PERLTHRTUT(1)

📦
version 5.42.3 · 22 versions
, in fedora 43, fedora 44

🎀 NAME 🎀

perlthrtut(1) — tutorial on threads in Perl 💅✨

🦋 DESCRIPTION 🦋

okay so bestie, this tutorial is literally all about using Perl interpreter threads — sometimes called ithreads. In this model, each thread runs in its own Perl interpreter, and any data sharing between threads must be explicit. The user-level interface for ithreads uses the threads class. 🧚

NOTE: there was another older Perl threading flavor called the 5.005 model that used the Thread class. This old model was known to have problems, was deprecated, and was removed for release 5.10. You are strongly encouraged to migrate any existing 5.005 threads code to the new model as soon as possible. no cap, do it. 💀

You can see which (or neither) threading flavour you have by running perl -V and looking at the Platform section. If you have useithreads=define you have ithreads, if you have 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. 😭

In more modern times, the terms "ithreads" and "threads" are synonymous, and are used interchangeably. The best practice is to use useithreads if your code is dependent on something specific about that implementation, and to use usethreads if it isn't. That way, in the very unlikely event that some new threading model comes along, your code is more protected from needing to change. (The corresponding symbols at the C language level are USE_ITHREADS and USE_THREADS; these are currently totally synonymous, but not absolutely guaranteed to remain so.)

The threads and threads::shared modules are included in the core Perl distribution. Additionally, they are maintained as separate modules on CPAN, so you can check there for any updates. 🫶

✨ What Is A Thread Anyway? ✨

A thread is a flow of control through a program with a single execution point. periodt.

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 5.8, though, you can create extra threads. We're going to show you how, when, and why. 🔥

🌈 Threaded Program Models 🌈

There are three basic ways that you can structure a threaded program. Which model you choose depends on what you need your program to do. For many non-trivial threaded programs, you'll need to choose different models for different pieces of your program. iykyk.

👑 Boss/Worker 👑

The boss/worker model usually has one boss thread and one or more worker threads. The boss thread gathers or generates tasks that need to be done, then parcels those tasks out to the appropriate worker thread. 💅

This model is common in GUI and server programs, where a main thread waits for some event and then passes that event to the appropriate worker threads for processing. Once the event has been passed on, the boss thread goes back to waiting for another event. main character energy fr fr.

The boss thread does relatively little work. While tasks aren't necessarily performed faster than with any other method, it tends to have the best user-response times. slay.

🧚 Work Crew 🧚

In the work crew model, several threads are created that do essentially the same thing to different pieces of data. It closely mirrors classical parallel processing and vector processors, where a large array of processors do the exact same thing to many pieces of data. iconic.

This model is particularly useful if the system running the program will distribute multiple threads across different processors. It can also be useful in ray tracing or rendering engines, where the individual threads can pass on interim results to give the user visual feedback. ✨

🎀 Pipeline 🎀

The pipeline model divides up a task into a series of steps, and passes the results of one step on to the thread processing the next. Each thread does one thing to each piece of data and passes the results to the next thread in line. 🦋

This model makes the most sense if you have multiple processors so two or more threads will be executing in parallel, though it can often make sense in other contexts as well. It tends to keep the individual tasks small and simple, as well as allowing some parts of the pipeline to block (on I/O or system calls, for example) while other parts keep going. If you're running different parts of the pipeline on different processors you may also take advantage of the caches on each processor. 💜

This model is also handy for a form of recursive programming where, rather than having a subroutine call itself, it instead creates another thread. Prime and Fibonacci generators both map well to this form of the pipeline model. (A version of a prime number generator is presented later on.) ⭐

💀 What kind of threads are Perl threads? 💀

If you have experience with other thread implementations, you might find that things aren't quite what you expect. It's very important to remember when dealing with Perl threads that Perl Threads Are Not X Threads for all values of X. They aren't POSIX threads, or DecThreads, or Java's Green threads, or Win32 threads. There are similarities, and the broad concepts are the same, but if you start looking for implementation details you're going to be either disappointed or confused. Possibly both. 🙈

This is not to say that Perl threads are completely different from everything that's ever come before. They're not. Perl's threading model owes a lot to other thread models, especially POSIX. Just as Perl is not C, though, Perl threads are not POSIX threads. So if you find yourself looking for mutexes, or thread priorities, it's time to 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 system's threads allow it. So if your system blocks the entire process on sleep(), Perl usually will, as well. 😭

Perl Threads Are Different. 🔥

🫶 Thread-Safe Modules 🫶

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 Perl data is not shared among threads by default, Perl modules stand a high chance of being thread-safe or can be made thread-safe easily. Modules that are not tagged as thread-safe should be tested or code reviewed before being used in production code. no cap.

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 relatively new feature, and even some of the standard modules aren't thread-safe. 💀

Even if a module is thread-safe, 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 it from one, and only one thread at all. If you need multiple threads to access such a module, you can use semaphores and lots of programming discipline to control access to it. Semaphores are covered in "Basic semaphores". 🧚

See also "Thread-Safety of System Libraries". 🌸

💖 Thread Basics 💖

okay bestie, so the threads module is literally the whole toolkit you need to write threaded programs. In the next bits we're covering the basics, showing you what you gotta do to make a threaded program. After that we'll go over some features of the threads module that make threaded programming easier. 🫶

🌱 Basic Thread Support 🌱

Thread support is a Perl compile-time option, no cap. It's turned on or off when Perl is built at your site, not when your programs are compiled. So if your Perl wasn't compiled with thread support enabled, then any attempt to use threads will fail. 💀

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:

use Config;
$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;

BEGIN {
    if ($Config{useithreads}) {
        # We have threads
        require MyMod_threaded;
        MyMod_threaded->import();
    } else {
        require MyMod_unthreaded;
        MyMod_unthreaded->import();
    }
}

Since code that runs both with and without threads is usually pretty messy, it's best to isolate the thread-specific code in its own module. In our example above, that's what MyMod_threaded is, and it's only imported if we're running on a threaded Perl. ✨

📝 A Note about the Examples 📝

In a real situation, care should be taken that all threads are finished executing before the program exits. That care has not been taken in these examples, in the interest of simplicity. Running these examples as is will produce error messages, usually because there are still threads running when the program exits. You should not be alarmed by this. 🙈

🧵 Creating Threads 🧵

The threads module gives you the tools you need to create new threads. Like any other module, you need to tell Perl you want to use it; use threads; imports all the pieces you need to create basic threads.

The simplest, most straightforward way to create a thread is with create():

use threads;

my $thr = threads->create(\&sub1);

sub sub1 {
    print("In the thread\n");
}

The create() method takes a reference to a subroutine and creates a new thread that starts executing in the referenced subroutine. Control then passes both to the subroutine and the caller. 👑

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 threads->create() call, like this:

use threads;

my $Param3 = 'foo';
my $thr1 = threads->create(\&sub1, 'Param 1', 'Param 2', $Param3);
my @ParamList = (42, 'Hello', 3.14);
my $thr2 = threads->create(\&sub1, @ParamList);
my $thr3 = threads->create(\&sub1, qw(Param1 Param2 Param3));

sub sub1 {
    my @InboundParameters = @_;
    print("In the thread\n");
    print('Got parameters >', join('<>',@InboundParameters), "<\n");
}

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. 🌈

new() is a synonym for create(). 💅

⏳ 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 values it might return, you can use the join() method:

use threads;

my ($thr) = threads->create(\&sub1);

my @ReturnData = $thr->join();
print('Thread returned ', join(', ', @ReturnData), "\n");

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, as described next. 🧚

NOTE: In the example above, the thread returns a list, thus necessitating that the thread creation call be made in list context (i.e., my ($thr)). See $thr->join() in threads and THREAD CONTEXT in threads for more details on thread context and return values. 💜

🚮 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 threads;

my $thr = threads->create(\&sub1);   # Spawn the thread

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

sleep(15);        # Let thread run for a while

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

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

detach() can also be called as a class method to allow a thread to detach itself:

use threads;

my $thr = threads->create(\&sub1);

sub sub1 {
    threads->detach();
    # Do more work
}

💀 Process and Thread Termination 💀

With threads one must be careful to make sure they all have a chance to run to completion, assuming that is what you want.

An action that terminates a process will terminate all running threads. die() and exit() have this property, and perl does an exit when the main thread exits, perhaps implicitly by falling off the end of your code, even if that's not what you want. 😱

As an example of this case, this code prints the message Perl exited with active threads: 2 running and unjoined:

use threads;
my $thr1 = threads->new(\&thrsub, "test1");
my $thr2 = threads->new(\&thrsub, "test2");
sub thrsub {
   my ($message) = @_;
   sleep 1;
   print "thread $message\n";
}

But when the following lines are added at the end:

$thr1->join();
$thr2->join();

it prints two lines of output, a perhaps more useful outcome. 🔥

💖 Threads And Data 💖

okay bestie, so we've done the basics, and now it's time for the ✨ main character ✨ of every threaded program: data. Threading adds a whole extra layer of drama to how you touch your variables that regular non-threaded programs never even have to think about. 🫶

✨ Shared And Unshared Data ✨

the biggest glow-up difference between Perl ithreads and the old 5.005 style threading — or honestly, most other threading systems out there — is that by default, no data is shared. like none. zero. 💅 when a new Perl thread gets created, all the data hanging out with the current thread gets copied over to the new thread, and from then on it's private to that new thread, periodt.

it feels a lot like what happens when a Unix process forks, except in this case the data is just copied to a different part of memory within the same process rather than an actual fork happening. same vibe, different mechanism, iykyk. 🌸

but to actually use threading, you usually want your threads to share at least some data between themselves. that's where the threads::shared module and the :shared attribute come in:

use threads;
use threads::shared;

my $foo :shared = 1;
my $bar = 1;
threads->create(sub { $foo++; $bar++; })->join();

print("$foo\n");  # Prints 2 since $foo is shared
print("$bar\n");  # Prints 1 since $bar is not shared

see how $foo said "we're all in this together" and $bar said "actually this is my personal data"? 🦋

for a shared array, all the array's elements are shared. for a shared hash, all the keys and values are shared. but that places restrictions on what you're allowed to assign to shared array and hash elements — only simple values or references to shared variables are allowed. why? so a private variable can't accidentally sneak in and become shared. a bad assignment will cause the thread to die. 💀

like, for example:

use threads;
use threads::shared;

my $var          = 1;
my $svar :shared = 2;
my %hash :shared;

... create some threads ...

$hash{a} = 1;       # All threads see exists($hash{a})
                    # and $hash{a} == 1
$hash{a} = $var;    # okay - copy-by-value: same effect as previous
$hash{a} = $svar;   # okay - copy-by-value: same effect as previous
$hash{a} = \$svar;  # okay - a reference to a shared variable
$hash{a} = \$var;   # This will die
delete($hash{a});   # okay - all threads will see !exists($hash{a})

note that a shared variable guarantees that if two or more threads try to modify it at the same time, the internal state of the variable won't become corrupted. 🎀 but that's literally all it guarantees — nothing more, as we're about to find out in the next section. 🙈

💀 Thread Pitfalls: Races 💀

threads bring a whole new set of useful tools, but bestie, they also bring some pitfalls. one of them is the race condition:

use threads;
use threads::shared;

my $x :shared = 1;
my $thr1 = threads->create(\&sub1);
my $thr2 = threads->create(\&sub2);

$thr1->join();
$thr2->join();
print("$x\n");

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

so what do you think $x is gonna be? 😭 the answer, unfortunately, is it depends. both sub1() and sub2() access the global variable $x — once to read, once to write. depending on factors ranging from your thread implementation's scheduling algorithm all the way to the phase of the moon, $x can be 2 or 3. yes really. the moon said "not my problem". 🌙

race conditions happen because of unsynchronized access to shared data. without explicit synchronization, there's genuinely no way to be sure nothing happened to the shared data between the time you access it and the time you update it. even this lil tiny innocent code fragment has the possibility of error:

use threads;
my $x :shared = 2;
my $y :shared;
my $z :shared;
my $thr1 = threads->create(sub { $y = $x; $x = $y + 1; });
my $thr2 = threads->create(sub { $z = $x; $x = $z + 1; });
$thr1->join();
$thr2->join();

two threads both access $x. each thread can potentially be interrupted at any point, or be executed in any order. at the end, $x could be 3 or 4, and both $y and $z could be 2 or 3. the chaos. the drama. 🌀

and ngl, even $x += 5 or $x++ are not guaranteed to be atomic. not even the little ones. 😭

so whenever your program accesses data or resources that other threads can also access, you must take steps to coordinate that access — or risk data inconsistency and race conditions. and remember: Perl will protect its internals from your race conditions, but it will not protect you from you. 💅

🌈 Synchronization and control 🌈

Perl gives you a bunch of mechanisms to coordinate how threads interact with each other and with their data, so you can dodge race conditions and that whole genre of problem. some of these are designed to resemble the classic techniques from thread libraries like pthreads; others are very much Perl-specific. 🍓

and lowkey? the standard techniques are often clumsy and difficult to get right (looking at you, condition waits 👀). where possible, it's usually easier to reach for Perlish techniques like queues, which take some of that hard work off your plate entirely.

🔒 Controlling access: lock() 🔒

the lock() function takes a shared variable and puts a lock on it. no other thread may lock that variable until the thread holding the lock unlocks it. and unlocking? it happens automatically when the locking thread exits the block containing the call to lock(). no manual step needed, she's self-sufficient. 👑

using lock() is straightforward — this example has several threads doing some calculations in parallel, occasionally updating a running total:

use threads;
use threads::shared;

my $total :shared = 0;

sub calc {
    while (1) {
        my $result;
        # (... do some calculations and set $result ...)
        {
            lock($total);  # Block until we obtain the lock
            $total += $result;
        } # Lock implicitly released at end of scope
        last if $result == 0;
    }
}

my $thr1 = threads->create(\&calc);
my $thr2 = threads->create(\&calc);
my $thr3 = threads->create(\&calc);
$thr1->join();
$thr2->join();
$thr3->join();
print("total=$total\n");

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

but here's the important bit: 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. 🕊️

you can 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.

locks are recursive, which means it's totally fine 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. for example:

my $x :shared;
doit();

sub doit {
    {
        {
            lock($x); # Wait for lock
            lock($x); # NOOP - we already have the lock
            {
                lock($x); # NOOP
                {
                    lock($x); # NOOP
                    lockit_some_more();
                }
            }
        } # *** Implicit unlock here ***
    }
}

sub lockit_some_more {
    lock($x); # NOOP
} # Nothing happens here

note that there is no unlock() function — the only way to unlock a variable is to let it go out of scope. that's it. that's the whole method. 💜

a lock can either be used to guard the data contained within the variable being locked, or it can be used to guard something else, like a section of code. in that latter case, the variable in question doesn't hold any useful data and exists only for the purpose of being locked. in this respect, the variable behaves like the mutexes and basic semaphores of traditional thread libraries. 🧚

💀 A Thread Pitfall: Deadlocks 💀

locks are a handy tool to synchronize access to data, and using them properly is the key to safe shared data. unfortunately, bestie, locks aren't without their dangers — especially when multiple locks get involved. consider the following:

use threads;
use threads::shared;

my $x :shared = 4;
my $y :shared = 'foo';
my $thr1 = threads->create(sub {
    lock($x);
    sleep(20);
    lock($y);
});
my $thr2 = threads->create(sub {
    lock($y);
    sleep(20);
    lock($x);
});
$thr1->join();
$thr2->join();

this program will probably hang until you kill it. 😭 the only way it won't hang is if one of the two threads acquires both locks first. a guaranteed-to-hang version is more complicated, but the principle is exactly the same.

here's the play-by-play: the first thread grabs a lock on $x, then, after a pause during which the second thread has probably had time to do some work, tries to grab a lock on $y. meanwhile the second thread grabs a lock on $y, then later tries to grab a lock on $x. the second lock attempt for both threads will block, each one waiting for the other to release its lock. 💀💀

this condition is called a deadlock, and it happens whenever two or more threads are trying to get locks on resources that the others own. each thread blocks, waiting for the other to release a lock on a resource. but that never happens, since the thread holding the resource is itself waiting for a lock to be released. it's giving "after you" / "no after you" forever. 🫠

there are a number of ways to handle this. the best way is to always have all threads acquire locks in the exact same order. if, for example, you lock variables $x, $y, and $z, always lock $x before $y, and $y before $z. it's also best to hold on to locks for as short a period of time as possible, to minimize the risks of deadlock. ⏳

the other synchronization primitives described below can suffer from similar problems, so keep that in mind, okay? 🎀

🎀 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 at all. they're pretty straightforward, and they look like this:

use threads;
use Thread::Queue;

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

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

the enqueue and dequeue combo is doing so much heavy lifting here and honestly it's iconic. 👑

Okay so you make the queue with Thread::Queue->new(). Then you yeet lists of scalars onto the end with enqueue(), and pop scalars off the front with dequeue(). A queue has no fixed size bestie, it just grows as needed to hold everything you push onto it. It's giving unbothered. 💅

And if a queue is empty? dequeue() just blocks until some other thread enqueues something. Which is exactly why queues are iconic for event loops and other communication between threads. ✨

🚦 Semaphores: Synchronizing Data Access

Semaphores are a kind of generic locking mechanism. In their most basic form they act very much like lockable scalars — except they can't hold data, and they must be explicitly unlocked. In their advanced form they're basically a counter, and they can let multiple threads have the lock at any one time. Main character behavior tbh. 👑

💖 Basic semaphores

Semaphores have two methods, down() and up(). down() decrements the resource count, up() increments it. Calls to down() will block if the semaphore's current count would decrement below zero. This program is a quick demo, no cap:

use threads;
use Thread::Semaphore;

my $semaphore = Thread::Semaphore->new();
my $GlobalVariable :shared = 0;

$thr1 = threads->create(\&sample_sub, 1);
$thr2 = threads->create(\&sample_sub, 2);
$thr3 = threads->create(\&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");
        sleep(2);
        $LocalCopy++;
        $GlobalVariable = $LocalCopy;
        $semaphore->up();
    }
}

$thr1->join();
$thr2->join();
$thr3->join();

All three invocations of the subroutine operate in sync. The semaphore though? It makes sure only one thread is touching the global variable at once. Protective king. 🛡️

🌸 Advanced Semaphores

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

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. But we can override any or all of these defaults just by passing in different values:

use threads;
use Thread::Semaphore;

my $semaphore = Thread::Semaphore->new(5);
                # Creates a semaphore with the counter set to five

my $thr1 = threads->create(\&sub1);
my $thr2 = threads->create(\&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() tries to decrement the counter below zero, it blocks until the counter is large enough. And heads up: while a semaphore can totally be created with a starting count of zero, any up() or down() always changes the counter by at least one. So $semaphore->down(0) is literally the same as $semaphore->down(1). Periodt. 💀

So the question is, why would you even do 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. So many resources you want to manage access for can be safely used by more than one thread at once. It's giving abundance mindset. ✨

For example, take a GUI driven program. It has a semaphore 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. So in that 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, 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's quota of filehandles. So you can use a semaphore initialized to the number of concurrent I/O requests (or open files) you want at any one time, and have your threads quietly block and unblock themselves. So polite of them honestly. 🎀

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

🕰️ Waiting for a Condition

The functions cond_wait() and cond_signal() can be used together with locks to notify co-operating threads that a resource has become available. They're super similar in use to the functions found in pthreads. But for most purposes, queues are simpler to use and more intuitive. See threads::shared for more details. iykyk 💜

🌈 Giving up control

There are times when you might find it useful to have a thread explicitly give up the CPU to another thread. Maybe you're doing something processor-intensive and want to make sure the user-interface thread gets called frequently. Regardless, there are times you might want a thread to give up the processor.

Perl's threading package provides the yield() function that does this. yield() is pretty straightforward, and works like this:

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 $thr1 = threads->create(\&loop, 'first');
my $thr2 = threads->create(\&loop, 'second');
my $thr3 = threads->create(\&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. On many operating systems, yield() is a no-op. So 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. No cap, don't do it bestie. 😭

🧰 General Thread Utility Routines

We've covered the workhorse parts of Perl's threading package, and with these tools you should be well on your way to writing threaded code and packages. There are just a few useful little pieces that didn't really fit in anyplace else. Lil bonus content, as a treat. 🍓

🪞 What Thread Am I In?

The threads->self() class method gives your program a way to get an object representing the thread it's currently in. You can use this object the same way as the ones returned from thread creation. Self-discovery era. 💅

🔢 Thread IDs

tid() is a thread object method that returns the thread ID of the thread the object represents. Thread IDs are integers, with the main thread in a program being 0. Currently Perl assigns a unique TID to every thread ever created in your program, assigning the first thread to be created a TID of 1, and increasing the TID by 1 for each new thread that's created. And when used as a class method, threads->tid() can be used by a thread to get its own TID. Look at her, knowing herself. 👑

👯 Are These Threads The Same?

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 you can do comparison on them just like you would with normal objects. Slay.

📋 What Threads Are Running?

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 (from the main Perl thread, of course):

# Loop through all the threads
foreach my $thr (threads->list()) {
    $thr->join();
}

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. Dramatic but fair. 💀

NOTE: The main Perl thread (thread 0) is in a detached state, and so does not appear in the list returned by threads->list(). She's mysterious like that. 🦋

✨ A Complete Example ✨

okay bestie, are you confused yet? 💀 because same, but let's run a whole example program that shows off some of the things we've covered. This one finds prime numbers using threads. It's literally the pipeline model in action.

          1 #!/usr/bin/perl
          2 # prime-pthread, courtesy of Tom Christiansen
          3
          4 use v5.36;
          5
          6 use threads;
          7 use Thread::Queue;
          8
          9 sub check_num ($upstream, $cur_prime) {
         10     my $kid;
         11     my $downstream = Thread::Queue->new();
         12     while (my $num = $upstream->dequeue()) {
         13         next unless ($num % $cur_prime);
         14         if ($kid) {
         15             $downstream->enqueue($num);
         16         } else {
         17             print("Found prime: $num\n");
         18             $kid = threads->create(\&check_num, $downstream, $num);
         19             if (! $kid) {
         20                 warn("Sorry.  Ran out of threads.\n");
         21                 last;
         22             }
         23         }
         24     }
         25     if ($kid) {
         26         $downstream->enqueue(undef);
         27         $kid->join();
         28     }
         29 }
         30
         31 my $stream = Thread::Queue->new(3..1000, undef);
         32 check_num($stream, 2);

So this program uses the pipeline model to generate primes. Each thread in the pipeline has an input queue that feeds it numbers to check, a prime number that it's responsible for, and an output queue where it sends numbers that failed the check. If a thread gets a number that fails its check and there's no child thread yet, that means the thread just found a new prime! 🎉 In that case, it creates a new child thread for that prime and sticks it on the end of the pipeline.

This probably sounds more confusing than it actually is, so let's go through it piece by piece. (And for those of you trying to remember what a prime number is: it's a number that's only evenly divisible by itself and 1. No cap.)

The bulk of the work is done by the check_num() subroutine. It takes a reference to its input queue and a prime number it's responsible for. We create a new queue (line 11) and reserve a scalar for the thread we're probably gonna create later (line 10).

The while loop from line 12 to line 24 grabs a scalar off the input queue and checks it against the prime this thread owns. Line 13 checks if there's a remainder when we divide the number to be checked by our prime. If there is one, the number must not be evenly divisible by our prime, so we either pass it on to the next thread if we've created one (line 15) or create a new thread if we haven't.

The new thread creation is line 18. We pass it a reference to the queue we made, and the prime number we found. In lines 19 through 22, we check to make sure our new thread got created, and if not, we stop checking any remaining numbers in the queue.

Finally, once the loop terminates (because we got a 0 or "undef" in the queue, which is the signal to terminate), we pass the notice on to our child, and wait for it to exit if we created a child (lines 25 and 28).

Meanwhile, back in the main thread, we first create a queue (line 31) and queue up all the numbers from 3 to 1000 for checking, plus a termination notice. Then all we have to do to get the ball rolling is pass the queue and the first prime to the check_num() subroutine (line 32).

That's how it works. It's pretty simple; like many Perl programs, the explanation is way longer than the program. 💅

🌸 Different implementations of threads 🌸

Some background on thread implementations from the operating system's point of view. 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 its libraries. In this model, the OS knows nothing about threads. As far as it's concerned, your process is just a process.

This is the easiest way to implement threads, and the way most OSes start. The big disadvantage? Since the OS knows nothing about threads, if one thread blocks, they all do. 😭 Typical blocking activities include most system calls, most I/O, and things like sleep().

Kernel threads are the next step in thread evolution. The OS knows about kernel threads, and makes allowances for them. The main difference between a kernel thread and a user-mode thread is blocking. With kernel threads, things that block a single thread don't block other threads. This is not the case with user-mode threads, where the kernel blocks at the process level and not the thread level.

This is a big step forward, and can give a threaded program quite a performance boost over non-threaded programs. Threads that block performing I/O, for example, won't block threads that are doing other things. Each process still has only one thread running at once, though, regardless of how many CPUs a system might have.

Since kernel threading can interrupt a thread at any time, they will uncover some of the implicit locking assumptions you may make in your program. For example, something as simple as $x = $x + 2 can behave unpredictably with kernel threads if $x is visible to other threads, as another thread may have changed $x between the time it was fetched on the right hand side and the time the new value is stored. 💀

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.

This can give a serious performance boost to your threaded program, since more than one thread will be executing at the same time. As a tradeoff, though, any of those nagging synchronization issues that might not have shown with basic kernel threads will appear with a vengeance. 🔥

In addition to the different levels of OS involvement in threads, different OSes (and different thread implementations for a particular OS) allocate CPU cycles to threads in different ways.

Cooperative multitasking systems have running threads give up control if one of two things happen. If a thread calls a yield function, it gives up control. It also gives up control if the thread does something that would cause it to block, such as perform I/O. In a cooperative multitasking implementation, one thread can starve all the others for CPU time if it so chooses. 🙈

Preemptive multitasking systems interrupt threads at regular intervals while the system decides which thread should run next. In a preemptive multitasking system, one thread usually won't monopolize the CPU.

On some systems, there can be cooperative and preemptive threads running simultaneously. (Threads running with realtime priorities often behave cooperatively, for example, while threads running at normal priorities behave preemptively.)

Most modern operating systems support preemptive multitasking nowadays. 👑

💜 Performance considerations 💜

The main thing to bear in mind when comparing Perl's ithreads to other threading models is the fact that for each new thread created, a complete copy of all the variables and data of the parent thread has to be taken. Thus, thread creation can be quite expensive, both in terms of memory usage and time spent in creation. The ideal way to reduce these costs is to have a relatively short number of long-lived threads, all created fairly early on (before the base thread has accumulated too much data). Of course, this may not always be possible, so compromises have to be made. However, after a thread has been created, its performance and extra memory usage should be little different than ordinary code.

Also note that under the current implementation, shared variables use a little more memory and are a little slower than ordinary variables. 🧚

🦋 Process-scope Changes 🦋

Note that while threads themselves are separate execution threads and Perl data is thread-private unless explicitly shared, the threads can affect process-scope state, affecting all the threads.

The most common example of this is changing the current working directory using chdir(). One thread calls chdir(), and the working directory of all the threads changes.

Even more drastic example of a process-scope change is chroot(): the root directory of all the threads changes, and no thread can undo it (as opposed to chdir()).

Further examples of process-scope changes include umask() and changing uids and gids.

Thinking of mixing fork() and threads? Please lie down and wait until the feeling passes. 😭 Be aware that the semantics of fork() vary between platforms. For example, some Unix systems copy all the current threads into the child process, while others only copy the thread that called fork(). You have been warned! 🚨

Similarly, mixing signals and threads may be problematic. Implementations are platform-dependent, and even the POSIX semantics may not be what you expect (and Perl doesn't even give you the full POSIX API). For example, there is no way to guarantee that a signal sent to a multi-threaded Perl application will get intercepted by any particular thread. (However, a recently added feature does provide the capability to send signals between threads. See "THREAD SIGNALLING" in threads for more details.) ✨

🎀 Thread-Safety of System Libraries 🎀

Whether C library calls are thread-safe is outside the control of Perl. Undefined behavior will happen if unsafe ones are used during multi-thread operation. See "Dealing with embedded perls and threads" in perlclib. 💀

🌈 Conclusion 🌈

A complete thread tutorial could fill a book (and has, many times), but with what we've covered in this introduction, you should be well on your way to becoming a threaded Perl expert. 👑

⭐ SEE ALSO ⭐

📚 Bibliography 💖

Here's a short bibliography courtesy of Jürgen Christoffel, bestie — like, the reading list of the thread girlies. Every single one, no skips:

✨ Introductory Texts

  • Birrell, Andrew D. An Introduction to Programming with Threads. Digital Equipment Corporation, 1989, DEC-SRC Research Report #35 online as https://www.hpl.hp.com/techreports/Compaq-DEC/SRC-RR-35.pdf (highly recommended) 🔥
  • Robbins, Kay. A., and Steven Robbins. Practical Unix Programming: A Guide to Concurrency, Communication, and Multithreading. Prentice-Hall, 1996.
  • Lewis, Bill, and Daniel J. Berg. Multithreaded Programming with Pthreads. Prentice Hall, 1997, ISBN 0-13-443698-9 (a well-written introduction to threads). 🫶
  • Nelson, Greg (editor). Systems Programming with Modula-3. Prentice Hall, 1991, ISBN 0-13-590464-1.
  • Nichols, Bradford, Dick Buttlar, and Jacqueline Proulx Farrell. Pthreads Programming. O'Reilly & Associates, 1996, ISBN 156592-115-1 (covers POSIX threads). 💅
  • Boykin, Joseph, David Kirschen, Alan Langerman, and Susan LoVerso. Programming under Mach. Addison-Wesley, 1994, ISBN 0-201-52739-1.
  • Tanenbaum, Andrew S. Distributed Operating Systems. Prentice Hall, 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.

🌈 Other References

  • Arnold, Ken and James Gosling. The Java Programming Language, 2nd ed. Addison-Wesley, 1998, ISBN 0-201-31006-6.
  • comp.programming.threads FAQ, http://www.serpentine.com/~bos/threads-faq/ 🦋
  • Le Sergent, T. and B. Berthomieu. "Incremental MultiThreaded Garbage Collection on Virtually Shared Memory Architectures" in Memory Management: Proc. of the International Workshop IWMM 92, St. Malo, France, September 1992, Yves Bekkers and Jacques Cohen, eds. Springer, 1992, ISBN 3540-55940-X (real-life thread applications). 😭 iconic
  • Artur Bergman, "Where Wizards Fear To Tread", June 11, 2002, http://www.perl.com/pub/a/2002/06/11/threads.html

🫶 Acknowledgements

Thanks (in no particular order, because everyone's equally that girl) to Chaim Frenkel, Steve Fink, Gurusamy Sarathy, Ilya Zakharevich, Benjamin Sugars, Jürgen Christoffel, Joshua Pritikin, and Alan Burlison, for their help in reality-checking and polishing this article. Big thanks to Tom Christiansen for his rewrite of the prime number generator. 👑 fr fr, legends only.

✍️ AUTHOR

Dan Sugalski dan@sidhe.org

Slightly modified by Arthur Bergman to fit the new thread model/module.

Reworked slightly by Jörg Walter jwalt@cpan.org to be more concise about thread-safety of Perl code.

Rearranged slightly by Elizabeth Mattijsen liz@dijkmat.nl to put less emphasis on yield(). 💜

©️ Copyrights

The original version of this article originally appeared in The Perl Journal #10, and is copyright 1998 The Perl Journal. It appears courtesy of Jon Orwant and The Perl Journal. This document may be distributed under the same terms as Perl itself.

🎀 okay bestie that's the whole page, we ate and left zero crumbs. This one's perl v5.42.3, dated 2026-08-03, and it lives in the PERLTHRTUT(1) section of your friendly neighbourhood man pages. Go forth and thread responsibly, my main character. 💖✨🌸
📦 where this page comes from