mbuf -- memory management in the kernel IPC subsystem
#include <sys/param.h>
#include <sys/systm.h>
#include <sys/mbuf.h>
Mbuf allocation macros [Toc] [Back]
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_args), void *opt_args, short flags, int type);
MEXTFREE(struct mbuf *mbuf);
MEXT_ADD_REF(struct mbuf *mbuf);
MEXT_REM_REF(struct mbuf *mbuf);
MFREE(struct mbuf *mbuf, struct mbuf *successor);
Mbuf utility macros [Toc] [Back]
void *
mtod(struct mbuf *mbuf, type);
int
MEXT_IS_REF(struct mbuf *mbuf);
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 [Toc] [Back]
struct mbuf *
m_get(int how, int type);
struct mbuf *
m_getm(struct mbuf *orig, int len, int how, int type);
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 [Toc] [Back]
void
m_adj(struct mbuf *mbuf, int len);
struct mbuf *
m_prepend(struct mbuf *mbuf, int len, int how);
struct mbuf *
m_pullup(struct mbuf *mbuf, int len);
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);
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 machine-dependent constant
defined in <machine/param.h>. The mbuf header includes:
m_next a pointer to the next mbuf in the mbuf chain
m_nextpkt a pointer to the next mbuf chain in the queue
m_data a pointer to the data
m_len the length of the data
m_type the type of data
m_flags the mbuf flags
The mbuf flag bits are defined as follows:
/* mbuf flags */
#define M_EXT 0x0001 /* has associated external storage */
#define M_PKTHDR 0x0002 /* start of record */
#define M_EOR 0x0004 /* end of record */
#define M_RDONLY 0x0008 /* associated data marked read-only */
#define M_PROTO1 0x0010 /* protocol-specific */
#define M_PROTO2 0x0020 /* protocol-specific */
#define M_PROTO3 0x0040 /* protocol-specific */
#define M_PROTO4 0x0080 /* protocol-specific */
#define M_PROTO5 0x0100 /* protocol-specific */
/* mbuf pkthdr flags, also in m_flags */
#define M_BCAST 0x0200 /* send/received as link-level broadcast */
#define M_MCAST 0x0400 /* send/received as link-level multicast */
#define M_FRAG 0x0800 /* packet is fragment of larger packet */
#define M_FIRSTFRAG 0x1000 /* packet is first fragment */
#define M_LASTFRAG 0x2000 /* packet is last fragment */
The available mbuf types are defined as follows:
/* mbuf types */
#define MT_FREE 0 /* should be on free list */
#define MT_DATA 1 /* dynamic (data) allocation */
#define MT_HEADER 2 /* packet header */
#define MT_SONAME 8 /* socket name */
#define MT_FTABLE 11 /* fragment reassembly header */
#define MT_CONTROL 14 /* extra-data protocol message */
#define MT_OOBDATA 15 /* expedited data */
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).
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 developer can check whether the reference count for the
external storage of a given mbuf is greater than 1 with the MEXT_IS_REF
macro. Similarly, the developer can directly add and remove references,
if absolutely necessary, with the use of the MEXT_ADD_REF and
MEXT_REM_REF macros.
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's equal to the sum of MLEN and MHLEN. 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 [Toc] [Back]
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_TRYWAIT or M_DONTWAIT.
It specifies whether the caller is willing to block if necessary.
If how is set to M_TRYWAIT, a failed allocation will result in the
caller being put to sleep for a designated kern.ipc.mbuf_wait
(sysctl(8) tunable) number of ticks. 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.
Programmers should be careful not to confuse the mbuf allocation
flag M_DONTWAIT with the malloc(9) allocation flag, M_NOWAIT. They
are not the same.
MGETHDR(mbuf, how, type)
Allocate an mbuf and initialize it to contain a packet header and
internal data. See MGET() for details.
MCLGET(mbuf, how)
Allocate and attach an mbuf cluster to mbuf. If the macro fails,
the M_EXT flag won't 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_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_getclr(how, type)
Allocate an mbuf and zero out the data region.
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_prepend(mbuf, len, how)
Allocate a new mbuf and prepend it to the mbuf chain, handle
M_PKTHDR properly. Note: It doesn't allocate any mbuf clusters, so
len must be less than MLEN or MHLEN, depending on the M_PKTHDR flag
setting.
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). Return the new mbuf chain on success, NULL on
failure (the mbuf chain is freed in this case). Note: It doesn't
allocate any mbuf clusters, so len must be less than MHLEN.
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 doesn't
allocate any mbuf clusters, just adds mbufs to the mbuf chain.
It's 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.
When running a kernel compiled with the option MBUF_STRESS_TEST, the following
sysctl(8)-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.
See above.
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.
The original mbuf man page was written by Yar Tikhiy.
FreeBSD 5.2.1 October 17, 2000 FreeBSD 5.2.1 [ Back ] |
