Add Py_fclose() function

Doc/c-api/sys.rst

@@ -226,13 +226,28 @@ Operating System Utilities
    On success, return the new file object.
    On error, set an exception and return ``NULL``.
 
+   The file must be closed by :c:func:`Py_fclose` rather than calling directly
+   ``fclose()``.
+
    The file descriptor is created non-inheritable (:pep:`446`).
 
    The caller must hold the GIL.
 
    .. versionadded:: next
 
 
+.. c:function:: int Py_fclose(FILE *file)
+
+   Call ``fclose(file)``.
+
+   This function is needed on Windows: ``FILE*`` files opened by
+   :c:func:`Py_fopen` in the Python DLL must be closed by the Python DLL to use
+   the same C runtime version. Otherwise, calling ``fclose()`` directly can
+   cause undefined behavior.
+
+   .. versionadded:: next
+
+
 .. _systemfunctions:
 
 System Functions

Doc/whatsnew/3.14.rst

@@ -1036,7 +1036,8 @@ New features
 
 * Add :c:func:`Py_fopen` function to open a file. Similar to the
   :c:func:`!fopen` function, but the *path* parameter is a Python object and an
-  exception is set on error.
+  exception is set on error. Add also :c:func:`Py_fclose` function to close a
+  file, function needed for Windows support.
   (Contributed by Victor Stinner in :gh:`127350`.)
 
 Porting to Python 3.14

Include/cpython/fileutils.h

@@ -5,3 +5,5 @@
 PyAPI_FUNC(FILE*) Py_fopen(
     PyObject *path,
     const char *mode);
+
+PyAPI_FUNC(int) Py_fclose(FILE *file);

Misc/NEWS.d/next/C_API/2024-12-11-13-01-26.gh-issue-127350.uEBZZ4.rst

@@ -1,3 +1,5 @@
 Add :c:func:`Py_fopen` function to open a file. Similar to the :c:func:`!fopen`
 function, but the *path* parameter is a Python object and an exception is set
-on error. Patch by Victor Stinner.
+on error. Add also :c:func:`Py_fclose` function to close a file, function
+needed for Windows support.
+Patch by Victor Stinner.

Modules/_testcapi/file.c

@@ -17,7 +17,7 @@ _testcapi.py_fopen
     mode: str
     /
 
-Call Py_fopen() and return fread(256).
+Call Py_fopen(), fread(256) and Py_fclose(). Return read bytes.
 [clinic start generated code]*/
 
 static PyObject *
@@ -31,7 +31,7 @@ _testcapi_py_fopen_impl(PyObject *module, PyObject *path, const char *mode)
 
     char buffer[256];
     size_t size = fread(buffer, 1, Py_ARRAY_LENGTH(buffer), fp);
-    fclose(fp);
+    Py_fclose(fp);
 
     return PyBytes_FromStringAndSize(buffer, size);
 }

Python/fileutils.c

@@ -1836,6 +1836,19 @@ Py_fopen(PyObject *path, const char *mode)
     return f;
 }
 
+
+// Call fclose().
+//
+// This function is needed on Windows: FILE* files opened by Py_fopen() in the
+// Python DLL must be closed by the Python DLL to use the same C runtime
+// version. Otherwise, calling fclose() directly can cause undefined behavior.
+int
+Py_fclose(FILE *file)
+{
+    return fclose(file);
+}
+
+
 /* Read count bytes from fd into buf.
 
    On success, return the number of read bytes, it can be lower than count.