as internal preprocessor:
It does not do macro processing, include file handling, or
anything else you may get from your C compiler’s preprocessor. You can
do include file processing with the
.include). You can use the GNU C compiler driver
to get other “CPP” style preprocessing by giving the input file a
‘.S’ suffix. See Options Controlling the Kind of
Output in Using GNU CC.
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 in
specific 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