Ocaml can't find Unix module
unix library is not linked by default, so you need to pass some linking flags, to make it work, e.g.,
ocamlc unix.cma fork.ml -o forkIf you don't want to know anything about cma, you can use ocamlbuild, instead of ocamlc:
ocamlbuild -lib unix fork.nativeOr even more general
ocamlbuild -pkg unix fork.nativeThe latter (with pkg option) would be a preferred way, since it will allow you to specify any package installed with opam. E.g., if you would ever try to use lwt, the you can just link with it with
ocamlbuild -pkg lwt fork.native