Mailslots

Chapter 19

Mailslots

Microsoft Windows NT, Windows 95, Windows 98, and Windows Me platforms (but not Windows CE) include a simple one-way interprocess communication (IPC) mechanism known as mailslots. In simplest terms, mailslots allow a client process to transmit or broadcast messages to one or more server processes. Mailslots can assist transmission of messages among processes on the same computer or among processes on different computers across a network. Developing applications using mailslots is simple, requiring no formal knowledge of underlying network transport protocols such as TCP/IP or IPX. Because mailslots are designed around a broadcast architecture, you can't expect reliable data transmissions using mailslots. They can be useful, nevertheless, in certain types of network programming situations in which delivery of data isn't mission-critical.

One possible scenario for using mailslots is developing a messaging system that includes everyone in your office. Imagine that your office environment has a large number of workstations. The office is suffering from a soda shortage, and every workstation user in your office is interested in knowing every few minutes how many sodas are available in the vending machine. Mailslots lend themselves well to this type of situation. You can easily implement a mailslot client application that monitors the soda count and broadcasts to every interested workstation user the total number of available sodas at five-minute intervals. Because mailslots don't guarantee delivery of a broadcast message, some workstation users might not receive all updates. A few transmission failures won't be a problem in this case because messages sent at five-minute intervals with occasional misses are still frequent enough to keep the workstation users well informed.

The major limitation of mailslots is that they permit only unreliable one-way data communication from a client to a server. The biggest advantage of mailslots is that they allow a client application to easily send broadcast messages to one or more server applications.

This chapter explains how to develop a mailslot client/server application. We'll describe mailslot naming conventions before we address the message sizing considerations that control the overall behavior of mailslots. Next we'll show you the details of developing a basic client/server application. At the end of this chapter, we'll tell you about known problems and limitations of mailslots and offer workaround solutions.