@node Task Documentation
@appendix Task Documentation

This chapter presents a sample assignment and a filled-in design
document for one possible implementation.  Its purpose is to give you an
idea of what we expect to see in your own design documents.

@menu
* Sample Assignment::           
* Sample Design Document::      
@end menu

@node Sample Assignment
@section Sample Assignment

Implement @func{thread_join}.

@deftypefun void thread_join (tid_t @var{tid})
Blocks the current thread until thread @var{tid} exits.  If @var{A} is
the running thread and @var{B} is the argument, then we say that
``@var{A} joins @var{B}.''

Incidentally, the argument is a thread id, instead of a thread pointer,
because a thread pointer is not unique over time.  That is, when a
thread dies, its memory may be, whether immediately or much later,
reused for another thread.  If thread @var{A} over time had two children
@var{B} and @var{C} that were stored at the same address, then
@code{thread_join(@var{B})} and @code{thread_join(@var{C})} would be
ambiguous.

A thread may only join its immediate children.  Calling
@func{thread_join} on a thread that is not the caller's child should
cause the caller to return immediately.  Children are not ``inherited,''
that is, if @var{A} has child @var{B} and @var{B} has child @var{C},
then @var{A} always returns immediately should it try to join @var{C},
even if @var{B} is dead.

A thread need not ever be joined.  Your solution should properly free
all of a thread's resources, including its @struct{thread},
whether it is ever joined or not, and regardless of whether the child
exits before or after its parent.  That is, a thread should be freed
exactly once in all cases.

Joining a given thread is idempotent.  That is, joining a thread
multiple times is equivalent to joining it once, because it has already
exited at the time of the later joins.  Thus, joins on a given thread
after the first should return immediately.

You must handle all the ways a join can occur: nested joins (@var{A}
joins @var{B}, then @var{B} joins @var{C}), multiple joins (@var{A}
joins @var{B}, then @var{A} joins @var{C}), and so on.
@end deftypefun

@node Sample Design Document
@section Sample Design Document

@example
@include sample.tmpl.texi
@end example