diff --git a/CMakeLists.txt b/CMakeLists.txt
index 706880d..39e57cd 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -9,6 +9,7 @@ set(CMAKE_CXX_FLAGS_RELEASE_INIT "-Wshadow -Wall")
 
 # generate release executables
 set(CMAKE_BUILD_TYPE "Release")
+set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_SOURCE_DIR}/module)
 
 add_library(fib SHARED ${CMAKE_SOURCE_DIR}/module/fib.cpp)
 target_include_directories(fib PRIVATE ${CMAKE_SOURCE_DIR})
@@ -24,4 +25,11 @@ target_include_directories(nasock PRIVATE ${CMAKE_SOURCE_DIR})
 
 add_executable(nasal main.cpp)
 target_link_libraries(nasal dl)
-target_include_directories(nasal PRIVATE ${CMAKE_SOURCE_DIR})
\ No newline at end of file
+target_include_directories(nasal PRIVATE ${CMAKE_SOURCE_DIR})
+
+add_custom_command(
+    TARGET nasal POST_BUILD
+    COMMAND ${CMAKE_COMMAND} -E copy
+            ${CMAKE_SOURCE_DIR}/build/nasal
+            ${CMAKE_SOURCE_DIR}/nasal
+)
\ No newline at end of file
diff --git a/module/fib.cpp b/module/fib.cpp
index b58ab8e..70c2219 100644
--- a/module/fib.cpp
+++ b/module/fib.cpp
@@ -1,6 +1,10 @@
+// module for test
+
 #include <iostream>
 #include "../nasal.h"
 
+namespace nasal_fib_module {
+
 double fibonaci(double x) {
     if (x<=2) {
         return x;
@@ -33,12 +37,56 @@ var quick_fib(var* args, usize size, gc* ngc) {
     return var::num(res);
 }
 
-mod_func func_tbl[]={
-    {"fib",fib},
-    {"quick_fib",quick_fib},
-    {nullptr, nullptr},
+u32 ghost_for_test;
+
+void ghost_for_test_destructor(void* ptr) {
+    std::cout<<"delete "<<ptr<<"\n";
+    delete (u32*)ptr;
+}
+
+var create_new_ghost(var* args, usize size, gc* ngc) {
+    var res=ngc->alloc(vm_obj);
+    res.obj().set(ghost_for_test, new u32);
+    return res;
+}
+
+var set_new_ghost(var* args, usize size, gc* ngc) {
+    var res=args[0];
+    if (!res.objchk(ghost_for_test)) {
+        std::cout<<"set_new_ghost: not ghost for test type.\n";
+        return nil;
+    }
+    std::cout<<"set_new_ghost: successfully set ghost.\n";
+    f64 num=args[1].num();
+    *((u32*)res.obj().ptr)=static_cast<u32>(num);
+    return nil;
+}
+
+var print_new_ghost(var* args, usize size, gc* ngc) {
+    var res=args[0];
+    if (!res.objchk(ghost_for_test)) {
+        std::cout<<"print_new_ghost: not ghost for test type.\n";
+        return nil;
+    }
+    std::cout<<"print_new_ghost: result = "<<*((u32*)res.obj().ptr)<<"\n";
+    return nil;
+}
+
+module_func_info func_tbl[]={
+    {"fib", fib},
+    {"quick_fib", quick_fib},
+    {"create_ghost", create_new_ghost},
+    {"set_ghost", set_new_ghost},
+    {"print_ghost", print_new_ghost},
+    {nullptr, nullptr}
 };
 
-extern "C" mod_func* get(ghost_register_table* table) {
-    return func_tbl;
+}
+
+extern "C" module_func_info* get(ghost_register_table* table) {
+    nasal_fib_module::ghost_for_test=table->register_ghost_type(
+        "fib_for_test",
+        nasal_fib_module::ghost_for_test_destructor
+    );
+    return nasal_fib_module::func_tbl;
 }
\ No newline at end of file
diff --git a/module/keyboard.cpp b/module/keyboard.cpp
index 6736385..f4b798d 100644
--- a/module/keyboard.cpp
+++ b/module/keyboard.cpp
@@ -88,13 +88,13 @@ var nas_noblock(var* args, usize size, gc* ngc) {
     return nil;
 }
 
-mod_func func_tbl[]={
+module_func_info func_tbl[]={
     {"nas_getch",nas_getch},
     {"nas_kbhit",nas_kbhit},
     {"nas_noblock",nas_noblock},
     {nullptr,nullptr}
 };
 
-extern "C" mod_func* get(ghost_register_table* table) {
+extern "C" module_func_info* get(ghost_register_table* table) {
     return func_tbl;
 }
\ No newline at end of file
diff --git a/module/libfib.nas b/module/libfib.nas
index b852ec0..7fe2a1c 100644
--- a/module/libfib.nas
+++ b/module/libfib.nas
@@ -2,9 +2,28 @@ var libfib=func(){
     var dl=dylib.dlopen("libfib."~(os.platform()=="windows"?"dll":"so"));
     var fib=dl.fib;
     var qfib=dl.quick_fib;
+    var create_ghost=dl.create_ghost;
+    var set_ghost=dl.set_ghost;
+    var print_ghost=dl.print_ghost;
+    var zero_call=dylib.limitcall(0);
     var call=dylib.limitcall(1);
-    return {
-        fib: func(x){return call(fib,x)},
-        qfib:func(x){return call(qfib,x)}
+    var test_call=dylib.limitcall(2);
+    var res={
+        fib: func(x) {return call(fib,x)},
+        qfib: func(x) {return call(qfib,x)},
+        create_ghost: func() {return zero_call(create_ghost)},
+        set_ghost: func(object, x) {return test_call(set_ghost, object, x)},
+        print_ghost: func(object) {return call(print_ghost, object)}
     };
+
+    res.test_ghost=func() {
+        var ghost=res.create_ghost();
+        res.print_ghost(nil); # err
+        res.print_ghost(ghost); # random
+        res.set_ghost(nil, 114); # err
+        res.set_ghost(ghost, 114); # success
+        res.print_ghost(ghost); # 114
+    }
+
+    return res;
 }();
\ No newline at end of file
diff --git a/module/matrix.cpp b/module/matrix.cpp
index d74431c..05e8454 100644
--- a/module/matrix.cpp
+++ b/module/matrix.cpp
@@ -266,7 +266,7 @@ var nas_vec3_dot(var* args, usize size, gc* ngc) {
     return var::num(v0[0].num()*v1[0].num()+v0[1].num()*v1[1].num()+v0[2].num()*v1[2].num());
 }
 
-mod_func func_tbl[]={
+module_func_info func_tbl[]={
     {"nas_vec2",nas_vec2},
     {"nas_vec2_add",nas_vec2_add},
     {"nas_vec2_sub",nas_vec2_sub},
@@ -291,6 +291,6 @@ mod_func func_tbl[]={
     {nullptr,nullptr}
 };
 
-extern "C" mod_func* get(ghost_register_table* table) {
+extern "C" module_func_info* get(ghost_register_table* table) {
     return func_tbl;
 }
\ No newline at end of file
diff --git a/module/nasocket.cpp b/module/nasocket.cpp
index 3935567..4234b11 100644
--- a/module/nasocket.cpp
+++ b/module/nasocket.cpp
@@ -190,7 +190,7 @@ var nas_errno(var* args, usize size, gc* ngc) {
     return ngc->newstr(strerror(errno));
 }
 
-mod_func func_tbl[]={
+module_func_info func_tbl[]={
     {"nas_socket",nas_socket},
     {"nas_closesocket",nas_closesocket},
     {"nas_shutdown",nas_shutdown},
@@ -206,6 +206,6 @@ mod_func func_tbl[]={
     {nullptr,nullptr}
 };
 
-extern "C" mod_func* get(ghost_register_table* table) {
+extern "C" module_func_info* get(ghost_register_table* table) {
     return func_tbl;
 }
\ No newline at end of file
diff --git a/nasal_builtin.h b/nasal_builtin.h
index a1c6c55..10b3071 100644
--- a/nasal_builtin.h
+++ b/nasal_builtin.h
@@ -544,13 +544,13 @@ var builtin_open(var* local, gc& ngc) {
         return nas_err("open", "failed to open file <"+name.str()+">");
     }
     var ret=ngc.alloc(vm_obj);
-    ret.obj().set(ghost_file, res);
+    ret.obj().set(global_ghost_type_table.ghost_file, res);
     return ret;
 }
 
 var builtin_close(var* local, gc& ngc) {
     var fd=local[1];
-    if (!fd.objchk(ghost_file)) {
+    if (!fd.objchk(global_ghost_type_table.ghost_file)) {
         return nas_err("close", "not a valid filehandle");
     }
     fd.obj().clear();
@@ -561,7 +561,7 @@ var builtin_read(var* local, gc& ngc) {
     var fd=local[1];
     var buf=local[2];
     var len=local[3];
-    if (!fd.objchk(ghost_file)) {
+    if (!fd.objchk(global_ghost_type_table.ghost_file)) {
         return nas_err("read", "not a valid filehandle");
     }
     if (buf.type!=vm_str || buf.val.gcobj->unmut) {
@@ -587,7 +587,7 @@ var builtin_read(var* local, gc& ngc) {
 var builtin_write(var* local, gc& ngc) {
     var fd=local[1];
     var str=local[2];
-    if (!fd.objchk(ghost_file)) {
+    if (!fd.objchk(global_ghost_type_table.ghost_file)) {
         return nas_err("write", "not a valid filehandle");
     }
     if (str.type!=vm_str) {
@@ -600,7 +600,7 @@ var builtin_seek(var* local, gc& ngc) {
     var fd=local[1];
     var pos=local[2];
     var whence=local[3];
-    if (!fd.objchk(ghost_file)) {
+    if (!fd.objchk(global_ghost_type_table.ghost_file)) {
         return nas_err("seek", "not a valid filehandle");
     }
     return var::num((f64)fseek((FILE*)fd.obj().ptr, pos.num(), whence.num()));
@@ -608,7 +608,7 @@ var builtin_seek(var* local, gc& ngc) {
 
 var builtin_tell(var* local, gc& ngc) {
     var fd=local[1];
-    if (!fd.objchk(ghost_file)) {
+    if (!fd.objchk(global_ghost_type_table.ghost_file)) {
         return nas_err("tell", "not a valid filehandle");
     }
     return var::num((f64)ftell((FILE*)fd.obj().ptr));
@@ -616,7 +616,7 @@ var builtin_tell(var* local, gc& ngc) {
 
 var builtin_readln(var* local, gc& ngc) {
     var fd=local[1];
-    if (!fd.objchk(ghost_file)) {
+    if (!fd.objchk(global_ghost_type_table.ghost_file)) {
         return nas_err("readln", "not a valid filehandle");
     }
     var str=ngc.alloc(vm_str);
@@ -664,7 +664,7 @@ var builtin_stat(var* local, gc& ngc) {
 
 var builtin_eof(var* local, gc& ngc) {
     var fd=local[1];
-    if (!fd.objchk(ghost_file)) {
+    if (!fd.objchk(global_ghost_type_table.ghost_file)) {
         return nas_err("readln", "not a valid filehandle");
     }
     return var::num((f64)feof((FILE*)fd.obj().ptr));
@@ -849,13 +849,13 @@ var builtin_opendir(var* local, gc& ngc) {
     }
 #endif
     var ret=ngc.alloc(vm_obj);
-    ret.obj().set(ghost_dir,p);
+    ret.obj().set(global_ghost_type_table.ghost_dir,p);
     return ret;
 }
 
 var builtin_readdir(var* local, gc& ngc) {
     var handle=local[1];
-    if (!handle.objchk(ghost_dir)) {
+    if (!handle.objchk(global_ghost_type_table.ghost_dir)) {
         return nas_err("readdir", "not a valid dir handle");
     }
 #ifdef _MSC_VER
@@ -872,7 +872,7 @@ var builtin_readdir(var* local, gc& ngc) {
 
 var builtin_closedir(var* local, gc& ngc) {
     var handle=local[1];
-    if (!handle.objchk(ghost_dir)) {
+    if (!handle.objchk(global_ghost_type_table.ghost_dir)) {
         return nas_err("closedir", "not a valid dir handle");
     }
     handle.obj().clear();
@@ -936,7 +936,7 @@ var builtin_dlopen(var* local, gc& ngc) {
     }
     var ret=ngc.temp=ngc.alloc(vm_hash);
     var lib=ngc.alloc(vm_obj);
-    lib.obj().set(ghost_dylib, ptr);
+    lib.obj().set(global_ghost_type_table.ghost_dylib, ptr);
     ret.hash().elems["lib"]=lib;
 
 #ifdef _WIN32
@@ -948,14 +948,14 @@ var builtin_dlopen(var* local, gc& ngc) {
         return nas_err("dlopen", "cannot find <get> function");
     }
     // get function pointer by name
-    mod_func* tbl=(mod_func*)((getptr)func)(&global_ghost_type_table);
+    module_func_info* tbl=((get_func_ptr)func)(&global_ghost_type_table);
     if (!tbl) {
         return nas_err("dlopen", "failed to get module functions");
     }
     for(u32 i=0;tbl[i].name;++i) {
         void* p=(void*)tbl[i].fd;
         var tmp=ngc.alloc(vm_obj);
-        tmp.obj().set(ghost_faddr, p);
+        tmp.obj().set(global_ghost_type_table.ghost_faddr, p);
         ret.hash().elems[tbl[i].name]=tmp;
     }
 
@@ -965,7 +965,7 @@ var builtin_dlopen(var* local, gc& ngc) {
 
 var builtin_dlclose(var* local, gc& ngc) {
     var libptr=local[1];
-    if (!libptr.objchk(ghost_dylib)) {
+    if (!libptr.objchk(global_ghost_type_table.ghost_dylib)) {
         return nas_err("dlclose", "\"lib\" is not a valid dynamic lib");
     }
     libptr.obj().clear();
@@ -975,23 +975,27 @@ var builtin_dlclose(var* local, gc& ngc) {
 var builtin_dlcallv(var* local, gc& ngc) {
     var fp=local[1];
     var args=local[2];
-    if (!fp.objchk(ghost_faddr)) {
+    if (!fp.objchk(global_ghost_type_table.ghost_faddr)) {
         return nas_err("dlcall", "\"ptr\" is not a valid function pointer");
     }
     auto& vec=args.vec().elems;
-    return ((mod)fp.obj().ptr)(vec.data(), vec.size(), &ngc);
+    return ((module_func)fp.obj().ptr)(
+        vec.data(),
+        vec.size(),
+        &ngc
+    );
 }
 
 var builtin_dlcall(var* local, gc& ngc) {
     var fp=local[1];
-    if (!fp.objchk(ghost_faddr)) {
+    if (!fp.objchk(global_ghost_type_table.ghost_faddr)) {
         return nas_err("dlcall", "\"ptr\" is not a valid function pointer");
     }
 
     var* local_frame_start=local+2;
     usize local_frame_size=ngc.rctx->top-local_frame_start;
     // arguments' stored place begins at local +2
-    return ((mod)fp.obj().ptr)(
+    return ((module_func)fp.obj().ptr)(
         local_frame_start,
         local_frame_size,
         &ngc
diff --git a/nasal_gc.h b/nasal_gc.h
index 63f37db..65c8414 100644
--- a/nasal_gc.h
+++ b/nasal_gc.h
@@ -55,13 +55,13 @@ enum class gc_status:u8 {
     found
 };
 
-struct nas_vec;  // vector
-struct nas_hash; // hashmap(dict)
-struct nas_func; // function(lambda)
-struct nas_upval;// upvalue
-struct nas_obj;  // special objects
-struct nas_co;   // coroutine
-struct nas_val;  // nas_val includes gc-managed types
+struct nas_vec;   // vector
+struct nas_hash;  // hashmap(dict)
+struct nas_func;  // function(lambda)
+struct nas_upval; // upvalue
+struct nas_obj;   // special objects
+struct nas_co;    // coroutine
+struct nas_val;   // nas_val includes gc-managed types
 
 struct var {
 public:
@@ -154,6 +154,7 @@ struct nas_func {
 };
 
 struct nas_upval {
+public:
     /* on stack, use these variables */
     bool onstk;
     u32 size;
@@ -162,19 +163,28 @@ struct nas_upval {
     /* not on stack, use this */
     std::vector<var> elems;
 
-    nas_upval() {onstk=true;stk=nullptr;size=0;}
-    var& operator[](usize n) {return onstk? stk[n]:elems[n];}
-    void clear() {onstk=true;elems.clear();size=0;}
+public:
+    nas_upval(): onstk(true), size(0), stk(nullptr) {}
+
+    var& operator[](usize n) {
+        return onstk? stk[n]:elems[n];
+    }
+
+    void clear() {
+        onstk=true;
+        elems.clear();
+        size=0;
+    }
 };
 
-void file_dtor(void* ptr) {
+void filehandle_destructor(void* ptr) {
     if ((FILE*)ptr==stdin) {
         return;
     }
     fclose((FILE*)ptr);
 }
 
-void dir_dtor(void* ptr) {
+void dir_entry_destructor(void* ptr) {
 #ifndef _MSC_VER
     closedir((DIR*)ptr);
 #else
@@ -182,7 +192,7 @@ void dir_dtor(void* ptr) {
 #endif
 }
 
-void dylib_dtor(void* ptr) {
+void dylib_destructor(void* ptr) {
 #ifdef _WIN32
     FreeLibrary((HMODULE)ptr);
 #else
@@ -190,9 +200,7 @@ void dylib_dtor(void* ptr) {
 #endif
 }
 
-void faddr_dtor(void* ptr) {}
-
-usize ghost_file, ghost_dir, ghost_dylib, ghost_faddr;
+void func_addr_destructor(void* ptr) {}
 
 struct ghost_register_table {
 private:
@@ -202,17 +210,25 @@ private:
     std::unordered_map<string,usize> mapper;
     std::vector<dtor> destructors;
 
+public:
+    // reserved ghost type only for native functions
+    usize ghost_file;
+    usize ghost_dir;
+    usize ghost_dylib;
+    usize ghost_faddr;
+
 public:
     ghost_register_table() {
-        ghost_file=register_ghost_type("file", file_dtor);
-        ghost_dir=register_ghost_type("dir", dir_dtor);
-        ghost_dylib=register_ghost_type("dylib", dylib_dtor);
-        ghost_faddr=register_ghost_type("faddr", faddr_dtor);
+        ghost_file=register_ghost_type("file", filehandle_destructor);
+        ghost_dir=register_ghost_type("dir", dir_entry_destructor);
+        ghost_dylib=register_ghost_type("dylib", dylib_destructor);
+        ghost_faddr=register_ghost_type("faddr", func_addr_destructor);
     }
 
     usize register_ghost_type(const std::string& name, dtor ptr) {
         if (mapper.count(name)) {
-            std::cerr<<"ghost type \""<<name<<"\" already exists.\n\n";
+            std::cerr<<"nasal_gc.h: ghost_register_table::register_ghost_type: ";
+            std::cerr<<"ghost type \""<<name<<"\" already exists.\n";
             std::exit(-1);
         }
         auto res=destructors.size();
@@ -578,11 +594,27 @@ public:
     void clear();
     void info();
     var alloc(const u8);
-    var newstr(char);
-    var newstr(const char*);
-    var newstr(const string&);
     void ctxchg(nas_co&);
     void ctxreserve();
+
+public:
+    var newstr(char c) {
+        var s=alloc(vm_str);
+        s.str()=c;
+        return s;
+    }
+
+    var newstr(const char* buff) {
+        var s=alloc(vm_str);
+        s.str()=buff;
+        return s;
+    }
+
+    var newstr(const string& buff) {
+        var s=alloc(vm_str);
+        s.str()=buff;
+        return s;
+    }
 };
 
 void gc::mark() {
@@ -689,7 +721,8 @@ void gc::extend(u8 type) {
         nas_val* tmp=new nas_val(type);
 
         if (!tmp) {
-            std::cerr<<"failed to allocate new memory\n";
+            std::cerr<<"nasal_gc.h: gc::extend: ";
+            std::cerr<<"failed to allocate memory\n";
             std::exit(-1);
         }
 
@@ -811,24 +844,6 @@ var gc::alloc(u8 type) {
     return ret;
 }
 
-var gc::newstr(char c) {
-    var s=alloc(vm_str);
-    s.str()=c;
-    return s;
-}
-
-var gc::newstr(const char* buff) {
-    var s=alloc(vm_str);
-    s.str()=buff;
-    return s;
-}
-
-var gc::newstr(const string& buff) {
-    var s=alloc(vm_str);
-    s.str()=buff;
-    return s;
-}
-
 void gc::ctxchg(nas_co& co) {
     // store running state to main context
     mctx=*rctx;
@@ -860,19 +875,19 @@ void gc::ctxreserve() {
 }
 
 // use to print error log and return error value
-var nas_err(const string& err_f, const string& info) {
-    std::cerr<<"[vm] "<<err_f<<": "<<info<<"\n";
+var nas_err(const string& error_function_name, const string& info) {
+    std::cerr<<"[vm] "<<error_function_name<<": "<<info<<"\n";
     return var::none();
 }
 
 // module function type
-typedef var (*mod)(var*, usize, gc*);
+typedef var (*module_func)(var*, usize, gc*);
 
 // module function stores in tables with this type, end with {nullptr,nullptr}
-struct mod_func {
+struct module_func_info {
     const char* name;
-    mod fd;
+    module_func fd;
 };
 
 // module function "get" type
-typedef mod_func* (*getptr)(ghost_register_table*);
+typedef module_func_info* (*get_func_ptr)(ghost_register_table*);