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Threads have been heavily updated in Perl 56 to form a significant, if still largely experimental, part of the Perl language In fact, in some circumstances, threads actually form a core part of the language s new architecture, and on the Windows platform threads are used to emulate the operation for fork, a function that is missing from the operating system itself Within Perl, the thread system is controlled using the Thread module, which provides an object-oriented interface for the creation and control of individual threads To create a new thread, you create a new Thread object and supply the name of a predefined subroutine, which forms the basis of the thread s execution sequence Once started, a thread can be paused, stopped, split into other threads, or bonded with other threads to create a superthread In all instances, the threads remain attached to the parent process it s not possible to convert a thread into a new process, although there s potentially no reason why you couldn t call fork!
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Creating a New Thread
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To create a new thread, import the Thread module and then create a new Thread object For example, to create a new thread that uses the subroutine process_queue:
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use Thread; $thread = new Thread \&process_queue,"/usr/local/queue";
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The object accepts the name of the subroutine to execute, and any further arguments are supplied as arguments to that subroutine The $thread variable in the preceding example contains a reference to the newly created thread and will provide a link from the main program to the thread The thread can obtain a reference to itself with the self method:
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$me = Thread->self;
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Each thread is given its own unique thread ID The main program has a thread ID of 0, and subsequent threads are given a sequential thread number up to a current maximum of 232 1 You can discover the thread ID using the tid method,
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$tid = $thread->tid;
or for a thread to find its own ID:
$mytid = Thread->self->tid; PROGRAMMING WITH PERL
You can also get a list of all the running and finished threads (providing the thread has not been joined see the section on the next page) by using the list method:
@threads = Thread->list;
You ll need to process the information yourself, but the list of object references should be enough for you to determine the current status of each thread using the methods we ve already seen If all you want is a unique identifier for a thread (perhaps for tracking or logging purposes), the best solution is to use the Thread::Specific module, which creates a thread-specific key To use it, call the key_create function within a thread:
use Thread::Specific; my $k = key_create Thread::Specific;
Creating a Thread Using an Anonymous Subroutine
You can supply an anonymous subroutine as the first argument to the new constructor when creating a new thread, although it looks a little bit like line noise:
$t = Thread->new(sub { print "I'm a thread" } );
Note that closures work as normal, so this
my $message = "I'm another thread"; $t = Thread->new(sub { display $message } );
does what you expect, and displays the message using whatever method display handles
Perl: The Complete Reference
As an alternative to the anonymous subroutine form, you can also import the special async function from Thread, which creates a new anonymous subroutine thread for you For example, we could rewrite the preceding example as
use Thread async ; $t = async { display $message; };
Note that the block is treated as an anonymous subroutine and must be terminated by a closing semicolon It returns a thread object, just like the new function/constructor
Controlling a Thread
Once you have created a new thread, it continues running until the subroutine that you supplied terminates However, there is the issue of what happens to the thread when it returns Unless the parent process is actually waiting for a return value (which defeats the object!), you need a way of harvesting the threads at some later date, in a similar fashion to harvesting child processes started with fork The method for handling this is called join, and it works in a similar fashion to waitpid if the thread has already terminated, then it returns immediately If the thread is still running, then join blocks the process until it has been completed For example,
$result = $thread->join;
Note that the join method actually does what it suggests, it joins the thread on which you apply the method to the current process, even if that happens to be another thread This allows you to spawn a number of threads that can be set to wait for each other s return values great for cooperative processing tasks where each thread is working on a different part of a calculation There are only two exceptions to the use of join; you cannot join a thread to itself, and you can t join a thread that has already been joined by another thread Note, as well, that the return value from the join method is always evaluated in a list context This means that the return value from the subroutine can be a scalar, array, hash, or other object, but it also means that in a scalar context, the last value of the list is assigned to the scalar
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