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 desy*
,w*
ous*
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()
orPyBuffer_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 bePyBUF_SIMPLE
orPyBUF_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-NULLformat
values.Important exception: If a consumer requests a buffer without the
PyBUF_FORMAT
flag,format
will be set to NULL, butitemsize
still has the value for the original format.If
shape
is present, the equalityproduct(shape) * itemsize == len
still holds and the consumer can useitemsize
to navigate the buffer.If
shape
is NULL as a result of aPyBUF_SIMPLE
or aPyBUF_WRITABLE
request, the consumer must disregarditemsize
and assumeitemsize == 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
andsuboffsets
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 toPyBUF_MAX_NDIM
dimensions.
-
Py_ssize_t *
shape
¶ An array of
Py_ssize_t
of lengthndim
indicating the shape of the memory as an n-dimensional array. Note thatshape[0] * ... * shape[ndim-1] * itemsize
MUST be equal tolen
.Shape values are restricted to
shape[n] >= 0
. The caseshape[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 lengthndim
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 lengthndim
. Ifsuboffsets[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.
-
void *
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
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 |
---|---|---|---|
|
oui | oui | if needed |
|
oui | oui | NULL |
|
oui | NULL | NULL |
|
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 |
---|---|---|---|---|
|
oui | oui | NULL | C |
|
oui | oui | NULL | F |
|
oui | oui | NULL | C or F |
|
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 |
---|---|---|---|---|---|---|
|
oui | oui | if needed | U | 0 | oui |
|
oui | oui | if needed | U | 1 or 0 | oui |
|
oui | oui | NULL | U | 0 | oui |
|
oui | oui | NULL | U | 1 or 0 | oui |
|
oui | oui | NULL | U | 0 | NULL |
|
oui | oui | NULL | U | 1 or 0 | NULL |
|
oui | NULL | NULL | C | 0 | NULL |
|
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;
}