gh-157741: Prevent NULL 'environ' pointer after os.environ.clear()

Lib/test/test_os/test_os.py

@@ -1693,6 +1693,17 @@ def test_clearenv(self):
 
         self.assertRaises(TypeError, os.environ.clear, None)
 
+    def test_clearenv_environ_not_null(self):
+        from test.support import import_helper
+        ctypes = import_helper.import_module('ctypes')
+
+        os.environ.clear()
+        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"
+        )
+
 
 class WalkTests(unittest.TestCase):
     """Tests for os.walk()."""

Misc/NEWS.d/next/Library/2026-09-18-18-07-38.gh-issue-157741.Bbt18o.rst

@@ -0,0 +1,2 @@
+Fix crash in C libraries after os.environ.clear() by ensuring environ
+pointer is not left as NULL.

Modules/posixmodule.c

@@ -13895,6 +13895,8 @@ os_unsetenv_impl(PyObject *module, PyObject *name)
 
 
 #ifdef HAVE_CLEARENV
+static char *empty_environ[] = { NULL };
+
 /*[clinic input]
 os._clearenv
 [clinic start generated code]*/
@@ -13912,6 +13914,12 @@ 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