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       | if needed    |
+-------------------------------+---------+-----------+--------------+
| 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   | readonly   | format   |
|=================================|=========|===========|==============|==========|============|==========|
| PyBUF_FULL                      | oui     | oui       | if needed    | U        | 0          | oui      |
+---------------------------------+---------+-----------+--------------+----------+------------+----------+
| PyBUF_FULL_RO                   | oui     | oui       | if needed    | U        | 1 or 0     | oui      |
+---------------------------------+---------+-----------+--------------+----------+------------+----------+
| PyBUF_RECORDS                   | oui     | oui       | NULL         | U        | 0          | oui      |
+---------------------------------+---------+-----------+--------------+----------+------------+----------+
| PyBUF_RECORDS_RO                | oui     | oui       | NULL         | U        | 1 or 0     | oui      |
+---------------------------------+---------+-----------+--------------+----------+------------+----------+
| PyBUF_STRIDED                   | oui     | oui       | NULL         | U        | 0          | NULL     |
+---------------------------------+---------+-----------+--------------+----------+------------+----------+
| PyBUF_STRIDED_RO                | oui     | oui       | NULL         | U        | 1 or 0     | NULL     |
+---------------------------------+---------+-----------+--------------+----------+------------+----------+
| PyBUF_CONTIG                    | oui     | NULL      | NULL         | C        | 0          | NULL     |
+---------------------------------+---------+-----------+--------------+----------+------------+----------+
| PyBUF_CONTIG_RO                 | oui     | NULL      | NULL         | C        | 1 or 0     | 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 *)

   Return the implied "itemsize" from "format". This function is not
   yet implemented.

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.

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'" (for C-style or Fortran-style
   ordering). "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.
