diff --git a/makefile b/makefile
index 7cde7a3..808bda3 100644
--- a/makefile
+++ b/makefile
@@ -330,6 +330,7 @@ test:nasal
 	@ ./nasal -t -d test/quick_sort.nas
 	@ ./nasal -t -d test/regex_test.nas
 	@ ./nasal -e test/scalar.nas hello world
+	@ ./nasal test/subprocess_test.nas
 	@ ./nasal -e test/trait.nas
 	@ ./nasal -t -d test/turingmachine.nas
 	@ ./nasal -d test/wavecollapse.nas
diff --git a/module/fib.cpp b/module/fib.cpp
index dace56b..36cee41 100644
--- a/module/fib.cpp
+++ b/module/fib.cpp
@@ -91,10 +91,10 @@ var set_new_ghost(var* args, usize size, gc* ngc) {
     }
     f64 num = args[1].num();
 
-    reinterpret_cast<ghost_obj*>(res.ghost().pointer)->number = static_cast<u32>(num);
+    res.ghost().get<ghost_obj>()->number = static_cast<u32>(num);
     std::cout << "set_new_ghost: successfully set ghost.number = " << num << "\n";
 
-    reinterpret_cast<ghost_obj*>(res.ghost().pointer)->test_string = ngc->newstr("just for test");
+    res.ghost().get<ghost_obj>()->test_string = ngc->newstr("just for test");
     std::cout << "set_new_ghost: successfully set ghost.test_string = just for test\n";
     return nil;
 }
@@ -106,9 +106,9 @@ var print_new_ghost(var* args, usize size, gc* ngc) {
         return nil;
     }
     std::cout << "print_new_ghost: " << res.ghost() << " number = "
-              << reinterpret_cast<ghost_obj*>(res.ghost().pointer)->number
+              << res.ghost().get<ghost_obj>()->number
               << " test_string = "
-              << reinterpret_cast<ghost_obj*>(res.ghost().pointer)->test_string
+              << res.ghost().get<ghost_obj>()->test_string
               << "\n";
     return nil;
 }
diff --git a/src/nasal_type.cpp b/src/nasal_type.cpp
index c163541..4d6b2a2 100644
--- a/src/nasal_type.cpp
+++ b/src/nasal_type.cpp
@@ -164,7 +164,7 @@ void nas_ghost::clear() {
 std::ostream& operator<<(std::ostream& out, const nas_ghost& ghost) {
     out << "<object " << ghost.get_ghost_name();
     out << " at 0x" << std::hex;
-    out << reinterpret_cast<u64>(ghost.pointer) << std::dec << ">";
+    out << ghost.convert<u64>() << std::dec << ">";
     return out;
 }
 
diff --git a/src/nasal_type.h b/src/nasal_type.h
index 208babb..5a759e5 100644
--- a/src/nasal_type.h
+++ b/src/nasal_type.h
@@ -213,6 +213,12 @@ public:
 
 public:
     const auto& get_ghost_name() const { return type_name; }
+
+public:
+    template<typename T>
+    T* get() { return static_cast<T*>(pointer); }
+    template<typename T>
+    T convert() const { return reinterpret_cast<T>(pointer); }
 };
 
 struct context {
diff --git a/src/natives/builtin.cpp b/src/natives/builtin.cpp
index 79c0f76..203fae8 100644
--- a/src/natives/builtin.cpp
+++ b/src/natives/builtin.cpp
@@ -615,7 +615,7 @@ var builtin_time_stamp(context* ctx, gc* ngc) {
     if (!object.object_check("nasal-time-stamp")) {
         return nil;
     }
-    auto stamp = static_cast<time_stamp*>(object.ghost().pointer);
+    auto stamp = object.ghost().get<time_stamp>();
     stamp->make_stamp();
     return nil;
 }
@@ -625,7 +625,7 @@ var builtin_elapsed_millisecond(context* ctx, gc* ngc) {
     if (!object.object_check("nasal-time-stamp")) {
         return var::num(-1);
     }
-    auto stamp = static_cast<time_stamp*>(object.ghost().pointer);
+    auto stamp = object.ghost().get<time_stamp>();
     return var::num(static_cast<f64>(stamp->elapsed_milliseconds()));
 }
 
@@ -634,7 +634,7 @@ var builtin_elapsed_microsecond(context* ctx, gc* ngc) {
     if (!object.object_check("nasal-time-stamp")) {
         return var::num(-1);
     }
-    auto stamp = static_cast<time_stamp*>(object.ghost().pointer);
+    auto stamp = object.ghost().get<time_stamp>();
     return var::num(static_cast<f64>(stamp->elapsed_microseconds()));
 }
 
@@ -717,7 +717,7 @@ var builtin_ghosttype(context* ctx, gc* ngc) {
     if (!name.length()) {
         std::stringstream ss;
         ss << "0x" << std::hex;
-        ss << reinterpret_cast<u64>(arg.ghost().pointer) << std::dec;
+        ss << arg.ghost().convert<u64>() << std::dec;
         return ngc->newstr(ss.str());
     }
     return ngc->newstr(name);
diff --git a/src/natives/dylib_lib.cpp b/src/natives/dylib_lib.cpp
index f72f700..329d8ed 100644
--- a/src/natives/dylib_lib.cpp
+++ b/src/natives/dylib_lib.cpp
@@ -55,7 +55,7 @@ var builtin_dlopen(context* ctx, gc* ngc) {
     // get "get" function, to get the register table
 #ifdef _WIN32
     void* register_table_get_function = reinterpret_cast<void*>(GetProcAddress(
-        static_cast<HMODULE>(library_object.ghost().pointer), "get"
+        library_object.ghost().convert<HMODULE>(), "get"
     ));
 #else
     void* register_table_get_function = dlsym(
@@ -105,7 +105,7 @@ var builtin_dlcallv(context* ctx, gc* ngc) {
         );
     }
     auto& vec = arguments.vec().elems;
-    return reinterpret_cast<module_func>(function_object.ghost().pointer)(
+    return function_object.ghost().convert<module_func>()(
         vec.data(),
         vec.size(),
         ngc
@@ -124,7 +124,7 @@ var builtin_dlcall(context* ctx, gc* ngc) {
     // so arguments starts from ctx->localr[2]
     var* local_frame_start = ctx->localr + 2;
     usize local_frame_size = ngc->running_context->top - local_frame_start;
-    return reinterpret_cast<module_func>(function_object.ghost().pointer)(
+    return function_object.ghost().convert<module_func>()(
         local_frame_start,
         local_frame_size,
         ngc
diff --git a/src/natives/io_lib.cpp b/src/natives/io_lib.cpp
index c5efb0b..83f81dc 100644
--- a/src/natives/io_lib.cpp
+++ b/src/natives/io_lib.cpp
@@ -101,7 +101,7 @@ var builtin_read(context* ctx, gc* ngc) {
     }
     auto read_size = fread(
         temp_buffer, 1, length.num(),
-        static_cast<FILE*>(file_descriptor.ghost().pointer)
+        file_descriptor.ghost().get<FILE>()
     );
     buffer.str() = temp_buffer;
     buffer.val.gcobj->immutable = true;
@@ -121,7 +121,7 @@ var builtin_write(context* ctx, gc* ngc) {
     }
     return var::num(static_cast<f64>(fwrite(
         source.str().c_str(), 1, source.str().length(),
-        static_cast<FILE*>(file_descriptor.ghost().pointer)
+        file_descriptor.ghost().get<FILE>()
     )));
 }
 
@@ -134,7 +134,7 @@ var builtin_seek(context* ctx, gc* ngc) {
         return nas_err("io::seek", "not a valid filehandle");
     }
     return var::num(static_cast<f64>(fseek(
-        static_cast<FILE*>(file_descriptor.ghost().pointer),
+        file_descriptor.ghost().get<FILE>(),
         position.num(),
         whence.num()
     )));
@@ -146,7 +146,7 @@ var builtin_tell(context* ctx, gc* ngc) {
         return nas_err("io::tell", "not a valid filehandle");
     }
     return var::num(static_cast<f64>(
-        ftell(static_cast<FILE*>(file_descriptor.ghost().pointer))
+        ftell(file_descriptor.ghost().get<FILE>())
     ));
 }
 
@@ -157,7 +157,7 @@ var builtin_readln(context* ctx, gc* ngc) {
     }
     auto result = ngc->alloc(vm_type::vm_str);
     char c;
-    while((c = fgetc(static_cast<FILE*>(file_descriptor.ghost().pointer)))!=EOF) {
+    while((c = fgetc(file_descriptor.ghost().get<FILE>()))!=EOF) {
         if (c=='\r') {
             continue;
         }
@@ -204,7 +204,7 @@ var builtin_eof(context* ctx, gc* ngc) {
         return nas_err("io::readln", "not a valid filehandle");
     }
     return var::num(static_cast<f64>(
-        feof(static_cast<FILE*>(file_descriptor.ghost().pointer))
+        feof(file_descriptor.ghost().get<FILE>())
     ));
 }
 
diff --git a/src/natives/json_lib.cpp b/src/natives/json_lib.cpp
index 719120c..b068f3b 100644
--- a/src/natives/json_lib.cpp
+++ b/src/natives/json_lib.cpp
@@ -369,7 +369,7 @@ var builtin_json_stringify(context* ctx, gc* ngc) {
     if (!json_object.object_check("nasal::json")) {
         return nas_err("json::stringify", "expect a json object.");
     }
-    auto json_ptr = static_cast<json*>(json_object.ghost().pointer);
+    auto json_ptr = json_object.ghost().get<json>();
     return ngc->newstr(json_ptr->stringify(object));
 }
 
@@ -382,7 +382,7 @@ var builtin_json_parse(context* ctx, gc* ngc) {
     if (!input_string.is_str()) {
         return nas_err("json::parse", "require string as the input.");
     }
-    auto json_ptr = static_cast<json*>(json_object.ghost().pointer);
+    auto json_ptr = json_object.ghost().get<json>();
     return json_ptr->parse(input_string.str(), ngc);
 }
 
@@ -391,7 +391,7 @@ var builtin_json_get_error(context* ctx, gc* ngc) {
     if (!json_object.object_check("nasal::json")) {
         return nas_err("json::get_error", "expect a json object.");
     }
-    auto json_ptr = static_cast<json*>(json_object.ghost().pointer);
+    auto json_ptr = json_object.ghost().get<json>();
     return ngc->newstr(json_ptr->get_error());
 }
 
diff --git a/src/natives/subprocess.cpp b/src/natives/subprocess.cpp
index a04038e..1e5394d 100644
--- a/src/natives/subprocess.cpp
+++ b/src/natives/subprocess.cpp
@@ -24,18 +24,16 @@
 
 namespace nasal {
 
-const auto subproc_desc_name = "subprocess_descriptor";
-
 void subprocess_desc_dtor(void* ptr) {
 #ifdef WIN32
-    auto pi = &static_cast<subprocess_descriptor*>(ptr)->pi;
+    auto pi = &static_cast<subprocess*>(ptr)->pi;
 
     WaitForSingleObject(pi->hProcess, 0);
     TerminateProcess(pi->hProcess, 0);
     CloseHandle(pi->hProcess);
     CloseHandle(pi->hThread);
 #else
-    auto pid = static_cast<subprocess_descriptor*>(ptr)->pid;
+    auto pid = static_cast<subprocess*>(ptr)->pid;
 
     int status;
     pid_t result = waitpid(pid, &status, WNOHANG);
@@ -50,26 +48,30 @@ void subprocess_desc_dtor(void* ptr) {
 var builtin_subprocess_create(context* ctx, gc* ngc) {
     auto cmd = ctx->localr[1];
     if (!cmd.is_vec()) {
-        return nas_err("subprocess::create", "expect a string as the command");
+        return nas_err("subprocess::create",
+            "expect string vector as the command"
+        );
     }
 
     for(const auto& v : cmd.vec().elems) {
         if (!v.is_str()) {
-            return nas_err("subprocess::create", "non-string arguments");
+            return nas_err("subprocess::create",
+                "non-string argument found"
+            );
         }
     }
 
     auto obj = ngc->alloc(vm_type::vm_ghost);
     obj.ghost().set(
-        subproc_desc_name,
+        subprocess::name(),
         subprocess_desc_dtor,
         nullptr,
-        new subprocess_descriptor
+        new subprocess
     );
 
 #ifdef WIN32
-    auto si = &static_cast<subprocess_descriptor*>(obj.ghost().pointer)->si;
-    auto pi = &static_cast<subprocess_descriptor*>(obj.ghost().pointer)->pi;
+    auto si = &obj.ghost().get<subprocess>()->si;
+    auto pi = &obj.ghost().get<subprocess>()->pi;
 
     // init STARTUPINFO
     ZeroMemory(si, sizeof(STARTUPINFOW));
@@ -135,7 +137,7 @@ var builtin_subprocess_create(context* ctx, gc* ngc) {
     }
 
     // parent process
-    static_cast<subprocess_descriptor*>(obj.ghost().pointer)->pid = pid;
+    obj.ghost().get<subprocess>()->pid = pid;
     for(usize i = 0; argv[i]; ++i) {
         delete argv[i];
     }
@@ -148,14 +150,14 @@ var builtin_subprocess_create(context* ctx, gc* ngc) {
 
 var builtin_subprocess_terminate(context* ctx, gc* ngc) {
     auto obj = ctx->localr[1];
-    if (!obj.object_check(subproc_desc_name)) {
+    if (!obj.object_check(subprocess::name())) {
         return nas_err("subprocess::terminate",
-            "need correct subprocess descriptor"
+            "need correct subprocess object"
         );
     }
 
 #ifdef WIN32
-    auto pi = &static_cast<subprocess_descriptor*>(obj.ghost().pointer)->pi;
+    auto pi = &obj.ghost().get<subprocess>()->pi;
 
     WaitForSingleObject(pi->hProcess, 0);
     TerminateProcess(pi->hProcess, -1);
@@ -166,7 +168,7 @@ var builtin_subprocess_terminate(context* ctx, gc* ngc) {
     CloseHandle(pi->hProcess);
     CloseHandle(pi->hThread);
 #else
-    auto pid = static_cast<subprocess_descriptor*>(obj.ghost().pointer)->pid;
+    auto pid = obj.ghost().get<subprocess>()->pid;
 
     int status;
     pid_t result = waitpid(pid, &status, WNOHANG);
diff --git a/src/natives/subprocess.h b/src/natives/subprocess.h
index 4e25f19..587527f 100644
--- a/src/natives/subprocess.h
+++ b/src/natives/subprocess.h
@@ -14,13 +14,18 @@
 
 namespace nasal {
 
-struct subprocess_descriptor {
+struct subprocess {
 #ifndef WIN32
     pid_t pid;
 #else
     STARTUPINFOW si;
     PROCESS_INFORMATION pi;
 #endif
+
+public:
+    static const char* name() {
+        return "nasal::subprocess";
+    }
 };
 
 void subprocess_desc_dtor(void*);
diff --git a/src/natives/unix_lib.cpp b/src/natives/unix_lib.cpp
index b791680..f394734 100644
--- a/src/natives/unix_lib.cpp
+++ b/src/natives/unix_lib.cpp
@@ -89,7 +89,7 @@ var builtin_readdir(context* ctx, gc* ngc) {
     }
     return ngc->newstr(data.cFileName);
 #else
-    dirent* p = readdir(static_cast<DIR*>(handle.ghost().pointer));
+    dirent* p = readdir(handle.ghost().get<DIR>());
     return p? ngc->newstr(p->d_name):nil;
 #endif
 }
diff --git a/test/for_subprocess_test/infinite_loop.nas b/test/for_subprocess_test/infinite_loop.nas
new file mode 100644
index 0000000..bfba824
--- /dev/null
+++ b/test/for_subprocess_test/infinite_loop.nas
@@ -0,0 +1,11 @@
+use std.unix;
+
+var count = 0;
+while(1) {
+    unix.sleep(0.1);
+    count += 0.1;
+    println("process running time: ", count);
+    if (count > 2) {
+        die("test failed");
+    }
+}
\ No newline at end of file
diff --git a/test/subprocess_test.nas b/test/subprocess_test.nas
new file mode 100644
index 0000000..12cd261
--- /dev/null
+++ b/test/subprocess_test.nas
@@ -0,0 +1,13 @@
+use std.subprocess;
+use std.unix;
+
+var process = subprocess.create([
+    "./nasal",
+    "test/for_subprocess_test/infinite_loop.nas"
+]);
+
+unix.sleep(1);
+
+println("kill subprocess...");
+var res = subprocess.terminate(process);
+println("subprocess killed, code ", res);
\ No newline at end of file