In every OS project too, Assembly is needed. It is really fast, but is not for everyone. All code must be commented. Indentation also makes a program easier to read, and all you need to do is to press this TAB key. Makefiles are very compiler-dependent, and with many source files it's harder to understand the connection between the modules and where is the main program.
Output is to filehandle.
Inc. This file is part of the GNU C Library. See the GNU Library General Public License for more details. Documentation Documentation is fundamental. But information should be always available for everyone who wants to know a little bit more about your program. Some 5 lines could be enough for a lot of things.