diff --git a/Test/Sample.AsyncSocket.h b/Test/Sample.AsyncSocket.h
index 7006f01..09833cb 100644
--- a/Test/Sample.AsyncSocket.h
+++ b/Test/Sample.AsyncSocket.h
@@ -38,7 +38,7 @@ void example_binding(const vu::Endpoint& endpoint)
   server.bind(endpoint);
   server.listen();
   server.run();
-  server.close();
+  // server.stop();
 }
 
 void example_inheritance(const vu::Endpoint& endpoint)
@@ -77,7 +77,7 @@ void example_inheritance(const vu::Endpoint& endpoint)
   server.bind(endpoint);
   server.listen();
   server.run();
-  server.close();
+  // server.stop();
 }
 
 #endif // VU_INET_ENABLED
diff --git a/include/Vutils.h b/include/Vutils.h
index 150e5d7..8f144b6 100644
--- a/include/Vutils.h
+++ b/include/Vutils.h
@@ -109,6 +109,7 @@
 #include <vector>
 #include <thread>
 #include <memory>
+#include <atomic>
 #include <numeric>
 #include <sstream>
 #include <cassert>
@@ -1295,7 +1296,6 @@ class Socket : public LastError
     const address_family_t af = AF_INET,
     const type_t type = SOCK_STREAM,
     const protocol_t proto = IPPROTO_IP,
-    const bool wsa = true,
     const Options* options = nullptr
   );
   Socket(const Socket& right);
@@ -1306,7 +1306,6 @@ class Socket : public LastError
   const Socket& operator=(const Socket& right);
 
   const SOCKET& vuapi handle() const;
-  const WSADATA& vuapi wsa_data() const;
   const address_family_t vuapi af() const;
   const type_t vuapi type() const;
   const protocol_t vuapi  protocol() const;
@@ -1342,9 +1341,9 @@ class Socket : public LastError
 
   IResult vuapi close();
 
-  const sockaddr_in vuapi get_local_sai() const;
-  const sockaddr_in vuapi get_remote_sai() const;
-  std::string vuapi get_host_name() const;
+  const sockaddr_in vuapi get_local_sai();
+  const sockaddr_in vuapi get_remote_sai();
+  std::string vuapi get_host_name();
 
   Options& options();
 
@@ -1358,7 +1357,6 @@ class Socket : public LastError
   std::string vuapi get_host_address(const std::string& name) const;
 
 private:
-  bool m_wsa;
   type_t m_type;
   WSADATA m_wsa_data;
   address_family_t m_af;
@@ -1366,7 +1364,7 @@ class Socket : public LastError
   sockaddr_in m_sai;
   SOCKET m_socket;
   Options m_options;
-  bool m_self;
+  bool m_attached;
 };
 
 class AsyncSocket : public LastError
@@ -1424,9 +1422,7 @@ class AsyncSocket : public LastError
   VUResult vuapi listen(const int maxcon = SOMAXCONN);
 
   VUResult vuapi run(const bool in_worker_thread = false);
-
-  VUResult vuapi stop();
-  IResult  vuapi close(const Socket::shutdowns_t flags = SD_BOTH, const bool cleanup = false);
+  VUResult vuapi stop(const Socket::shutdowns_t flags = SD_BOTH, const bool cleanup = false);
 
   void vuapi get_connections(std::set<SOCKET>& connections);
   VUResult vuapi disconnect_connections(const Socket::shutdowns_t flags = SD_BOTH, const bool cleanup = false);
@@ -1454,16 +1450,18 @@ class AsyncSocket : public LastError
   IResult vuapi do_close(WSANETWORKEVENTS& events, SOCKET& connection);
 
 protected:
+  HANDLE m_thread;
+  std::atomic<bool> m_running;
+
+  side_type  m_side;
   vu::Socket m_socket;
-  side_type m_side;
-  bool m_running;
-  DWORD m_n_events;
+
+  DWORD  m_n_events;
   SOCKET m_connections[WSA_MAXIMUM_WAIT_EVENTS];
-  std::recursive_mutex m_mutex_client_list;
   WSAEVENT m_events[WSA_MAXIMUM_WAIT_EVENTS];
+  std::recursive_mutex m_mutex_client_list;
+
   fn_prototype_t m_functions[function::UNDEFINED];
-  std::mutex m_mutex;
-  HANDLE m_thread;
 };
 
 #endif // VU_INET_ENABLED
diff --git a/src/details/asyncsocket.cpp b/src/details/asyncsocket.cpp
index 5adbef1..d6b2afd 100644
--- a/src/details/asyncsocket.cpp
+++ b/src/details/asyncsocket.cpp
@@ -19,7 +19,7 @@ AsyncSocket::AsyncSocket(
   const vu::Socket::type_t type,
   const vu::Socket::protocol_t proto,
   const vu::Socket::Options* options
-) : m_socket(af, type, proto, true, options), m_thread(INVALID_HANDLE_VALUE), LastError()
+) : m_socket(af, type, proto, options), m_thread(INVALID_HANDLE_VALUE), LastError()
 {
   UNREFERENCED_PARAMETER(m_side);
 
@@ -33,15 +33,12 @@ AsyncSocket::AsyncSocket(
 
 AsyncSocket::~AsyncSocket()
 {
-  this->close();
 }
 
 void vuapi AsyncSocket::initialze()
 {
   m_n_events = 0;
 
-  std::lock_guard<std::recursive_mutex> lg(m_mutex_client_list);
-
   memset(m_connections, int(INVALID_SOCKET), sizeof(m_connections));
   memset(m_events, int(0), sizeof(m_events));
 
@@ -97,8 +94,6 @@ VUResult vuapi AsyncSocket::listen(const int maxcon)
     return 2;
   }
 
-  std::lock_guard<std::recursive_mutex> lg(m_mutex_client_list);
-
   m_connections[m_n_events] = m_socket.handle();
   m_events[m_n_events] = event;
   m_n_events++;
@@ -110,9 +105,14 @@ VUResult vuapi AsyncSocket::listen(const int maxcon)
   return result;
 }
 
-IResult vuapi AsyncSocket::close(const Socket::shutdowns_t flags, const bool cleanup)
+IResult vuapi AsyncSocket::stop(const Socket::shutdowns_t flags, const bool cleanup)
 {
-  this->stop();
+  if (!m_socket.available())
+  {
+    return 1;
+  }
+
+  m_running = false;
 
   this->disconnect_connections(flags, cleanup);
 
@@ -121,18 +121,6 @@ IResult vuapi AsyncSocket::close(const Socket::shutdowns_t flags, const bool cle
     TerminateThread(m_thread, 0); // CloseHandle(m_thread);
   }
 
-  auto result =  m_socket.close();
-
-  m_last_error_code = GetLastError();
-
-  return result;
-}
-
-VUResult vuapi AsyncSocket::stop()
-{
-  m_mutex.lock();
-  m_running = false;
-  m_mutex.unlock();
   return VU_OK;
 }
 
@@ -152,8 +140,6 @@ VUResult vuapi AsyncSocket::connect(const Endpoint& endpoint)
     return 2;
   }
 
-  std::lock_guard<std::recursive_mutex> lg(m_mutex_client_list);
-
   auto result = m_socket.connect(endpoint);
   if (result == VU_OK)
   {
@@ -179,31 +165,29 @@ void vuapi AsyncSocket::get_connections(std::set<SOCKET>& connections)
 {
   connections.clear();
 
-  if (m_socket.available())
+  if (!m_socket.available())
   {
-    std::lock_guard<std::recursive_mutex> lg(m_mutex_client_list);
+    return;
+  }
 
-    for (auto& socket : m_connections)
+  for (auto& socket : m_connections)
+  {
+    if (socket == INVALID_SOCKET) // ignore invalid socket handle
     {
-      if (socket == INVALID_SOCKET) // ignore invalid socket handle
-      {
-        continue;
-      }
-
-      if (m_side == side_type::SERVER && socket == m_socket.handle()) // ignore server socket handle
-      {
-        continue;
-      }
+      continue;
+    }
 
-      connections.insert(socket);
+    if (m_side == side_type::SERVER && socket == m_socket.handle()) // ignore server socket handle
+    {
+      continue;
     }
+
+    connections.insert(socket);
   }
 }
 
 VUResult vuapi AsyncSocket::disconnect_connections(const Socket::shutdowns_t flags, const bool cleanup)
 {
-  std::lock_guard<std::recursive_mutex> lg(m_mutex_client_list);
-
   std::set<SOCKET> connections;
   this->get_connections(connections);
   for (const auto& connection : connections)
@@ -365,8 +349,6 @@ IResult vuapi AsyncSocket::do_open(WSANETWORKEVENTS& events, SOCKET& connection)
     return events.iErrorCode[FD_ACCEPT_BIT];
   }
 
-  std::lock_guard<std::recursive_mutex> lg(m_mutex_client_list);
-
   Socket::Handle obj = { 0 };
   int n = static_cast<int>(sizeof(obj.sai));
 
@@ -398,8 +380,6 @@ IResult vuapi AsyncSocket::do_recv(WSANETWORKEVENTS& events, SOCKET& connection)
     return events.iErrorCode[FD_READ_BIT];
   }
 
-  std::lock_guard<std::recursive_mutex> lg(m_mutex_client_list);
-
   Socket socket(m_socket);
   socket.attach(connection);
   this->on_recv(socket);
@@ -415,8 +395,6 @@ IResult vuapi AsyncSocket::do_send(WSANETWORKEVENTS& events, SOCKET& connection)
     return events.iErrorCode[FD_WRITE_BIT];
   }
 
-  std::lock_guard<std::recursive_mutex> lg(m_mutex_client_list);
-
   Socket socket(m_socket);
   socket.attach(connection);
   this->on_send(socket);
@@ -427,7 +405,7 @@ IResult vuapi AsyncSocket::do_send(WSANETWORKEVENTS& events, SOCKET& connection)
 
 IResult vuapi AsyncSocket::do_close(WSANETWORKEVENTS& events, SOCKET& connection)
 {
-  // TODO: In certain cases(e.g., user - mode drivers), it crashes.
+  // TODO: Vic. In certain cases(e.g., user - mode drivers), it crashes.
   //  I'm not sure why, so temporarily comment out these codes.
   // 
   // if (events.iErrorCode[FD_CLOSE_BIT] != 0)
@@ -435,8 +413,6 @@ IResult vuapi AsyncSocket::do_close(WSANETWORKEVENTS& events, SOCKET& connection
   //   return events.iErrorCode[FD_CLOSE_BIT];
   // }
 
-  std::lock_guard<std::recursive_mutex> lg(m_mutex_client_list);
-
   std::vector<std::pair<SOCKET, HANDLE>> in_used_connections;
 
   for (int i = 0; i < WSA_MAXIMUM_WAIT_EVENTS; i++)
diff --git a/src/details/socket.cpp b/src/details/socket.cpp
index 2289e2f..8b196e7 100644
--- a/src/details/socket.cpp
+++ b/src/details/socket.cpp
@@ -103,29 +103,30 @@ Socket::Socket(
   const address_family_t af,
   const type_t type,
   const protocol_t proto,
-  const bool wsa,
   const Options* options
-) : LastError(), m_af(af), m_type(type), m_proto(proto), m_wsa(wsa), m_self(false)
+) : LastError(), m_af(af), m_type(type), m_proto(proto), m_attached(false)
 {
   ZeroMemory(&m_wsa_data, sizeof(m_wsa_data));
-  ZeroMemory(&m_sai, sizeof(m_sai));
+  if (WSAStartup(MAKEWORD(2, 2), &m_wsa_data) == INVALID_SOCKET)
+  {
+    m_last_error_code = GetLastError();
+    assert("start wsa failed.");
+  }
 
   if (options != nullptr)
   {
     m_options = *options;
   }
 
-  if (m_wsa)
-  {
-    if (WSAStartup(MAKEWORD(2, 2), &m_wsa_data) != 0)
-    {
-      m_last_error_code = GetLastError();
-    }
-  }
+  ZeroMemory(&m_sai, sizeof(m_sai));
+  m_sai.sin_family = m_af;
 
   m_socket = ::socket(m_af, m_type, m_proto);
-
-  m_sai.sin_family = m_af;
+  if (m_socket == INVALID_SOCKET)
+  {
+    m_last_error_code = GetLastError();
+    assert("open socket failed.");
+  }
 }
 
 Socket::Socket(const Socket& right)
@@ -135,14 +136,23 @@ Socket::Socket(const Socket& right)
 
 Socket::~Socket()
 {
-  this->close();
+  if (m_attached)
+  {
+    return; // ignore if the connection is attached from outside
+  }
 
-  if (m_wsa)
+  if (::closesocket(m_socket) == INVALID_SOCKET)
   {
-    WSACleanup();
+    assert("close socket failed.");
+    m_last_error_code = GetLastError();
+    m_socket = INVALID_SOCKET;
   }
 
-  m_last_error_code = GetLastError();
+  if (WSACleanup() == INVALID_SOCKET)
+  {
+    assert("clean wsa failed.");
+    m_last_error_code = GetLastError();
+  }
 }
 
 bool Socket::operator==(const Socket& right)
@@ -162,17 +172,13 @@ const vu::Socket& Socket::operator=(const Socket& right)
     return *this;
   }
 
-  m_wsa = false;
-  // m_wsa = right.m_wsa;
-  // m_wsa_data = right.m_wsa_data;
-
   m_type = right.m_type;
   m_af = right.m_af;
   m_proto = right.m_proto;
   m_sai = right.m_sai;
   m_socket = right.m_socket;
   m_options = right.m_options;
-  m_self = right.m_self;
+  m_attached = right.m_attached;
 
   return *this;
 }
@@ -198,6 +204,7 @@ void vuapi Socket::attach(const Handle& socket)
 {
   m_socket = socket.s;
   m_sai = socket.sai;
+  m_attached = true;
 }
 
 void vuapi Socket::detach()
@@ -211,11 +218,6 @@ Socket::Options& Socket::options()
   return m_options;
 }
 
-const WSADATA& vuapi Socket::wsa_data() const
-{
-  return m_wsa_data;
-}
-
 const Socket::address_family_t vuapi Socket::af() const
 {
   return m_af;
@@ -236,7 +238,7 @@ const SOCKET& vuapi Socket::handle() const
   return m_socket;
 }
 
-const sockaddr_in vuapi Socket::get_local_sai() const
+const sockaddr_in vuapi Socket::get_local_sai()
 {
   sockaddr_in result = { 0 };
 
@@ -244,12 +246,13 @@ const sockaddr_in vuapi Socket::get_local_sai() const
   {
     auto size = int(sizeof(result));
     ::getsockname(m_socket, (struct sockaddr*)&result, &size);
+    m_last_error_code = GetLastError();
   }
 
   return result;
 }
 
-const sockaddr_in vuapi Socket::get_remote_sai() const
+const sockaddr_in vuapi Socket::get_remote_sai()
 {
   sockaddr_in result = { 0 };
 
@@ -257,6 +260,7 @@ const sockaddr_in vuapi Socket::get_remote_sai() const
   {
     auto size = int(sizeof(result));
     ::getpeername(m_socket, (struct sockaddr*)&result, &size);
+    m_last_error_code = GetLastError();
   }
 
   return result;
@@ -273,7 +277,7 @@ VUResult vuapi Socket::set_option(
     return 1;
   }
 
-  if (::setsockopt(m_socket, level, option, static_cast<const char*>(value), size) != 0)
+  if (::setsockopt(m_socket, level, option, static_cast<const char*>(value), size) == INVALID_SOCKET)
   {
     m_last_error_code = GetLastError();
     return 3;
@@ -292,6 +296,7 @@ VUResult vuapi Socket::enable_non_blocking(bool state)
   ulong non_block = state ? 1 : 0;
   if (::ioctlsocket(m_socket, FIONBIO, &non_block) == SOCKET_ERROR)
   {
+    m_last_error_code = GetLastError();
     return 2;
   }
 
@@ -394,8 +399,6 @@ VUResult vuapi Socket::connect(const Endpoint& endpoint)
     return m_last_error_code == WSAEWOULDBLOCK ? VU_OK : 2;
   }
 
-  m_self = true;
-
   return VU_OK;
 }
 
@@ -620,10 +623,7 @@ VUResult vuapi Socket::close()
     return 1;
   }
 
-  if (m_self)
-  {
-    ::closesocket(m_socket);
-  }
+  ::closesocket(m_socket);
 
   m_socket = INVALID_SOCKET;
 
@@ -649,16 +649,18 @@ VUResult vuapi Socket::disconnect(const shutdowns_t flags, const bool cleanup)
     return 2;
   }
 
-  if (closesocket(m_socket) == SOCKET_ERROR)
+  if (::closesocket(m_socket) == SOCKET_ERROR)
   {
     m_last_error_code = GetLastError();
     return 3;
   }
 
+  m_socket = INVALID_SOCKET;
+
   return VU_OK;
 }
 
-std::string vuapi Socket::get_host_name() const
+std::string vuapi Socket::get_host_name()
 {
   std::string result = "";
 
@@ -676,7 +678,7 @@ std::string vuapi Socket::get_host_name() const
   ZeroMemory(h.get(), MAXBYTE);
   if (::gethostname(h.get(), MAXBYTE) == SOCKET_ERROR)
   {
-    //m_last_error_code = GetLastError();
+    m_last_error_code = GetLastError();
     return result;
   }