Issue #13053: Added section on migrating from CObject to Capsule
to howto/cporting.rst.
diff --git a/Doc/howto/cporting.rst b/Doc/howto/cporting.rst
index 46482ab..5ba5df7 100644
--- a/Doc/howto/cporting.rst
+++ b/Doc/howto/cporting.rst
@@ -209,6 +209,58 @@
}
+CObject replaced with Capsule
+=============================
+
+The :ctype:`Capsule` object was introduced in Python 3.1 and 2.7 to replace
+:ctype:`CObject`. CObjects were useful,
+but the :ctype:`CObject` API was problematic: it didn't permit distinguishing
+between valid CObjects, which allowed mismatched CObjects to crash the
+interpreter, and some of its APIs relied on undefined behavior in C.
+(For further reading on the rationale behind Capsules, please see :issue:`5630`.)
+
+If you're currently using CObjects, and you want to migrate to 3.1 or newer,
+you'll need to switch to Capsules.
+:ctype:`CObject` was deprecated in 3.1 and 2.7 and completely removed in
+Python 3.2. If you only support 2.7, or 3.1 and above, you
+can simply switch to :ctype:`Capsule`. If you need to support 3.0 or
+versions of Python earlier than 2.7 you'll have to support both CObjects
+and Capsules.
+
+The following example header file :file:`capsulethunk.h` may
+solve the problem for you;
+simply write your code against the :ctype:`Capsule` API, include
+this header file after ``"Python.h"``, and you'll automatically use CObjects
+in Python 3.0 or versions earlier than 2.7.
+
+:file:`capsulethunk.h` simulates Capsules using CObjects. However,
+:ctype:`CObject` provides no place to store the capsule's "name". As a
+result the simulated :ctype:`Capsule` objects created by :file:`capsulethunk.h`
+behave slightly differently from real Capsules. Specifically:
+
+ * The name parameter passed in to :cfunc:`PyCapsule_New` is ignored.
+
+ * The name parameter passed in to :cfunc:`PyCapsule_IsValid` and
+ :cfunc:`PyCapsule_GetPointer` is ignored, and no error checking
+ of the name is performed.
+
+ * :cfunc:`PyCapsule_GetName` always returns NULL.
+
+ * :cfunc:`PyCapsule_SetName` always throws an exception and
+ returns failure. (Since there's no way to store a name
+ in a CObject, noisy failure of :cfunc:`PyCapsule_SetName`
+ was deemed preferable to silent failure here. If this is
+ inconveient, feel free to modify your local
+ copy as you see fit.)
+
+You can find :file:`capsulethunk.h` in the Python source distribution
+in the :file:`Doc/includes` directory. We also include it here for
+your reference; here is :file:`capsulethunk.h`:
+
+.. literalinclude:: ../includes/capsulethunk.h
+
+
+
Other options
=============
diff --git a/Doc/includes/capsulethunk.h b/Doc/includes/capsulethunk.h
new file mode 100644
index 0000000..6b20564
--- /dev/null
+++ b/Doc/includes/capsulethunk.h
@@ -0,0 +1,134 @@
+#ifndef __CAPSULETHUNK_H
+#define __CAPSULETHUNK_H
+
+#if ( (PY_VERSION_HEX < 0x02070000) \
+ || ((PY_VERSION_HEX >= 0x03000000) \
+ && (PY_VERSION_HEX < 0x03010000)) )
+
+#define __PyCapsule_GetField(capsule, field, default_value) \
+ ( PyCapsule_CheckExact(capsule) \
+ ? (((PyCObject *)capsule)->field) \
+ : (default_value) \
+ ) \
+
+#define __PyCapsule_SetField(capsule, field, value) \
+ ( PyCapsule_CheckExact(capsule) \
+ ? (((PyCObject *)capsule)->field = value), 1 \
+ : 0 \
+ ) \
+
+
+#define PyCapsule_Type PyCObject_Type
+
+#define PyCapsule_CheckExact(capsule) (PyCObject_Check(capsule))
+#define PyCapsule_IsValid(capsule, name) (PyCObject_Check(capsule))
+
+
+#define PyCapsule_New(pointer, name, destructor) \
+ (PyCObject_FromVoidPtr(pointer, destructor))
+
+
+#define PyCapsule_GetPointer(capsule, name) \
+ (PyCObject_AsVoidPtr(capsule))
+
+/* Don't call PyCObject_SetPointer here, it fails if there's a destructor */
+#define PyCapsule_SetPointer(capsule, pointer) \
+ __PyCapsule_SetField(capsule, cobject, pointer)
+
+
+#define PyCapsule_GetDestructor(capsule) \
+ __PyCapsule_GetField(capsule, destructor)
+
+#define PyCapsule_SetDestructor(capsule, dtor) \
+ __PyCapsule_SetField(capsule, destructor, dtor)
+
+
+/*
+ * Sorry, there's simply no place
+ * to store a Capsule "name" in a CObject.
+ */
+#define PyCapsule_GetName(capsule) NULL
+
+static int
+PyCapsule_SetName(PyObject *capsule, const char *unused)
+{
+ unused = unused;
+ PyErr_SetString(PyExc_NotImplementedError,
+ "can't use PyCapsule_SetName with CObjects");
+ return 1;
+}
+
+
+
+#define PyCapsule_GetContext(capsule) \
+ __PyCapsule_GetField(capsule, descr)
+
+#define PyCapsule_SetContext(capsule, context) \
+ __PyCapsule_SetField(capsule, descr, context)
+
+
+static void *
+PyCapsule_Import(const char *name, int no_block)
+{
+ PyObject *object = NULL;
+ void *return_value = NULL;
+ char *trace;
+ size_t name_length = (strlen(name) + 1) * sizeof(char);
+ char *name_dup = (char *)PyMem_MALLOC(name_length);
+
+ if (!name_dup) {
+ return NULL;
+ }
+
+ memcpy(name_dup, name, name_length);
+
+ trace = name_dup;
+ while (trace) {
+ char *dot = strchr(trace, '.');
+ if (dot) {
+ *dot++ = '\0';
+ }
+
+ if (object == NULL) {
+ if (no_block) {
+ object = PyImport_ImportModuleNoBlock(trace);
+ } else {
+ object = PyImport_ImportModule(trace);
+ if (!object) {
+ PyErr_Format(PyExc_ImportError,
+ "PyCapsule_Import could not "
+ "import module \"%s\"", trace);
+ }
+ }
+ } else {
+ PyObject *object2 = PyObject_GetAttrString(object, trace);
+ Py_DECREF(object);
+ object = object2;
+ }
+ if (!object) {
+ goto EXIT;
+ }
+
+ trace = dot;
+ }
+
+ if (PyCObject_Check(object)) {
+ PyCObject *cobject = (PyCObject *)object;
+ return_value = cobject->cobject;
+ } else {
+ PyErr_Format(PyExc_AttributeError,
+ "PyCapsule_Import \"%s\" is not valid",
+ name);
+ }
+
+EXIT:
+ Py_XDECREF(object);
+ if (name_dup) {
+ PyMem_FREE(name_dup);
+ }
+ return return_value;
+}
+
+#endif /* #if PY_VERSION_HEX < 0x02070000 */
+
+#endif /* __CAPSULETHUNK_H */