=head1 DESCRIPTION
-One of the most prominent new features of Perl 5.005 is the inclusion
-of threads. Threads make a number of things a lot easier, and are a
-very useful addition to your bag of programming tricks.
+B<NOTE>: this tutorial describes the new Perl threading flavour
+introduced in Perl 5.6.0 called interpreter threads, or ithreads
+for short. There is another older perl threading flavour called
+the 5.005 model, unsurprisingly for 5.005 versions of Perl.
+
+You can see which (or neither) threading flavour you have by
+running C<perl -V> and looking at the C<Platform> section.
+If you have C<useithreads=define> you have ithreads, if you
+have C<use5005threads=define> you have 5.005 threads.
+If you have neither, you don't have any thread support built in.
+If you have both, you are in trouble.
+
=head1 What Is A Thread Anyway?
Sounds an awful lot like a process, doesn't it? Well, it should.
Threads are one of the pieces of a process. Every process has at least
one thread and, up until now, every process running Perl had only one
-thread. With 5.005, though, you can create extra threads. We're going
+thread. With 5.8, though, you can create extra threads. We're going
to show you how, when, and why.
=head1 Threaded Program Models
While the information in this section is useful, it's not necessary,
so you can skip it if you don't feel up to it.
-There are three basic categories of threads-user-mode threads, kernel
+There are three basic categories of threads: user-mode threads, kernel
threads, and multiprocessor kernel threads.
User-mode threads are threads that live entirely within a program and
was fetched on the right hand side and the time the new value is
stored.
-Multiprocessor Kernel Threads are the final step in thread
+Multiprocessor kernel threads are the final step in thread
support. With multiprocessor kernel threads on a machine with multiple
CPUs, the OS may schedule two or more threads to run simultaneously on
different CPUs.
step back a bit and think about what you want to do and how Perl can
do it.
+However it is important to remember that Perl threads cannot magically
+do things unless your operating systems threads allows it. So if your
+system blocks the entire process on sleep(), perl usually will as well.
+
=head1 Threadsafe Modules
-The addition of threads has changed Perl's internals
-substantially. There are implications for people who write
-modules--especially modules with XS code or external libraries. While
-most modules won't encounter any problems, modules that aren't
-explicitly tagged as thread-safe should be tested before being used in
-production code.
+The addition of threads has changed Perl's internals
+substantially. There are implications for people who write
+modules with XS code or external libraries. However, since the threads
+do not share data, pure Perl modules that don't interact with external
+systems should be safe. Modules that are not tagged as thread-safe should
+be tested or code reviewed before being used in production code.
Not all modules that you might use are thread-safe, and you should
always assume a module is unsafe unless the documentation says
otherwise. This includes modules that are distributed as part of the
-core. Threads are a beta feature, and even some of the standard
-modules aren't thread-safe.
+core. Threads are a new feature, and even some of the standard
+modules aren't thread-safe. (*** I think ActiveState checked this for
+psuedofork, check with GSAR)
+
+Even if a module is threadsafe, it doesn't mean that the module is optimized
+to work well with threads. A module could possibly be rewritten to utilize
+the new features in threaded Perl to increase performance in a threaded
+environment.
If you're using a module that's not thread-safe for some reason, you
can protect yourself by using semaphores and lots of programming
=head1 Thread Basics
-The core Thread module provides the basic functions you need to write
+The core L<threads> module provides the basic functions you need to write
threaded programs. In the following sections we'll cover the basics,
showing you what you need to do to create a threaded program. After
-that, we'll go over some of the features of the Thread module that
+that, we'll go over some of the features of the L<threads> module that
make threaded programming easier.
=head2 Basic Thread Support
-Thread support is a Perl compile-time option-it's something that's
+Thread support is a Perl compile-time option - it's something that's
turned on or off when Perl is built at your site, rather than when
your programs are compiled. If your Perl wasn't compiled with thread
support enabled, then any attempt to use threads will fail.
-Remember that the threading support in 5.005 is in beta release, and
-should be treated as such. You should expect that it may not function
-entirely properly, and the thread interface may well change some
-before it is a fully supported, production release. The beta version
-shouldn't be used for mission-critical projects. Having said that,
-threaded Perl is pretty nifty, and worth a look.
-
Your programs can use the Config module to check whether threads are
enabled. If your program can't run without them, you can say something
like:
- $Config{usethreads} or die "Recompile Perl with threads to run this program.";
+ $Config{useithreads} or die "Recompile Perl with threads to run this program.";
A possibly-threaded program using a possibly-threaded module might
have code like this:
use Config;
use MyMod;
- if ($Config{usethreads}) {
+ if ($Config{useithreads}) {
# We have threads
require MyMod_threaded;
import MyMod_threaded;
=head2 Creating Threads
-The Thread package provides the tools you need to create new
+The L<threads> package provides the tools you need to create new
threads. Like any other module, you need to tell Perl you want to use
-it; use Thread imports all the pieces you need to create basic
+it; C<use threads> imports all the pieces you need to create basic
threads.
The simplest, straightforward way to create a thread is with new():
- use Thread;
+ use threads;
- $thr = new Thread \&sub1;
+ $thr = threads->new(\&sub1);
sub sub1 {
print "In the thread\n";
If you need to, your program can pass parameters to the subroutine as
part of the thread startup. Just include the list of parameters as
-part of the C<Thread::new> call, like this:
+part of the C<threads::new> call, like this:
- use Thread;
+ use threads;
$Param3 = "foo";
- $thr = new Thread \&sub1, "Param 1", "Param 2", $Param3;
- $thr = new Thread \&sub1, @ParamList;
- $thr = new Thread \&sub1, qw(Param1 Param2 $Param3);
+ $thr = threads->new(\&sub1, "Param 1", "Param 2", $Param3);
+ $thr = threads->new(\&sub1, @ParamList);
+ $thr = threads->new(\&sub1, qw(Param1 Param2 $Param3));
sub sub1 {
my @InboundParameters = @_;
}
-The subroutine runs like a normal Perl subroutine, and the call to new
-Thread returns whatever the subroutine returns.
-
The last example illustrates another feature of threads. You can spawn
off several threads using the same subroutine. Each thread executes
the same subroutine, but in a separate thread with a separate
environment and potentially separate arguments.
-The other way to spawn a new thread is with async(), which is a way to
-spin off a chunk of code like eval(), but into its own thread:
-
- use Thread qw(async);
-
- $LineCount = 0;
-
- $thr = async {
- while(<>) {$LineCount++}
- print "Got $LineCount lines\n";
- };
-
- print "Waiting for the linecount to end\n";
- $thr->join;
- print "All done\n";
-
-You'll notice we did a use Thread qw(async) in that example. async is
-not exported by default, so if you want it, you'll either need to
-import it before you use it or fully qualify it as
-Thread::async. You'll also note that there's a semicolon after the
-closing brace. That's because async() treats the following block as an
-anonymous subroutine, so the semicolon is necessary.
-
-Like eval(), the code executes in the same context as it would if it
-weren't spun off. Since both the code inside and after the async start
-executing, you need to be careful with any shared resources. Locking
-and other synchronization techniques are covered later.
-
=head2 Giving up control
There are times when you may find it useful to have a thread
Perl's threading package provides the yield() function that does
this. yield() is pretty straightforward, and works like this:
- use Thread qw(yield async);
- async {
- my $foo = 50;
- while ($foo--) { print "first async\n" }
- yield;
- $foo = 50;
- while ($foo--) { print "first async\n" }
- };
- async {
- my $foo = 50;
- while ($foo--) { print "second async\n" }
- yield;
- $foo = 50;
- while ($foo--) { print "second async\n" }
- };
+ use threads;
+
+ sub loop {
+ my $thread = shift;
+ my $foo = 50;
+ while($foo--) { print "in thread $thread\n" }
+ threads->yield();
+ $foo = 50;
+ while($foo--) { print "in thread $thread\n" }
+ }
+
+ my $thread1 = threads->new(\&loop, 'first');
+ my $thread2 = threads->new(\&loop, 'second');
+ my $thread3 = threads->new(\&loop, 'third');
+
+It is important to remember that yield() is only a hint to give up the CPU,
+it depends on your hardware, OS and threading libraries what actually happens.
+Therefore it is important to note that one should not build the scheduling of
+the threads around yield() calls. It might work on your platform but it won't
+work on another platform.
=head2 Waiting For A Thread To Exit
Since threads are also subroutines, they can return values. To wait
-for a thread to exit and extract any scalars it might return, you can
-use the join() method.
+for a thread to exit and extract any values it might return, you can
+use the join() method:
- use Thread;
- $thr = new Thread \&sub1;
+ use threads;
+ $thr = threads->new(\&sub1);
@ReturnData = $thr->join;
print "Thread returned @ReturnData";
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
+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
it'll run until it's finished, then Perl will clean up after it
automatically.
- use Thread;
- $thr = new Thread \&sub1; # Spawn the thread
+ use threads;
+ $thr = threads->new(\&sub1); # Spawn the thread
$thr->detach; # Now we officially don't care any more
=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 my
-variables even more important. Remember--if your variables aren't
-lexically scoped (declared with C<my>) you're probably sharing it between
-threads.
+The biggest difference between perl threading and the old 5.005 style
+threading, or most other threading systems out there, is that all data
+is not shared. When a new perl thread is created all data is cloned
+and is private to that thread!
+
+To make use of threading however, one usually want the threads to share
+data between each other. This is done with the L<threads::shared> module
+and the C< : shared> attribute:
-=head2 Thread Pitfall: Races
+ use threads;
+ use threads::shared;
+ my $foo : shared = 1;
+ my $bar = 1;
+ threads->new(sub { $foo++; $bar++ })->join;
+
+ print "$foo\n"; #prints 2 since $foo is shared
+ print "$bar\n"; #prints 1 since $bar is not shared
+
+=head2 Thread Pitfalls: 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);
+ use threads;
+ use threads::shared;
+ my $a : shared = 1;
+ $thr1 = threads->new(\&sub1);
+ $thr2 = threads->new(\&sub2);
- sleep 10;
+ $thr1->join;
+ $thr2->join;
print "$a\n";
sub sub1 { $foo = $a; $a = $foo + 1; }
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; };
+ use threads;
+ my $a : shared = 2;
+ my $b : shared;
+ my $c : shared;
+ my $thr1 = threads->create(sub { $b = $a; $a = $b + 1; });
+ my $thr2 = threads->create(sub { $c = $a; $a = $c + 1; });
+ $thr1->join();
+ $thr2->join();
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
=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:
+The lock() function takes a shared variable 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 {
+ use threads;
+ my $a : shared = 4;
+ $thr1 = threads->new(sub {
$foo = 12;
{
lock ($a); # Block until we get access to $a
$a = $b * $foo;
}
print "\$foo was $foo\n";
- };
- $thr2 = async {
+ });
+ $thr2 = threads->new(sub {
$bar = 7;
{
lock ($a); # Block until we can get access to $a
$a = $c * $bar;
}
print "\$bar was $bar\n";
- };
+ });
$thr1->join;
$thr2->join;
print "\$a is $a\n";
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.
+locking that flock() gives you.
You may lock arrays and hashes as well as scalars. Locking an array,
though, will not block subsequent locks on array elements, just lock
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 {
+ use threads;
+ my $a : shared = 4;
+ my $b : shared = "foo";
+ my $thr1 = threads->new(sub {
lock($a);
yield;
sleep 20;
lock ($b);
- };
- async {
+ });
+ my $thr2 = threads->new(sub {
lock($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
synchronization issues. They're pretty straightforward, and look like
this:
- use Thread qw(async);
- use Thread::Queue;
+ use threads;
+ use threads::shared::queue;
- my $DataQueue = new Thread::Queue;
- $thr = async {
+ my $DataQueue = new threads::shared::queue;
+ $thr = threads->new(sub {
while ($DataElement = $DataQueue->dequeue) {
print "Popped $DataElement off the queue\n";
}
- };
+ });
$DataQueue->enqueue(12);
$DataQueue->enqueue("A", "B", "C");
$DataQueue->enqueue(\$thr);
sleep 10;
$DataQueue->enqueue(undef);
+ $thr->join();
-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.
+You create the queue with C<new threads::shared::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
semaphore's current count would decrement below zero. This program
gives a quick demonstration:
- use Thread qw(yield);
- use Thread::Semaphore;
- my $semaphore = new Thread::Semaphore;
+ use threads qw(yield);
+ use threads::shared::semaphore;
+ my $semaphore = new threads::shared::semaphore;
$GlobalVariable = 0;
- $thr1 = new Thread \&sample_sub, 1;
- $thr2 = new Thread \&sample_sub, 2;
- $thr3 = new Thread \&sample_sub, 3;
+ $thr1 = new threads \&sample_sub, 1;
+ $thr2 = new threads \&sample_sub, 2;
+ $thr3 = new threads \&sample_sub, 3;
sub sample_sub {
my $SubNumber = shift @_;
}
}
+ $thr1->join();
+ $thr2->join();
+ $thr3->join();
+
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.
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).
+Each semaphore has a counter attached to it. By default, semaphores are
+created with the counter set to one, down() decrements the counter by
+one, and up() increments by one. However, we can override any or all
+of these defaults simply by passing in different values:
+
+ use threads;
+ use threads::shared::semaphore;
+ my $semaphore = threads::shared::semaphore->new(5);
+ # Creates a semaphore with the counter set to five
+
+ $thr1 = threads->new(\&sub1);
+ $thr2 = threads->new(\&sub1);
+
+ sub sub1 {
+ $semaphore->down(5); # Decrements the counter by five
+ # Do stuff here
+ $semaphore->up(5); # Increment the counter by five
+ }
+
+ $thr1->detach();
+ $thr2->detach();
+
+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, and so $semaphore->down(0) is the same as
+$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
=back
-=head2 Attributes: Restricting Access To Subroutines
-
-In addition to synchronizing access to data or resources, you might
-find it useful to synchronize access to subroutines. You may be
-accessing a singular machine resource (perhaps a vector processor), or
-find it easier to serialize calls to a particular subroutine than to
-have a set of locks and sempahores.
-
-One of the additions to Perl 5.005 is subroutine attributes. The
-Thread package uses these to provide several flavors of
-serialization. It's important to remember that these attributes are
-used in the compilation phase of your program so you can't change a
-subroutine's behavior while your program is actually running.
-
-=head2 Subroutine Locks
-
-The basic subroutine lock looks like this:
-
- sub test_sub :locked {
- }
-
-This ensures that only one thread will be executing this subroutine at
-any one time. Once a thread calls this subroutine, any other thread
-that calls it will block until the thread in the subroutine exits
-it. A more elaborate example looks like this:
-
- use Thread qw(yield);
-
- new Thread \&thread_sub, 1;
- new Thread \&thread_sub, 2;
- new Thread \&thread_sub, 3;
- new Thread \&thread_sub, 4;
-
- sub sync_sub :locked {
- my $CallingThread = shift @_;
- print "In sync_sub for thread $CallingThread\n";
- yield;
- sleep 3;
- print "Leaving sync_sub for thread $CallingThread\n";
- }
-
- sub thread_sub {
- my $ThreadID = shift @_;
- print "Thread $ThreadID calling sync_sub\n";
- sync_sub($ThreadID);
- print "$ThreadID is done with sync_sub\n";
- }
-
-The C<locked> attribute tells perl to lock sync_sub(), and if you run
-this, you can see that only one thread is in it at any one time.
-
-=head2 Methods
-
-Locking an entire subroutine can sometimes be overkill, especially
-when dealing with Perl objects. When calling a method for an object,
-for example, you want to serialize calls to a method, so that only one
-thread will be in the subroutine for a particular object, but threads
-calling that subroutine for a different object aren't blocked. The
-method attribute indicates whether the subroutine is really a method.
-
- use Thread;
-
- sub tester {
- my $thrnum = shift @_;
- my $bar = new Foo;
- foreach (1..10) {
- print "$thrnum calling per_object\n";
- $bar->per_object($thrnum);
- print "$thrnum out of per_object\n";
- yield;
- print "$thrnum calling one_at_a_time\n";
- $bar->one_at_a_time($thrnum);
- print "$thrnum out of one_at_a_time\n";
- yield;
- }
- }
-
- foreach my $thrnum (1..10) {
- new Thread \&tester, $thrnum;
- }
-
- package Foo;
- sub new {
- my $class = shift @_;
- return bless [@_], $class;
- }
-
- sub per_object :locked :method {
- my ($class, $thrnum) = @_;
- print "In per_object for thread $thrnum\n";
- yield;
- sleep 2;
- print "Exiting per_object for thread $thrnum\n";
- }
-
- sub one_at_a_time :locked {
- my ($class, $thrnum) = @_;
- print "In one_at_a_time for thread $thrnum\n";
- yield;
- sleep 2;
- print "Exiting one_at_a_time for thread $thrnum\n";
- }
-
-As you can see from the output (omitted for brevity; it's 800 lines)
-all the threads can be in per_object() simultaneously, but only one
-thread is ever in one_at_a_time() at once.
-
-=head2 Locking A Subroutine
-
-You can lock a subroutine as you would lock a variable. Subroutine locks
-work the same as specifying a C<locked> attribute for the subroutine,
-and block all access to the subroutine for other threads until the
-lock goes out of scope. When the subroutine isn't locked, any number
-of threads can be in it at once, and getting a lock on a subroutine
-doesn't affect threads already in the subroutine. Getting a lock on a
-subroutine looks like this:
-
- lock(\&sub_to_lock);
-
-Simple enough. Unlike the C<locked> attribute, which is a compile time
-option, locking and unlocking a subroutine can be done at runtime at your
-discretion. There is some runtime penalty to using lock(\&sub) instead
-of the C<locked> attribute, so make sure you're choosing the proper
-method to do the locking.
-
-You'd choose lock(\&sub) when writing modules and code to run on both
-threaded and unthreaded Perl, especially for code that will run on
-5.004 or earlier Perls. In that case, it's useful to have subroutines
-that should be serialized lock themselves if they're running threaded,
-like so:
-
- package Foo;
- use Config;
- $Running_Threaded = 0;
-
- BEGIN { $Running_Threaded = $Config{'usethreads'} }
-
- sub sub1 { lock(\&sub1) if $Running_Threaded }
-
-
-This way you can ensure single-threadedness regardless of which
-version of Perl you're running.
-
=head1 General Thread Utility Routines
We've covered the workhorse parts of Perl's threading package, and
=head2 What Thread Am I In?
-The Thread->self method provides your program with a way to get an
+The C<threads->self> method provides your program with a way to get an
object representing the thread it's currently in. You can use this
-object in the same way as the ones returned from the thread creation.
+object in the same way as the ones returned from thread creation.
=head2 Thread IDs
The equal() method takes two thread objects and returns true
if the objects represent the same thread, and false if they don't.
+Thread objects also have an overloaded == comparison so that you can do
+comparison on them as you would with normal objects.
+
=head2 What Threads Are Running?
-Thread->list returns a list of thread objects, one for each thread
-that's currently running. Handy for a number of things, including
-cleaning up at the end of your program:
+threads->list returns a list of thread objects, one for each thread
+that's currently running and not detached. Handy for a number of things,
+including cleaning up at the end of your program:
# Loop through all the threads
- foreach $thr (Thread->list) {
+ foreach $thr (threads->list) {
# Don't join the main thread or ourselves
- if ($thr->tid && !Thread::equal($thr, Thread->self)) {
+ if ($thr->tid && !threads::equal($thr, threads->self)) {
$thr->join;
}
}
-The example above is just for illustration. It isn't strictly
-necessary to join all the threads you create, since Perl detaches all
-the threads before it exits.
+If some threads have not finished running when the main perl thread
+ends, perl will warn you about it and die, since it is impossible for perl
+to clean up itself while other threads are running
=head1 A Complete Example
3
4 use strict;
5
- 6 use Thread;
- 7 use Thread::Queue;
+ 6 use threads;
+ 7 use threads::shared::queue;
8
- 9 my $stream = new Thread::Queue;
- 10 my $kid = new Thread(\&check_num, $stream, 2);
+ 9 my $stream = new threads::shared::queue;
+ 10 my $kid = new threads(\&check_num, $stream, 2);
11
12 for my $i ( 3 .. 1000 ) {
13 $stream->enqueue($i);
19 sub check_num {
20 my ($upstream, $cur_prime) = @_;
21 my $kid;
- 22 my $downstream = new Thread::Queue;
+ 22 my $downstream = new threads::shared::queue;
23 while (my $num = $upstream->dequeue) {
24 next unless $num % $cur_prime;
25 if ($kid) {
26 $downstream->enqueue($num);
27 } else {
28 print "Found prime $num\n";
- 29 $kid = new Thread(\&check_num, $downstream, $num);
+ 29 $kid = new threads(\&check_num, $downstream, $num);
30 }
31 }
32 $downstream->enqueue(undef) if $kid;
This program uses the pipeline model to generate prime numbers. Each
thread in the pipeline has an input queue that feeds numbers to be
checked, a prime number that it's responsible for, and an output queue
-that it funnels numbers that have failed the check into. If the thread
+that into which it funnels numbers that have failed the check. If the thread
has a number that's failed its check and there's no child thread, then
the thread must have found a new prime number. In that case, a new
child thread is created for that prime and stuck on the end of the
pipeline.
-This probably sounds a bit more confusing than it really is, so lets
+This probably sounds a bit more confusing than it really is, so let's
go through this program piece by piece and see what it does. (For
those of you who might be trying to remember exactly what a prime
number is, it's a number that's only evenly divisible by itself and 1)
Finally, once the loop terminates (because we got a 0 or undef in the
queue, which serves as a note to die), we pass on the notice to our
-child and wait for it to exit if we've created a child (Lines 32 and
+child and wait for it to exit if we've created a child (lines 32 and
37).
Meanwhile, back in the main thread, we create a queue (line 9) and the
=head1 Conclusion
A complete thread tutorial could fill a book (and has, many times),
-but this should get you well on your way. The final authority on how
-Perl's threads behave is the documention bundled with the Perl
-distribution, but with what we've covered in this article, you should
-be well on your way to becoming a threaded Perl expert.
+but with what we've covered in this introduction, you should be well
+on your way to becoming a threaded Perl expert.
=head1 Bibliography
Birrell, Andrew D. An Introduction to Programming with
Threads. Digital Equipment Corporation, 1989, DEC-SRC Research Report
#35 online as
-http://www.research.digital.com/SRC/staff/birrell/bib.html (highly
-recommended)
+http://gatekeeper.dec.com/pub/DEC/SRC/research-reports/abstracts/src-rr-035.html
+(highly recommended)
Robbins, Kay. A., and Steven Robbins. Practical Unix Programming: A
Guide to Concurrency, Communication, and
0-201-52739-1.
Tanenbaum, Andrew S. Distributed Operating Systems. Prentice Hall,
-1995, ISBN 0-13-143934-0 (great textbook).
+1995, ISBN 0-13-219908-4 (great textbook).
Silberschatz, Abraham, and Peter B. Galvin. Operating System Concepts,
4th ed. Addison-Wesley, 1995, ISBN 0-201-59292-4
Dan Sugalski E<lt>sugalskd@ous.eduE<gt>
+Slightly modified by Arthur Bergman to fit the new thread model/module.
+
=head1 Copyrights
This article originally appeared in The Perl Journal #10, and is
as Perl itself.
+For more information please see L<threads> and L<threads::shared>.
+
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