perlothrtut.pod

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for a thread to exit and extract any scalars it might return, you can
use the join() method.

    use Thread; 
    $thr = Thread->new( \&sub1 );

    @ReturnData = $thr->join; 
    print "Thread returned @ReturnData"; 

    sub sub1 { return "Fifty-six", "foo", 2; }

In the example above, the join() method returns as soon as the thread
ends.  In addition to waiting for a thread to finish and gathering up
any values that the thread might have returned, join() also performs
any OS cleanup necessary for the thread.  That cleanup might be
important, especially for long-running programs that spawn lots of
threads.  If you don't want the return values and don't want to wait
for the thread to finish, you should call the detach() method
instead. detach() is covered later in the article.

=head2 Errors In Threads

So what happens when an error occurs in a thread? Any errors that
could be caught with eval() are postponed until the thread is
joined.  If your program never joins, the errors appear when your
program exits.

Errors deferred until a join() can be caught with eval():

    use Thread qw(async); 
    $thr = async {$b = 3/0};   # Divide by zero error
    $foo = eval {$thr->join}; 
    if ($@) { 
        print "died with error $@\n"; 
    } else { 
        print "Hey, why aren't you dead?\n"; 
    }

eval() passes any results from the joined thread back unmodified, so
if you want the return value of the thread, this is your only chance
to get them.

=head2 Ignoring A Thread

join() does three things: it waits for a thread to exit, cleans up
after it, and returns any data the thread may have produced.  But what
if you're not interested in the thread's return values, and you don't
really care when the thread finishes? All you want is for the thread
to get cleaned up after when it's done.

In this case, you use the detach() method.  Once a thread is detached,
it'll run until it's finished, then Perl will clean up after it
automatically.

    use Thread; 
    $thr = Thread->new( \&sub1 ); # Spawn the thread

    $thr->detach; # Now we officially don't care any more

    sub sub1 { 
        $a = 0; 
        while (1) { 
            $a++; 
            print "\$a is $a\n"; 
            sleep 1; 
        } 
    }


Once a thread is detached, it may not be joined, and any output that
it might have produced (if it was done and waiting for a join) is
lost.

=head1 Threads And Data

Now that we've covered the basics of threads, it's time for our next
topic: data.  Threading introduces a couple of complications to data
access that non-threaded programs never need to worry about.

=head2 Shared And Unshared Data

The single most important thing to remember when using threads is that
all threads potentially have access to all the data anywhere in your
program.  While this is true with a nonthreaded Perl program as well,
it's especially important to remember with a threaded program, since
more than one thread can be accessing this data at once.

Perl's scoping rules don't change because you're using threads.  If a
subroutine (or block, in the case of async()) could see a variable if
you weren't running with threads, it can see it if you are.  This is
especially important for the subroutines that create, and makes C<my>
variables even more important.  Remember--if your variables aren't
lexically scoped (declared with C<my>) you're probably sharing them
between threads.

=head2 Thread Pitfall: Races

While threads bring a new set of useful tools, they also bring a
number of pitfalls.  One pitfall is the race condition:

    use Thread; 
    $a = 1; 
    $thr1 = Thread->new(\&sub1); 
    $thr2 = Thread->new(\&sub2); 

    sleep 10; 
    print "$a\n";

    sub sub1 { $foo = $a; $a = $foo + 1; }
    sub sub2 { $bar = $a; $a = $bar + 1; }

What do you think $a will be? The answer, unfortunately, is "it
depends." Both sub1() and sub2() access the global variable $a, once
to read and once to write.  Depending on factors ranging from your
thread implementation's scheduling algorithm to the phase of the moon,
$a can be 2 or 3.

Race conditions are caused by unsynchronized access to shared
data.  Without explicit synchronization, there's no way to be sure that
nothing has happened to the shared data between the time you access it
and the time you update it.  Even this simple code fragment has the
possibility of error:

    use Thread qw(async); 
    $a = 2; 
    async{ $b = $a; $a = $b + 1; }; 
    async{ $c = $a; $a = $c + 1; };

Two threads both access $a.  Each thread can potentially be interrupted
at any point, or be executed in any order.  At the end, $a could be 3
or 4, and both $b and $c could be 2 or 3.

Whenever your program accesses data or resources that can be accessed
by other threads, you must take steps to coordinate access or risk
data corruption and race conditions.

=head2 Controlling access: lock()

The lock() function takes a variable (or subroutine, but we'll get to
that later) and puts a lock on it.  No other thread may lock the
variable until the locking thread exits the innermost block containing
the lock.  Using lock() is straightforward:

    use Thread qw(async); 
    $a = 4; 
    $thr1 = async { 
        $foo = 12; 
        { 
            lock ($a); # Block until we get access to $a 
            $b = $a; 
            $a = $b * $foo; 
        } 
        print "\$foo was $foo\n";
    }; 
    $thr2 = async { 
        $bar = 7; 
        { 
            lock ($a); # Block until we can get access to $a
            $c = $a; 
            $a = $c * $bar; 
        } 
        print "\$bar was $bar\n";
    }; 
    $thr1->join; 
    $thr2->join; 
    print "\$a is $a\n";

lock() blocks the thread until the variable being locked is
available.  When lock() returns, your thread can be sure that no other
thread can lock that variable until the innermost block containing the
lock exits.

It's important to note that locks don't prevent access to the variable
in question, only lock attempts.  This is in keeping with Perl's
longstanding tradition of courteous programming, and the advisory file
locking that flock() gives you.  Locked subroutines behave differently,
however.  We'll cover that later in the article.

You may lock arrays and hashes as well as scalars.  Locking an array,
though, will not block subsequent locks on array elements, just lock
attempts on the array itself.

Finally, locks are recursive, which means it's okay for a thread to
lock a variable more than once.  The lock will last until the outermost
lock() on the variable goes out of scope.

=head2 Thread Pitfall: Deadlocks

Locks are a handy tool to synchronize access to data.  Using them
properly is the key to safe shared data.  Unfortunately, locks aren't
without their dangers.  Consider the following code:

    use Thread qw(async yield); 
    $a = 4; 
    $b = "foo"; 
    async { 
        lock($a); 
        yield; 
        sleep 20; 
        lock ($b); 
    }; 
    async { 
        lock($b); 
        yield; 
        sleep 20; 
        lock ($a); 
    };

This program will probably hang until you kill it.  The only way it
won't hang is if one of the two async() routines acquires both locks
first.  A guaranteed-to-hang version is more complicated, but the
principle is the same.

The first thread spawned by async() will grab a lock on $a then, a
second or two later, try to grab a lock on $b.  Meanwhile, the second
thread grabs a lock on $b, then later tries to grab a lock on $a.  The
second lock attempt for both threads will block, each waiting for the
other to release its lock.

This condition is called a deadlock, and it occurs whenever two or
more threads are trying to get locks on resources that the others
own.  Each thread will block, waiting for the other to release a lock
on a resource.  That never happens, though, since the thread with the
resource is itself waiting for a lock to be released.

There are a number of ways to handle this sort of problem.  The best
way is to always have all threads acquire locks in the exact same
order.  If, for example, you lock variables $a, $b, and $c, always lock
$a before $b, and $b before $c.  It's also best to hold on to locks for
as short a period of time to minimize the risks of deadlock.

=head2 Queues: Passing Data Around

A queue is a special thread-safe object that lets you put data in one
end and take it out the other without having to worry about
synchronization issues.  They're pretty straightforward, and look like
this:

    use Thread qw(async); 
    use Thread::Queue;

    my $DataQueue = Thread::Queue->new();
    $thr = async { 
        while ($DataElement = $DataQueue->dequeue) { 
            print "Popped $DataElement off the queue\n";
        } 
    }; 

    $DataQueue->enqueue(12); 
    $DataQueue->enqueue("A", "B", "C"); 
    sleep 10; 
    $DataQueue->enqueue(undef);

You create the queue with new Thread::Queue.  Then you can add lists of
scalars onto the end with enqueue(), and pop scalars off the front of
it with dequeue().  A queue has no fixed size, and can grow as needed
to hold everything pushed on to it.

If a queue is empty, dequeue() blocks until another thread enqueues
something.  This makes queues ideal for event loops and other
communications between threads.

=head1 Threads And Code

In addition to providing thread-safe access to data via locks and
queues, threaded Perl also provides general-purpose semaphores for
coarser synchronization than locks provide and thread-safe access to
entire subroutines.

=head2 Semaphores: Synchronizing Data Access

Semaphores are a kind of generic locking mechanism.  Unlike lock, which
gets a lock on a particular scalar, Perl doesn't associate any
particular thing with a semaphore so you can use them to control
access to anything you like.  In addition, semaphores can allow more
than one thread to access a resource at once, though by default
semaphores only allow one thread access at a time.

=over 4

=item Basic semaphores

Semaphores have two methods, down and up. down decrements the resource
count, while up increments it.  down calls will block if the
semaphore's current count would decrement below zero.  This program
gives a quick demonstration:

    use Thread qw(yield); 
    use Thread::Semaphore; 
    my $semaphore = Thread::Semaphore->new();
    $GlobalVariable = 0;

    $thr1 = Thread->new( \&sample_sub, 1 );
    $thr2 = Thread->new( \&sample_sub, 2 );
    $thr3 = Thread->new( \&sample_sub, 3 );

    sub sample_sub { 
        my $SubNumber = shift @_; 
        my $TryCount = 10; 
        my $LocalCopy; 
        sleep 1; 
        while ($TryCount--) { 
            $semaphore->down; 
            $LocalCopy = $GlobalVariable; 
            print "$TryCount tries left for sub $SubNumber (\$GlobalVariable is $GlobalVariable)\n"; 
            yield; 
            sleep 2; 
            $LocalCopy++; 
            $GlobalVariable = $LocalCopy; 
            $semaphore->up; 
        } 
    }

The three invocations of the subroutine all operate in sync.  The
semaphore, though, makes sure that only one thread is accessing the
global variable at once.

=item Advanced Semaphores

By default, semaphores behave like locks, letting only one thread
down() them at a time.  However, there are other uses for semaphores.

Each semaphore has a counter attached to it. down() decrements the
counter and up() increments the counter.  By default, semaphores are
created with the counter set to one, down() decrements by one, and
up() increments by one.  If down() attempts to decrement the counter
below zero, it blocks until the counter is large enough.  Note that
while a semaphore can be created with a starting count of zero, any
up() or down() always changes the counter by at least
one. $semaphore->down(0) is the same as $semaphore->down(1).

The question, of course, is why would you do something like this? Why
create a semaphore with a starting count that's not one, or why
decrement/increment it by more than one? The answer is resource
availability.  Many resources that you want to manage access for can be
safely used by more than one thread at once.

For example, let's take a GUI driven program.  It has a semaphore that
it uses to synchronize access to the display, so only one thread is
ever drawing at once.  Handy, but of course you don't want any thread
to start drawing until things are properly set up.  In this case, you
can create a semaphore with a counter set to zero, and up it when
things are ready for drawing.

Semaphores with counters greater than one are also useful for
establishing quotas.  Say, for example, that you have a number of
threads that can do I/O at once.  You don't want all the threads
reading or writing at once though, since that can potentially swamp
your I/O channels, or deplete your process' quota of filehandles.  You
can use a semaphore initialized to the number of concurrent I/O
requests (or open files) that you want at any one time, and have your
threads quietly block and unblock themselves.

Larger increments or decrements are handy in those cases where a
thread needs to check out or return a number of resources at once.