how to not #include malloc only on freebsd, b/c it wants stdlib instead.

some portability issues: what if openbsd came along?

where to find cpp symbols
- compiler/cpp hardcoded
- header files
- gcc -v

what to comment: functions, globals, vars, algs, data structures, and other
important items.

when docuemnting functions, cover: pre/post conditions, input/output args,
type/names/meaning of args, return values and errors, what side effects a
function has, what to expect after it has run, caller's responsibility,
calee's duties, etc.

makefile intro: why they are needed. how they work.