Q185728: MFCSocs.exe Avoids Two Common MFC Socket Mistakes
Article: Q185728
Product(s): Microsoft C Compiler
Version(s): 5.0,6.0
Operating System(s):
Keyword(s): kbfile kbnetwork kbSample kbAPI kbMFC kbSDKPlatform kbVC500 kbVC600 kbWinsock kbGrpDSNe
Last Modified: 15-MAY-2002
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The information in this article applies to:
- The Microsoft Foundation Classes (MFC), used with:
- Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0
- Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0
- Microsoft Visual C++.NET (2002)
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NOTE: Microsoft Visual C++ NET (2002) supported both the managed code model that is provided by the .NET Framework and the unmanaged native Windows code model. The information in this article applies to unmanaged Visual C++ code only.
SUMMARY
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The MFCSocs sample is a minimal MFC application that demonstrates how to use the
CSocket and CAsyncSocket classes to communicate in a Transmission Control
Protocol (TCP) connection. This sample is a dialog-based application. Depending
on the selection in the user interface, the application can run either as a TCP
server that listens on a certain TCP port, or as a client that connects to a
server on the same TCP port.
When running as a client, the application uses the CSocket class to communicate
with the server. When running as a server, the application uses CAsyncSocket to
listen to connection requests and communicate with the client after the
connection is established.
MORE INFORMATION
================
The following files are available for download from the Microsoft Download
Center:
Mfcsocs.exe
For additional information about how to download Microsoft Support files, click
the article number below to view the article in the Microsoft Knowledge Base:
Q119591 How to Obtain Microsoft Support Files from Online Services
Microsoft used the most current virus detection software available on the date of
posting to scan this file for viruses. After it is posted, the file is housed on
secure servers that prevent any unauthorized changes to the file.
This sample addresses two common mistakes that MFC socket programmers can make:
1. Issuing more than one Receive call in the OnReceive notification function.
The CSocket class implements blocking for the Receive and Send calls by
looping CAsyncSocket Receive and Send calls until all the data has been
received or completely sent. Therefore, the socket type of both Csocket and
CAysncSocket is nonblocking. For a nonblocking socket, an FD_READ
notification that the TCP stack sends to the application indicates data
availability. MFC then maps the FD_READ notification to the OnReceive
notification call.
In Windows Sockets, you should not make multiple recv calls within an FD_READ
notification unless you are willing to disable FD_READ notifications prior to
calling recv. However, CSocket and CAsyncSocket make no provision for doing
so. Therefore, you should make only one Receive call per OnReceive function.
Under high data transmission rate, if you make more than one Receive call in
the OnReceive function, the application might lose FD_READ, have fake
FD_READ, or have no FD_READ (hanging).
You can use CSocket with CArchive and CSocketFile to directly receive and send
MFC CObject-derived objects. However, under high data transmission rates, you
should not use CSocket with CArchive and CSocketFile within the OnReceive
function because they might internally generate multiple Receive calls.
2. Not issuing enough Send calls to make sure all data has been sent when using
the CAsyncSocket::Send function.
When you issue an asynchronous Send call to send a buffer, there are three
possible outcomes:
1. The call returns the number of bytes sent, and the buffer is completely
sent
2. The call returns the number of bytes sent, and the buffer is partially
sent
3. The call returns the WSAEWOULDBLOCK error, and the buffer is not sent.
In the second case, the program should continue to issue calls to Send until
either all the data has been sent or WSAEWOULDBLOCK occurs.
In the third case, when the TCP/IP stack frees more buffer space, it
dispatches an FD_WRITE notification to the application that is mapped to the
OnSend notification function by MFC. At this point, the program should
continue to send data until either all the data has been sent or
WSAEWOULDBLOCK occurs.
When you use a CSocket Receive and Send outside OnReceive and OnSend (in other
words, when you use a default do-nothing OnSend and OnReceive), it is okay to
use Receive and Send in a loop or use them with CArchive and CSocketFile.
Steps to Run the Sample
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1. Run the .exe file (exe1) and select Server. This puts the .exe instance in
server listening state.
2. Run the .exe again (exe2) either on the same machine or a remote machine.
Select Client and type in the name of the machine on which exe1 is running.
3. In exe2, type anything in the Send box. Click Send to make the connection and
send it. Click Yes to accept the incoming connection on the server (exe1).
Now you can send messages from either exe1 or exe2. Note that once connected,
the application does not reconnect when you click Send.
4. If either exe1 or exe2 wants to quit, type "bye" in either application and
click Send. To restart communication, go to step 3.
Additional query words:
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Keywords : kbfile kbnetwork kbSample kbAPI kbMFC kbSDKPlatform kbVC500 kbVC600 kbWinsock kbGrpDSNet
Technology : kbAudDeveloper kbMFC kbVCNET
Version : :5.0,6.0
Issue type : kbhowto
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