python · search1ofall-maker · Sep 18, 2026 · Sep 18, 2026 · Sep 18, 2026 · Sep 18, 2026
diff --git a/Lib/test/test_os/test_os.py b/Lib/test/test_os/test_os.py
@@ -1693,6 +1693,18 @@ def test_clearenv(self):
 
         self.assertRaises(TypeError, os.environ.clear, None)
 
+    def test_clearenv_environ_not_null(self):
+        import ctypes
+        os.environ.clear()
+        try:
+            c_environ = ctypes.c_void_p.in_dll(ctypes.CDLL(None), "environ")
+            self.assertIsNotNone(
+                c_environ.value,
+                "os.environ.clear() set the C 'environ' pointer to NULL"
+            )
+        except AttributeError:
+            pass    
+
 
 class WalkTests(unittest.TestCase):
     """Tests for os.walk()."""

diff --git a/Misc/NEWS.d/next/Library/2026-09-18-15-30-00.GH-157741.rst b/Misc/NEWS.d/next/Library/2026-09-18-15-30-00.GH-157741.rst
@@ -0,0 +1 @@
+Fix crash in third-party C libraries (such as Tcl/Tk) after :func:`os.environ.clear` by ensuring the C ``environ`` pointer is not left as ``NULL``.
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
@@ -13893,7 +13893,8 @@ os_unsetenv_impl(PyObject *module, PyObject *name)
 }
 #endif /* !MS_WINDOWS */
 
-
+static char *empty_environ[] = { NULL };
+/*NR8889*/
 #ifdef HAVE_CLEARENV
 /*[clinic input]
 os._clearenv
@@ -13912,6 +13913,12 @@ os__clearenv_impl(PyObject *module)
     if (err) {
         return posix_error();
     }
+
+    /* glibc's clearenv() sets 'environ' to NULL. Point it to a static empty
+       array to prevent crashes in third-party C libraries (e.g. Tcl/Tk)
+       that access 'environ' without a NULL check. */
+    environ = empty_environ;
+
     Py_RETURN_NONE;
 }
 #endif
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Fix crash in third-party C libraries (such as Tcl/Tk) after :func:`os.environ.clear` by ensuring the C ``environ`` pointer is not left as ``NULL``.