AtomsThis chapter describes how to use the Dynamic Data Exchange Management Library (DDEML). The DDEML is a dynamic-link library (DLL) that applications running with the Microsoft Windows operating system can use to share data.
The following topics are related to the information in this chapter:
Atoms
Memory management
Clipboard
Dynamic-link libraries
Object linking and embedding (OLE)
Dynamic data exchange (DDE) is a form of interprocess communication that uses shared memory to exchange data between applications. Applications can use DDE for one-time data transfers and for ongoing exchanges in which the applications send updates to one another as new data becomes available.
Dynamic data exchange differs from the clipboard data-transfer mechanism that is also part of the Windows operating system. One difference is that the clipboard is almost always used as a one-time response to a specific action by the user—such as choosing the Paste command from a menu. Although DDE may also be initiated by a user, it typically continues without the user's further involvement.
The DDEML provides an application programming interface (API) that simplifies the task of adding DDE capability to a Windows application. Instead of sending, posting, and processing DDE messages directly, an application uses the functions provided by the DDEML to manage DDE conversations. (A DDE conversation is the interaction between client and server applications.) The DDEML also provides a facility for managing the strings and data that are shared among DDE applications. Instead of using atoms and pointers to shared memory objects, DDE applications create and exchange string handles, which identify strings, and data handles, which identify global memory objects. DDEML provides a service that makes it possible for a server application to register the service names that it supports. The names are broadcast to other applications in the system, which can then use the names to connect to the server. The DDEML also ensures compatibility among DDE applications by forcing them to implement the DDE protocol in a consistent manner.
Existing applications that use the message-based DDE protocol are fully compatible with those that use the DDEML. That is, an application that uses message-based DDE can establish conversations and perform transactions with applications that use the DDEML. Because of the many advantages of the DDEML, new applications should use it rather than the DDE messages.
The DDEML can run on systems that have Microsoft Windows version 3.0 or later installed. The DDEML does not support real mode. To use the API elements of the DDE management library, you must include the DDEML.H header file in your source files, link with DDEML.LIB, and ensure that DDEML.DLL resides in the system's path.