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Version: 365019 (ubuntu - 25/10/10)

Section: 9 (Appels noyau Linux)


BSD mandoc

NAME

mbuf - memory management in the kernel IPC subsystem

SYNOPSIS

In sys/param.h In sys/systm.h In sys/mbuf.h

Mbuf allocation macros

Fn MGET struct mbuf *mbuf int how short type Fn MGETHDR struct mbuf *mbuf int how short type Fn MCLGET struct mbuf *mbuf int how Fo MEXTADD Fa struct mbuf *mbuf Fa caddr_t buf Fa u_int size Fa void (*free)(void *opt_arg1, void *opt_arg2) Fa void *opt_arg1 Fa void *opt_arg2 Fa short flags Fa int type Fc Fn MEXTFREE struct mbuf *mbuf Fn MFREE struct mbuf *mbuf struct mbuf *successor

Mbuf utility macros

Fn mtod struct mbuf *mbuf type Fn M_ALIGN struct mbuf *mbuf u_int len Fn MH_ALIGN struct mbuf *mbuf u_int len Ft int Fn M_LEADINGSPACE struct mbuf *mbuf Ft int Fn M_TRAILINGSPACE struct mbuf *mbuf Fn M_MOVE_PKTHDR struct mbuf *to struct mbuf *from Fn M_PREPEND struct mbuf *mbuf int len int how Fn MCHTYPE struct mbuf *mbuf u_int type Ft int Fn M_WRITABLE struct mbuf *mbuf

Mbuf allocation functions

Ft struct mbuf * Fn m_get int how int type Ft struct mbuf * Fn m_getm struct mbuf *orig int len int how int type Ft struct mbuf * Fn m_getcl int how short type int flags Ft struct mbuf * Fn m_getclr int how int type Ft struct mbuf * Fn m_gethdr int how int type Ft struct mbuf * Fn m_free struct mbuf *mbuf Ft void Fn m_freem struct mbuf *mbuf

Mbuf utility functions

Ft void Fn m_adj struct mbuf *mbuf int len Ft void Fn m_align struct mbuf *mbuf int len Ft int Fn m_append struct mbuf *mbuf int len c_caddr_t cp Ft struct mbuf * Fn m_prepend struct mbuf *mbuf int len int how Ft struct mbuf * Fn m_copyup struct mbuf *mbuf int len int dstoff Ft struct mbuf * Fn m_pullup struct mbuf *mbuf int len Ft struct mbuf * Fn m_pulldown struct mbuf *mbuf int offset int len int *offsetp Ft struct mbuf * Fn m_copym struct mbuf *mbuf int offset int len int how Ft struct mbuf * Fn m_copypacket struct mbuf *mbuf int how Ft struct mbuf * Fn m_dup struct mbuf *mbuf int how Ft void Fn m_copydata const struct mbuf *mbuf int offset int len caddr_t buf Ft void Fn m_copyback struct mbuf *mbuf int offset int len caddr_t buf Ft struct mbuf * Fo m_devget Fa char *buf Fa int len Fa int offset Fa struct ifnet *ifp Fa void (*copy)(char *from, caddr_t to, u_int len) Fc Ft void Fn m_cat struct mbuf *m struct mbuf *n Ft u_int Fn m_fixhdr struct mbuf *mbuf Ft void Fn m_dup_pkthdr struct mbuf *to struct mbuf *from Ft void Fn m_move_pkthdr struct mbuf *to struct mbuf *from Ft u_int Fn m_length struct mbuf *mbuf struct mbuf **last Ft struct mbuf * Fn m_split struct mbuf *mbuf int len int how Ft int Fn m_apply struct mbuf *mbuf int off int len int (*f)(void *arg, void *data, u_int len) void *arg Ft struct mbuf * Fn m_getptr struct mbuf *mbuf int loc int *off Ft struct mbuf * Fn m_defrag struct mbuf *m0 int how Ft struct mbuf * Fn m_unshare struct mbuf *m0 int how

DESCRIPTION

An Vt mbuf is a basic unit of memory management in the kernel IPC subsystem. Network packets and socket buffers are stored in Vt mbufs . A network packet may span multiple Vt mbufs arranged into a Vt mbuf chain (linked list), which allows adding or trimming network headers with little overhead.

While a developer should not bother with Vt mbuf internals without serious reason in order to avoid incompatibilities with future changes, it is useful to understand the general structure of an Vt mbuf .

An Vt mbuf consists of a variable-sized header and a small internal buffer for data. The total size of an Vt mbuf , MSIZE is a constant defined in In sys/param.h . The Vt mbuf header includes:

m_next
(Vt struct mbuf ) A pointer to the next Vt mbuf in the Vt mbuf chain .
m_nextpkt
(Vt struct mbuf ) A pointer to the next Vt mbuf chain in the queue.
m_data
(Vt caddr_t ) A pointer to data attached to this Vt mbuf .
m_len
(Vt int ) The length of the data.
m_type
(Vt short ) The type of the data.
m_flags
(Vt int ) The Vt mbuf flags.

The Vt 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 Vt 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 */
 

If the M_PKTHDR flag is set, a Vt struct pkthdr Va m_pkthdr is added to the Vt mbuf header. It contains a pointer to the interface the packet has been received from (Vt struct ifnet *rcvif ) and the total packet length (Vt int len ) Optionally, it may also contain an attached list of packet tags (Vt struct m_tag ) See mbuf_tags9 for details. Fields used in offloading checksum calculation to the hardware are kept in m_pkthdr as well. See Sx HARDWARE-ASSISTED CHECKSUM CALCULATION for details.

If small enough, data is stored in the internal data buffer of an Vt mbuf . If the data is sufficiently large, another Vt mbuf may be added to the Vt mbuf chain , or external storage may be associated with the Vt mbuf . MHLEN bytes of data can fit into an Vt mbuf with the M_PKTHDR flag set, MLEN bytes can otherwise.

If external storage is being associated with an Vt 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 Vt 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 Vt mbuf cluster . Vt Mbuf clusters can be allocated and configured with the use of the MCLGET macro. Each Vt 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 Vt mbuf cluster . It is equal to the sum of MLEN and MHLEN It is typically preferable to store data into the data region of an Vt mbuf , if size permits, as opposed to allocating a separate Vt mbuf cluster to hold the same data.

Macros and Functions

There are numerous predefined macros and functions that provide the developer with common utilities.
Fn mtod mbuf type
Convert an Fa mbuf pointer to a data pointer. The macro expands to the data pointer cast to the pointer of the specified Fa type . Note It is advisable to ensure that there is enough contiguous data in Fa mbuf . See Fn m_pullup for details.
Fn MGET mbuf how type
Allocate an Vt mbuf and initialize it to contain internal data. Fa mbuf will point to the allocated Vt mbuf on success, or be set to NULL on failure. The Fa how argument is to be set to M_WAIT or M_DONTWAIT It specifies whether the caller is willing to block if necessary. A number of other functions and macros related to Vt mbufs have the same argument because they may at some point need to allocate new Vt mbufs .

Programmers should be careful not to confuse the Vt mbuf allocation flag M_DONTWAIT with the malloc(9) allocation flag, M_NOWAIT They are not the same.

Fn MGETHDR mbuf how type
Allocate an Vt mbuf and initialize it to contain a packet header and internal data. See Fn MGET for details.
Fn MCLGET mbuf how
Allocate and attach an Vt mbuf cluster to Fa mbuf . If the macro fails, the M_EXT flag will not be set in Fa mbuf .
Fn M_ALIGN mbuf len
Set the pointer Fa mbuf->m_data to place an object of the size Fa len at the end of the internal data area of Fa mbuf , long word aligned. Applicable only if Fa mbuf is newly allocated with Fn MGET or Fn m_get .
Fn MH_ALIGN mbuf len
Serves the same purpose as Fn M_ALIGN does, but only for Fa mbuf newly allocated with Fn MGETHDR or Fn m_gethdr , or initialized by Fn m_dup_pkthdr or Fn m_move_pkthdr .
Fn m_align mbuf len
Services the same purpose as Fn M_ALIGN but handles any type of mbuf.
Fn M_LEADINGSPACE mbuf
Returns the number of bytes available before the beginning of data in Fa mbuf .
Fn M_TRAILINGSPACE mbuf
Returns the number of bytes available after the end of data in Fa mbuf .
Fn M_PREPEND mbuf len how
This macro operates on an Vt mbuf chain . It is an optimized wrapper for Fn m_prepend that can make use of possible empty space before data (e.g. left after trimming of a link-layer header). The new Vt mbuf chain pointer or NULL is in Fa mbuf after the call.
Fn M_MOVE_PKTHDR to from
Using this macro is equivalent to calling Fn m_move_pkthdr to from .
Fn M_WRITABLE mbuf
This macro will evaluate true if Fa mbuf is not marked M_RDONLY and if either Fa 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 Fa mbuf->m_flags . This can be achieved during setup of the external storage, by passing the M_RDONLY bit as a Fa flags argument to the Fn MEXTADD macro, or can be directly set in individual Vt mbufs .
Fn MCHTYPE mbuf type
Change the type of Fa mbuf to Fa type . This is a relatively expensive operation and should be avoided.

The functions are:

Fn m_get how type
A function version of Fn MGET for non-critical paths.
Fn m_getm orig len how type
Allocate Fa len bytes worth of Vt mbufs and Vt mbuf clusters if necessary and append the resulting allocated Vt mbuf chain to the Vt mbuf chain Fa orig , if it is non- NULL If the allocation fails at any point, free whatever was allocated and return NULL If Fa orig is non- NULL it will not be freed. It is possible to use Fn m_getm to either append Fa len bytes to an existing Vt mbuf or Vt mbuf chain (for example, one which may be sitting in a pre-allocated ring) or to simply perform an all-or-nothing Vt mbuf and Vt mbuf cluster allocation.
Fn m_gethdr how type
A function version of Fn MGETHDR for non-critical paths.
Fn m_getcl how type flags
Fetch an Vt mbuf with a Vt 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 Vt mbuf and Vt mbuf cluster together, as it avoids having to unlock/relock between allocations. Returns NULL on failure.
Fn m_getclr how type
Allocate an Vt mbuf and zero out the data region.
Fn m_free mbuf
Frees Vt mbuf . Returns m_next of the freed Vt mbuf .

The functions below operate on Vt mbuf chains .

Fn m_freem mbuf
Free an entire Vt mbuf chain , including any external storage.
Fn m_adj mbuf len
Trim Fa len bytes from the head of an Vt mbuf chain if Fa len is positive, from the tail otherwise.
Fn m_append mbuf len cp
Append Vt len bytes of data Vt cp to the Vt mbuf chain . Extend the mbuf chain if the new data does not fit in existing space.
Fn m_prepend mbuf len how
Allocate a new Vt mbuf and prepend it to the Vt mbuf chain , handle M_PKTHDR properly. Note It does not allocate any Vt mbuf clusters , so Fa len must be less than MLEN or MHLEN depending on the M_PKTHDR flag setting.
Fn m_copyup mbuf len dstoff
Similar to Fn m_pullup but copies Fa len bytes of data into a new mbuf at Fa dstoff bytes into the mbuf. The Fa dstoff argument aligns the data and leaves room for a link layer header. Returns the new Vt mbuf chain on success, and frees the Vt mbuf chain and returns NULL on failure. Note The function does not allocate Vt mbuf clusters , so Fa len + dstoff must be less than MHLEN
Fn m_pullup mbuf len
Arrange that the first Fa len bytes of an Vt mbuf chain are contiguous and lay in the data area of Fa mbuf , so they are accessible with Fn 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 Vt mbuf chain on success, NULL on failure (the Vt mbuf chain is freed in this case). Note It does not allocate any Vt mbuf clusters , so Fa len must be less than MHLEN
Fn m_pulldown mbuf offset len offsetp
Arrange that Fa len bytes between Fa offset and Fa offset + len in the Vt mbuf chain are contiguous and lay in the data area of Fa mbuf , so they are accessible with Fn mtod mbuf type . Fa len must be smaller than, or equal to, the size of an Vt mbuf cluster . Return a pointer to an intermediate Vt mbuf in the chain containing the requested region; the offset in the data region of the Vt mbuf chain to the data contained in the returned mbuf is stored in Fa *offsetp . If Fa offp is NULL, the region may be accessed using Fn mtod mbuf type . If Fa offp is non-NULL, the region may be accessed using Fn mtod mbuf uint8_t + *offsetp . The region of the mbuf chain between its beginning and Fa off is not modified, therefore it is safe to hold pointers to data within this region before calling Fn m_pulldown .
Fn m_copym mbuf offset len how
Make a copy of an Vt mbuf chain starting Fa offset bytes from the beginning, continuing for Fa len bytes. If Fa len is M_COPYALL copy to the end of the Vt mbuf chain . Note The copy is read-only, because the Vt mbuf clusters are not copied, only their reference counts are incremented.
Fn m_copypacket mbuf how
Copy an entire packet including header, which must be present. This is an optimized version of the common case Fn m_copym mbuf 0 M_COPYALL how . Note the copy is read-only, because the Vt mbuf clusters are not copied, only their reference counts are incremented.
Fn m_dup mbuf how
Copy a packet header Vt mbuf chain into a completely new Vt mbuf chain , including copying any Vt mbuf clusters . Use this instead of Fn m_copypacket when you need a writable copy of an Vt mbuf chain .
Fn m_copydata mbuf offset len buf
Copy data from an Vt mbuf chain starting Fa off bytes from the beginning, continuing for Fa len bytes, into the indicated buffer Fa buf .
Fn m_copyback mbuf offset len buf
Copy Fa len bytes from the buffer Fa buf back into the indicated Vt mbuf chain , starting at Fa offset bytes from the beginning of the Vt mbuf chain , extending the Vt mbuf chain if necessary. Note It does not allocate any Vt mbuf clusters , just adds Vt mbufs to the Vt mbuf chain . It is safe to set Fa offset beyond the current Vt mbuf chain end: zeroed Vt mbufs will be allocated to fill the space.
Fn m_length mbuf last
Return the length of the Vt mbuf chain , and optionally a pointer to the last Vt mbuf .
Fn m_dup_pkthdr to from how
Upon the function's completion, the Vt mbuf Fa to will contain an identical copy of Fa from->m_pkthdr and the per-packet attributes found in the Vt mbuf chain Fa from . The Vt mbuf Fa from must have the flag M_PKTHDR initially set, and Fa to must be empty on entry.
Fn m_move_pkthdr to from
Move m_pkthdr and the per-packet attributes from the Vt mbuf chain Fa from to the Vt mbuf Fa to . The Vt mbuf Fa from must have the flag M_PKTHDR initially set, and Fa to must be empty on entry. Upon the function's completion, Fa from will have the flag M_PKTHDR and the per-packet attributes cleared.
Fn m_fixhdr mbuf
Set the packet-header length to the length of the Vt mbuf chain .
Fn m_devget buf len offset ifp copy
Copy data from a device local memory pointed to by Fa buf to an Vt mbuf chain . The copy is done using a specified copy routine Fa copy , or Fn bcopy if Fa copy is NULL
Fn m_cat m n
Concatenate Fa n to Fa m . Both Vt mbuf chains must be of the same type. Fa N is still valid after the function returned. Note It does not handle M_PKTHDR and friends.
Fn m_split mbuf len how
Partition an Vt mbuf chain in two pieces, returning the tail: all but the first Fa len bytes. In case of failure, it returns NULL and attempts to restore the Vt mbuf chain to its original state.
Fn m_apply mbuf off len f arg
Apply a function to an Vt mbuf chain , at offset Fa off , for length Fa len bytes. Typically used to avoid calls to Fn m_pullup which would otherwise be unnecessary or undesirable. Fa arg is a convenience argument which is passed to the callback function Fa f .

Each time Fn f is called, it will be passed Fa arg , a pointer to the Fa data in the current mbuf, and the length Fa 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 Fn m_apply will return the error and stop iterating through the Vt mbuf chain .

Fn m_getptr mbuf loc off
Return a pointer to the mbuf containing the data located at Fa loc bytes from the beginning of the Vt mbuf chain . The corresponding offset into the mbuf will be stored in Fa *off .
Fn 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. Fa how should be either M_WAIT or M_DONTWAIT 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.

Fn 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. Fa how should be either M_WAIT or M_DONTWAIT 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 Vt mbuf of a packet contains two fields used for that purpose, Vt int Va csum_flags and Vt int Va 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 Vt mbuf in the Vt 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.

For outbound packets which have been fragmented by the host CPU, the following will also be true, regardless of the checksum flag settings:

The last rule for fragmented packets takes precedence over the one for a TCP or UDP checksum. Nevertheless, offloading a TCP or UDP checksum is possible for a fragmented packet if the flag CSUM_IP_FRAGS is set in the field ifnet.if_data.ifi_hwassist associated with the network interface. However, in this case the interface is expected to figure out the location of the checksum field within the sequence of fragments by itself because csum_data contains a fragment count instead of a checksum offset value.

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.

For inbound packets which are IP fragments, all csum_data fields will be summed during reassembly to obtain the final checksum value passed to an upper layer in the csum_data field of the reassembled packet. The csum_flags fields of all fragments will be consolidated using logical AND to obtain the final value for csum_flags Thus, in order to successfully offload checksum computation for fragmented data, all fragments should have the same value of csum_flags

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 Vt mbufs .
net.inet.ip.mbuf_frag_size
Causes Fn ip_output to fragment outgoing Vt mbuf chains into fragments of the specified size. Setting this variable to 1 is an excellent way to test the long Vt mbuf chain handling ability of network drivers.
kern.ipc.m_defragrandomfailures
Causes the function Fn m_defrag to randomly fail, returning NULL Any piece of code which uses Fn m_defrag should be tested with this feature.

RETURN VALUES

See above.

SEE ALSO

ifnet(9), mbuf_tags9

HISTORY

Vt 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 Fx use of Vt mbufs is almost entirely limited to packet storage, with uma(9) zones being used directly to store other network-related memory.

Historically, the Vt 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 Fx 5.3 , the Vt mbuf allocator is a wrapper around uma(9), allowing caching of Vt mbufs , clusters, and Vt mbuf + cluster pairs in per-CPU caches, as well as bringing other benefits of slab allocation.

AUTHORS

The original manual page was written by Yar Tikhiy. The uma(9) Vt mbuf allocator was written by Bosko Milekic.
> on ne peut pas paramétrer l'apprence de cette cochonnerie comme on veut.
Ouais, franchement c'est naze, je peux même pas assortir le menu
à mon vernis à ongles. Aux chiottes MacOS.
-+- M in GFA : "De la bonne tenue des secrétaires" -+-