looked up these flags, since I didn't know all of them:

-C: compile but do not link (I think this is covered by -E)

-E: stop after running the preprocessor

-H: print the name of each header file included, indenting child #includes (what do you use this for? is it printed to stderr?)

-P: inhibit the generation of line markers from the preprocessor (I've used this before, but can't recall the problem it solves? Not expanding __LINE__?)

-nostdinc: do not search standard system directories for header files (/usr/include, /usr/local/include, ..., this is standard in the Linux kernel, since there is no libc)

-undef: do not predefine any system-specific or GCC macros (probably to not expand occurrences of sequences like "__GNUC__" and others from unexpectedly being expanded).

-x <language>: specify the language, rather then letting the compiler guess based on the file extension (is this required?)