[3.14] gh-141004: document curses C API (GH-141254) (#141292)

gh-141004: document `curses` C API (GH-141254)
(cherry picked from commit dbe4090)

Co-authored-by: Bénédikt Tran <[email protected]>

Author: miss-islington

Doc/c-api/concrete.rst

@@ -115,5 +115,13 @@ Other Objects
    gen.rst
    coro.rst
    contextvars.rst
-   datetime.rst
    typehints.rst
+
+
+C API for extension modules
+===========================
+
+.. toctree::
+
+   curses.rst
+   datetime.rst

Doc/c-api/curses.rst

@@ -0,0 +1,138 @@
+.. highlight:: c
+
+Curses C API
+------------
+
+:mod:`curses` exposes a small C interface for extension modules.
+Consumers must include the header file :file:`py_curses.h` (which is not
+included by default by :file:`Python.h`) and :c:func:`import_curses` must
+be invoked, usually as part of the module initialisation function, to populate
+:c:var:`PyCurses_API`.
+
+.. warning::
+
+   Neither the C API nor the pure Python :mod:`curses` module are compatible
+   with subinterpreters.
+
+.. c:macro:: import_curses()
+
+   Import the curses C API. The macro does not need a semi-colon to be called.
+
+   On success, populate the :c:var:`PyCurses_API` pointer.
+
+   On failure, set :c:var:`PyCurses_API` to NULL and set an exception.
+   The caller must check if an error occurred via :c:func:`PyErr_Occurred`:
+
+   .. code-block::
+
+      import_curses();  // semi-colon is optional but recommended
+      if (PyErr_Occurred()) { /* cleanup */ }
+
+
+.. c:var:: void **PyCurses_API
+
+   Dynamically allocated object containing the curses C API.
+   This variable is only available once :c:macro:`import_curses` succeeds.
+
+   ``PyCurses_API[0]`` corresponds to :c:data:`PyCursesWindow_Type`.
+
+   ``PyCurses_API[1]``, ``PyCurses_API[2]``, and ``PyCurses_API[3]``
+   are pointers to predicate functions of type ``int (*)(void)``.
+
+   When called, these predicates return whether :func:`curses.setupterm`,
+   :func:`curses.initscr`, and :func:`curses.start_color` have been called
+   respectively.
+
+   See also the convenience macros :c:macro:`PyCursesSetupTermCalled`,
+   :c:macro:`PyCursesInitialised`, and :c:macro:`PyCursesInitialisedColor`.
+
+   .. note::
+
+      The number of entries in this structure is subject to changes.
+      Consider using :c:macro:`PyCurses_API_pointers` to check if
+      new fields are available or not.
+
+
+.. c:macro:: PyCurses_API_pointers
+
+   The number of accessible fields (``4``) in :c:var:`PyCurses_API`.
+   This number is incremented whenever new fields are added.
+
+
+.. c:var:: PyTypeObject PyCursesWindow_Type
+
+   The :ref:`heap type <heap-types>` corresponding to :class:`curses.window`.
+
+
+.. c:function:: int PyCursesWindow_Check(PyObject *op)
+
+   Return true if *op* is a :class:`curses.window` instance, false otherwise.
+
+
+The following macros are convenience macros expanding into C statements.
+In particular, they can only be used as ``macro;`` or ``macro``, but not
+``macro()`` or ``macro();``.
+
+.. c:macro:: PyCursesSetupTermCalled
+
+   Macro checking if :func:`curses.setupterm` has been called.
+
+   The macro expansion is roughly equivalent to:
+
+   .. code-block::
+
+      {
+          typedef int (*predicate_t)(void);
+          predicate_t was_setupterm_called = (predicate_t)PyCurses_API[1];
+          if (!was_setupterm_called()) {
+              return NULL;
+          }
+      }
+
+
+.. c:macro:: PyCursesInitialised
+
+   Macro checking if :func:`curses.initscr` has been called.
+
+   The macro expansion is roughly equivalent to:
+
+   .. code-block::
+
+      {
+          typedef int (*predicate_t)(void);
+          predicate_t was_initscr_called = (predicate_t)PyCurses_API[2];
+          if (!was_initscr_called()) {
+              return NULL;
+          }
+      }
+
+
+.. c:macro:: PyCursesInitialisedColor
+
+   Macro checking if :func:`curses.start_color` has been called.
+
+   The macro expansion is roughly equivalent to:
+
+   .. code-block::
+
+      {
+          typedef int (*predicate_t)(void);
+          predicate_t was_start_color_called = (predicate_t)PyCurses_API[3];
+          if (!was_start_color_called()) {
+              return NULL;
+          }
+      }
+
+
+Internal data
+-------------
+
+The following objects are exposed by the C API but should be considered
+internal-only.
+
+.. c:macro:: PyCurses_CAPSULE_NAME
+
+   Name of the curses capsule to pass to :c:func:`PyCapsule_Import`.
+
+   Internal usage only. Use :c:macro:`import_curses` instead.
+