path: root/share/man/man9/mbuf.9

.\"     $OpenBSD: mbuf.9,v 1.4 2002/02/16 21:27:39 millert Exp $
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.\" Copyright (c) 2001 Jean-Jacques Bernard-Gundol <jjbg@openbsd.org>
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.Dd December 4, 2001
.Dt MBUF 9
.Os
.Sh NAME
.Nm mbuf
.Nd Kernel memory management for networking protocols
.Sh SYNOPSIS
.Fd #include <sys/mbuf.h>
.Ft struct mbuf *
.Fn m_copym2 "struct mbuf *m" "int off0" "int len" "int wait"
.Ft struct mbuf *
.Fn m_copym "struct mbuf *m" "int off0" "int len" "int wait"
.Ft struct mbuf *
.Fn m_free "struct mbuf *m"
.Fn MFREE "struct mbuf *m" "struct mbuf *n"
.Ft struct mbuf *
.Fn m_get "int nowait" "int type"
.Fn MGET "struct mbuf *m" "int how" "int type"
.Ft struct mbuf *
.Fn m_getclr "int nowait" "int type"
.Ft struct mbuf *
.Fn m_gethdr "int nowait" "int type"
.Fn MGETHDR "struct mbuf *m" "int how" "int type"
.Ft struct mbuf *
.Fn m_prepend "struct mbuf *m" "int len" "int how"
.Fn M_PREPEND "struct mbuf *m" "int plen" "int how"
.Ft struct mbuf *
.Fn m_pulldown "struct mbuf *m" "int off" "int len" "int *offp"
.Ft struct mbuf *
.Fn m_pullup "struct mbuf *n" "int len"
.Ft struct mbuf *
.Fn m_pullup2 "struct mbuf *n" "int len"
.Ft struct mbuf *
.Fn m_retry "int i" "int t"
.Ft struct mbuf *
.Fn m_retryhdr "int i" "int t"
.Ft struct mbuf *
.Fn m_split "struct mbuf *m0" "int len0" "int wait"
.Ft struct mbuf *
.Fn m_inject "struct mbuf *m0" "int len0" "int siz" "int wait"
.Ft struct mbuf *
.Fn m_getptr "struct mbuf *m" "int loc" "int *off"
.Ft void
.Fn m_adj "struct mbuf *mp" "int req_len"
.Ft void
.Fn m_copyback "struct mbuf *m0" "int off" "int len" "caddr_t cp"
.Ft void
.Fn m_freem "struct mbuf *m"
.Ft void 
.Fn m_reclaim "void"
.Ft void
.Fn m_copydata "struct mbuf *m" "int off" "int len" "caddr_t cp"
.Ft void
.Fn m_cat "struct mbuf *m" "struct mbuf *n"
.Ft struct mbuf *
.Fn m_devget "char *buf" "int totlen" "int off0" "struct ifnet *ifp" \
"void (*func)(const void *, void *, size_t)"
.Ft void
.Fn m_zero "struct mbuf *m"
.Ft int
.Fn m_apply "struct mbuf *m" "int off" "int len" \
"int (*func)(caddr_t, caddr_t, unsigned int)" "caddr_t fstate"
.Fn MEXTALLOC "struct mbuf *m" "int size" "int how"
.Fn MCLGET "struct mbuf *m" "int how"
.Fn MEXTADD "struct mbuf *m" "caddr_t buf" "int type" \
"void (*free)(caddr_t, u_int, void *)" "void *arg"
.Fn M_ALIGN "struct mbuf *m" "int len"
.Fn MH_ALIGN "struct mbuf *m" "int len"
.Fn M_READONLY "struct mbuf *m"
.Fn M_LEADINGSPACE "struct mbuf *m"
.Fn M_TRAILINGSPACE "struct mbuf *m"
.Fn MCHTYPE "struct mbuf *m" "int type"
.Pp
.Bd -literal
#define MLEN            (MSIZE - sizeof(struct m_hdr))
#define MHLEN           (MLEN - sizeof(struct pkthdr))

#define MINCLSIZE       (MHLEN + 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;
        short   mh_flags;
};

struct  pkthdr {
        struct  ifnet *rcvif;
        int     len;
        void    *tdbi;

};

struct m_ext {
        caddr_t ext_buf;
        void    (*ext_free)
(struct mbuf *);
        u_int   ext_size;
        void    (*ext_ref)
(struct mbuf *);
        void    *ext_handle;
};

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;
};

union mcluster {
        union   mcluster *mcl_next;
        char    mcl_buf[MCLBYTES];
};

#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
.Ed
.Sh DESCRIPTION
The
.Nm
functions provide a way to manage the memory buffers used by the
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
.Nm
is MSIZE
.Pq defined in Aq Pa machine/param.h .
.Pp
An
.Nm
structure is defined as an
.Fa m_hdr
structure followed by a
union.
The header contains the following elements:
.Bl -tag -width foobarmoocow
.It Fa mh_next
A pointer to the next mbuf in the mbuf chain.
.It Fa mh_nextpkt
A pointer to the next mbuf chain (i.e., packet) in the queue.
.It Fa mh_data
Indicates the address of the beginning of data in the mbuf.
.It Fa mh_type
Indicates the type of data contained in the mbuf (see below).
.It Fa mh_flags
Flags (see below).
.El
.Pp
The
.Fa mh_type
variable can take the following values:
.Bl -tag -compact -offset indent -width XXXXXXXXXXXXXXXXXX
.It Dv MT_FREE
the mbuf should be on the free list.
.It Dv MT_DATA
the data in the mbuf was dynamically allocated.
.It Dv MT_HEADER
the data contains a packet header.
.It Dv MT_SONAME
the data is a socket name.
.It Dv MT_SOOPTS
the data are socket options.
.It Dv MT_FTABLE
the data is a fragment reassembly header.
.It Dv MT_CONTROL
the mbuf contains extra-data protocol message.
.It Dv MT_OOBDATA
the data consists of out-of-banddata.
.El
.Pp
The
.Fa mh_flags
variable can take the following values:
.Bl -tag -compact -offset indent -width XXXXXXXXXXXXXXXXXX
.It Dv M_EXT
mbuf has associated external storage.
.It Dv M_PKTHDR
the mbuf is the first that forms a packet.
.It Dv M_EOR
end of record.
.It Dv M_CLUSTER
the external storage is a cluster.
.It Dv M_PROTO1
protocol-specific.
.It Dv M_BCAST
packet send/received as link-level broadcast.
.It Dv M_MCAST
packet send/received as link-level multicast.
.It Dv M_CONF
packet was encrypted (ESP-transport).
.It Dv M_AUTH
packet was authenticated (AH).
.It Dv M_COMP
packet was compressed (IPCOMP).
.It Dv M_IPV4_CSUM_OUT
IPv4 checksum needed.
.It Dv M_TCPV4_CSUM_OUT
TCP checksum needed.
.It Dv M_UDPV4_CSUM_OUT
UDP checksum needed.
.It Dv M_IPV4_CSUM_IN_OK
IPv4 checksum verified.
.It Dv M_IPV4_CSUM_IN_BAD
IPv4 checksum bad.
.It Dv M_TCP_CSUM_IN_OK
TCP/IPv4 checksum verified.
.It Dv M_TCP_CSUM_IN_BAD
TCP/IPv4 checksum bad.
.It Dv M_UDP_CSUM_IN_OK
UDP/IPv4 checksum verified.
.It Dv M_UDP_CSUM_IN_BAD
UDP/IPv4 checksum bad.
.It Dv M_ANYCAST6
received as IPv6 anycast.
.It Dv M_LOOP
for mbuf statistics.
.El
.Pp
An external cluster is used when the data to hold in the mbuf is
large.
The size of an external cluster is MCLBYTES
.Pq also defined in Aq Pa machine/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).
.Pp
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:
.Bl -tag -width foobarmoocow
.It Fa m_dat
buffer holding the data (size MLEN).
.El
.Pp
When the M_PKTHDR is set, it means that the data contained in the mbuf
is a packet header.
The data itself is contained in the mbuf (just like the previous case)
but 8 bytes are taken from the data portion to store a packet
header.
The data part has then the following elements:
.Bl -tag -width foobarmoocow
.It Fa m_pkthdr
packet header, containing the length of the data, a pointer to the
interface onwhich the data was received and a generic pointer to a
structure containing information for IPsec processing. 
.It Fa m_pktdat
buffer holding the data (size MHLEN).
.El
.Pp
When the M_EXT flag is set, it means that an external storage buffer
is used to hold the data, which is no more stored in the mbuf.
The data part of the mbuf has now the following elements:
.Bl -tag -width foobarmoocow
.It Fa m_pkthdr
a packet header, just like the previous case, but it is empty.
No information is stored here
.It Fa 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.
.El
.Pp
When both the M_EXT and M_PKTHDR flags are set, it means that an
external storage buffer is used to store the data and this data
contains a packet header.
The structure used is the same than the previous one except that the
.Fa m_pkthdr
element is not empty, it contains the same information than when
M_PKTHDR is used alone.
.Pp
.Bl -tag -width compact
.It Fn m_copym "struct mbuf *m" "int off0" "int len" "int wait"
Make a copy of an mbuf chain starting at
.Fa off0
bytes from the beginning
and continuing for
.Fa len
bytes.
The
.Fa wait
parameter can be M_WAIT or
M_DONTWAIT.
It does not copy clusters, it just increases their reference count.
.It Fn m_copym2 "struct mbuf *m" "int off0" "int len" "int wait"
The same as
.Fn m_copym
except that it copies cluster mbufs, where
.Fn m_copym
just increase the reference count of the clusters.
.It Fn m_free "struct mbuf *m"
Free the mbuf pointed to by
.Fa m .
A pointer to the successor of the mbuf,
if it exists, is returned by the function.
.It Fn MFREE "m" "n"
Free the mbuf pointed to by
.Fa m
and use
.Fa n
to point to the next mbuf in
the chain if it exists.
See
.Fn m_free .
.It Fn m_get "int nowait" "int type"
Return a pointer to an mbuf of the type specified.
.It Fn MGET "m" "how" "type"
Return a pointer to an mbuf in
.Fa m
of the type specified.
See
.Fn m_get .
.It Fn m_getclr "int nowait" "int type"
Return a pointer to an mbuf of the type specified, and clear the data
area of the mbuf.
.It Fn m_gethdr "int nowait" "int type"
Return a pointer to an mbuf after initializing it to contain a packet
header.
The type is specified.
.It Fn MGETHDR "m" "int how" "int type"
Return a pointer to an mbuf in m after initializing it to contain a
packet header.
See
.Fn m_gethdr .
.It Fn m_prepend "struct mbuf *m" "int len" "int how"
Allocates a new mbuf and prepend it to the mbuf chain pointed to by
.Fa m .
If
.Fa 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.
.It Fn M_PREPEND "m" "plen" "how"
Prepend space of size
.Fa plen
to the mbuf pointed to by
.Fa m .
If a new mbuf must be allocated,
.Fa how
specifies whether to wait or not.
.It Fn m_pulldown "struct mbuf *m" "int off" "int len" "int *offp"
Ensure that the data in the mbuf chain starting at
.Fa off
and ending at
.Fa off+len
will be put in a continuous memory region.
.Fa len
must be smaller or equal than MCLBYTES.
The pointer returned points to an mbuf in the chain and the new offset
for data in this mbuf is
.Fa *offp .
.It Fn 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
.Fa len
will be put in continuous memory region.
The
.Fa len
argument must be smaller or equal than MHLEN.
.It Fn m_pullup2 "struct mbuf *n" "int len"
Just like
.Fn m_pullup ,
ensure that the data starting at the beginning of the mbuf chain and
ending at
.Fa len
will be put in continuous memory region.
The
.Fa len
argument can be up to MCLBYTES.
.Fn m_pullup2
will simply call
.Fn m_pullup
if
.Fa len
is smaller or equal to MHLEN.
.It Fn m_retry "int i" "int t"
Used when
.Fn MGET
fails.
.Fn m_retry
asks protocols to free memory space and re-attempts to allocate an mbuf.
.It Fn m_retryhdr "int i" "int t"
Same as
.Fn m_retry
except it retries a failed
.Fn MRETRYHDR .
.It Fn m_split "struct mbuf *m0" "int len0" "int wait"
Attempts to split an mbuf chain in 2 pieces, returning a pointer on
the tail (which is made of the previous mbuf chain but the first
.Fa len0
bytes).
.It Fn m_inject "struct mbuf *m0" "int len0" "int siz" "int wait"
Injects a new mbuf chain of length
.Fa siz
in the mbuf chain pointed to by
.Fa m0
at position
.Fa len0 .
If there is enough space for an object of size
.Fa 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.
.It Fn m_getptr "struct mbuf *m" "int loc" "int *off"
Returns a pointer to the mbuf containing the data located at
.Fa loc
bytes of the beginning.
The offset in the new mbuf is pointed to by
.Fa off .
.It Fn m_adj "struct mbuf *mp" "int req_len"
Trims
.Fa req_len
bytes of data from the mbuf chain pointed to by
.Fa mp .
If
.Fa 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.
.It Fn m_copyback "struct mbuf *m0" "int off" "int len" "caddr_t cp"
Copy data from a buffer back into the mbuf chain pointed to by
.Fa m0
starting at
.Fa off
bytes from the beginning and extending the mbuf chain if
necessary.
The mbufs need to be initialized properly, including the setting of
.Fa m_len .
.It Fn m_freem "struct mbuf *m"
Free the mbuf chain pointed to by
.Fa m .
.It Fn m_reclaim "void"
Ask protocols to free unused memory space.
.It Fn m_copydata "struct mbuf *m" "int off" "int len" "caddr_t cp"
Copy data from the mbuf chain pointed to by
.Fa m
starting at
.Fa off
bytes from the beginning and continuing for
.Fa len
bytes into the buffer pointed to by
.Fa cp .
.It Fn m_cat "struct mbuf *m" "struct mbuf *n"
Concatenate the mbuf chain pointed to by
.Fa n
to the mbuf chain pointed to by
.Fa m .
The mbuf chains must be of the same type.
.It Fn m_devget "char *buf" "int totlen" "int off0" "struct ifnet *ifp" \
"void (*func)(const void *, void *, size_t)"
Copy
.Fa totlen
bytes of data from device local memory pointed to by
.Fa buf
using the function
.Fa func .
The data is copied into an mbuf chain and a pointer to the head of it
is returned.
If
.Fa off0
is non-zero, it means the packet is trailer-encapsulated and
.Fa off0
bytes plus the type and length fields will be skipped before doing the
copy.
.It Fn m_zero "struct mbuf *m"
Zeroize the data part of the mbufs in the mbuf chain pointed to by
.Fa m
.It Fn m_apply "struct mbuf *m" "int off" "int len" \
"int (*func)(caddr_t, caddr_t, unsigned int)" "caddr_t fstate"
Apply the function
.Fa func
to the data in the mbuf chain pointed to by
.Fa m
starting at
.Fa off
bytes from the beginning and continuing for
.Fa len
bytes.
.It Fn mtod "struct mbuf *m" "datatype"
Return a pointer to the data contained in the specified mbuf
.Fa m
casted to
.Fa datatype .
.It Fn MCLGET "struct mbuf *m" "int how"
Allocate and add an mbuf cluster to the mbuf pointed to by
.Fa m .
On success, the flag M_EXT is set in the mbuf.
.It Fn MEXTALLOC "struct mbuf *m" "int size" "int how"
Allocate external storage of size
.Fa size
and add it to the mbuf pointed to by
.Fa m .
On success, the flag M_EXT is set in the mbuf.
.It Fn MEXTADD "struct mbuf *m" "caddr_t buf" "int type" \
"void (*free)(caddr_t, u_int, void *)" "void *arg"
Add pre-allocated storage to the mbuf pointed to by
.Fa m .
On success, the flag M_EXT is set in the mbuf.
.It Fn M_ALIGN "m" "len"
Set the
.Fa m_data
pointer of the newly allocated mbuf with
.Fn m_get
or
.Fn MGET
pointed to by
.Fa m
to an object of the specified size
.Fa len
at the end of the mbuf, longword aligned.
.It Fn MH_ALIGN "m" "len"
Same as
.Fn M_ALIGN
except it is for an mbuf allocated with
.Fn m_gethdr
or
.Fn MGETHDR .
.It Fn M_READONLY "m"
Check if the data of the mbuf pointed to by
.Fa m
is read-only.
This is true for non-cluster external storage and for clusters that
are being referenced by more than one mbuf.
.It Fn M_LEADINGSPACE "m"
Compute the amount of space available before the current start of data
in the mbuf pointed to by
.Fa m .
.It Fn M_TRAILINGSPACE "m"
Compute the amount of space available after the end of data in the
mbuf pointed to by
.Fa m .
.It Fn MCHTYPE "m" "type"
Change the type of the mbuf pointed to by
.Fa m
to
.Fa type .
.El
.Sh SEE ALSO
.Xr netstat 1 ,
.Xr mbuf_tags 9 ,
.Pa /usr/share/doc/smm/18.net.
.Rs
.%A Jun-Ichiro Hagino
.%T "Mbuf issues in 4.4BSD IPv6/IPsec support (experiences from KAME IPv6/IPsec implementation)"
.%B "Proceedings of the freenix track: 2000 USENIX annual technical conference"
.%D June 2000
.Re
.Sh CODE REFERENCES
The mbuf management functions are implemented in the files
.Pa sys/kern/uipc_mbuf.c
and
.Pa sys/kern/uipc_mbuf2.c .
The function prototypes and the macros are located in
.Pa sys/sys/mbuf.h .