• Adjusts and removes extra whitespace. It leaves one space or tab before the keywords on a line, and turns any other whitespace on the line into a single space.
• Removes all comments, replacing them with a single space, or an appropriate number of newlines.
• Converts character constants into the appropriate numeric values.

Excess whitespace, comments, and character constants cannot be used in the portions of the input text that are not preprocessed.

If the first line of an input file is #NO_APP or if you use the -f option, whitespace and comments are not removed from the input file. Within an input file, you can ask for whitespace and comment removal inspecific portions of the by putting a line that says #APP before the text that may contain whitespace or comments, and putting a line that says #NO_APP after this text. This feature is mainly intend to support asm statements in compilers whose output is otherwise free of comments and whitespace.