4.26 MFL Preprocessor

Before compiling the script file, mailfromd preprocesses it. The built-in preprocessor handles only file inclusion (see include), while the rest of traditional facilities, such as macro expansion, are supported via m4, which is used as an external preprocessor.

The detailed description of m4 facilities lies far beyond the scope of this document. You will find a complete user manual in GNU M4 in GNU M4 macro processor. For the rest of this section we assume the reader is sufficiently acquainted with m4 macro processor.

The external preprocessor is invoked with -s flag, instructing it to include line synchronization information in its output, which is subsequently used by MFL compiler for purposes of error reporting. The initial set of macro definitions is supplied in preprocessor setup file pp-setup, located in the library search path²⁰, which is fed to the preprocessor input before the script file itself. The default pp-setup file renames all m4 built-in macro names so they all start with the prefix ‘m4_’²¹. It changes comment characters to ‘/*’, ‘*/’ pair, and leaves the default quoting characters, grave (‘`’) and acute (‘'’) accents without change. Finally, pp-setup defines several useful macros (see m4 macros).