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Simple Instant Messaging using MailSlots

, 3 Aug 2004 38.4K 1.8K 21
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An article on using Windows MailSlots to create a basic Instant Message Client


Instant messaging is more than just a way for some 18 year old girls to keep track what they are all going to wear the next day at school. IM is being used by businesses for internal communication. IM can be a very affective tool for getting information quickly back from a coworker.

IM can enhance many existing business applications. The real problem is how to add this support while not writing too much actual code. One solution would be to use some sort of TCP/IP server, and have a bunch of TCP/IP clients connect to the server. That type of solution might prove to be somewhat difficult. Luckily there is a real easy way to send messages from one computer to another, “mailslots”.

Mailslots are a message passing algorithm for one directional communications. Using a small amount of code you can get a good start on an IM client, without ever writing a server. My solutions presented here is a rough prototype for building a mailslot based chat program. A good amount of work will still need to go into it to really make an affective tool. But, if you are willing to spend some time you could easily integrate a mailslot chat program into an already existing MFC application.

Running the sample

After building the solution you will need to have all users of the software run it from the same directory. So you will need to create a shared directory that all users of the mailslot program can access. To start MailSlotApp.exe you need to pass your username as the only command line parameter. For example, "MailSlotApp Chad" you start the program with the user "Chad". When the program starts the user "Chad" would be added to the file "userlist.dat".

When the UserList dialog starts up it will list the contents of the "userlist.dat" file. As users login there names will be added to the "userlist.dat" file and a notification will be sent to all active mailslots. (currently no notification is included for logout).

To send a message to another user just double click on their name in the list and a MailSlotDlg will appear. From the MailSlotDlg you can enter a new message and send it of to the other user. When the other user receives the message the m_mailSlot object in their MailSlotApp.exe will read the message and notify the UserList that a new message has arrived.

After receiving notification of the new MailSlot message their UserList dialog will create a new non-modal dialog to manage the conversation with that particular user.

Source Code Organization

Serializable CObject. Base for MailSlotMessage.
MailSlot and MailSlotMessage classes.
Test Application.
IM dialog.
User selection dialog. Spawns MailSlotDlg's as needed.

The most useful classes to reuse in your own projects will probably be found in Package.h/cpp and MailSlot.h/cpp. The actual dialogs in UserList and MailSlotDlg need some work before they could really be useful.

The project contains two dialogs. The first dialog is the UserList dialog. This is kind of like your buddy list in a normal IM client. Here you should be presented with a list of all users that are running the program from the same directory as you.

The second dialog is the MailSlotDlg. This dialog will display your conversation history with a particular user, and it will allow you to enter new messages. The MailSlotDlg class never does directly interact with the MailSlot.

All communication between users occurs through the use of a MailSlot object m_mailSlot within the UserList dialog. When you send a message from the MailSlotDlg that message will actually get forwarded to the UserList dialog which will then finish sending the message. When a new message has been received it is the responsibility of the UserList dialog to process the message and determine where to forward the message.

Send Message:       MailSlotDlg-->UserList-->MailSlot
Receive Message:    MailSlot->UserList-->MailSlotDlg

MailSlot Class

The MailSlot class can be used to abstract away the details of actually sending and receiving data from an actual mailslot. To use the MailSlot class all you need to do is create an instance of MailSlot and call its create method.

// Create a MailSlot class. This will open a mailslot  
// on this computer with the name m_userName. The "this" 
// and "MAILSLOT_NOTIFY_MSG" parameters are needed so 
// the mailslot class can notify the "this" dialog 
// when the MailSlot has read a new message. 
m_mailSlot.Create(m_userName, this, MAILSLOT_NOTIFY_MSG);         

Sending an message using the mailslot class is real easy. Just call MailSlot::Send(..)

// To send a message just create a MailSlotMessage 
// and pass it to Send method for the mailslot.

Reading from a mailslot involves setting up a method to handle the messages sent to MAILSLOT_NOTIFY_MSG

// Receive message from a mailslot is a little more complicated. 
// This method will be called
// by the MailSlot class when the MailSlot class reads a new message.
LRESULT UserList::OnMailSlotNotify(WPARAM w, LPARAM l)
 MailSlotMessage msg;
 while (m_mailSlot.Read(msg)) {
     if (msg.MsgType() == MailSlotMessage::TEXT) {
         // If the type == MailSlotMessage::TEXT then we
         // should display the message in the
         // correct MailSlotDlg. Calling GetDlg(..) will
         // create the new dialog if it does
         // not already exist.
         MailSlotDlg * pWnd = GetDlg(msg.FromComputerName(), 
         if (pWnd)
             pWnd->AddMessageToHistory(msg); // add message to history
     } else {
         // A special message is send when a new user 
         // logins in. This will cause us to
         // refresh the list.
         if (msg.MsgText() == LOGIN_MSG) {
 return 1;


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Comments and Discussions

GeneralApp not working correctly. Pin
BigDick29-May-08 8:49
memberBigDick29-May-08 8:49 

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