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CSocket Class

Derives from CAsyncSocket, inherits its encapsulation of the Windows Sockets API, and represents a higher level of abstraction than that of a CAsyncSocket object.


class CSocket : public CAsyncSocket


Public Constructors

Name Description
CSocket::CSocket Constructs a CSocket object.

Public Methods

Name Description
CSocket::Attach Attaches a SOCKET handle to a CSocket object.
CSocket::CancelBlockingCall Cancels a blocking call that is currently in progress.
CSocket::Create Creates a socket.
CSocket::FromHandle Returns a pointer to a CSocket object, given a SOCKET handle.
CSocket::IsBlocking Determines whether a blocking call is in progress.

Protected Methods

Name Description
CSocket::OnMessagePending Called to process pending messages while waiting for a blocking call to complete.


CSocket works with classes CSocketFile and CArchive to manage the sending and receiving of data.

A CSocket object also provides blocking, which is essential to the synchronous operation of CArchive. Blocking functions, such as Receive, Send, ReceiveFrom, SendTo, and Accept (all inherited from CAsyncSocket), do not return a WSAEWOULDBLOCK error in CSocket. Instead, these functions wait until the operation completes. Additionally, the original call will terminate with the error WSAEINTR if CancelBlockingCall is called while one of these functions is blocking.

To use a CSocket object, call the constructor, then call Create to create the underlying SOCKET handle (type SOCKET). The default parameters of Create create a stream socket, but if you are not using the socket with a CArchive object, you can specify a parameter to create a datagram socket instead, or bind to a specific port to create a server socket. Connect to a client socket using Connect on the client side and Accept on the server side. Then create a CSocketFile object and associate it to the CSocket object in the CSocketFile constructor. Next, create a CArchive object for sending and one for receiving data (as needed), then associate them with the CSocketFile object in the CArchive constructor. When communications are complete, destroy the CArchive, CSocketFile, and CSocket objects. The SOCKET data type is described in the article Windows Sockets: Background.

When you use CArchive with CSocketFile and CSocket, you might encounter a situation where CSocket::Receive enters a loop (by PumpMessages(FD_READ)) waiting for the requested amount of bytes. This is because Windows sockets allow only one recv call per FD_READ notification, but CSocketFile and CSocket allow multiple recv calls per FD_READ. If you get an FD_READ when there is no data to read, the application hangs. If you never get another FD_READ, the application stops communicating over the socket.

You can resolve this problem as follows. In the OnReceive method of your socket class, call CAsyncSocket::IOCtl(FIONREAD, ...) before you call the Serialize method of your message class when the expected data to be read from the socket exceeds the size of one TCP packet (maximum transmission unit of the network medium, usually at least 1096 bytes). If the size of the available data is less than needed, wait for all the data to be received and only then start the read operation.

In the following example, m_dwExpected is the approximate number of bytes that the user expects to receive. It is assumed that you declare it elsewhere in your code.

void CChatSocket::OnReceive(int nErrorCode)

   DWORD dwReceived;

   if (IOCtl(FIONREAD, &dwReceived))
      if (dwReceived >= m_dwExpected) // Process only if you have enough data
      // Error handling here


When using MFC sockets in secondary threads in a statically linked MFC application, you must call AfxSocketInit in each thread that uses sockets to initialize the socket libraries. By default, AfxSocketInit is called only in the primary thread.

For more information, see Windows Sockets in MFC, Windows Sockets: Using Sockets with Archives, Windows Sockets: How Sockets with Archives Work, Windows Sockets: Sequence of Operations, Windows Sockets: Example of Sockets Using Archives.

Inheritance Hierarchy





Header: afxsock.h


Call this member function to attach the hSocket handle to a CSocket object.

BOOL Attach(SOCKET hSocket);


Contains a handle to a socket.

Return Value

Nonzero if the function is successful.


The SOCKET handle is stored in the object's m_hSocket data member.

For more information, see Windows Sockets: Using Sockets with Archives.


class CSockThread : public CWinThread
   SOCKET m_hConnected;

   CChatSocket m_sConnected;

   // remainder of class declaration omitted.


BOOL CSockThread::InitInstance()
   // Attach the socket object to the socket handle
   // in the context of this thread.
   m_hConnected = NULL;

   return TRUE;


// This listening socket has been constructed
// in the primary thread.
void CListeningSocket::OnAccept(int nErrorCode)

   // This CSocket object is used just temporarily
   // to accept the incoming connection.
   CSocket sConnected;

   // Start the other thread.
   CSockThread *pSockThread = (CSockThread*)AfxBeginThread(
   if (NULL != pSockThread)
      // Detach the newly accepted socket and save
      // the SOCKET handle in our new thread object.
      // After detaching it, it should no longer be
      // used in the context of this thread.
      pSockThread->m_hConnected = sConnected.Detach();


Call this member function to cancel a blocking call currently in progress.

void CancelBlockingCall();


This function cancels any outstanding blocking operation for this socket. The original blocking call will terminate as soon as possible with the error WSAEINTR.

In the case of a blocking Connect operation, the Windows Sockets implementation will terminate the blocking call as soon as possible, but it may not be possible for the socket resources to be released until the connection has completed (and then been reset) or timed out. This is likely to be noticeable only if the application immediately tries to open a new socket (if no sockets are available), or to connect to the same peer.

Canceling any operation other than Accept can leave the socket in an indeterminate state. If an application cancels a blocking operation on a socket, the only operation that the application can depend on being able to perform on the socket is a call to Close, although other operations may work on some Windows Sockets implementations. If you desire maximum portability for your application, you must be careful not to depend on performing operations after a cancel.

For more information, see Windows Sockets: Using Sockets with Archives.


Call the Create member function after constructing a socket object to create the Windows socket and attach it.

BOOL Create(
    UINT nSocketPort = 0,
    int nSocketType = SOCK_STREAM,
    LPCTSTR lpszSocketAddress = NULL);


A particular port to be used with the socket, or 0 if you want MFC to select a port.


A pointer to a string containing the network address of the connected socket, a dotted number such as "". Passing the NULL string for this parameter indicates the CSocket instance should listen for client activity on all network interfaces.

Return Value

Nonzero if the function is successful; otherwise 0, and a specific error code can be retrieved by calling GetLastError.


Create then calls Bind to bind the socket to the specified address. The following socket types are supported:

  • SOCK_STREAM Provides sequenced, reliable, two-way, connection-based byte streams. Uses Transmission Control Protocol (TCP) for the Internet address family.

  • SOCK_DGRAM Supports datagrams, which are connectionless, unreliable buffers of a fixed (typically small) maximum length. Uses User Datagram Protocol (UDP) for the Internet address family. To use this option, you must not use the socket with a CArchive object.


    The Accept member function takes a reference to a new, empty CSocket object as its parameter. You must construct this object before you call Accept. Keep in mind that if this socket object goes out of scope, the connection closes. Do not call Create for this new socket object.

For more information about stream and datagram sockets, see the articles Windows Sockets: Background, Windows Sockets: Ports and Socket Addresses, and Windows Sockets: Using Sockets with Archives.


Constructs a CSocket object.



After construction, you must call the Create member function.

For more information, see Windows Sockets: Using Sockets with Archives.


Returns a pointer to a CSocket object.

static CSocket* PASCAL FromHandle(SOCKET hSocket);


Contains a handle to a socket.

Return Value

A pointer to a CSocket object, or NULL if there is no CSocket object attached to hSocket.


When given a SOCKET handle, if a CSocket object is not attached to the handle, the member function returns NULL and does not create a temporary object.

For more information, see Windows Sockets: Using Sockets with Archives.


Call this member function to determine if a blocking call is in progress.

BOOL IsBlocking();

Return Value

Nonzero if the socket is blocking; otherwise 0.


For more information, see Windows Sockets: Using Sockets with Archives.


Override this member function to look for particular messages from Windows and respond to them in your socket.

virtual BOOL OnMessagePending();

Return Value

Nonzero if the message was handled; otherwise 0.


This is an advanced overridable.

The framework calls OnMessagePending while the socket is pumping Windows messages to give you an opportunity to deal with messages of interest to your application. For examples of how you might use OnMessagePending, see the article Windows Sockets: Deriving from Socket Classes.

For more information, see Windows Sockets: Using Sockets with Archives.

See also

CAsyncSocket Class
Hierarchy Chart
CAsyncSocket Class
CSocketFile Class