Mbuf allocation macros

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

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

MCLGET(struct mbuf *mbuf, int how);

MEXTADD(struct mbuf *mbuf, caddr_t buf, u_int size, void (*free)(void *opt_arg1, void *opt_arg2), void *opt_arg1, void *opt_arg2, short 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, u_int type);

int
M_WRITABLE(struct mbuf *mbuf);

Mbuf allocation functions

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

struct mbuf *
m_getm(struct mbuf *orig, int len, int how, int type);

struct mbuf *
m_getcl(int how, short type, int flags);

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

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

u_int
m_fixhdr(struct mbuf *mbuf);

void
m_dup_pkthdr(struct mbuf *to, struct mbuf *from);

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);

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 pointer of 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.

Historical mbuf allocator (See HISTORY section) used allocation flags M_WAIT and M_DONTWAIT. These constants are kept for compatibility and their use in new code is discouraged.

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 passed unmodified to free. 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. If the macro fails, the M_EXT flag will not be set in mbuf.

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_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_getclr(how, type)

Allocate an mbuf and zero out the data region.

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 offp is NULL, the region may be accessed using mtod(mbuf, type). If offp is non-NULL, the region may be accessed using mtod(mbuf, uint8_t, +, *offsetp). The region of the mbuf chain between its beginning and off 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 still valid after the function returned. Note: It does not handle M_PKTHDR and friends.

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.