diff --git a/include/pocketpy/vm.h b/include/pocketpy/vm.h
index 3e5451d6..c2b23252 100644
--- a/include/pocketpy/vm.h
+++ b/include/pocketpy/vm.h
@@ -141,7 +141,7 @@ public:
     struct{
         PyObject* error;
         stack_no_copy<ArgsView> s_view;
-    } _c;
+    } __c;
 
     PyObject *None, *True, *False, *NotImplemented;
     PyObject *StopIteration, *Ellipsis;
diff --git a/src/pocketpy_c.cpp b/src/pocketpy_c.cpp
index 005ba852..e5a33f82 100644
--- a/src/pocketpy_c.cpp
+++ b/src/pocketpy_c.cpp
@@ -16,15 +16,15 @@ typedef int (*LuaStyleFuncC)(VM*);
     }
 
 #define PK_ASSERT_NO_ERROR() \
-    if(vm->_c.error != nullptr) \
+    if(vm->__c.error != nullptr) \
         return false;
 
 static int count_extra_elements(VM* vm, int n){
     if(vm->callstack.empty()){
         return vm->s_data.size();
     }
-    PK_ASSERT(!vm->_c.s_view.empty());
-    return vm->s_data._sp - vm->_c.s_view.top().end();
+    PK_ASSERT(!vm->__c.s_view.empty());
+    return vm->s_data._sp - vm->__c.s_view.top().end();
 }
 
 static PyObject* stack_item(VM* vm, int index){
@@ -33,8 +33,8 @@ static PyObject* stack_item(VM* vm, int index){
     if(vm->callstack.empty()){
         begin = vm->s_data.begin();
     }else{
-        PK_ASSERT(!vm->_c.s_view.empty());
-        begin = vm->_c.s_view.top().begin();
+        PK_ASSERT(!vm->__c.s_view.empty());
+        begin = vm->__c.s_view.top().begin();
     }
     int size = end - begin;
     if(index < 0) index += size;
@@ -47,11 +47,11 @@ static PyObject* stack_item(VM* vm, int index){
 #define PK_PROTECTED(__B) \
     try{ __B }  \
     catch(const Exception& e ) { \
-        vm->_c.error = e.self(); \
+        vm->__c.error = e.self(); \
         return false; \
     } catch(const std::exception& re){ \
         PyObject* e_t = vm->_t(vm->tp_exception); \
-        vm->_c.error = vm->call(e_t, VAR(re.what())); \
+        vm->__c.error = vm->call(e_t, VAR(re.what())); \
         return false; \
     }
 
@@ -144,8 +144,8 @@ int pkpy_stack_size(pkpy_vm* vm_handle){
     if(vm->callstack.empty()){
         return vm->s_data.size();
     }
-    if(vm->_c.s_view.empty()) exit(127);
-    return vm->s_data._sp - vm->_c.s_view.top().begin();
+    if(vm->__c.s_view.empty()) exit(127);
+    return vm->s_data._sp - vm->__c.s_view.top().begin();
 }
 
 // int
@@ -325,7 +325,7 @@ struct TempViewPopper{
 
     void restore() noexcept{
         if(used) return;
-        vm->_c.s_view.pop();
+        vm->__c.s_view.pop();
         used = true;
     }
 
@@ -337,15 +337,15 @@ static PyObject* c_function_wrapper(VM* vm, ArgsView args) {
     LuaStyleFuncC f = lambda_get_userdata<LuaStyleFuncC>(args.begin());
     PyObject** curr_sp = vm->s_data._sp;
 
-    vm->_c.s_view.push(args);
+    vm->__c.s_view.push(args);
     TempViewPopper _tvp(vm);
     int retc = f(vm);       // may raise, _tvp will handle this via RAII
     _tvp.restore();
 
     // propagate_if_errored
-    if (vm->_c.error != nullptr){
-        PyObject* e_obj = PK_OBJ_GET(Exception, vm->_c.error).self();
-        vm->_c.error = nullptr;
+    if (vm->__c.error != nullptr){
+        PyObject* e_obj = PK_OBJ_GET(Exception, vm->__c.error).self();
+        vm->__c.error = nullptr;
         vm->_error(e_obj);
         return nullptr;
     }
@@ -503,30 +503,30 @@ bool pkpy_error(pkpy_vm* vm_handle, const char* name, pkpy_CString message) {
             std::cerr << "[warning] pkpy_error(): " << Str(name).escape() << " not found, fallback to 'Exception'" << std::endl;
         }
     }
-    vm->_c.error = vm->call(e_t, VAR(message));
+    vm->__c.error = vm->call(e_t, VAR(message));
     return false;
 }
 
 bool pkpy_check_error(pkpy_vm* vm_handle) {
     VM* vm = (VM*) vm_handle;
-    return vm->_c.error != nullptr;
+    return vm->__c.error != nullptr;
 }
 
 bool pkpy_clear_error(pkpy_vm* vm_handle, char** message) {
     VM* vm = (VM*) vm_handle;
     // no error
-    if (vm->_c.error == nullptr) return false;
-    Exception& e = PK_OBJ_GET(Exception, vm->_c.error);
+    if (vm->__c.error == nullptr) return false;
+    Exception& e = PK_OBJ_GET(Exception, vm->__c.error);
     if (message != nullptr)
         *message = strdup(e.summary().c_str());
     else
         std::cout << e.summary() << std::endl;
-    vm->_c.error = nullptr;
+    vm->__c.error = nullptr;
     if(vm->callstack.empty()){
         vm->s_data.clear();
     }else{
-        if(vm->_c.s_view.empty()) exit(127);
-        vm->s_data.reset(vm->_c.s_view.top().end());
+        if(vm->__c.s_view.empty()) exit(127);
+        vm->s_data.reset(vm->__c.s_view.top().end());
     }
     return true;
 }
diff --git a/src/vm.cpp b/src/vm.cpp
index dd9b5490..77239a1d 100644
--- a/src/vm.cpp
+++ b/src/vm.cpp
@@ -74,7 +74,7 @@ namespace pkpy{
 
     VM::VM(bool enable_os) : heap(this), enable_os(enable_os) {
         this->vm = this;
-        this->_c.error = nullptr;
+        this->__c.error = nullptr;
         _stdout = [](const char* buf, int size) { std::cout.write(buf, size); };
         _stderr = [](const char* buf, int size) { std::cerr.write(buf, size); };
         _main = nullptr;
@@ -1336,7 +1336,7 @@ void ManagedHeap::mark() {
     for(auto [_, co]: vm->__cached_codes) co->_gc_mark();
     if(vm->__last_exception) PK_OBJ_MARK(vm->__last_exception);
     if(vm->__curr_class) PK_OBJ_MARK(vm->__curr_class);
-    if(vm->_c.error != nullptr) PK_OBJ_MARK(vm->_c.error);
+    if(vm->__c.error != nullptr) PK_OBJ_MARK(vm->__c.error);
     if(_gc_marker_ex) _gc_marker_ex(vm);
 }