Windows Sockets: How Sockets with Archives Work


For the latest documentation on Visual Studio 2017 RC, see Visual Studio 2017 RC Documentation.

This article explains how a CSocket object, a CSocketFile object, and a CArchive object are combined to simplify sending and receiving data through a Windows Socket.

The article Windows Sockets: Example of Sockets Using Archives presents the PacketSerialize function. The archive object in the PacketSerialize example works much like an archive object passed to an MFC Serialize function. The essential difference is that for sockets, the archive is attached not to a standard CFile object (typically associated with a disk file) but to a CSocketFile object. Rather than connecting to a disk file, the CSocketFile object connects to a CSocket object.

A CArchive object manages a buffer. When the buffer of a storing (sending) archive is full, an associated CFile object writes out the buffer's contents. Flushing the buffer of an archive attached to a socket is equivalent to sending a message. When the buffer of a loading (receiving) archive is full, the CFile object stops reading until the buffer is available again.

Class CSocketFile derives from CFile, but it does not support CFile member functions such as the positioning functions (Seek, GetLength, SetLength, and so on), the locking functions (LockRange, UnlockRange), or the GetPosition function. All the CSocketFile object must do is write or read sequences of bytes to or from the associated CSocket object. Because a file is not involved, operations such as Seek and GetPosition make no sense. CSocketFile is derived from CFile, so it would normally inherit all of these member functions. To prevent this, the unsupported CFile member functions are overridden in CSocketFile to throw a CNotSupportedException.

The CSocketFile object calls member functions of its CSocket object to send or receive data.

The following figure shows the relationships among these objects on both sides of the communication.

CArchive, CSocketFile, and CSocket
CArchive, CSocketFile, and CSocket

The purpose of this apparent complexity is to shield you from the necessity of managing the details of the socket yourself. You create the socket, the file, and the archive, and then begin sending or receiving data by inserting it to the archive or extracting it from the archive. CArchive, CSocketFile, and CSocket manage the details behind the scenes.

A CSocket object is actually a two-state object: sometimes asynchronous (the usual state) and sometimes synchronous. In its asynchronous state, a socket can receive asynchronous notifications from the framework. However, during an operation such as receiving or sending data the socket becomes synchronous. This means the socket will receive no further asynchronous notifications until the synchronous operation has completed. Because it switches modes, you can, for example, do something like the following:

void CMySocket::OnReceive(int nErrorCode)
   if (0 == nErrorCode)
      CSocketFile file(this);
      CArchive ar(&file, CArchive::load);
      CString str;

      ar >> str;

If CSocket were not implemented as a two-state object, it might be possible to receive additional notifications for the same kind of event while you were processing a previous notification. For example, you might get an OnReceive notification while processing an OnReceive. In the code fragment above, extracting str from the archive might lead to recursion. By switching states, CSocket prevents recursion by preventing additional notifications. The general rule is no notifications within notifications.

System_CAPS_ICON_note.jpg Note

A CSocketFile can also be used as a (limited) file without a CArchive object. By default, the CSocketFile constructor's bArchiveCompatible parameter is TRUE. This specifies that the file object is for use with an archive. To use the file object without an archive, pass FALSE in the bArchiveCompatible parameter.

In its "archive compatible" mode, a CSocketFile object provides better performance and reduces the danger of a "deadlock." A deadlock occurs when both the sending and receiving sockets are waiting on each other, or waiting for a common resource. This situation might occur if the CArchive object worked with the CSocketFile the way it does with a CFile object. With CFile, the archive can assume that if it receives fewer bytes than it requested, the end of file has been reached. With CSocketFile, however, data is message based; the buffer can contain multiple messages, so receiving fewer than the number of bytes requested does not imply end of file. The application does not block in this case as it might with CFile, and it can continue reading messages from the buffer until the buffer is empty. The IsBufferEmpty function in CArchive is useful for monitoring the state of the archive's buffer in such a case.

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

Windows Sockets in MFC