diff --git a/Python/pystate.c b/Python/pystate.c
index 2fc563b..0a6d035 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -159,6 +159,12 @@
 #define HEAD_UNLOCK(runtime) \
     PyThread_release_lock((runtime)->interpreters.mutex)
 
+int
+_Py_IsMainInterpreter(PyThreadState* tstate)
+{
+    return (tstate->interp == tstate->interp->runtime->interpreters.main);
+}
+
 /* Forward declaration */
 static void _PyGILState_NoteThreadState(
     struct _gilstate_runtime_state *gilstate, PyThreadState* tstate);