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array, hash, scalar ref, array ref or hash ref.  C<share> will return
the shared rvalue but always as a reference.

C<share> will traverse up references exactly I<one> level.
C<share(\$a)> is equivalent to C<share($a)>, while C<share(\\$a)> is not.
This means that you must create nested shared data structures by first
creating individual shared leaf notes, then adding them to a shared hash
or array.

A variable can also be marked as shared at compile time by using the
C<shared> attribute: C<my $var : shared>.

If you want to share a newly created reference unfortunately you
need to use C<&share([])> and C<&share({})> syntax due to problems
with Perl's prototyping.

The only values that can be assigned to a shared scalar are other scalar
values, or shared refs, eg

    my $var : shared;
    $var = 1;              # ok
    $var = &share([]);     # ok
    $var = [];             # error
    $var = A->new;         # error
    $var = &share(A->new); # ok as long as the A object is not nested

Note that it is often not wise to share an object unless the class itself
has been written to support sharing; for example, an object's destructor
may get called multiple times, one for each thread's scope exit.

=item lock VARIABLE

C<lock> places a lock on a variable until the lock goes out of scope.
If the variable is locked by another thread, the C<lock> call will
block until it's available. C<lock> is recursive, so multiple calls
to C<lock> are safe -- the variable will remain locked until the
outermost lock on the variable goes out of scope.

If a container object, such as a hash or array, is locked, all the
elements of that container are not locked. For example, if a thread
does a C<lock @a>, any other thread doing a C<lock($a[12])> won't block.

C<lock> will traverse up references exactly I<one> level.
C<lock(\$a)> is equivalent to C<lock($a)>, while C<lock(\\$a)> is not.

Note that you cannot explicitly unlock a variable; you can only wait
for the lock to go out of scope. If you need more fine-grained
control, see L<Thread::Semaphore>.

=item cond_wait VARIABLE

=item cond_wait CONDVAR, LOCKVAR

The C<cond_wait> function takes a B<locked> variable as a parameter,
unlocks the variable, and blocks until another thread does a
C<cond_signal> or C<cond_broadcast> for that same locked variable.
The variable that C<cond_wait> blocked on is relocked after the
C<cond_wait> is satisfied.  If there are multiple threads
C<cond_wait>ing on the same variable, all but one will reblock waiting
to reacquire the lock on the variable. (So if you're only using
C<cond_wait> for synchronisation, give up the lock as soon as
possible). The two actions of unlocking the variable and entering the
blocked wait state are atomic, the two actions of exiting from the
blocked wait state and relocking the variable are not.

In its second form, C<cond_wait> takes a shared, B<unlocked> variable
followed by a shared, B<locked> variable.  The second variable is
unlocked and thread execution suspended until another thread signals
the first variable.

It is important to note that the variable can be notified even if
no thread C<cond_signal> or C<cond_broadcast> on the variable.
It is therefore important to check the value of the variable and
go back to waiting if the requirement is not fulfilled.  For example,
to pause until a shared counter drops to zero:

    { lock($counter); cond_wait($count) until $counter == 0; }

=item cond_timedwait VARIABLE, ABS_TIMEOUT

=item cond_timedwait CONDVAR, ABS_TIMEOUT, LOCKVAR

In its two-argument form, C<cond_timedwait> takes a B<locked> variable
and an absolute timeout as parameters, unlocks the variable, and blocks
until the timeout is reached or another thread signals the variable.  A
false value is returned if the timeout is reached, and a true value
otherwise.  In either case, the variable is re-locked upon return.

Like C<cond_wait>, this function may take a shared, B<locked> variable
as an additional parameter; in this case the first parameter is an
B<unlocked> condition variable protected by a distinct lock variable.

Again like C<cond_wait>, waking up and reacquiring the lock are not
atomic, and you should always check your desired condition after this
function returns.  Since the timeout is an absolute value, however, it
does not have to be recalculated with each pass:

    lock($var);
    my $abs = time() + 15;
    until ($ok = desired_condition($var)) {
      last if !cond_timedwait($var, $abs);
    }
    # we got it if $ok, otherwise we timed out!

=item cond_signal VARIABLE

The C<cond_signal> function takes a B<locked> variable as a parameter
and unblocks one thread that's C<cond_wait>ing on that variable. If
more than one thread is blocked in a C<cond_wait> on that variable,
only one (and which one is indeterminate) will be unblocked.

If there are no threads blocked in a C<cond_wait> on the variable,
the signal is discarded. By always locking before signaling, you can
(with care), avoid signaling before another thread has entered cond_wait().

C<cond_signal> will normally generate a warning if you attempt to use it
on an unlocked variable. On the rare occasions where doing this may be
sensible, you can skip the warning with

    { no warnings 'threads'; cond_signal($foo) }