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C Preprocessor Output

The output from the C preprocessor looks much like the input, except that all preprocessing directive lines have been replaced with blank lines and all comments with spaces. Whitespace within a line is not altered; however, a space is inserted after the expansions of most macro calls. Source file name and line number information is conveyed by lines of the following form.

    # linenum filename flags

Spaces are inserted as needed into the middle of the input (but never within a string or character constant). Such a line means that the following line originated in file filename at line linenum. After the file name comes zero or more flags, which are 1, 2, 3, or 4. If there are multiple flags, spaces separate them. Here is what the flags mean:

- 1 indicates the start of a new file.
- 2 indicates returning to a file (after having included another file).
- 3 indicates that the text following comes from a system header file, so certain warnings should be suppressed.
- 4 indicates that the text following should be treated as C.