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 form

# linenum filename flags

which 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' or `3'. If there are multiple flags, spaces separate them. Here is what the flags mean:

`1'

This indicates the start of a new file.

`2'

This indicates returning to a file (after having included another file).

`3'

This indicates that the following text comes from a system header file, so certain warnings should be suppressed.