--- name: "MGET" section: "9freebsd" platform: "linux" package: "freebsd-manpages" version: "12.2-2" release: "Debian 13 (Trixie)" platforms: ["linux"] url: "https://new.carta.tech/MCHTYPE/9freebsd" --- # MGET(9freebsd) memory management in the kernel IPC subsystem Part of [MCHTYPE(9freebsd)](/MCHTYPE/9freebsd.md). ## NAME **mbuf** – memory management in the kernel IPC subsystem ## SYNOPSIS **#include \** **#include \** **#include \** ### Mbuf allocation macros **MGET**(*struct mbuf \*mbuf*, *int how*, *short type*); **MGETHDR**(*struct mbuf \*mbuf*, *int how*, *short type*); *int*\ **MCLGET**(*struct mbuf \*mbuf*, *int how*); **MEXTADD**(*struct mbuf \*mbuf*, *char \*buf*, *u\_int size*, *void (\*free)(struct mbuf \*)*, *void \*opt\_arg1*, *void \*opt\_arg2*, *int 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*); *int*\ **M\_LEADINGSPACE**(*struct mbuf \*mbuf*); *int*\ **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*); *int*\ **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\_gethdr**(*int how*, *short type*); *struct* *mbuf* *\**\ **m\_free**(*struct mbuf \*mbuf*); *void*\ **m\_freem**(*struct mbuf \*mbuf*); ### Mbuf utility functions *void*\ **m\_adj**(*struct mbuf \*mbuf*, *int len*); *void*\ **m\_align**(*struct mbuf \*mbuf*, *int len*); *int*\ **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**(*const struct mbuf \*mbuf*, *int how*); *void*\ **m\_copydata**(*const struct mbuf \*mbuf*, *int offset*, *int len*, *caddr\_t buf*); *void*\ **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)*); *void*\ **m\_cat**(*struct mbuf \*m*, *struct mbuf \*n*); *void*\ **m\_catpkt**(*struct mbuf \*m*, *struct mbuf \*n*); *u\_int*\ **m\_fixhdr**(*struct mbuf \*mbuf*); *int*\ **m\_dup\_pkthdr**(*struct mbuf \*to*, *const struct mbuf \*from*, *int how*); *void*\ **m\_move\_pkthdr**(*struct mbuf \*to*, *struct mbuf \*from*); *u\_int*\ **m\_length**(*struct mbuf \*mbuf*, *struct mbuf \*\*last*); *struct* *mbuf* *\**\ **m\_split**(*struct mbuf \*mbuf*, *int len*, *int how*); *int*\ **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*); ## DESCRIPTION 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: - *m\_next* (*struct* *mbuf* *\**) A pointer to the next *mbuf* in the *mbuf* *chain*. - *m\_nextpkt* (*struct* *mbuf* *\**) A pointer to the next *mbuf* *chain* in the queue. - *m\_data* (*caddr\_t*) A pointer to data attached to this *mbuf*. - *m\_len* (*int*) The length of the data. - *m\_type* (*short*) The type of the data. - *m\_flags* (*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 */ #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)](/mbuf_tags/9.md) 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. - **mtod**(*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 **m\_pullup**() 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*. - **MGETHDR**(*mbuf*, *how*, *type*) Allocate an *mbuf* and initialize it to contain a packet header and internal data. See **MGET**() for details. - **MEXTADD**(*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 saved in *ext\_arg1* and *ext\_arg2* fields of the *struct* *m\_ext* of the mbuf. 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. - **MCLGET**(*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. - **M\_ALIGN**(*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**(). - **MH\_ALIGN**(*mbuf*, *len*) Serves the same purpose as **M\_ALIGN**() does, but only for *mbuf* newly allocated with **MGETHDR**() or **m\_gethdr**(), or initialized by **m\_dup\_pkthdr**() or **m\_move\_pkthdr**(). - **m\_align**(*mbuf*, *len*) Services the same purpose as **M\_ALIGN**() but handles any type of mbuf. - **M\_LEADINGSPACE**(*mbuf*) Returns the number of bytes available before the beginning of data in *mbuf*. - **M\_TRAILINGSPACE**(*mbuf*) Returns the number of bytes available after the end of data in *mbuf*. - **M\_PREPEND**(*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. - **M\_MOVE\_PKTHDR**(*to*, *from*) Using this macro is equivalent to calling **m\_move\_pkthdr**(*to*, *from*). - **M\_WRITABLE**(*mbuf*) 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*. - **MCHTYPE**(*mbuf*, *type*) Change the type of *mbuf* to *type*. This is a relatively expensive operation and should be avoided. The functions are: - **m\_get**(*how*, *type*) A function version of **MGET**() for non-critical paths. - **m\_get2**(*size*, *how*, *type*, *flags*) Allocate an *mbuf* with enough space to hold specified amount of data. - **m\_getm**(*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. - **m\_gethdr**(*how*, *type*) A function version of **MGETHDR**() for non-critical paths. - **m\_getcl**(*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. - **m\_getjcl**(*how*, *type*, *flags*, *size*) This is like **m\_getcl**() but it the size of the cluster allocated will be large enough for *size* bytes. - **m\_free**(*mbuf*) Frees *mbuf*. Returns *m\_next* of the freed *mbuf*. The functions below operate on *mbuf* *chains*. - **m\_freem**(*mbuf*) Free an entire *mbuf* *chain*, including any external storage. - **m\_adj**(*mbuf*, *len*) Trim *len* bytes from the head of an *mbuf* *chain* if *len* is positive, from the tail otherwise. - **m\_append**(*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. - **m\_prepend**(*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. - **m\_copyup**(*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. - **m\_pulldown**(*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**(). - **m\_copym**(*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. - **m\_copypacket**(*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. - **m\_dup**(*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*. - **m\_copydata**(*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*. - **m\_copyback**(*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. - **m\_length**(*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. - **m\_fixhdr**(*mbuf*) Set the packet-header length to the length of the *mbuf* *chain*. - **m\_devget**(*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 **bcopy**() if *copy* is NULL. - **m\_cat**(*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\_catpkt**(*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. - **m\_split**(*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 **f**() 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 **m\_apply**() will return the error and stop iterating through the *mbuf* *chain*. - **m\_getptr**(*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. - **m\_collapse**(*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 **m\_defrag**(), *how* should be one of M\_WAITOK or M\_NOWAIT. - **m\_unshare**(*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. ## HARDWARE-ASSISTED CHECKSUM CALCULATION 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)](/ifnet/9.md)) 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: - CSUM\_IP 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. - CSUM\_TCP The TCP checksum is to be computed. (See below.) - CSUM\_UDP 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: - CSUM\_IP\_CHECKED The IP header checksum has been computed. - CSUM\_IP\_VALID The IP header has a valid checksum. This flag can appear only in combination with CSUM\_IP\_CHECKED. - CSUM\_DATA\_VALID The checksum of the data portion of the IP packet has been computed and stored in the field *csum\_data* in network byte order. - CSUM\_PSEUDO\_HDR 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. ## STRESS TESTING When running a kernel compiled with the option MBUF\_STRESS\_TEST, the following[sysctl(8)](/sysctl/8.md) -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*. - *net.inet.ip.mbuf\_frag\_size* Causes **ip\_output**() 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. - *kern.ipc.m\_defragrandomfailures* Causes the function **m\_defrag**() to randomly fail, returning NULL. Any piece of code which uses **m\_defrag**() should be tested with this feature. ## RETURN VALUES See above. ## SEE ALSO [ifnet(9)](/ifnet/9.md), [mbuf\_tags(9)](/mbuf_tags/9.md) ## HISTORY *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)](/uma/9.md) 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)](/uma/9.md), allowing caching of *mbufs*, clusters, and *mbuf* + cluster pairs in per-CPU caches, as well as bringing other benefits of slab allocation. ## AUTHORS The original **mbuf** manual page was written by Yar Tikhiy. The [uma(9)](/uma/9.md) *mbuf* allocator was written by Bosko Milekic.