cpython/Doc/c-api/capsule.rst

.. highlightlang:: c

.. _capsules:

Capsules
--------

.. index:: object: Capsule

Refer to :ref:`using-capsules` for more information on using these objects.


.. ctype:: PyCapsule

   This subtype of :ctype:`PyObject` represents an opaque value, useful for C
   extension modules who need to pass an opaque value (as a :ctype:`void\*`
   pointer) through Python code to other C code.  It is often used to make a C
   function pointer defined in one module available to other modules, so the
   regular import mechanism can be used to access C APIs defined in dynamically
   loaded modules.

.. ctype:: PyCapsule_Destructor

   The type of a destructor callback for a capsule.  Defined as::

      typedef void (*PyCapsule_Destructor)(PyObject *);

   See :cfunc:`PyCapsule_New` for the semantics of PyCapsule_Destructor
   callbacks.


.. cfunction:: int PyCapsule_CheckExact(PyObject *p)

   Return true if its argument is a :ctype:`PyCapsule`.


.. cfunction:: PyObject* PyCapsule_New(void *pointer, const char *name, PyCapsule_Destructor destructor)

   Create a :ctype:`PyCapsule` encapsulating the *pointer*.  The *pointer*
   argument may not be *NULL*.

   On failure, set an exception and return *NULL*.

   The *name* string may either be *NULL* or a pointer to a valid C string.  If
   non-*NULL*, this string must outlive the capsule.  (Though it is permitted to
   free it inside the *destructor*.)

   If the *destructor* argument is not *NULL*, it will be called with the
   capsule as its argument when it is destroyed.

   If this capsule will be stored as an attribute of a module, the *name* should
   be specified as ``modulename.attributename``.  This will enable other modules
   to import the capsule using :cfunc:`PyCapsule_Import`.


.. cfunction:: void* PyCapsule_GetPointer(PyObject *capsule, const char *name)

   Retrieve the *pointer* stored in the capsule.  On failure, set an exception
   and return *NULL*.

   The *name* parameter must compare exactly to the name stored in the capsule.
   If the name stored in the capsule is *NULL*, the *name* passed in must also
   be *NULL*.  Python uses the C function :cfunc:`strcmp` to compare capsule
   names.


.. cfunction:: PyCapsule_Destructor PyCapsule_GetDestructor(PyObject *capsule)

   Return the current destructor stored in the capsule.  On failure, set an
   exception and return *NULL*.

   It is legal for a capsule to have a *NULL* destructor.  This makes a *NULL*
   return code somewhat ambiguous; use :cfunc:`PyCapsule_IsValid` or
   :cfunc:`PyErr_Occurred` to disambiguate.


.. cfunction:: void* PyCapsule_GetContext(PyObject *capsule)

   Return the current context stored in the capsule.  On failure, set an
   exception and return *NULL*.

   It is legal for a capsule to have a *NULL* context.  This makes a *NULL*
   return code somewhat ambiguous; use :cfunc:`PyCapsule_IsValid` or
   :cfunc:`PyErr_Occurred` to disambiguate.


.. cfunction:: const char* PyCapsule_GetName(PyObject *capsule)

   Return the current name stored in the capsule.  On failure, set an exception
   and return *NULL*.

   It is legal for a capsule to have a *NULL* name.  This makes a *NULL* return
   code somewhat ambiguous; use :cfunc:`PyCapsule_IsValid` or
   :cfunc:`PyErr_Occurred` to disambiguate.


.. cfunction:: void* PyCapsule_Import(const char *name, int no_block)

   Import a pointer to a C object from a capsule attribute in a module.  The
   *name* parameter should specify the full name to the attribute, as in
   ``module.attribute``.  The *name* stored in the capsule must match this
   string exactly.  If *no_block* is true, import the module without blocking
   (using :cfunc:`PyImport_ImportModuleNoBlock`).  If *no_block* is false,
   import the module conventionally (using :cfunc:`PyImport_ImportModule`).

   Return the capsule's internal *pointer* on success.  On failure, set an
   exception and return *NULL*.  However, if :cfunc:`PyCapsule_Import` failed to
   import the module, and *no_block* was true, no exception is set.

.. cfunction:: int PyCapsule_IsValid(PyObject *capsule, const char *name)

   Determines whether or not *capsule* is a valid capsule.  A valid capsule is
   non-*NULL*, passes :cfunc:`PyCapsule_CheckExact`, has a non-*NULL* pointer
   stored in it, and its internal name matches the *name* parameter.  (See
   :cfunc:`PyCapsule_GetPointer` for information on how capsule names are
   compared.)

   In other words, if :cfunc:`PyCapsule_IsValid` returns a true value, calls to
   any of the accessors (any function starting with :cfunc:`PyCapsule_Get`) are
   guaranteed to succeed.

   Return a nonzero value if the object is valid and matches the name passed in.
   Return 0 otherwise.  This function will not fail.

.. cfunction:: int PyCapsule_SetContext(PyObject *capsule, void *context)

   Set the context pointer inside *capsule* to *context*.

   Return 0 on success.  Return nonzero and set an exception on failure.

.. cfunction:: int PyCapsule_SetDestructor(PyObject *capsule, PyCapsule_Destructor destructor)

   Set the destructor inside *capsule* to *destructor*.

   Return 0 on success.  Return nonzero and set an exception on failure.

.. cfunction:: int PyCapsule_SetName(PyObject *capsule, const char *name)

   Set the name inside *capsule* to *name*.  If non-*NULL*, the name must
   outlive the capsule.  If the previous *name* stored in the capsule was not
   *NULL*, no attempt is made to free it.

   Return 0 on success.  Return nonzero and set an exception on failure.

.. cfunction:: int PyCapsule_SetPointer(PyObject *capsule, void *pointer)

   Set the void pointer inside *capsule* to *pointer*.  The pointer may not be
   *NULL*.

   Return 0 on success.  Return nonzero and set an exception on failure.