bpo-45459: Add Py_buffer to limited API (GH-29991)

- [x] ``Py_buffer`` struct
- [x] ``PyBuffer_*()`` API functions
- [x] ``PyBUF_*`` constants
- [x] ``Py_bf_getbuffer`` and ``Py_bf_releasebuffer`` type slots
- [x] ``PyMemoryView_FromBuffer()`` API
- [x] tests for limited API
- [x] ``make regen-limited-abi``
- [x] documentation update
- [ ] export ``PyPickleBuffer*()`` API ???

17 files changed, 308 insertions, 131 deletions

diff --git a/Doc/c-api/buffer.rst b/Doc/c-api/buffer.rst
index 820a3a6..05e131d 100644
--- a/Doc/c-api/buffer.rst
+++ b/Doc/c-api/buffer.rst
@@ -499,6 +499,13 @@ Buffer-related functions
    This function fails if *len* != *src->len*.
 
 
+.. c:function:: int PyObject_CopyData(Py_buffer *dest, Py_buffer *src)
+
+   Copy data from *src* to *dest* buffer. Can convert between C-style and
+   or Fortran-style buffers.
+
+   ``0`` is returned on success, ``-1`` on error.
+
 .. c:function:: 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 :term:`contiguous` (C-style if
diff --git a/Doc/c-api/type.rst b/Doc/c-api/type.rst
index f968869..97a818a 100644
--- a/Doc/c-api/type.rst
+++ b/Doc/c-api/type.rst
@@ -272,12 +272,6 @@ The following functions and structs are used to create
       * :c:member:`~PyTypeObject.tp_vectorcall_offset`
         (see :ref:`PyMemberDef <pymemberdef-offsets>`)
 
-      The following fields cannot be set using :c:type:`PyType_Spec` and
-      :c:type:`PyType_Slot` under the limited API:
-
-      * :c:member:`~PyBufferProcs.bf_getbuffer`
-      * :c:member:`~PyBufferProcs.bf_releasebuffer`
-
       Setting :c:data:`Py_tp_bases` or :c:data:`Py_tp_base` may be
       problematic on some platforms.
       To avoid issues, use the *bases* argument of
@@ -287,6 +281,11 @@ The following functions and structs are used to create
 
         Slots in :c:type:`PyBufferProcs` in may be set in the unlimited API.
 
+     .. versionchanged:: 3.11
+        :c:member:`~PyBufferProcs.bf_getbuffer` and
+        :c:member:`~PyBufferProcs.bf_releasebuffer` are now available
+        under limited API.
+
    .. c:member:: void *PyType_Slot.pfunc
 
       The desired value of the slot. In most cases, this is a pointer
diff --git a/Doc/data/stable_abi.dat b/Doc/data/stable_abi.dat
index 02e54e5..18bbf03 100644
--- a/Doc/data/stable_abi.dat
+++ b/Doc/data/stable_abi.dat
@@ -10,6 +10,14 @@ function,PyArg_ValidateKeywordArguments,3.2,
 var,PyBaseObject_Type,3.2,
 function,PyBool_FromLong,3.2,
 var,PyBool_Type,3.2,
+function,PyBuffer_FillContiguousStrides,3.11,
+function,PyBuffer_FillInfo,3.11,
+function,PyBuffer_FromContiguous,3.11,
+function,PyBuffer_GetPointer,3.11,
+function,PyBuffer_IsContiguous,3.11,
+function,PyBuffer_Release,3.11,
+function,PyBuffer_SizeFromFormat,3.11,
+function,PyBuffer_ToContiguous,3.11,
 var,PyByteArrayIter_Type,3.2,
 function,PyByteArray_AsString,3.2,
 function,PyByteArray_Concat,3.2,
@@ -375,6 +383,7 @@ function,PyMem_Malloc,3.2,
 function,PyMem_Realloc,3.2,
 type,PyMemberDef,3.2,
 var,PyMemberDescr_Type,3.2,
+function,PyMemoryView_FromBuffer,3.11,
 function,PyMemoryView_FromMemory,3.7,
 function,PyMemoryView_FromObject,3.2,
 function,PyMemoryView_GetContiguous,3.2,
@@ -476,8 +485,10 @@ function,PyObject_CallMethodObjArgs,3.2,
 function,PyObject_CallNoArgs,3.10,
 function,PyObject_CallObject,3.2,
 function,PyObject_Calloc,3.7,
+function,PyObject_CheckBuffer,3.11,
 function,PyObject_CheckReadBuffer,3.2,
 function,PyObject_ClearWeakRefs,3.2,
+function,PyObject_CopyData,3.11,
 function,PyObject_DelItem,3.2,
 function,PyObject_DelItemString,3.2,
 function,PyObject_Dir,3.2,
@@ -495,6 +506,7 @@ function,PyObject_GenericSetDict,3.7,
 function,PyObject_GetAIter,3.10,
 function,PyObject_GetAttr,3.2,
 function,PyObject_GetAttrString,3.2,
+function,PyObject_GetBuffer,3.11,
 function,PyObject_GetItem,3.2,
 function,PyObject_GetIter,3.2,
 function,PyObject_HasAttr,3.2,
@@ -832,6 +844,7 @@ var,Py_UTF8Mode,3.8,
 function,Py_VaBuildValue,3.2,
 var,Py_Version,3.11,
 function,Py_XNewRef,3.10,
+type,Py_buffer,3.11,
 type,Py_intptr_t,3.2,
 type,Py_ssize_t,3.2,
 type,Py_uintptr_t,3.2,
diff --git a/Doc/whatsnew/3.11.rst b/Doc/whatsnew/3.11.rst
index e7f3dab..3458ad6 100644
--- a/Doc/whatsnew/3.11.rst
+++ b/Doc/whatsnew/3.11.rst
@@ -657,6 +657,26 @@ New Features
   :c:macro:`PY_VERSION_HEX`.
   (Contributed by  Gabriele N. Tornetta in :issue:`43931`.)
 
+* :c:type:`Py_buffer` and APIs are now part of the limited API and the stable
+  ABI:
+
+  * :c:func:`PyObject_CheckBuffer`
+  * :c:func:`PyObject_GetBuffer`
+  * :c:func:`PyBuffer_GetPointer`
+  * :c:func:`PyBuffer_SizeFromFormat`
+  * :c:func:`PyBuffer_ToContiguous`
+  * :c:func:`PyBuffer_FromContiguous`
+  * :c:func:`PyBuffer_CopyData`
+  * :c:func:`PyBuffer_IsContiguous`
+  * :c:func:`PyBuffer_FillContiguousStrides`
+  * :c:func:`PyBuffer_FillInfo`
+  * :c:func:`PyBuffer_Release`
+  * :c:func:`PyMemoryView_FromBuffer`
+  * :c:member:`~PyBufferProcs.bf_getbuffer` and
+    :c:member:`~PyBufferProcs.bf_releasebuffer` type slots
+
+  (Contributed by Christian Heimes in :issue:`45459`.)
+
 
 Porting to Python 3.11
 ----------------------
diff --git a/Include/Python.h b/Include/Python.h
index 7260ae5..5416b04 100644
--- a/Include/Python.h
+++ b/Include/Python.h
@@ -50,6 +50,7 @@
 #include "longobject.h"
 #include "cpython/longintrepr.h"
 #include "boolobject.h"
+#include "buffer.h"
 #include "floatobject.h"
 #include "complexobject.h"
 #include "rangeobject.h"
diff --git a/Include/buffer.h b/Include/buffer.h
new file mode 100644
index 0000000..6893505
--- /dev/null
+++ b/Include/buffer.h
@@ -0,0 +1,142 @@
+/* Public Py_buffer API */
+
+#ifndef Py_BUFFER_H
+#define Py_BUFFER_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= 0x030b0000
+
+/* === New Buffer API ============================================
+ * Limited API and stable ABI since Python 3.11
+ *
+ * Py_buffer struct layout and size is now part of the stable abi3. The
+ * struct layout and size must not be changed in any way, as it would
+ * break the ABI.
+ *
+ */
+
+typedef struct {
+    void *buf;
+    PyObject *obj;        /* owned reference */
+    Py_ssize_t len;
+    Py_ssize_t itemsize;  /* This is Py_ssize_t so it can be
+                             pointed to by strides in simple case.*/
+    int readonly;
+    int ndim;
+    char *format;
+    Py_ssize_t *shape;
+    Py_ssize_t *strides;
+    Py_ssize_t *suboffsets;
+    void *internal;
+} Py_buffer;
+
+/* Return 1 if the getbuffer function is available, otherwise return 0. */
+PyAPI_FUNC(int) PyObject_CheckBuffer(PyObject *obj);
+
+/* This is a C-API version of the getbuffer function call.  It checks
+   to make sure object has the required function pointer and issues the
+   call.
+
+   Returns -1 and raises an error on failure and returns 0 on success. */
+PyAPI_FUNC(int) PyObject_GetBuffer(PyObject *obj, Py_buffer *view,
+                                   int flags);
+
+/* Get the memory area pointed to by the indices for the buffer given.
+   Note that view->ndim is the assumed size of indices. */
+PyAPI_FUNC(void *) PyBuffer_GetPointer(const Py_buffer *view, const Py_ssize_t *indices);
+
+/* Return the implied itemsize of the data-format area from a
+   struct-style description. */
+PyAPI_FUNC(Py_ssize_t) PyBuffer_SizeFromFormat(const char *format);
+
+/* Implementation in memoryobject.c */
+PyAPI_FUNC(int) PyBuffer_ToContiguous(void *buf, const Py_buffer *view,
+                                      Py_ssize_t len, char order);
+
+PyAPI_FUNC(int) PyBuffer_FromContiguous(const Py_buffer *view, const void *buf,
+                                        Py_ssize_t len, char order);
+
+/* Copy len bytes of data from the contiguous chunk of memory
+   pointed to by buf into the buffer exported by obj.  Return
+   0 on success and return -1 and raise a PyBuffer_Error on
+   error (i.e. the object does not have a buffer interface or
+   it is not working).
+
+   If fort is 'F', then if the object is multi-dimensional,
+   then the data will be copied into the array in
+   Fortran-style (first dimension varies the fastest).  If
+   fort is 'C', then the data will be copied into the array
+   in C-style (last dimension varies the fastest).  If fort
+   is 'A', then it does not matter and the copy will be made
+   in whatever way is more efficient. */
+PyAPI_FUNC(int) PyObject_CopyData(PyObject *dest, PyObject *src);
+
+/* Copy the data from the src buffer to the buffer of destination. */
+PyAPI_FUNC(int) PyBuffer_IsContiguous(const Py_buffer *view, char fort);
+
+/*Fill the strides array with byte-strides of a contiguous
+  (Fortran-style if fort is 'F' or C-style otherwise)
+  array of the given shape with the given number of bytes
+  per element. */
+PyAPI_FUNC(void) PyBuffer_FillContiguousStrides(int ndims,
+                                               Py_ssize_t *shape,
+                                               Py_ssize_t *strides,
+                                               int itemsize,
+                                               char fort);
+
+/* Fills in a buffer-info structure correctly for an exporter
+   that can only share a contiguous chunk of memory of
+   "unsigned bytes" of the given length.
+
+   Returns 0 on success and -1 (with raising an error) on error. */
+PyAPI_FUNC(int) PyBuffer_FillInfo(Py_buffer *view, PyObject *o, void *buf,
+                                  Py_ssize_t len, int readonly,
+                                  int flags);
+
+/* Releases a Py_buffer obtained from getbuffer ParseTuple's "s*". */
+PyAPI_FUNC(void) PyBuffer_Release(Py_buffer *view);
+
+/* Maximum number of dimensions */
+#define PyBUF_MAX_NDIM 64
+
+/* Flags for getting buffers */
+#define PyBUF_SIMPLE 0
+#define PyBUF_WRITABLE 0x0001
+
+#ifndef Py_LIMITED_API
+/*  we used to include an E, backwards compatible alias */
+#define PyBUF_WRITEABLE PyBUF_WRITABLE
+#endif
+
+#define PyBUF_FORMAT 0x0004
+#define PyBUF_ND 0x0008
+#define PyBUF_STRIDES (0x0010 | PyBUF_ND)
+#define PyBUF_C_CONTIGUOUS (0x0020 | PyBUF_STRIDES)
+#define PyBUF_F_CONTIGUOUS (0x0040 | PyBUF_STRIDES)
+#define PyBUF_ANY_CONTIGUOUS (0x0080 | PyBUF_STRIDES)
+#define PyBUF_INDIRECT (0x0100 | PyBUF_STRIDES)
+
+#define PyBUF_CONTIG (PyBUF_ND | PyBUF_WRITABLE)
+#define PyBUF_CONTIG_RO (PyBUF_ND)
+
+#define PyBUF_STRIDED (PyBUF_STRIDES | PyBUF_WRITABLE)
+#define PyBUF_STRIDED_RO (PyBUF_STRIDES)
+
+#define PyBUF_RECORDS (PyBUF_STRIDES | PyBUF_WRITABLE | PyBUF_FORMAT)
+#define PyBUF_RECORDS_RO (PyBUF_STRIDES | PyBUF_FORMAT)
+
+#define PyBUF_FULL (PyBUF_INDIRECT | PyBUF_WRITABLE | PyBUF_FORMAT)
+#define PyBUF_FULL_RO (PyBUF_INDIRECT | PyBUF_FORMAT)
+
+
+#define PyBUF_READ  0x100
+#define PyBUF_WRITE 0x200
+
+#endif /* !Py_LIMITED_API || Py_LIMITED_API >= 3.11 */
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* Py_BUFFER_H */
diff --git a/Include/cpython/abstract.h b/Include/cpython/abstract.h
index 2876a7b..b5a3139 100644
--- a/Include/cpython/abstract.h
+++ b/Include/cpython/abstract.h
@@ -168,74 +168,6 @@ PyAPI_FUNC(int) _PyObject_HasLen(PyObject *o);
    value.  If one of the calls fails, this function returns -1. */
 PyAPI_FUNC(Py_ssize_t) PyObject_LengthHint(PyObject *o, Py_ssize_t);
 
-/* === New Buffer API ============================================ */
-
-/* Return 1 if the getbuffer function is available, otherwise return 0. */
-PyAPI_FUNC(int) PyObject_CheckBuffer(PyObject *obj);
-
-/* This is a C-API version of the getbuffer function call.  It checks
-   to make sure object has the required function pointer and issues the
-   call.
-
-   Returns -1 and raises an error on failure and returns 0 on success. */
-PyAPI_FUNC(int) PyObject_GetBuffer(PyObject *obj, Py_buffer *view,
-                                   int flags);
-
-/* Get the memory area pointed to by the indices for the buffer given.
-   Note that view->ndim is the assumed size of indices. */
-PyAPI_FUNC(void *) PyBuffer_GetPointer(const Py_buffer *view, const Py_ssize_t *indices);
-
-/* Return the implied itemsize of the data-format area from a
-   struct-style description. */
-PyAPI_FUNC(Py_ssize_t) PyBuffer_SizeFromFormat(const char *format);
-
-/* Implementation in memoryobject.c */
-PyAPI_FUNC(int) PyBuffer_ToContiguous(void *buf, const Py_buffer *view,
-                                      Py_ssize_t len, char order);
-
-PyAPI_FUNC(int) PyBuffer_FromContiguous(const Py_buffer *view, const void *buf,
-                                        Py_ssize_t len, char order);
-
-/* Copy len bytes of data from the contiguous chunk of memory
-   pointed to by buf into the buffer exported by obj.  Return
-   0 on success and return -1 and raise a PyBuffer_Error on
-   error (i.e. the object does not have a buffer interface or
-   it is not working).
-
-   If fort is 'F', then if the object is multi-dimensional,
-   then the data will be copied into the array in
-   Fortran-style (first dimension varies the fastest).  If
-   fort is 'C', then the data will be copied into the array
-   in C-style (last dimension varies the fastest).  If fort
-   is 'A', then it does not matter and the copy will be made
-   in whatever way is more efficient. */
-PyAPI_FUNC(int) PyObject_CopyData(PyObject *dest, PyObject *src);
-
-/* Copy the data from the src buffer to the buffer of destination. */
-PyAPI_FUNC(int) PyBuffer_IsContiguous(const Py_buffer *view, char fort);
-
-/*Fill the strides array with byte-strides of a contiguous
-  (Fortran-style if fort is 'F' or C-style otherwise)
-  array of the given shape with the given number of bytes
-  per element. */
-PyAPI_FUNC(void) PyBuffer_FillContiguousStrides(int ndims,
-                                               Py_ssize_t *shape,
-                                               Py_ssize_t *strides,
-                                               int itemsize,
-                                               char fort);
-
-/* Fills in a buffer-info structure correctly for an exporter
-   that can only share a contiguous chunk of memory of
-   "unsigned bytes" of the given length.
-
-   Returns 0 on success and -1 (with raising an error) on error. */
-PyAPI_FUNC(int) PyBuffer_FillInfo(Py_buffer *view, PyObject *o, void *buf,
-                                  Py_ssize_t len, int readonly,
-                                  int flags);
-
-/* Releases a Py_buffer obtained from getbuffer ParseTuple's "s*". */
-PyAPI_FUNC(void) PyBuffer_Release(Py_buffer *view);
-
 /* === Sequence protocol ================================================ */
 
 /* Assume tp_as_sequence and sq_item exist and that 'i' does not
diff --git a/Include/cpython/object.h b/Include/cpython/object.h
index 7b9f3ac..1554ac8 100644
--- a/Include/cpython/object.h
+++ b/Include/cpython/object.h
@@ -2,6 +2,8 @@
 #  error "this header file must not be included directly"
 #endif
 
+#include "buffer.h" // for Py_buffer, included after PyObject has been defined
+
 PyAPI_FUNC(void) _Py_NewReference(PyObject *op);
 
 #ifdef Py_TRACE_REFS
@@ -45,61 +47,12 @@ typedef struct _Py_Identifier {
 #define _Py_static_string(varname, value)  static _Py_Identifier varname = _Py_static_string_init(value)
 #define _Py_IDENTIFIER(varname) _Py_static_string(PyId_##varname, #varname)
 
-/* buffer interface */
-typedef struct bufferinfo {
-    void *buf;
-    PyObject *obj;        /* owned reference */
-    Py_ssize_t len;
-    Py_ssize_t itemsize;  /* This is Py_ssize_t so it can be
-                             pointed to by strides in simple case.*/
-    int readonly;
-    int ndim;
-    char *format;
-    Py_ssize_t *shape;
-    Py_ssize_t *strides;
-    Py_ssize_t *suboffsets;
-    void *internal;
-} Py_buffer;
-
 typedef int (*getbufferproc)(PyObject *, Py_buffer *, int);
 typedef void (*releasebufferproc)(PyObject *, Py_buffer *);
 
 typedef PyObject *(*vectorcallfunc)(PyObject *callable, PyObject *const *args,
                                     size_t nargsf, PyObject *kwnames);
 
-/* Maximum number of dimensions */
-#define PyBUF_MAX_NDIM 64
-
-/* Flags for getting buffers */
-#define PyBUF_SIMPLE 0
-#define PyBUF_WRITABLE 0x0001
-/*  we used to include an E, backwards compatible alias  */
-#define PyBUF_WRITEABLE PyBUF_WRITABLE
-#define PyBUF_FORMAT 0x0004
-#define PyBUF_ND 0x0008
-#define PyBUF_STRIDES (0x0010 | PyBUF_ND)
-#define PyBUF_C_CONTIGUOUS (0x0020 | PyBUF_STRIDES)
-#define PyBUF_F_CONTIGUOUS (0x0040 | PyBUF_STRIDES)
-#define PyBUF_ANY_CONTIGUOUS (0x0080 | PyBUF_STRIDES)
-#define PyBUF_INDIRECT (0x0100 | PyBUF_STRIDES)
-
-#define PyBUF_CONTIG (PyBUF_ND | PyBUF_WRITABLE)
-#define PyBUF_CONTIG_RO (PyBUF_ND)
-
-#define PyBUF_STRIDED (PyBUF_STRIDES | PyBUF_WRITABLE)
-#define PyBUF_STRIDED_RO (PyBUF_STRIDES)
-
-#define PyBUF_RECORDS (PyBUF_STRIDES | PyBUF_WRITABLE | PyBUF_FORMAT)
-#define PyBUF_RECORDS_RO (PyBUF_STRIDES | PyBUF_FORMAT)
-
-#define PyBUF_FULL (PyBUF_INDIRECT | PyBUF_WRITABLE | PyBUF_FORMAT)
-#define PyBUF_FULL_RO (PyBUF_INDIRECT | PyBUF_FORMAT)
-
-
-#define PyBUF_READ  0x100
-#define PyBUF_WRITE 0x200
-/* End buffer interface */
-
 
 typedef struct {
     /* Number implementations must check *both*
diff --git a/Include/memoryobject.h b/Include/memoryobject.h
index 0298cc9..154397c 100644
--- a/Include/memoryobject.h
+++ b/Include/memoryobject.h
@@ -25,7 +25,7 @@ PyAPI_FUNC(PyObject *) PyMemoryView_FromObject(PyObject *base);
 PyAPI_FUNC(PyObject *) PyMemoryView_FromMemory(char *mem, Py_ssize_t size,
                                                int flags);
 #endif
-#ifndef Py_LIMITED_API
+#if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= 0x030b0000
 PyAPI_FUNC(PyObject *) PyMemoryView_FromBuffer(const Py_buffer *info);
 #endif
 PyAPI_FUNC(PyObject *) PyMemoryView_GetContiguous(PyObject *base,
diff --git a/Include/typeslots.h b/Include/typeslots.h
index 5800d01..506b055 100644
--- a/Include/typeslots.h
+++ b/Include/typeslots.h
@@ -1,12 +1,6 @@
 /* Do not renumber the file; these numbers are part of the stable ABI. */
-#if defined(Py_LIMITED_API)
-/* Disabled, see #10181 */
-#undef Py_bf_getbuffer
-#undef Py_bf_releasebuffer
-#else
 #define Py_bf_getbuffer 1
 #define Py_bf_releasebuffer 2
-#endif
 #define Py_mp_ass_subscript 3
 #define Py_mp_length 4
 #define Py_mp_subscript 5
diff --git a/Lib/test/test_stable_abi_ctypes.py b/Lib/test/test_stable_abi_ctypes.py
index 9fd6b14..a49235b 100644
--- a/Lib/test/test_stable_abi_ctypes.py
+++ b/Lib/test/test_stable_abi_ctypes.py
@@ -28,6 +28,14 @@ SYMBOL_NAMES = (
     "PyBaseObject_Type",
     "PyBool_FromLong",
     "PyBool_Type",
+    "PyBuffer_FillContiguousStrides",
+    "PyBuffer_FillInfo",
+    "PyBuffer_FromContiguous",
+    "PyBuffer_GetPointer",
+    "PyBuffer_IsContiguous",
+    "PyBuffer_Release",
+    "PyBuffer_SizeFromFormat",
+    "PyBuffer_ToContiguous",
     "PyByteArrayIter_Type",
     "PyByteArray_AsString",
     "PyByteArray_Concat",
@@ -381,6 +389,7 @@ SYMBOL_NAMES = (
     "PyMemberDescr_Type",
     "PyMember_GetOne",
     "PyMember_SetOne",
+    "PyMemoryView_FromBuffer",
     "PyMemoryView_FromMemory",
     "PyMemoryView_FromObject",
     "PyMemoryView_GetContiguous",
@@ -470,8 +479,10 @@ SYMBOL_NAMES = (
     "PyObject_CallNoArgs",
     "PyObject_CallObject",
     "PyObject_Calloc",
+    "PyObject_CheckBuffer",
     "PyObject_CheckReadBuffer",
     "PyObject_ClearWeakRefs",
+    "PyObject_CopyData",
     "PyObject_DelItem",
     "PyObject_DelItemString",
     "PyObject_Dir",
@@ -489,6 +500,7 @@ SYMBOL_NAMES = (
     "PyObject_GetAIter",
     "PyObject_GetAttr",
     "PyObject_GetAttrString",
+    "PyObject_GetBuffer",
     "PyObject_GetItem",
     "PyObject_GetIter",
     "PyObject_HasAttr",
diff --git a/Lib/test/test_xxlimited.py b/Lib/test/test_xxlimited.py
index e3f521d..6dbfb3f 100644
--- a/Lib/test/test_xxlimited.py
+++ b/Lib/test/test_xxlimited.py
@@ -58,6 +58,17 @@ class TestXXLimited(CommonTests, unittest.TestCase):
         with self.assertRaises(self.module.Error):
             raise self.module.Error
 
+    def test_buffer(self):
+        xxo = self.module.Xxo()
+        self.assertEqual(xxo.x_exports, 0)
+        b1 = memoryview(xxo)
+        self.assertEqual(xxo.x_exports, 1)
+        b2 = memoryview(xxo)
+        self.assertEqual(xxo.x_exports, 2)
+        b1[0] = 1
+        self.assertEqual(b1[0], 1)
+        self.assertEqual(b2[0], 1)
+
 
 class TestXXLimited35(CommonTests, unittest.TestCase):
     module = xxlimited_35
diff --git a/Makefile.pre.in b/Makefile.pre.in
index edc5fc3..4dcedd6 100644
--- a/Makefile.pre.in
+++ b/Makefile.pre.in
@@ -1439,6 +1439,7 @@ PYTHON_HEADERS= \
 		$(srcdir)/Include/abstract.h \
 		$(srcdir)/Include/bltinmodule.h \
 		$(srcdir)/Include/boolobject.h \
+		$(srcdir)/Include/buffer.h \
 		$(srcdir)/Include/bytearrayobject.h \
 		$(srcdir)/Include/bytesobject.h \
 		$(srcdir)/Include/ceval.h \
diff --git a/Misc/NEWS.d/next/C API/2021-10-18-16-54-24.bpo-45459.Y1pEZs.rst b/Misc/NEWS.d/next/C API/2021-10-18-16-54-24.bpo-45459.Y1pEZs.rst
new file mode 100644
index 0000000..a8d9322
--- /dev/null
+++ b/Misc/NEWS.d/next/C API/2021-10-18-16-54-24.bpo-45459.Y1pEZs.rst
@@ -0,0 +1,2 @@
+:c:type:`Py_buffer` and various ``Py_buffer`` related functions are now
+part of the limited API and stable ABI.
diff --git a/Misc/stable_abi.txt b/Misc/stable_abi.txt
index c4f5318..cc3cc56 100644
--- a/Misc/stable_abi.txt
+++ b/Misc/stable_abi.txt
@@ -2191,6 +2191,34 @@ function PyType_GetQualName
 data PyStructSequence_UnnamedField
     added 3.11
 
+# Add stable Py_buffer API in Python 3.11 (https://bugs.python.org/issue45459)
+struct Py_buffer
+    added 3.11
+function PyObject_CheckBuffer
+    added 3.11
+function PyObject_GetBuffer
+    added 3.11
+function PyBuffer_GetPointer
+    added 3.11
+function PyBuffer_SizeFromFormat
+    added 3.11
+function PyBuffer_ToContiguous
+    added 3.11
+function PyBuffer_FromContiguous
+    added 3.11
+function PyObject_CopyData
+    added 3.11
+function PyBuffer_IsContiguous
+    added 3.11
+function PyBuffer_FillContiguousStrides
+    added 3.11
+function PyBuffer_FillInfo
+    added 3.11
+function PyBuffer_Release
+    added 3.11
+function PyMemoryView_FromBuffer
+    added 3.11
+
 # (Detailed comments aren't really needed for further entries: from here on
 #  we can use version control logs.)
 
diff --git a/Modules/xxlimited.c b/Modules/xxlimited.c
index 93895c4..16d1b83 100644
--- a/Modules/xxlimited.c
+++ b/Modules/xxlimited.c
@@ -19,6 +19,7 @@
           def __init__(self):
               # In the C class, "_x_attr" is not accessible from Python code
               self._x_attr = {}
+              self._x_exports = 0
 
           def __getattr__(self, name):
               return self._x_attr[name]
@@ -29,6 +30,13 @@
           def __delattr__(self, name):
               del self._x_attr[name]
 
+          @property
+          def x_exports(self):
+              """Return the number of times an internal buffer is exported."""
+              # Each Xxo instance has a 10-byte buffer that can be
+              # accessed via the buffer interface (e.g. `memoryview`).
+              return self._x_exports
+
           def demo(o, /):
               if isinstance(o, str):
                   return o
@@ -57,6 +65,9 @@
 #define Py_LIMITED_API 0x030b0000
 
 #include "Python.h"
+#include <string.h>
+
+#define BUFSIZE 10
 
 // Module state
 typedef struct {
@@ -70,7 +81,9 @@ typedef struct {
 // Instance state
 typedef struct {
     PyObject_HEAD
-    PyObject            *x_attr;        /* Attributes dictionary */
+    PyObject            *x_attr;           /* Attributes dictionary */
+    char                x_buffer[BUFSIZE]; /* buffer for Py_buffer */
+    Py_ssize_t          x_exports;         /* how many buffer are exported */
 } XxoObject;
 
 // XXX: no good way to do this yet
@@ -89,6 +102,8 @@ newXxoObject(PyObject *module)
         return NULL;
     }
     self->x_attr = NULL;
+    memset(self->x_buffer, 0, BUFSIZE);
+    self->x_exports = 0;
     return self;
 }
 
@@ -212,11 +227,43 @@ static PyMethodDef Xxo_methods[] = {
     {NULL,              NULL}           /* sentinel */
 };
 
+/* Xxo buffer interface */
+
+static int
+Xxo_getbuffer(XxoObject *self, Py_buffer *view, int flags)
+{
+    int res = PyBuffer_FillInfo(view, (PyObject*)self,
+                               (void *)self->x_buffer, BUFSIZE,
+                               0, flags);
+    if (res == 0) {
+        self->x_exports++;
+    }
+    return res;
+}
+
+static void
+Xxo_releasebuffer(XxoObject *self, Py_buffer *view)
+{
+    self->x_exports--;
+}
+
+static PyObject *
+Xxo_get_x_exports(XxoObject *self, void *c)
+{
+    return PyLong_FromSsize_t(self->x_exports);
+}
+
 /* Xxo type definition */
 
 PyDoc_STRVAR(Xxo_doc,
              "A class that explicitly stores attributes in an internal dict");
 
+static PyGetSetDef Xxo_getsetlist[] = {
+    {"x_exports", (getter) Xxo_get_x_exports, NULL, NULL},
+    {NULL},
+};
+
+
 static PyType_Slot Xxo_Type_slots[] = {
     {Py_tp_doc, (char *)Xxo_doc},
     {Py_tp_traverse, Xxo_traverse},
@@ -226,6 +273,9 @@ static PyType_Slot Xxo_Type_slots[] = {
     {Py_tp_getattro, Xxo_getattro},
     {Py_tp_setattro, Xxo_setattro},
     {Py_tp_methods, Xxo_methods},
+    {Py_bf_getbuffer, Xxo_getbuffer},
+    {Py_bf_releasebuffer, Xxo_releasebuffer},
+    {Py_tp_getset, Xxo_getsetlist},
     {0, 0},  /* sentinel */
 };
 
diff --git a/PC/python3dll.c b/PC/python3dll.c
index b2bb170..70f11dc 100755
--- a/PC/python3dll.c
+++ b/PC/python3dll.c
@@ -94,6 +94,14 @@ EXPORT_FUNC(PyArg_ValidateKeywordArguments)
 EXPORT_FUNC(PyArg_VaParse)
 EXPORT_FUNC(PyArg_VaParseTupleAndKeywords)
 EXPORT_FUNC(PyBool_FromLong)
+EXPORT_FUNC(PyBuffer_FillContiguousStrides)
+EXPORT_FUNC(PyBuffer_FillInfo)
+EXPORT_FUNC(PyBuffer_FromContiguous)
+EXPORT_FUNC(PyBuffer_GetPointer)
+EXPORT_FUNC(PyBuffer_IsContiguous)
+EXPORT_FUNC(PyBuffer_Release)
+EXPORT_FUNC(PyBuffer_SizeFromFormat)
+EXPORT_FUNC(PyBuffer_ToContiguous)
 EXPORT_FUNC(PyByteArray_AsString)
 EXPORT_FUNC(PyByteArray_Concat)
 EXPORT_FUNC(PyByteArray_FromObject)
@@ -352,6 +360,7 @@ EXPORT_FUNC(PyMem_Malloc)
 EXPORT_FUNC(PyMem_Realloc)
 EXPORT_FUNC(PyMember_GetOne)
 EXPORT_FUNC(PyMember_SetOne)
+EXPORT_FUNC(PyMemoryView_FromBuffer)
 EXPORT_FUNC(PyMemoryView_FromMemory)
 EXPORT_FUNC(PyMemoryView_FromObject)
 EXPORT_FUNC(PyMemoryView_GetContiguous)
@@ -426,8 +435,10 @@ EXPORT_FUNC(PyObject_CallMethodObjArgs)
 EXPORT_FUNC(PyObject_CallNoArgs)
 EXPORT_FUNC(PyObject_CallObject)
 EXPORT_FUNC(PyObject_Calloc)
+EXPORT_FUNC(PyObject_CheckBuffer)
 EXPORT_FUNC(PyObject_CheckReadBuffer)
 EXPORT_FUNC(PyObject_ClearWeakRefs)
+EXPORT_FUNC(PyObject_CopyData)
 EXPORT_FUNC(PyObject_DelItem)
 EXPORT_FUNC(PyObject_DelItemString)
 EXPORT_FUNC(PyObject_Dir)
@@ -445,6 +456,7 @@ EXPORT_FUNC(PyObject_GenericSetDict)
 EXPORT_FUNC(PyObject_GetAIter)
 EXPORT_FUNC(PyObject_GetAttr)
 EXPORT_FUNC(PyObject_GetAttrString)
+EXPORT_FUNC(PyObject_GetBuffer)
 EXPORT_FUNC(PyObject_GetItem)
 EXPORT_FUNC(PyObject_GetIter)
 EXPORT_FUNC(PyObject_HasAttr)