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MBUF(9) Kernel Developer's Manual MBUF(9)

mbuf
kernel memory management for networking protocols

#include <sys/mbuf.h>

struct mbuf *
m_copym2(struct mbuf *m, int off, int len, int wait);

struct mbuf *
m_copym(struct mbuf *m, int off, int len, int wait);

struct mbuf *
m_free(struct mbuf *m);

MFREE(struct mbuf *m, struct mbuf *n);

struct mbuf *
m_get(int how, int type);

MGET(struct mbuf *m, int how, int type);

struct mbuf *
m_getclr(int how, int type);

struct mbuf *
m_gethdr(int how, int type);

MGETHDR(struct mbuf *m, int how, int type);

struct mbuf *
m_prepend(struct mbuf *m, int len, int how);

M_PREPEND(struct mbuf *m, int plen, int how);

struct mbuf *
m_pulldown(struct mbuf *m, int off, int len, int *offp);

struct mbuf *
m_pullup(struct mbuf *n, int len);

struct mbuf *
m_split(struct mbuf *m0, int len0, int wait);

struct mbuf *
m_inject(struct mbuf *m0, int len0, int siz, int wait);

struct mbuf *
m_getptr(struct mbuf *m, int loc, int *off);

void
m_adj(struct mbuf *mp, int req_len);

int
m_copyback(struct mbuf *m0, int off, int len, caddr_t cp, int wait);

void
m_freem(struct mbuf *m);

void
m_reclaim(void);

void
m_copydata(struct mbuf *m, int off, int len, caddr_t cp);

void
m_cat(struct mbuf *m, struct mbuf *n);

struct mbuf *
m_devget(char *buf, int totlen, int off, struct ifnet *ifp, void (*func)(const void *, void *, size_t));

void
m_zero(struct mbuf *m);

int
m_apply(struct mbuf *m, int off, int len, int (*func)(caddr_t, caddr_t, unsigned int), caddr_t fstate);

MCLGET(struct mbuf *m, int how);

struct mbuf *
MCLGETI(struct mbuf *m, int how, struct ifnet *ifp, int len);

MEXTADD(struct mbuf *m, caddr_t buf, u_int size, int type, void (*free)(caddr_t, u_int, void *), void *arg);

M_ALIGN(struct mbuf *m, int len);

MH_ALIGN(struct mbuf *m, int len);

M_READONLY(struct mbuf *m);

M_LEADINGSPACE(struct mbuf *m);

M_TRAILINGSPACE(struct mbuf *m);

int
m_dup_pkthdr(struct mbuf *to, struct mbuf *from, int how);

#define MLEN            (MSIZE - sizeof(struct m_hdr))
#define MHLEN           (MLEN - sizeof(struct pkthdr))

#define MINCLSIZE       (MHLEN + MLEN + 1)
#define M_MAXCOMPRESS   (MHLEN / 2)

#define mtod(m,t)       ((t)((m)->m_data))

struct m_hdr {
        struct  mbuf *mh_next;
        struct  mbuf *mh_nextpkt;
        caddr_t mh_data;
        u_int   mh_len;
        short   mh_type;
        u_short mh_flags;
};

struct pkthdr {
        struct  ifnet *rcvif;
        SLIST_HEAD(packet_tags, m_tag) tags;
        int     len;
        u_int16_t csum_flags;
        u_int16_t ether_vtag;
	struct	pkthdr_pf pf;
};

struct pkthdr_pf {
	void	 *hdr;
	u_int	  rtableid;
	u_int32_t qid;
	u_int16_t tag;
	u_int8_t  flags;
	u_int8_t  routed;
};

struct m_ext {
        caddr_t ext_buf;
        void    (*ext_free)(caddr_t, u_int, void *);
        void    *ext_arg;
        u_int   ext_size;
        int     ext_type;
        struct mbuf *ext_nextref;
        struct mbuf *ext_prevref;
};

struct mbuf {
        struct  m_hdr m_hdr;
        union {
                struct {
                        struct  pkthdr MH_pkthdr;
                        union {
                                struct  m_ext MH_ext;
                                char    MH_databuf[MHLEN];
                        } MH_dat;
                } MH;
                char    M_databuf[MLEN];
        } M_dat;
};

#define m_next          m_hdr.mh_next
#define m_len           m_hdr.mh_len
#define m_data          m_hdr.mh_data
#define m_type          m_hdr.mh_type
#define m_flags         m_hdr.mh_flags
#define m_nextpkt       m_hdr.mh_nextpkt
#define m_act           m_nextpkt
#define m_pkthdr        M_dat.MH.MH_pkthdr
#define m_ext           M_dat.MH.MH_dat.MH_ext
#define m_pktdat        M_dat.MH.MH_dat.MH_databuf
#define m_dat           M_dat.M_databuf

The mbuf functions provide a way to manage the memory buffers used by the kernel's networking subsystem. Several functions and macros are used to allocate and deallocate mbufs, but also to get, inject, remove, copy, modify, prepend or append data inside these mbufs. The size of an mbuf is MSIZE (defined in ⟨sys/param.h⟩).

An mbuf structure is defined as an m_hdr structure followed by a union. The header contains the following elements:

mh_next
A pointer to the next mbuf in the mbuf chain.
mh_nextpkt
A pointer to the next mbuf chain (i.e., packet) in the queue.
mh_data
Indicates the address of the beginning of data in the mbuf.
mh_len
Indicates the amount of data in the mbuf.
mh_type
Indicates the type of data contained in the mbuf (see below).
mh_flags
Flags (see below).

The mh_type variable can take the following values:

the mbuf should be on the free list.
the data in the mbuf was dynamically allocated.
the data contains a packet header.
the data is a socket name.
the data are socket options.
the data is a fragment reassembly header.
the mbuf contains extra-data protocol message.
the data consists of out-of-band data.

The mh_flags variable can take the following values:

mbuf has associated external storage.
the mbuf is the first that forms a packet.
end of record.
the external storage is a cluster.
protocol-specific.
packet send/received as link-level broadcast.
packet send/received as link-level multicast.
packet was encrypted (ESP-transport).
packet was authenticated (AH or ESP).
header was authenticated (AH).
header was IP-in-IP encapsulated by tunnel mode IPsec.
link layer specific flag.
for mbuf statistics.
dropped by bpf(4) filter.
m_pkthdr.ether_vtag variable is valid.

An external cluster is used when the data to hold in the mbuf is large. The size of an external cluster is between MCLBYTES and MAXMCLBYTES (also defined in ⟨sys/param.h⟩). A cluster should be used when the size of the data reach MINCLSIZE (the minimum size to be held by an external cluster).

The combination of the M_EXT and M_PKTHDR flags give four types of mbuf. When none of these constants are in use, the mbuf is a "normal" one, where the data part of the mbuf has the following elements:

m_dat
buffer holding the data (size MLEN).

When only M_PKTHDR is set, the data contained in the mbuf is a packet header. The data itself is contained in the mbuf (just like the previous case), but part of the mbuf is used to store a packet header. The data part has then the following elements:

m_pkthdr
packet header, containing the length of the data, a pointer to the interface on which the data was received, checksum information and list of mbuf_tags(9).
m_pktdat
buffer holding the data (size MHLEN).

The m_pkthdr.csum variable can take the following values:

IPv4 checksum needed.
TCP checksum needed.
UDP checksum needed.
IPv4 checksum verified.
IPv4 checksum bad.
TCP/IPv4 checksum verified.
TCP/IPv4 checksum bad.
UDP/IPv4 checksum verified.
UDP/IPv4 checksum bad.

When only M_EXT flag is set, an external storage buffer is being used to hold the data, which is no longer stored in the mbuf. The data part of the mbuf has now the following elements:

m_pkthdr
a packet header, just like the previous case, but it is empty. No information is stored here
m_ext
a structure containing information about the external storage buffer. The information consists of the address of the external buffer, a pointer to the function used to free the buffer, a pointer to the arguments of the function, the size of the buffer, the type of the buffer, and pointers to the previous and next mbufs using this cluster.

When both the M_EXT and M_PKTHDR flags are set, an external storage buffer is being used to store the data and this data contains a packet header. The structure used is the same as the previous one except that the m_pkthdr element is not empty, it contains the same information as when M_PKTHDR is used alone.

m_copym(struct mbuf *m, int off, int len, int wait)
Copy an mbuf chain starting at off bytes from the beginning and continuing for len bytes. If off is zero and m has the M_PKTHDR flag set, the header is copied. If len is M_COPYALL the whole mbuf is copied. The wait parameter can be M_WAIT or M_DONTWAIT. It does not copy clusters, it just increases their reference count.
m_copym2(struct mbuf *m, int off, int len, int wait)
The same as m_copym() except that it copies cluster mbufs, whereas m_copym() just increases the reference count of the clusters.
m_free(struct mbuf *m)
Free the mbuf pointed to by m. A pointer to the successor of the mbuf, if it exists, is returned by the function.
MFREE(struct mbuf *m, struct mbuf *n)
Free the mbuf pointed to by m and use n to point to the next mbuf in the chain if it exists. See m_free().
m_get(int how, int type)
Return a pointer to an mbuf of the type specified. If the how argument is M_WAITOK, the function may call tsleep(9) to await resources. If how is M_DONTWAIT and resources are not available, m_get() returns NULL.
MGET(struct mbuf *m, int how, int type)
Return a pointer to an mbuf in m of the type specified. See m_get() for a description of how.
m_getclr(int how, int type)
Return a pointer to an mbuf of the type specified, and clear the data area of the mbuf. See m_get() for a description of how.
m_gethdr(int how, int type)
Return a pointer to an mbuf of the type specified after initializing it to contain a packet header. See m_get() for a description of how.
MGETHDR(struct mbuf *m, int how, int type)
Return a pointer to an mbuf of the type specified after initializing it to contain a packet header. See m_get() for a description of how.
m_prepend(struct mbuf *m, int len, int how)
Allocate a new mbuf and prepend it to the mbuf chain pointed to by m. If m points to an mbuf with a packet header, it is moved to the new mbuf that has been prepended. The return value is a pointer on the new mbuf chain. If this function fails to allocate a new mbuf, m is freed. See m_get() for a description of how.

m_prepend() should never be called directly. Use M_PREPEND() instead.

M_PREPEND(struct mbuf *m, int plen, int how)
Prepend space of size plen to the mbuf pointed to by m. If a new mbuf must be allocated, how specifies whether to wait or not. If this function fails to allocate a new mbuf, m is freed.
m_pulldown(struct mbuf *m, int off, int len, int *offp)
Ensure that the data in the mbuf chain starting at off and ending at off+len will be put in a continuous memory region. If memory must be allocated, then it will fail if the len argument is greater than MAXMCLBYTES. The pointer returned points to an mbuf in the chain and the new offset for data in this mbuf is *offp. If this function fails, m is freed.
m_pullup(struct mbuf *n, int len)
Ensure that the data in the mbuf chain starting at the beginning of the chain and ending at len will be put in continuous memory region. If memory must be allocated, then it will fail if the len argument is greater than MAXMCLBYTES. If this function fails, n is freed.
m_split(struct mbuf *m0, int len0, int wait)
Split an mbuf chain in two pieces, returning a pointer to the tail (which is made of the previous mbuf chain except the first len0 bytes).
m_inject(struct mbuf *m0, int len0, int siz, int wait)
Inject a new mbuf chain of length siz into the mbuf chain pointed to by m0 at position len0. If there is enough space for an object of size siz in the appropriate location, no memory will be allocated. On failure, the function returns NULL (the mbuf is left untouched) and on success, a pointer to the first injected mbuf is returned.
m_getptr(struct mbuf *m, int loc, int *off)
Returns a pointer to the mbuf containing the data located at loc bytes of the beginning. The offset in the new mbuf is pointed to by off.
m_adj(struct mbuf *mp, int req_len)
Trims req_len bytes of data from the mbuf chain pointed to by mp. If req_len is positive, the data will be trimmed from the head of the mbuf chain and if it is negative, it will be trimmed from the tail of the mbuf chain.
m_copyback(struct mbuf *m0, int off, int len, caddr_t cp, int wait)
Copy data from a buffer pointed to by cp back into the mbuf chain pointed to by m0 starting at off bytes from the beginning, extending the mbuf chain if necessary, sleeping for mbufs if wait is M_WAIT. If M_NOWAIT is set and no mbufs are available, m_copyback() returns ENOBUFS. The mbuf chain must be initialized properly, including setting m_len.
m_freem(struct mbuf *m)
Free the mbuf chain pointed to by m.
m_reclaim(void)
Ask protocols to free unused memory space.
m_copydata(struct mbuf *m, int off, int len, caddr_t cp)
Copy data from the mbuf chain pointed to by m starting at off bytes from the beginning and continuing for len bytes into the buffer pointed to by cp.
m_cat(struct mbuf *m, struct mbuf *n)
Concatenate the mbuf chain pointed to by n to the mbuf chain pointed to by m. The mbuf chains must be of the same type.
m_devget(char *buf, int totlen, int off, struct ifnet *ifp, void (*func)(const void *, void *, size_t))
Copy totlen bytes of data from device local memory pointed to by buf using the function func or bcopy() if func is NULL. The data is copied into an mbuf chain at offset off and a pointer to the head of the chain is returned. Returns NULL on failure.
m_zero(struct mbuf *m)
Zeroize the data part of the mbufs in the mbuf chain pointed to by m.
m_apply(struct mbuf *m, int off, int len, int (*func)(caddr_t, caddr_t, unsigned int), caddr_t fstate)
Apply the function func to the data in the mbuf chain pointed to by m starting at off bytes from the beginning and continuing for len bytes.
mtod(struct mbuf *m, datatype)
Return a pointer to the data contained in the specified mbuf m cast to datatype.
MCLGET(struct mbuf *m, int how)
Allocate and add an mbuf cluster to the mbuf pointed to by m. On success, the flag M_EXT is set in the mbuf. See m_get() for a description of how.
MCLGETI(struct mbuf *m, int how, struct ifnet *ifp, int len)
If m is NULL, allocate it. Then allocate and add an mbuf cluster of length len to the mbuf pointed to by m. If ifp is passed in, then per-interface accounting for the mbuf will occur, and thus mbuf allocation can fail when limits are reached. Returns either the mbuf m that was passed in, or the newly allocated one which was allocated; in either case the flag M_EXT is set in the mbuf. See m_get() for a description of how.
MEXTADD(struct mbuf *m, caddr_t buf, u_int size, int type, void (*free)(caddr_t, u_int, void *), void *arg)
Add pre-allocated storage to the mbuf pointed to by m. On success, the flag M_EXT is set in the mbuf.
M_ALIGN(struct mbuf *m, int len)
Set the m_data pointer of the newly allocated mbuf with m_get() or MGET() pointed to by m to an object of the specified size len at the end of the mbuf, longword aligned.
MH_ALIGN(m, len)
Same as M_ALIGN() except it is for an mbuf allocated with m_gethdr() or MGETHDR().
M_READONLY(struct mbuf *m)
Check if the data of the mbuf pointed to by m is read-only. This is true for non-cluster external storage and for clusters that are being referenced by more than one mbuf.
M_LEADINGSPACE(struct mbuf *m)
Compute the amount of space available before the current start of data in the mbuf pointed to by m.
M_TRAILINGSPACE(struct mbuf *m)
Compute the amount of space available after the end of data in the mbuf pointed to by m.
m_dup_pkthdr(struct mbuf *to, struct mbuf *from, int how)
Copy mbuf packet header, including mbuf tags, from from to to. See m_get() for a description of how.

The mbuf management functions are implemented in the files sys/kern/uipc_mbuf.c and sys/kern/uipc_mbuf2.c. The function prototypes and the macros are located in sys/sys/mbuf.h.

netstat(1), mbuf_tags(9)

Jun-Ichiro Hagino, Mbuf issues in 4.4BSD IPv6/IPsec support (experiences from KAME IPv6/IPsec implementation), Proceedings of the Freenix Track: 2000 USENIX Annual Technical Conference, June 2000.

December 2, 2011 OpenBSD-5.1