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);
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);
MEXTFREE
(
struct
mbuf *mbuf);
MFREE
(
struct
mbuf *mbuf,
struct mbuf
*successor);
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_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 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 */
#define M_PROTO6 0x4000 /* protocol-specific (avoid M_BCAST conflict) */
#define M_FREELIST 0x8000 /* mbuf is on the free list */
/* mbuf pkthdr flags (also stored 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_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 (M_IOVEC) */
#define EXT_NET_DRV 100 /* custom ext_buf provided by net driver(s) */
#define EXT_MOD_TYPE 200 /* custom module's ext_buf type */
#define EXT_DISPOSABLE 300 /* can throw this buffer away w/page flipping */
#define EXT_EXTREF 400 /* 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) 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 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_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)) 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)-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),
mbuf_tags(9)
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) 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), 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)
mbuf allocator was written by Bosko
Milekic.