 545.Vt mbuf clusters 546if necessary and append the resulting allocated 547.Vt mbuf chain 548to the 549.Vt mbuf chain 550.Fa orig , 551if it is 552.No non- Ns Dv NULL . 553If the allocation fails at any point, 554free whatever was allocated and return 555.Dv NULL . 556If 557.Fa orig 558is 559.No non- Ns Dv NULL , 560it will not be freed. 561It is possible to use 562.Fn m_getm 563to either append 564.Fa len 565bytes to an existing 566.Vt mbuf 567or 568.Vt mbuf chain 569(for example, one which may be sitting in a pre-allocated ring) 570or to simply perform an all-or-nothing 571.Vt mbuf 572and 573.Vt mbuf cluster 574allocation. 575.It Fn m_gethdr how type 576A function version of 577.Fn MGETHDR 578for non-critical paths. 579.It Fn m_getcl how type flags 580Fetch an 581.Vt mbuf 582with a 583.Vt mbuf cluster 584attached to it. 585If one of the allocations fails, the entire allocation fails. 586This routine is the preferred way of fetching both the 587.Vt mbuf 588and 589.Vt mbuf cluster 590together, as it avoids having to unlock/relock between allocations. 591Returns 592.Dv NULL 593on failure. 594.It Fn m_getjcl how type flags size 595This is like 596.Fn m_getcl 597but it the size of the cluster allocated will be large enough for 598.Fa size 599bytes. 600.It Fn m_getclr how type 601Allocate an 602.Vt mbuf 603and zero out the data region. 604.It Fn m_free mbuf 605Frees 606.Vt mbuf . 607Returns 608.Va m_next 609of the freed 610.Vt mbuf . 611.El 612.Pp 613The functions below operate on 614.Vt mbuf chains . 615.Bl -ohang -offset indent 616.It Fn m_freem mbuf 617Free an entire 618.Vt mbuf chain , 619including any external storage. 620.\" 621.It Fn m_adj mbuf len 622Trim 623.Fa len 624bytes from the head of an 625.Vt mbuf chain 626if 627.Fa len 628is positive, from the tail otherwise. 629.\" 630.It Fn m_append mbuf len cp 631Append 632.Vt len 633bytes of data 634.Vt cp 635to the 636.Vt mbuf chain . 637Extend the mbuf chain if the new data does not fit in 638existing space. 639.\" 640.It Fn m_prepend mbuf len how 641Allocate a new 642.Vt mbuf 643and prepend it to the 644.Vt mbuf chain , 645handle 646.Dv M_PKTHDR 647properly. 648.Sy Note : 649It does not allocate any 650.Vt mbuf clusters , 651so 652.Fa len 653must be less than 654.Dv MLEN 655or 656.Dv MHLEN , 657depending on the 658.Dv M_PKTHDR 659flag setting. 660.\" 661.It Fn m_copyup mbuf len dstoff 662Similar to 663.Fn m_pullup 664but copies 665.Fa len 666bytes of data into a new mbuf at 667.Fa dstoff 668bytes into the mbuf. 669The 670.Fa dstoff 671argument aligns the data and leaves room for a link layer header. 672Returns the new 673.Vt mbuf chain 674on success, 675and frees the 676.Vt mbuf chain 677and returns 678.Dv NULL 679on failure. 680.Sy Note : 681The function does not allocate 682.Vt mbuf clusters , 683so 684.Fa len + dstoff 685must be less than 686.Dv MHLEN . 687.\" 688.It Fn m_pullup mbuf len 689Arrange that the first 690.Fa len 691bytes of an 692.Vt mbuf chain 693are contiguous and lay in the data area of 694.Fa mbuf , 695so they are accessible with 696.Fn mtod mbuf type . 697It is important to remember that this may involve 698reallocating some mbufs and moving data so all pointers 699referencing data within the old mbuf chain 700must be recalculated or made invalid. 701Return the new 702.Vt mbuf chain 703on success, 704.Dv NULL 705on failure 706(the 707.Vt mbuf chain 708is freed in this case). 709.Sy Note : 710It does not allocate any 711.Vt mbuf clusters , 712so 713.Fa len 714must be less than or equal to 715.Dv MHLEN . 716.\" 717.It Fn m_pulldown mbuf offset len offsetp 718Arrange that 719.Fa len 720bytes between 721.Fa offset 722and 723.Fa offset + len 724in the 725.Vt mbuf chain 726are contiguous and lay in the data area of 727.Fa mbuf , 728so they are accessible with 729.Fn mtod mbuf type . 730.Fa len 731must be smaller than, or equal to, the size of an 732.Vt mbuf cluster . 733Return a pointer to an intermediate 734.Vt mbuf 735in the chain containing the requested region; 736the offset in the data region of the 737.Vt mbuf chain 738to the data contained in the returned mbuf is stored in 739.Fa *offsetp . 740If 741.Fa offp 742is NULL, the region may be accessed using 743.Fn mtod mbuf type . 744If 745.Fa offp 746is non-NULL, the region may be accessed using 747.Fn mtod mbuf uint8_t + *offsetp . 748The region of the mbuf chain between its beginning and 749.Fa off 750is not modified, therefore it is safe to hold pointers to data within 751this region before calling 752.Fn m_pulldown . 753.\" 754.It Fn m_copym mbuf offset len how 755Make a copy of an 756.Vt mbuf chain 757starting 758.Fa offset 759bytes from the beginning, continuing for 760.Fa len 761bytes. 762If 763.Fa len 764is 765.Dv M_COPYALL , 766copy to the end of the 767.Vt mbuf chain . 768.Sy Note : 769The copy is read-only, because the 770.Vt mbuf clusters 771are not copied, only their reference counts are incremented. 772.\" 773.It Fn m_copypacket mbuf how 774Copy an entire packet including header, which must be present. 775This is an optimized version of the common case 776.Fn m_copym mbuf 0 M_COPYALL how . 777.Sy Note : 778the copy is read-only, because the 779.Vt mbuf clusters 780are not copied, only their reference counts are incremented. 781.\" 782.It Fn m_dup mbuf how 783Copy a packet header 784.Vt mbuf chain 785into a completely new 786.Vt mbuf chain , 787including copying any 788.Vt mbuf clusters . 789Use this instead of 790.Fn m_copypacket 791when you need a writable copy of an 792.Vt mbuf chain . 793.\" 794.It Fn m_copydata mbuf offset len buf 795Copy data from an 796.Vt mbuf chain 797starting 798.Fa off 799bytes from the beginning, continuing for 800.Fa len 801bytes, into the indicated buffer 802.Fa buf . 803.\" 804.It Fn m_copyback mbuf offset len buf 805Copy 806.Fa len 807bytes from the buffer 808.Fa buf 809back into the indicated 810.Vt mbuf chain , 811starting at 812.Fa offset 813bytes from the beginning of the 814.Vt mbuf chain , 815extending the 816.Vt mbuf chain 817if necessary. 818.Sy Note : 819It does not allocate any 820.Vt mbuf clusters , 821just adds 822.Vt mbufs 823to the 824.Vt mbuf chain . 825It is safe to set 826.Fa offset 827beyond the current 828.Vt mbuf chain 829end: zeroed 830.Vt mbufs 831will be allocated to fill the space. 832.\" 833.It Fn m_length mbuf last 834Return the length of the 835.Vt mbuf chain , 836and optionally a pointer to the last 837.Vt mbuf . 838.\" 839.It Fn m_dup_pkthdr to from how 840Upon the function's completion, the 841.Vt mbuf 842.Fa to 843will contain an identical copy of 844.Fa from->m_pkthdr 845and the per-packet attributes found in the 846.Vt mbuf chain 847.Fa from . 848The 849.Vt mbuf 850.Fa from 851must have the flag 852.Dv M_PKTHDR 853initially set, and 854.Fa to 855must be empty on entry. 856.\" 857.It Fn m_move_pkthdr to from 858Move 859.Va m_pkthdr 860and the per-packet attributes from the 861.Vt mbuf chain 862.Fa from 863to the 864.Vt mbuf 865.Fa to . 866The 867.Vt mbuf 868.Fa from 869must have the flag 870.Dv M_PKTHDR 871initially set, and 872.Fa to 873must be empty on entry. 874Upon the function's completion, 875.Fa from 876will have the flag 877.Dv M_PKTHDR 878and the per-packet attributes cleared. 879.\" 880.It Fn m_fixhdr mbuf 881Set the packet-header length to the length of the 882.Vt mbuf chain . 883.\" 884.It Fn m_devget buf len offset ifp copy 885Copy data from a device local memory pointed to by 886.Fa buf 887to an 888.Vt mbuf chain . 889The copy is done using a specified copy routine 890.Fa copy , 891or 892.Fn bcopy 893if 894.Fa copy 895is 896.Dv NULL . 897.\" 898.It Fn m_cat m n 899Concatenate 900.Fa n 901to 902.Fa m . 903Both 904.Vt mbuf chains 905must be of the same type. 906.Fa N 907is still valid after the function returned. 908.Sy Note : 909It does not handle 910.Dv M_PKTHDR 911and friends. 912.\" 913.It Fn m_split mbuf len how 914Partition an 915.Vt mbuf chain 916in two pieces, returning the tail: 917all but the first 918.Fa len 919bytes. 920In case of failure, it returns 921.Dv NULL 922and attempts to restore the 923.Vt mbuf chain 924to its original state. 925.\" 926.It Fn m_apply mbuf off len f arg 927Apply a function to an 928.Vt mbuf chain , 929at offset 930.Fa off , 931for length 932.Fa len 933bytes. 934Typically used to avoid calls to 935.Fn m_pullup 936which would otherwise be unnecessary or undesirable. 937.Fa arg 938is a convenience argument which is passed to the callback function 939.Fa f . 940.Pp 941Each time 942.Fn f 943is called, it will be passed 944.Fa arg , 945a pointer to the 946.Fa data 947in the current mbuf, and the length 948.Fa len 949of the data in this mbuf to which the function should be applied. 950.Pp 951The function should return zero to indicate success; 952otherwise, if an error is indicated, then 953.Fn m_apply 954will return the error and stop iterating through the 955.Vt mbuf chain . 956.\" 957.It Fn m_getptr mbuf loc off 958Return a pointer to the mbuf containing the data located at 959.Fa loc 960bytes from the beginning of the 961.Vt mbuf chain . 962The corresponding offset into the mbuf will be stored in 963.Fa *off . 964.It Fn m_defrag m0 how 965Defragment an mbuf chain, returning the shortest possible 966chain of mbufs and clusters. 967If allocation fails and this can not be completed, 968.Dv NULL 969will be returned and the original chain will be unchanged. 970Upon success, the original chain will be freed and the new 971chain will be returned. 972.Fa how 973should be either 974.Dv M_WAITOK 975or 976.Dv M_NOWAIT , 977depending on the caller's preference. 978.Pp 979This function is especially useful in network drivers, where 980certain long mbuf chains must be shortened before being added 981to TX descriptor lists. 982.It Fn m_unshare m0 how 983Create a version of the specified mbuf chain whose 984contents can be safely modified without affecting other users. 985If allocation fails and this operation can not be completed, 986.Dv NULL 987will be returned. 988The original mbuf chain is always reclaimed and the reference 989count of any shared mbuf clusters is decremented. 990.Fa how 991should be either 992.Dv M_WAITOK 993or 994.Dv M_NOWAIT , 995depending on the caller's preference. 996As a side-effect of this process the returned 997mbuf chain may be compacted. 998.Pp 999This function is especially useful in the transmit path of 1000network code, when data must be encrypted or otherwise 1001altered prior to transmission. 1002.El 1003.Sh HARDWARE-ASSISTED CHECKSUM CALCULATION 1004This section currently applies to TCP/IP only. 1005In order to save the host CPU resources, computing checksums is 1006offloaded to the network interface hardware if possible. 1007The 1008.Va m_pkthdr 1009member of the leading 1010.Vt mbuf 1011of a packet contains two fields used for that purpose, 1012.Vt int Va csum_flags 1013and 1014.Vt int Va csum_data . 1015The meaning of those fields depends on the direction a packet flows in, 1016and on whether the packet is fragmented. 1017Henceforth, 1018.Va csum_flags 1019or 1020.Va csum_data 1021of a packet 1022will denote the corresponding field of the 1023.Va m_pkthdr 1024member of the leading 1025.Vt mbuf 1026in the 1027.Vt mbuf chain 1028containing the packet. 1029.Pp 1030On output, checksum offloading is attempted after the outgoing 1031interface has been determined for a packet. 1032The interface-specific field 1033.Va ifnet.if_data.ifi_hwassist 1034(see 1035.Xr ifnet 9 ) 1036is consulted for the capabilities of the interface to assist in 1037computing checksums. 1038The 1039.Va csum_flags 1040field of the packet header is set to indicate which actions the interface 1041is supposed to perform on it. 1042The actions unsupported by the network interface are done in the 1043software prior to passing the packet down to the interface driver; 1044such actions will never be requested through 1045.Va csum_flags . 1046.Pp 1047The flags demanding a particular action from an interface are as follows: 1048.Bl -tag -width ".Dv CSUM_TCP" -offset indent 1049.It Dv CSUM_IP 1050The IP header checksum is to be computed and stored in the 1051corresponding field of the packet. 1052The hardware is expected to know the format of an IP header 1053to determine the offset of the IP checksum field. 1054.It Dv CSUM_TCP 1055The TCP checksum is to be computed. 1056(See below.) 1057.It Dv CSUM_UDP 1058The UDP checksum is to be computed. 1059(See below.) 1060.El 1061.Pp 1062Should a TCP or UDP checksum be offloaded to the hardware, 1063the field 1064.Va csum_data 1065will contain the byte offset of the checksum field relative to the 1066end of the IP header. 1067In this case, the checksum field will be initially 1068set by the TCP/IP module to the checksum of the pseudo header 1069defined by the TCP and UDP specifications. 1070.Pp 1071On input, an interface indicates the actions it has performed 1072on a packet by setting one or more of the following flags in 1073.Va csum_flags 1074associated with the packet: 1075.Bl -tag -width ".Dv CSUM_IP_CHECKED" -offset indent 1076.It Dv CSUM_IP_CHECKED 1077The IP header checksum has been computed. 1078.It Dv CSUM_IP_VALID 1079The IP header has a valid checksum. 1080This flag can appear only in combination with 1081.Dv CSUM_IP_CHECKED . 1082.It Dv CSUM_DATA_VALID 1083The checksum of the data portion of the IP packet has been computed 1084and stored in the field 1085.Va csum_data 1086in network byte order. 1087.It Dv CSUM_PSEUDO_HDR 1088Can be set only along with 1089.Dv CSUM_DATA_VALID 1090to indicate that the IP data checksum found in 1091.Va csum_data 1092allows for the pseudo header defined by the TCP and UDP specifications. 1093Otherwise the checksum of the pseudo header must be calculated by 1094the host CPU and added to 1095.Va csum_data 1096to obtain the final checksum to be used for TCP or UDP validation purposes. 1097.El 1098.Pp 1099If a particular network interface just indicates success or 1100failure of TCP or UDP checksum validation without returning 1101the exact value of the checksum to the host CPU, its driver can mark 1102.Dv CSUM_DATA_VALID 1103and 1104.Dv CSUM_PSEUDO_HDR 1105in 1106.Va csum_flags , 1107and set 1108.Va csum_data 1109to 1110.Li 0xFFFF 1111hexadecimal to indicate a valid checksum. 1112It is a peculiarity of the algorithm used that the Internet checksum 1113calculated over any valid packet will be 1114.Li 0xFFFF 1115as long as the original checksum field is included. 1116.Sh STRESS TESTING 1117When running a kernel compiled with the option 1118.Dv MBUF_STRESS_TEST , 1119the following 1120.Xr sysctl 8 Ns 1121-controlled options may be used to create 1122various failure/extreme cases for testing of network drivers 1123and other parts of the kernel that rely on 1124.Vt mbufs . 1125.Bl -tag -width ident 1126.It Va net.inet.ip.mbuf_frag_size 1127Causes 1128.Fn ip_output 1129to fragment outgoing 1130.Vt mbuf chains 1131into fragments of the specified size. 1132Setting this variable to 1 is an excellent way to 1133test the long 1134.Vt mbuf chain 1135handling ability of network drivers. 1136.It Va kern.ipc.m_defragrandomfailures 1137Causes the function 1138.Fn m_defrag 1139to randomly fail, returning 1140.Dv NULL . 1141Any piece of code which uses 1142.Fn m_defrag 1143should be tested with this feature. 1144.El 1145.Sh RETURN VALUES 1146See above. 1147.Sh SEE ALSO 1148.Xr ifnet 9 , 1149.Xr mbuf_tags 9 1150.Sh HISTORY 1151.\" Please correct me if I'm wrong 1152.Vt Mbufs 1153appeared in an early version of 1154.Bx . 1155Besides being used for network packets, they were used 1156to store various dynamic structures, such as routing table 1157entries, interface addresses, protocol control blocks, etc. 1158In more recent 1159.Fx 1160use of 1161.Vt mbufs 1162is almost entirely limited to packet storage, with 1163.Xr uma 9 1164zones being used directly to store other network-related memory. 1165.Pp 1166Historically, the 1167.Vt mbuf 1168allocator has been a special-purpose memory allocator able to run in 1169interrupt contexts and allocating from a special kernel address space map. 1170As of 1171.Fx 5.3 , 1172the 1173.Vt mbuf 1174allocator is a wrapper around 1175.Xr uma 9 , 1176allowing caching of 1177.Vt mbufs , 1178clusters, and 1179.Vt mbuf 1180+ cluster pairs in per-CPU caches, as well as bringing other benefits of 1181slab allocation. 1182.Sh AUTHORS 1183The original 1184.Nm 1185manual page was written by Yar Tikhiy. 1186The 1187.Xr uma 9 1188.Vt mbuf 1189allocator was written by Bosko Milekic. 1190