# Protocole tampon

Certains objets Python enveloppent l'accès à un tableau de mémoire sous-jacente (nommée zone tampon ou simplement tampon, buffer en anglais). Les objets natifs `bytes` et `bytearray` en sont des exemples, ainsi que quelques types d'extension comme `array.array`. Les bibliothèques tierces peuvent définir leurs propres types à des fins spéciales, telles que le traitement d'image ou l'analyse numérique.

Alors que chacun de ces types a sa propre sémantique, ils partagent la caractéristique commune d'être soutenus par un tampon de mémoire important. Il est donc souhaitable, dans certains cas, d'accéder à cette mémoire directement sans l'étape intermédiaire de copie.

Python fournit une telle facilité au niveau du C sous la forme de protocole tampon. Ce protocole comporte deux aspects :

- du côté producteur, un type peut exporter une "interface tampon" qui permet aux objets de ce type d'exposer des informations concernant leur tampon sous-jacent. Cette interface est décrite dans la section Buffer Object Structures ;

- du côté consommateur, plusieurs moyens sont disponibles pour obtenir un pointeur vers les données sous-jacentes brutes d'un objet (par exemple un paramètre de méthode).

Des objets simples tels que `bytes` et `bytearray` exposent leur tampon sous-jacent dans un format orienté octet. D'autres formes sont possibles ; par exemple, les éléments exposés par un `array.array` peuvent être des valeurs multi-octets.

Un exemple de consommateur de l'interface tampon est la méthode `write()` des objets fichiers : tout objet qui peut exporter une série d'octets à travers l'interface tampon peut être écrit dans un fichier. Alors que `write()` n'a besoin que d'un accès lecture au contenu interne de l'objet qui lui est passé, d'autres méthodes telles que `readinto()` nécessitent un accès écriture au contenu de leur argument. L'interface buffer permet aux objets d'autoriser ou de rejeter sélectivement l'exportation de tampons en mode lecture-écriture et en mode lecture seule.

Un consommateur de l'interface tampon peut acquérir un tampon sur un objet cible de deux manières :

- appelez `PyObject_GetBuffer()` avec les paramètres appropriés ;

- appelez `PyArg_ParseTuple()` (ou l'un de ses fonctions sœurs) avec l'un des `y*`, `w*` ou `s*` format codes.

Dans les deux cas, `PyBuffer_Release()` doit être appelée quand le tampon n'est plus nécessaire. Ne pas le faire peut conduire à divers problèmes tels que des fuites de ressources.

## La structure buffer

Les structures tampons (ou simplement les "tampons", buffers en anglais) sont utiles pour exposer les données binaires d'un autre objet au programmeur Python. Elles peuvent également être utilisées comme un mécanisme de découpage sans copie. En utilisant leur capacité à référencer un bloc de mémoire, il est possible d'exposer toutes les données au programmeur Python assez facilement. La mémoire peut être un grand tableau constant dans une extension C, il peut s'agir d'un bloc brut de mémoire à manipuler avant de passer à une bibliothèque de système d'exploitation ou être utilisé pour transmettre des données structurées dans son format natif en mémoire.

Contrairement à la plupart des types de données exposés par l'interpréteur Python, les tampons ne sont pas de simples pointeurs vers `PyObject` mais plutôt des structures C simples. Cela leur permet d'être créés et copiés très simplement. lorsque vous avez besoin d'une enveloppe générique (wrapper en anglais) pour un tampon, un objet memoryview peut être créé.

For short instructions how to write an exporting object, see Buffer Object Structures. For obtaining a buffer, see `PyObject_GetBuffer()`.

`Py_buffer`
  void *`buf`

    A pointer to the start of the logical structure described by the buffer fields. This can be any location within the underlying physical memory block of the exporter. For example, with negative `strides` the value may point to the end of the memory block.

    For contiguous arrays, the value points to the beginning of the memory block.

  void *`obj`

    A new reference to the exporting object. The reference is owned by the consumer and automatically decremented and set to `NULL` by `PyBuffer_Release()`. The field is the equivalent of the return value of any standard C-API function.

    As a special case, for temporary buffers that are wrapped by `PyMemoryView_FromBuffer()` or `PyBuffer_FillInfo()` this field is `NULL`. In general, exporting objects MUST NOT use this scheme.

  Py_ssize_t `len`

    `product(shape) * itemsize`. For contiguous arrays, this is the length of the underlying memory block. For non-contiguous arrays, it is the length that the logical structure would have if it were copied to a contiguous representation.

    Accessing `((char *)buf)[0] up to ((char *)buf)[len-1]` is only valid if the buffer has been obtained by a request that guarantees contiguity. In most cases such a request will be `PyBUF_SIMPLE` or `PyBUF_WRITABLE`.

  int `readonly`

    An indicator of whether the buffer is read-only. This field is controlled by the `PyBUF_WRITABLE` flag.

  Py_ssize_t `itemsize`

    Item size in bytes of a single element. Same as the value of `struct.calcsize()` called on non-`NULL` `format` values.

    Important exception: If a consumer requests a buffer without the `PyBUF_FORMAT` flag, `format` will be set to `NULL`, but `itemsize` still has the value for the original format.

    If `shape` is present, the equality `product(shape) * itemsize == len` still holds and the consumer can use `itemsize` to navigate the buffer.

    If `shape` is `NULL` as a result of a `PyBUF_SIMPLE` or a `PyBUF_WRITABLE` request, the consumer must disregard `itemsize` and assume `itemsize == 1`.

  const char *`format`

    A NUL terminated string in `struct` module style syntax describing the contents of a single item. If this is `NULL`, `"B"` (unsigned bytes) is assumed.

    This field is controlled by the `PyBUF_FORMAT` flag.

  int `ndim`

    The number of dimensions the memory represents as an n-dimensional array. If it is `0`, `buf` points to a single item representing a scalar. In this case, `shape`, `strides` and `suboffsets` MUST be `NULL`.

    The macro `PyBUF_MAX_NDIM` limits the maximum number of dimensions to 64. Exporters MUST respect this limit, consumers of multi-dimensional buffers SHOULD be able to handle up to `PyBUF_MAX_NDIM` dimensions.

  Py_ssize_t *`shape`

    An array of `Py_ssize_t` of length `ndim` indicating the shape of the memory as an n-dimensional array. Note that `shape[0] * ... * shape[ndim-1] * itemsize` MUST be equal to `len`.

    Shape values are restricted to `shape[n] >= 0`. The case `shape[n] == 0` requires special attention. See complex arrays for further information.

    The shape array is read-only for the consumer.

  Py_ssize_t *`strides`

    An array of `Py_ssize_t` of length `ndim` giving the number of bytes to skip to get to a new element in each dimension.

    Stride values can be any integer. For regular arrays, strides are usually positive, but a consumer MUST be able to handle the case `strides[n] <= 0`. See complex arrays for further information.

    The strides array is read-only for the consumer.

  Py_ssize_t *`suboffsets`

    An array of `Py_ssize_t` of length `ndim`. If `suboffsets[n] >= 0`, the values stored along the nth dimension are pointers and the suboffset value dictates how many bytes to add to each pointer after de-referencing. A suboffset value that is negative indicates that no de-referencing should occur (striding in a contiguous memory block).

    If all suboffsets are negative (i.e. no de-referencing is needed), then this field must be `NULL` (the default value).

    This type of array representation is used by the Python Imaging Library (PIL). See complex arrays for further information how to access elements of such an array.

    The suboffsets array is read-only for the consumer.

  void *`internal`

    This is for use internally by the exporting object. For example, this might be re-cast as an integer by the exporter and used to store flags about whether or not the shape, strides, and suboffsets arrays must be freed when the buffer is released. The consumer MUST NOT alter this value.

## Buffer request types

Buffers are usually obtained by sending a buffer request to an exporting object via `PyObject_GetBuffer()`. Since the complexity of the logical structure of the memory can vary drastically, the consumer uses the flags argument to specify the exact buffer type it can handle.

All `Py_buffer` fields are unambiguously defined by the request type.

### request-independent fields

The following fields are not influenced by flags and must always be filled in with the correct values: `obj`, `buf`, `len`, `itemsize`, `ndim`.

### readonly, format

> `PyBUF_WRITABLE`

>   Controls the `readonly` field. If set, the exporter MUST provide a writable buffer or else report failure. Otherwise, the exporter MAY provide either a read-only or writable buffer, but the choice MUST be consistent for all consumers.

> `PyBUF_FORMAT`

>   Controls the `format` field. If set, this field MUST be filled in correctly. Otherwise, this field MUST be `NULL`.

`PyBUF_WRITABLE` can be |'d to any of the flags in the next section. Since `PyBUF_SIMPLE` is defined as 0, `PyBUF_WRITABLE` can be used as a stand-alone flag to request a simple writable buffer.

`PyBUF_FORMAT` can be |'d to any of the flags except `PyBUF_SIMPLE`. The latter already implies format `B` (unsigned bytes).

### shape, strides, suboffsets

The flags that control the logical structure of the memory are listed in decreasing order of complexity. Note that each flag contains all bits of the flags below it.

Request | shape | strides | suboffsets
--- | --- | --- | ---
`PyBUF_INDIRECT` | oui | oui | si nécessaire
`PyBUF_STRIDES` | oui | oui | NULL
`PyBUF_ND` | oui | NULL | NULL
`PyBUF_SIMPLE` | NULL | NULL | NULL

### contiguity requests

C or Fortran contiguity can be explicitly requested, with and without stride information. Without stride information, the buffer must be C-contiguous.

Request | shape | strides | suboffsets | contig
--- | --- | --- | --- | ---
`PyBUF_C_CONTIGUOUS` | oui | oui | NULL | C
`PyBUF_F_CONTIGUOUS` | oui | oui | NULL | F
`PyBUF_ANY_CONTIGUOUS` | oui | oui | NULL | C or F
`PyBUF_ND` | oui | NULL | NULL | C

### compound requests

All possible requests are fully defined by some combination of the flags in the previous section. For convenience, the buffer protocol provides frequently used combinations as single flags.

In the following table U stands for undefined contiguity. The consumer would have to call `PyBuffer_IsContiguous()` to determine contiguity.

Request | shape | strides | suboffsets | contig | lecture seule | format
--- | --- | --- | --- | --- | --- | ---
`PyBUF_FULL` | oui | oui | si nécessaire | U | 0 | oui
`PyBUF_FULL_RO` | oui | oui | si nécessaire | U | 0 ou 1 | oui
`PyBUF_RECORDS` | oui | oui | NULL | U | 0 | oui
`PyBUF_RECORDS_RO` | oui | oui | NULL | U | 0 ou 1 | oui
`PyBUF_STRIDED` | oui | oui | NULL | U | 0 | NULL
`PyBUF_STRIDED_RO` | oui | oui | NULL | U | 0 ou 1 | NULL
`PyBUF_CONTIG` | oui | NULL | NULL | C | 0 | NULL
`PyBUF_CONTIG_RO` | oui | NULL | NULL | C | 0 ou 1 | NULL

## Complex arrays

### NumPy-style: shape and strides

The logical structure of NumPy-style arrays is defined by `itemsize`, `ndim`, `shape` and `strides`.

If `ndim == 0`, the memory location pointed to by `buf` is interpreted as a scalar of size `itemsize`. In that case, both `shape` and `strides` are `NULL`.

If `strides` is `NULL`, the array is interpreted as a standard n-dimensional C-array. Otherwise, the consumer must access an n-dimensional array as follows:

    ptr = (char *)buf + indices[0] * strides[0] + ... + indices[n-1] * strides[n-1];
    item = *((typeof(item) *)ptr);

As noted above, `buf` can point to any location within the actual memory block. An exporter can check the validity of a buffer with this function:

    def verify_structure(memlen, itemsize, ndim, shape, strides, offset):
        """Verify that the parameters represent a valid array within
           the bounds of the allocated memory:
               char *mem: start of the physical memory block
               memlen: length of the physical memory block
               offset: (char *)buf - mem
        """
        if offset % itemsize:
            return False
        if offset < 0 or offset+itemsize > memlen:
            return False
        if any(v % itemsize for v in strides):
            return False

        if ndim <= 0:
            return ndim == 0 and not shape and not strides
        if 0 in shape:
            return True

        imin = sum(strides[j]*(shape[j]-1) for j in range(ndim)
                   if strides[j] <= 0)
        imax = sum(strides[j]*(shape[j]-1) for j in range(ndim)
                   if strides[j] > 0)

        return 0 <= offset+imin and offset+imax+itemsize <= memlen

### PIL-style: shape, strides and suboffsets

In addition to the regular items, PIL-style arrays can contain pointers that must be followed in order to get to the next element in a dimension. For example, the regular three-dimensional C-array `char v[2][2][3]` can also be viewed as an array of 2 pointers to 2 two-dimensional arrays: `char (*v[2])[2][3]`. In suboffsets representation, those two pointers can be embedded at the start of `buf`, pointing to two `char x[2][3]` arrays that can be located anywhere in memory.

Here is a function that returns a pointer to the element in an N-D array pointed to by an N-dimensional index when there are both non-`NULL` strides and suboffsets:

    void *get_item_pointer(int ndim, void *buf, Py_ssize_t *strides,
                           Py_ssize_t *suboffsets, Py_ssize_t *indices) {
        char *pointer = (char*)buf;
        int i;
        for (i = 0; i < ndim; i++) {
            pointer += strides[i] * indices[i];
            if (suboffsets[i] >=0 ) {
                pointer = *((char**)pointer) + suboffsets[i];
            }
        }
        return (void*)pointer;
    }

## Fonctions relatives aux tampons

int `PyObject_CheckBuffer`(PyObject *obj)

  Return `1` if obj supports the buffer interface otherwise `0`. When `1` is returned, it doesn't guarantee that `PyObject_GetBuffer()` will succeed. This function always succeeds.

int `PyObject_GetBuffer`(PyObject *exporter, Py_buffer *view, int flags)

  Send a request to exporter to fill in view as specified by flags. If the exporter cannot provide a buffer of the exact type, it MUST raise `PyExc_BufferError`, set `view->obj` to `NULL` and return `-1`.

  On success, fill in view, set `view->obj` to a new reference to exporter and return 0. In the case of chained buffer providers that redirect requests to a single object, `view->obj` MAY refer to this object instead of exporter (See Buffer Object Structures).

  Successful calls to `PyObject_GetBuffer()` must be paired with calls to `PyBuffer_Release()`, similar to `malloc()` and `free()`. Thus, after the consumer is done with the buffer, `PyBuffer_Release()` must be called exactly once.

void `PyBuffer_Release`(Py_buffer *view)

  Release the buffer view and decrement the reference count for `view->obj`. This function MUST be called when the buffer is no longer being used, otherwise reference leaks may occur.

  It is an error to call this function on a buffer that was not obtained via `PyObject_GetBuffer()`.

Py_ssize_t `PyBuffer_SizeFromFormat`(const char *format)

  Retourne l'`itemsize` du `format`. En cas d'erreur, lève une exception et retourne -1.

  Nouveau dans la version 3.9.

int `PyBuffer_IsContiguous`(Py_buffer *view, char order)

  Return `1` if the memory defined by the view is C-style (order is `'C'`) or Fortran-style (order is `'F'`) contiguous or either one (order is `'A'`). Return `0` otherwise. This function always succeeds.

void* `PyBuffer_GetPointer`(Py_buffer *view, Py_ssize_t *indices)

  Get the memory area pointed to by the indices inside the given view. indices must point to an array of `view->ndim` indices.

int `PyBuffer_FromContiguous`(Py_buffer *view, void *buf, Py_ssize_t len, char fort)

  Copy contiguous len bytes from buf to view. fort can be `'C'` or `'F'` (for C-style or Fortran-style ordering). `0` is returned on success, `-1` on error.

int `PyBuffer_ToContiguous`(void *buf, Py_buffer *src, Py_ssize_t len, char order)

  Copy len bytes from src to its contiguous representation in buf. order can be `'C'` or `'F'` or `'A'` (for C-style or Fortran-style ordering or either one). `0` is returned on success, `-1` on error.

  This function fails if len != src->len.

void `PyBuffer_FillContiguousStrides`(int ndims, Py_ssize_t *shape, Py_ssize_t *strides, int itemsize, char order)

  Fill the strides array with byte-strides of a contiguous (C-style if order is `'C'` or Fortran-style if order is `'F'`) array of the given shape with the given number of bytes per element.

int `PyBuffer_FillInfo`(Py_buffer *view, PyObject *exporter, void *buf, Py_ssize_t len, int readonly, int flags)

  Handle buffer requests for an exporter that wants to expose buf of size len with writability set according to readonly. buf is interpreted as a sequence of unsigned bytes.

  The flags argument indicates the request type. This function always fills in view as specified by flags, unless buf has been designated as read-only and `PyBUF_WRITABLE` is set in flags.

  On success, set `view->obj` to a new reference to exporter and return 0. Otherwise, raise `PyExc_BufferError`, set `view->obj` to `NULL` and return `-1`;

  If this function is used as part of a getbufferproc, exporter MUST be set to the exporting object and flags must be passed unmodified. Otherwise, exporter MUST be `NULL`.