MBUF(9) BSD Kernel Developer's Manual MBUF(9)NAME
mbuf — memory management in the kernel IPC subsystem
SYNOPSIS
#include <sys/param.h>
#include <sys/systm.h>
#include <sys/mbuf.h>
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);
Mbuf utility macros
void *
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_PREPEND(struct mbuf *mbuf, int len, int how);
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_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);
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);
struct mbuf *
m_split(struct mbuf *mbuf, int len, 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 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 mbuf's general structure.
An mbuf consists of a variable-sized header and a small internal buffer
for data. The mbuf's total size, MSIZE, is a machine-dependent constant
defined in <machine/param.h>. The mbuf header includes:
m_next a pointer to the next buffer in the chain
m_nextpkt a pointer to the next 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_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 mbuf's internal data buffer. If
the data is sufficiently large, another mbuf may be added to the 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 other‐
wise.
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 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 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 a clus‐
ter. It's equal to the sum of MLEN and MHLEN. It is typically prefer‐
able to store data into an mbuf's data region, 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 pointer of the specified type. Note:
It is advisable to ensure that there is enough contiguous data in
the 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 MB_WAIT or MB_DONTWAIT.
It specifies whether the caller is willing to block if necessary.
If how is set to MB_WAIT, 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 mbuf-
related functions and macros 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 MB_DONTWAIT with the kmalloc(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 an mbuf. If the macro
fails, the M_EXT flag won't be set in the 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
chain pointer or NULL is in mbuf after the call.
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 chain to the orig mbuf chain, if
it is non-NULL. If the allocation fails at any point, free what‐
ever 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 chain, handle M_PKTHDR
properly. Note: It doesn't allocate any 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 chain on success, NULL on failure
(the chain is freed in this case). Note: It doesn't allocate any
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 begin‐
ning, continuing for len bytes. If len is M_COPYALL, copy to the
end of the mbuf chain. Note: The copy is read-only, because clus‐
ters 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 clusters are
not copied, only their reference counts are incremented.
m_dup(mbuf, how)
Copy a packet header mbuf chain into a completely new 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 chain,
extending the mbuf chain if necessary. Note: It doesn't allocate
any clusters, just adds mbufs to the chain. It's safe to set
offset beyond the current chain end: zeroed mbufs will be allocated
to fill the space.
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 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 chain to its original state.
STRESS TESTING
When running a kernel compiled with the option MBUF_STRESS_TEST, the fol‐
lowing sysctl(8)-controlled options may be used to create various fail‐
ure/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 driv‐
ers.
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.
HISTORY
Mbufs appeared in an early version of BSD. Besides for being used for
network packets, they were used to store various dynamic structures, such
as routing table entries, interface addresses, protocol control blocks,
etc.
AUTHORS
The original mbuf man page was written by Yar Tikhiy.
BSD August 18, 2007 BSD
