Understanding the preprocessor

This is an introduction to the code preprocessor.

The preprocessor is used to transform your sources before compilation. It allows you to include other files and to define macros that will be expanded when used in the source. It behaves similar to the C preprocessor, with some differences.
Important: It is recommended to avoid using the preprocessor if there is an alternative in the native language. For example, instead of defining program constants with an &define macro, use the CONSTANT instruction. Other language features such as IMPORT FGL increase code readability and modular programming, without the need of a preprocessor. The preprocessor might be desupported in a future version.

The preprocessor transforms files as follows:

  • The source file is read and split into lines.
  • Continued lines are merged into one long line if it is part of a preprocessor definition.
  • Comments are not removed unless they appear in a macro definition.
  • Each line is split into a list of lexical tokens.

The preprocessor implements the following features:

  1. File inclusion
  2. Conditional compilation
  3. Macro definition and expansion

There are two kind of macros:

  1. Simple macros
  2. Function macros

If a preprocessing directive is invalid, the compilers will generate an .err file with the preprocessing error included in the source file at the line position where the problem exists. When using the -M option, preprocessor errors will be printed to stderr, like regular compiler errors.