10.5. Interprocess Communication: DDE

When one of these operator interface methods is used, the operator interface will usually run as a separate program, distinct from the C++ control program (or the portion of the control program that is running in the computer that is used for the operator interface). Although this is not absolutely necessary, it is more convenient to keep them as separate entities than to try to use various multi-language facilities to combine them into a single program. Separate programs represent independent processes in the terminology used here. Independent processes do not share an address space so interprocess communication must be done explicitly rather than using language syntax. All of the operator interface programs used here run in Windows so a communication method appropriate to Windows will be used. Interprocess communication is one of those areas where it is difficult to find portable techniques.

Of the several methods for interprocess communication that Windows supports we will use one of the simplest and oldest, dynamic data exchange (DDE). This protocol only allows for exchange of character strings so is not a particularly efficient way to exchange numeric information. On the other hand, it is available very widely and is easily adapted to our needs.