The full scope of the macro facility can only be appreciated with some knowledge of how RFPP stores, retrieves and expands macro definitions. There are two fundamental macro operations; the macro definition and the macro invocation. The macro definition is achieved via the inbuilt macro: command, and the invocation occurs when the macro name is subsequently used in the code.

The macro: command stores the macro name and definition in the RFPP macro table and sets various pointers for its efficient extraction. The macro definition process is performed in two separate steps. The first occurs when the macro: instruction is encountered and the definition string is stored in the RFPP macro table, and the second when the string is placed in the Fortran output. During both these steps the definition is searched for any macro names and these are 'expanded' (i.e. the name is replaced by the definition) provided the appropriate definitions exist in the macro table. In other words the preprocessor has two (and only two!) opportunities to substitute the macro definition for a macro name. If a macro definition is not available by step two, the macro name is output into the Fortran.

Occasionally, it is necessary to prevent macro expansion during step 1, and in special circumstances even during step 2. In these cases macro expansion can be prevented by surrounding the definition with either square brackets or apostrophes. Understanding the function of macro expansion inhibitors is important for the advanced use of this facility.