This entry point is used before the source file reading starts. The pointer p points to the ReadObject used to read the source files. There is few uses of this entry point. You may alter the file handling function pointers or the memory allocation function pointers in the ReadObject.
This entry point is invoked only when the preprocessor load was initiated by external conditions, like command line option `-i' and never if the source code contained the preprocessor loading directive. This is because when the reader realizes that the preprocessor has to be loaded it is far over this point.
The function has to return zero or the error code and should set the parameter *pCmd to PreprocessorContinue, PreprocessorDone, or PreprocessorUnload.