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3.1 Preprocessing

The as internal preprocessor:

It does not do macro processing, include file handling, or anything else you may get from your C compilers preprocessor. You can do include file processing with the .include directive (see .include). You can use the GNU C compiler driver to get other CPP style preprocessing by giving the input file a .S suffix. See the Options Controlling the Kind of Output section of the GCC manual for more details

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 file 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 intended to support asm statements in compilers whose output is otherwise free of comments and whitespace.