* Any outstanding write must have finished before the file can be closed.

* The "queue" data structure actually implements a set anyway and will
  be renamed subsequently.

* The "index" is now a member of the "enqueued" struct.
* The search operation uses a predicate passed as function pointer.

* This is mostly to shut up the compiler.

* This prevents accidentally closing the file early.
* Also: Replace cryptic reference notation.
* A newly opened file finish state is set to false by default.
** The write process must wait to receive a close command from each collector
   to prevent file truncation.

* An object may not be accessed via the array because of the strict
  aliasing rule of C99.

* It's guaranteed to be initialized later, but the compiler does not know.