1PERLTHRTUT(1)          Perl Programmers Reference Guide          PERLTHRTUT(1)
2
3
4

NAME

6       perlthrtut - Tutorial on threads in Perl
7

DESCRIPTION

9       This tutorial describes the use of Perl interpreter threads (sometimes
10       referred to as ithreads).  In this model, each thread runs in its own
11       Perl interpreter, and any data sharing between threads must be
12       explicit.  The user-level interface for ithreads uses the threads
13       class.
14
15       NOTE: There was another older Perl threading flavor called the 5.005
16       model that used the threads class.  This old model was known to have
17       problems, is deprecated, and was removed for release 5.10.  You are
18       strongly encouraged to migrate any existing 5.005 threads code to the
19       new model as soon as possible.
20
21       You can see which (or neither) threading flavour you have by running
22       "perl -V" and looking at the "Platform" section.  If you have
23       "useithreads=define" you have ithreads, if you have
24       "use5005threads=define" you have 5.005 threads.  If you have neither,
25       you don't have any thread support built in.  If you have both, you are
26       in trouble.
27
28       The threads and threads::shared modules are included in the core Perl
29       distribution.  Additionally, they are maintained as a separate modules
30       on CPAN, so you can check there for any updates.
31

What Is A Thread Anyway?

33       A thread is a flow of control through a program with a single execution
34       point.
35
36       Sounds an awful lot like a process, doesn't it? Well, it should.
37       Threads are one of the pieces of a process.  Every process has at least
38       one thread and, up until now, every process running Perl had only one
39       thread.  With 5.8, though, you can create extra threads.  We're going
40       to show you how, when, and why.
41

Threaded Program Models

43       There are three basic ways that you can structure a threaded program.
44       Which model you choose depends on what you need your program to do.
45       For many non-trivial threaded programs, you'll need to choose different
46       models for different pieces of your program.
47
48   Boss/Worker
49       The boss/worker model usually has one boss thread and one or more
50       worker threads.  The boss thread gathers or generates tasks that need
51       to be done, then parcels those tasks out to the appropriate worker
52       thread.
53
54       This model is common in GUI and server programs, where a main thread
55       waits for some event and then passes that event to the appropriate
56       worker threads for processing.  Once the event has been passed on, the
57       boss thread goes back to waiting for another event.
58
59       The boss thread does relatively little work.  While tasks aren't
60       necessarily performed faster than with any other method, it tends to
61       have the best user-response times.
62
63   Work Crew
64       In the work crew model, several threads are created that do essentially
65       the same thing to different pieces of data.  It closely mirrors
66       classical parallel processing and vector processors, where a large
67       array of processors do the exact same thing to many pieces of data.
68
69       This model is particularly useful if the system running the program
70       will distribute multiple threads across different processors.  It can
71       also be useful in ray tracing or rendering engines, where the
72       individual threads can pass on interim results to give the user visual
73       feedback.
74
75   Pipeline
76       The pipeline model divides up a task into a series of steps, and passes
77       the results of one step on to the thread processing the next.  Each
78       thread does one thing to each piece of data and passes the results to
79       the next thread in line.
80
81       This model makes the most sense if you have multiple processors so two
82       or more threads will be executing in parallel, though it can often make
83       sense in other contexts as well.  It tends to keep the individual tasks
84       small and simple, as well as allowing some parts of the pipeline to
85       block (on I/O or system calls, for example) while other parts keep
86       going.  If you're running different parts of the pipeline on different
87       processors you may also take advantage of the caches on each processor.
88
89       This model is also handy for a form of recursive programming where,
90       rather than having a subroutine call itself, it instead creates another
91       thread.  Prime and Fibonacci generators both map well to this form of
92       the pipeline model. (A version of a prime number generator is presented
93       later on.)
94

What kind of threads are Perl threads?

96       If you have experience with other thread implementations, you might
97       find that things aren't quite what you expect.  It's very important to
98       remember when dealing with Perl threads that Perl Threads Are Not X
99       Threads for all values of X.  They aren't POSIX threads, or DecThreads,
100       or Java's Green threads, or Win32 threads.  There are similarities, and
101       the broad concepts are the same, but if you start looking for
102       implementation details you're going to be either disappointed or
103       confused.  Possibly both.
104
105       This is not to say that Perl threads are completely different from
106       everything that's ever come before. They're not.  Perl's threading
107       model owes a lot to other thread models, especially POSIX.  Just as
108       Perl is not C, though, Perl threads are not POSIX threads.  So if you
109       find yourself looking for mutexes, or thread priorities, it's time to
110       step back a bit and think about what you want to do and how Perl can do
111       it.
112
113       However, it is important to remember that Perl threads cannot magically
114       do things unless your operating system's threads allow it. So if your
115       system blocks the entire process on "sleep()", Perl usually will, as
116       well.
117
118       Perl Threads Are Different.
119

Thread-Safe Modules

121       The addition of threads has changed Perl's internals substantially.
122       There are implications for people who write modules with XS code or
123       external libraries. However, since Perl data is not shared among
124       threads by default, Perl modules stand a high chance of being thread-
125       safe or can be made thread-safe easily.  Modules that are not tagged as
126       thread-safe should be tested or code reviewed before being used in
127       production code.
128
129       Not all modules that you might use are thread-safe, and you should
130       always assume a module is unsafe unless the documentation says
131       otherwise.  This includes modules that are distributed as part of the
132       core.  Threads are a relatively new feature, and even some of the
133       standard modules aren't thread-safe.
134
135       Even if a module is thread-safe, it doesn't mean that the module is
136       optimized to work well with threads. A module could possibly be
137       rewritten to utilize the new features in threaded Perl to increase
138       performance in a threaded environment.
139
140       If you're using a module that's not thread-safe for some reason, you
141       can protect yourself by using it from one, and only one thread at all.
142       If you need multiple threads to access such a module, you can use
143       semaphores and lots of programming discipline to control access to it.
144       Semaphores are covered in "Basic semaphores".
145
146       See also "Thread-Safety of System Libraries".
147

Thread Basics

149       The threads module provides the basic functions you need to write
150       threaded programs.  In the following sections, we'll cover the basics,
151       showing you what you need to do to create a threaded program.   After
152       that, we'll go over some of the features of the threads module that
153       make threaded programming easier.
154
155   Basic Thread Support
156       Thread support is a Perl compile-time option. It's something that's
157       turned on or off when Perl is built at your site, rather than when your
158       programs are compiled. If your Perl wasn't compiled with thread support
159       enabled, then any attempt to use threads will fail.
160
161       Your programs can use the Config module to check whether threads are
162       enabled. If your program can't run without them, you can say something
163       like:
164
165           use Config;
166           $Config{useithreads} or
167               die('Recompile Perl with threads to run this program.');
168
169       A possibly-threaded program using a possibly-threaded module might have
170       code like this:
171
172           use Config;
173           use MyMod;
174
175           BEGIN {
176               if ($Config{useithreads}) {
177                   # We have threads
178                   require MyMod_threaded;
179                   import MyMod_threaded;
180               } else {
181                   require MyMod_unthreaded;
182                   import MyMod_unthreaded;
183               }
184           }
185
186       Since code that runs both with and without threads is usually pretty
187       messy, it's best to isolate the thread-specific code in its own module.
188       In our example above, that's what "MyMod_threaded" is, and it's only
189       imported if we're running on a threaded Perl.
190
191   A Note about the Examples
192       In a real situation, care should be taken that all threads are finished
193       executing before the program exits.  That care has not been taken in
194       these examples in the interest of simplicity.  Running these examples
195       as is will produce error messages, usually caused by the fact that
196       there are still threads running when the program exits.  You should not
197       be alarmed by this.
198
199   Creating Threads
200       The threads module provides the tools you need to create new threads.
201       Like any other module, you need to tell Perl that you want to use it;
202       "use threads;" imports all the pieces you need to create basic threads.
203
204       The simplest, most straightforward way to create a thread is with
205       "create()":
206
207           use threads;
208
209           my $thr = threads->create(\&sub1);
210
211           sub sub1 {
212               print("In the thread\n");
213           }
214
215       The "create()" method takes a reference to a subroutine and creates a
216       new thread that starts executing in the referenced subroutine.  Control
217       then passes both to the subroutine and the caller.
218
219       If you need to, your program can pass parameters to the subroutine as
220       part of the thread startup.  Just include the list of parameters as
221       part of the "threads->create()" call, like this:
222
223           use threads;
224
225           my $Param3 = 'foo';
226           my $thr1 = threads->create(\&sub1, 'Param 1', 'Param 2', $Param3);
227           my @ParamList = (42, 'Hello', 3.14);
228           my $thr2 = threads->create(\&sub1, @ParamList);
229           my $thr3 = threads->create(\&sub1, qw(Param1 Param2 Param3));
230
231           sub sub1 {
232               my @InboundParameters = @_;
233               print("In the thread\n");
234               print('Got parameters >', join('<>',@InboundParameters), "<\n");
235           }
236
237       The last example illustrates another feature of threads.  You can spawn
238       off several threads using the same subroutine.  Each thread executes
239       the same subroutine, but in a separate thread with a separate
240       environment and potentially separate arguments.
241
242       "new()" is a synonym for "create()".
243
244   Waiting For A Thread To Exit
245       Since threads are also subroutines, they can return values.  To wait
246       for a thread to exit and extract any values it might return, you can
247       use the "join()" method:
248
249           use threads;
250
251           my ($thr) = threads->create(\&sub1);
252
253           my @ReturnData = $thr->join();
254           print('Thread returned ', join(', ', @ReturnData), "\n");
255
256           sub sub1 { return ('Fifty-six', 'foo', 2); }
257
258       In the example above, the "join()" method returns as soon as the thread
259       ends.  In addition to waiting for a thread to finish and gathering up
260       any values that the thread might have returned, "join()" also performs
261       any OS cleanup necessary for the thread.  That cleanup might be
262       important, especially for long-running programs that spawn lots of
263       threads.  If you don't want the return values and don't want to wait
264       for the thread to finish, you should call the "detach()" method
265       instead, as described next.
266
267       NOTE: In the example above, the thread returns a list, thus
268       necessitating that the thread creation call be made in list context
269       (i.e., "my ($thr)").  See "$thr->join()" in threads and "THREAD
270       CONTEXT" in threads for more details on thread context and return
271       values.
272
273   Ignoring A Thread
274       "join()" does three things: it waits for a thread to exit, cleans up
275       after it, and returns any data the thread may have produced.  But what
276       if you're not interested in the thread's return values, and you don't
277       really care when the thread finishes? All you want is for the thread to
278       get cleaned up after when it's done.
279
280       In this case, you use the "detach()" method.  Once a thread is
281       detached, it'll run until it's finished; then Perl will clean up after
282       it automatically.
283
284           use threads;
285
286           my $thr = threads->create(\&sub1);   # Spawn the thread
287
288           $thr->detach();   # Now we officially don't care any more
289
290           sleep(15);        # Let thread run for awhile
291
292           sub sub1 {
293               my $count = 0;
294               while (1) {
295                   $count++;
296                   print("\$count is $count\n");
297                   sleep(1);
298               }
299           }
300
301       Once a thread is detached, it may not be joined, and any return data
302       that it might have produced (if it was done and waiting for a join) is
303       lost.
304
305       "detach()" can also be called as a class method to allow a thread to
306       detach itself:
307
308           use threads;
309
310           my $thr = threads->create(\&sub1);
311
312           sub sub1 {
313               threads->detach();
314               # Do more work
315           }
316
317   Process and Thread Termination
318       With threads one must be careful to make sure they all have a chance to
319       run to completion, assuming that is what you want.
320
321       An action that terminates a process will terminate all running threads.
322       die() and exit() have this property, and perl does an exit when the
323       main thread exits, perhaps implicitly by falling off the end of your
324       code, even if that's not what you want.
325
326       As an example of this case, this code prints the message "Perl exited
327       with active threads: 2 running and unjoined":
328
329           use threads;
330           my $thr1 = threads->new(\&thrsub, "test1");
331           my $thr2 = threads->new(\&thrsub, "test2");
332           sub thrsub {
333              my ($message) = @_;
334              sleep 1;
335              print "thread $message\n";
336           }
337
338       But when the following lines are added at the end:
339
340           $thr1->join();
341           $thr2->join();
342
343       it prints two lines of output, a perhaps more useful outcome.
344

Threads And Data

346       Now that we've covered the basics of threads, it's time for our next
347       topic: Data.  Threading introduces a couple of complications to data
348       access that non-threaded programs never need to worry about.
349
350   Shared And Unshared Data
351       The biggest difference between Perl ithreads and the old 5.005 style
352       threading, or for that matter, to most other threading systems out
353       there, is that by default, no data is shared. When a new Perl thread is
354       created, all the data associated with the current thread is copied to
355       the new thread, and is subsequently private to that new thread!  This
356       is similar in feel to what happens when a Unix process forks, except
357       that in this case, the data is just copied to a different part of
358       memory within the same process rather than a real fork taking place.
359
360       To make use of threading, however, one usually wants the threads to
361       share at least some data between themselves. This is done with the
362       threads::shared module and the ":shared" attribute:
363
364           use threads;
365           use threads::shared;
366
367           my $foo :shared = 1;
368           my $bar = 1;
369           threads->create(sub { $foo++; $bar++; })->join();
370
371           print("$foo\n");  # Prints 2 since $foo is shared
372           print("$bar\n");  # Prints 1 since $bar is not shared
373
374       In the case of a shared array, all the array's elements are shared, and
375       for a shared hash, all the keys and values are shared. This places
376       restrictions on what may be assigned to shared array and hash elements:
377       only simple values or references to shared variables are allowed - this
378       is so that a private variable can't accidentally become shared. A bad
379       assignment will cause the thread to die. For example:
380
381           use threads;
382           use threads::shared;
383
384           my $var          = 1;
385           my $svar :shared = 2;
386           my %hash :shared;
387
388           ... create some threads ...
389
390           $hash{a} = 1;       # All threads see exists($hash{a})
391                               # and $hash{a} == 1
392           $hash{a} = $var;    # okay - copy-by-value: same effect as previous
393           $hash{a} = $svar;   # okay - copy-by-value: same effect as previous
394           $hash{a} = \$svar;  # okay - a reference to a shared variable
395           $hash{a} = \$var;   # This will die
396           delete($hash{a});   # okay - all threads will see !exists($hash{a})
397
398       Note that a shared variable guarantees that if two or more threads try
399       to modify it at the same time, the internal state of the variable will
400       not become corrupted. However, there are no guarantees beyond this, as
401       explained in the next section.
402
403   Thread Pitfalls: Races
404       While threads bring a new set of useful tools, they also bring a number
405       of pitfalls.  One pitfall is the race condition:
406
407           use threads;
408           use threads::shared;
409
410           my $x :shared = 1;
411           my $thr1 = threads->create(\&sub1);
412           my $thr2 = threads->create(\&sub2);
413
414           $thr1->join();
415           $thr2->join();
416           print("$x\n");
417
418           sub sub1 { my $foo = $x; $x = $foo + 1; }
419           sub sub2 { my $bar = $x; $x = $bar + 1; }
420
421       What do you think $x will be? The answer, unfortunately, is it depends.
422       Both "sub1()" and "sub2()" access the global variable $x, once to read
423       and once to write.  Depending on factors ranging from your thread
424       implementation's scheduling algorithm to the phase of the moon, $x can
425       be 2 or 3.
426
427       Race conditions are caused by unsynchronized access to shared data.
428       Without explicit synchronization, there's no way to be sure that
429       nothing has happened to the shared data between the time you access it
430       and the time you update it.  Even this simple code fragment has the
431       possibility of error:
432
433           use threads;
434           my $x :shared = 2;
435           my $y :shared;
436           my $z :shared;
437           my $thr1 = threads->create(sub { $y = $x; $x = $y + 1; });
438           my $thr2 = threads->create(sub { $z = $x; $x = $z + 1; });
439           $thr1->join();
440           $thr2->join();
441
442       Two threads both access $x.  Each thread can potentially be interrupted
443       at any point, or be executed in any order.  At the end, $x could be 3
444       or 4, and both $y and $z could be 2 or 3.
445
446       Even "$x += 5" or "$x++" are not guaranteed to be atomic.
447
448       Whenever your program accesses data or resources that can be accessed
449       by other threads, you must take steps to coordinate access or risk data
450       inconsistency and race conditions. Note that Perl will protect its
451       internals from your race conditions, but it won't protect you from you.
452

Synchronization and control

454       Perl provides a number of mechanisms to coordinate the interactions
455       between themselves and their data, to avoid race conditions and the
456       like.  Some of these are designed to resemble the common techniques
457       used in thread libraries such as "pthreads"; others are Perl-specific.
458       Often, the standard techniques are clumsy and difficult to get right
459       (such as condition waits). Where possible, it is usually easier to use
460       Perlish techniques such as queues, which remove some of the hard work
461       involved.
462
463   Controlling access: lock()
464       The "lock()" function takes a shared variable and puts a lock on it.
465       No other thread may lock the variable until the variable is unlocked by
466       the thread holding the lock. Unlocking happens automatically when the
467       locking thread exits the block that contains the call to the "lock()"
468       function.  Using "lock()" is straightforward: This example has several
469       threads doing some calculations in parallel, and occasionally updating
470       a running total:
471
472           use threads;
473           use threads::shared;
474
475           my $total :shared = 0;
476
477           sub calc {
478               while (1) {
479                   my $result;
480                   # (... do some calculations and set $result ...)
481                   {
482                       lock($total);  # Block until we obtain the lock
483                       $total += $result;
484                   } # Lock implicitly released at end of scope
485                   last if $result == 0;
486               }
487           }
488
489           my $thr1 = threads->create(\&calc);
490           my $thr2 = threads->create(\&calc);
491           my $thr3 = threads->create(\&calc);
492           $thr1->join();
493           $thr2->join();
494           $thr3->join();
495           print("total=$total\n");
496
497       "lock()" blocks the thread until the variable being locked is
498       available.  When "lock()" returns, your thread can be sure that no
499       other thread can lock that variable until the block containing the lock
500       exits.
501
502       It's important to note that locks don't prevent access to the variable
503       in question, only lock attempts.  This is in keeping with Perl's
504       longstanding tradition of courteous programming, and the advisory file
505       locking that "flock()" gives you.
506
507       You may lock arrays and hashes as well as scalars.  Locking an array,
508       though, will not block subsequent locks on array elements, just lock
509       attempts on the array itself.
510
511       Locks are recursive, which means it's okay for a thread to lock a
512       variable more than once.  The lock will last until the outermost
513       "lock()" on the variable goes out of scope. For example:
514
515           my $x :shared;
516           doit();
517
518           sub doit {
519               {
520                   {
521                       lock($x); # Wait for lock
522                       lock($x); # NOOP - we already have the lock
523                       {
524                           lock($x); # NOOP
525                           {
526                               lock($x); # NOOP
527                               lockit_some_more();
528                           }
529                       }
530                   } # *** Implicit unlock here ***
531               }
532           }
533
534           sub lockit_some_more {
535               lock($x); # NOOP
536           } # Nothing happens here
537
538       Note that there is no "unlock()" function - the only way to unlock a
539       variable is to allow it to go out of scope.
540
541       A lock can either be used to guard the data contained within the
542       variable being locked, or it can be used to guard something else, like
543       a section of code. In this latter case, the variable in question does
544       not hold any useful data, and exists only for the purpose of being
545       locked. In this respect, the variable behaves like the mutexes and
546       basic semaphores of traditional thread libraries.
547
548   A Thread Pitfall: Deadlocks
549       Locks are a handy tool to synchronize access to data, and using them
550       properly is the key to safe shared data.  Unfortunately, locks aren't
551       without their dangers, especially when multiple locks are involved.
552       Consider the following code:
553
554           use threads;
555
556           my $x :shared = 4;
557           my $y :shared = 'foo';
558           my $thr1 = threads->create(sub {
559               lock($x);
560               sleep(20);
561               lock($y);
562           });
563           my $thr2 = threads->create(sub {
564               lock($y);
565               sleep(20);
566               lock($x);
567           });
568
569       This program will probably hang until you kill it.  The only way it
570       won't hang is if one of the two threads acquires both locks first.  A
571       guaranteed-to-hang version is more complicated, but the principle is
572       the same.
573
574       The first thread will grab a lock on $x, then, after a pause during
575       which the second thread has probably had time to do some work, try to
576       grab a lock on $y.  Meanwhile, the second thread grabs a lock on $y,
577       then later tries to grab a lock on $x.  The second lock attempt for
578       both threads will block, each waiting for the other to release its
579       lock.
580
581       This condition is called a deadlock, and it occurs whenever two or more
582       threads are trying to get locks on resources that the others own.  Each
583       thread will block, waiting for the other to release a lock on a
584       resource.  That never happens, though, since the thread with the
585       resource is itself waiting for a lock to be released.
586
587       There are a number of ways to handle this sort of problem.  The best
588       way is to always have all threads acquire locks in the exact same
589       order.  If, for example, you lock variables $x, $y, and $z, always lock
590       $x before $y, and $y before $z.  It's also best to hold on to locks for
591       as short a period of time to minimize the risks of deadlock.
592
593       The other synchronization primitives described below can suffer from
594       similar problems.
595
596   Queues: Passing Data Around
597       A queue is a special thread-safe object that lets you put data in one
598       end and take it out the other without having to worry about
599       synchronization issues.  They're pretty straightforward, and look like
600       this:
601
602           use threads;
603           use Thread::Queue;
604
605           my $DataQueue = Thread::Queue->new();
606           my $thr = threads->create(sub {
607               while (my $DataElement = $DataQueue->dequeue()) {
608                   print("Popped $DataElement off the queue\n");
609               }
610           });
611
612           $DataQueue->enqueue(12);
613           $DataQueue->enqueue("A", "B", "C");
614           sleep(10);
615           $DataQueue->enqueue(undef);
616           $thr->join();
617
618       You create the queue with "Thread::Queue->new()".  Then you can add
619       lists of scalars onto the end with "enqueue()", and pop scalars off the
620       front of it with "dequeue()".  A queue has no fixed size, and can grow
621       as needed to hold everything pushed on to it.
622
623       If a queue is empty, "dequeue()" blocks until another thread enqueues
624       something.  This makes queues ideal for event loops and other
625       communications between threads.
626
627   Semaphores: Synchronizing Data Access
628       Semaphores are a kind of generic locking mechanism. In their most basic
629       form, they behave very much like lockable scalars, except that they
630       can't hold data, and that they must be explicitly unlocked. In their
631       advanced form, they act like a kind of counter, and can allow multiple
632       threads to have the lock at any one time.
633
634   Basic semaphores
635       Semaphores have two methods, "down()" and "up()": "down()" decrements
636       the resource count, while "up()" increments it. Calls to "down()" will
637       block if the semaphore's current count would decrement below zero.
638       This program gives a quick demonstration:
639
640           use threads;
641           use Thread::Semaphore;
642
643           my $semaphore = Thread::Semaphore->new();
644           my $GlobalVariable :shared = 0;
645
646           $thr1 = threads->create(\&sample_sub, 1);
647           $thr2 = threads->create(\&sample_sub, 2);
648           $thr3 = threads->create(\&sample_sub, 3);
649
650           sub sample_sub {
651               my $SubNumber = shift(@_);
652               my $TryCount = 10;
653               my $LocalCopy;
654               sleep(1);
655               while ($TryCount--) {
656                   $semaphore->down();
657                   $LocalCopy = $GlobalVariable;
658                   print("$TryCount tries left for sub $SubNumber "
659                        ."(\$GlobalVariable is $GlobalVariable)\n");
660                   sleep(2);
661                   $LocalCopy++;
662                   $GlobalVariable = $LocalCopy;
663                   $semaphore->up();
664               }
665           }
666
667           $thr1->join();
668           $thr2->join();
669           $thr3->join();
670
671       The three invocations of the subroutine all operate in sync.  The
672       semaphore, though, makes sure that only one thread is accessing the
673       global variable at once.
674
675   Advanced Semaphores
676       By default, semaphores behave like locks, letting only one thread
677       "down()" them at a time.  However, there are other uses for semaphores.
678
679       Each semaphore has a counter attached to it. By default, semaphores are
680       created with the counter set to one, "down()" decrements the counter by
681       one, and "up()" increments by one. However, we can override any or all
682       of these defaults simply by passing in different values:
683
684           use threads;
685           use Thread::Semaphore;
686
687           my $semaphore = Thread::Semaphore->new(5);
688                           # Creates a semaphore with the counter set to five
689
690           my $thr1 = threads->create(\&sub1);
691           my $thr2 = threads->create(\&sub1);
692
693           sub sub1 {
694               $semaphore->down(5); # Decrements the counter by five
695               # Do stuff here
696               $semaphore->up(5); # Increment the counter by five
697           }
698
699           $thr1->detach();
700           $thr2->detach();
701
702       If "down()" attempts to decrement the counter below zero, it blocks
703       until the counter is large enough.  Note that while a semaphore can be
704       created with a starting count of zero, any "up()" or "down()" always
705       changes the counter by at least one, and so "$semaphore->down(0)" is
706       the same as "$semaphore->down(1)".
707
708       The question, of course, is why would you do something like this? Why
709       create a semaphore with a starting count that's not one, or why
710       decrement or increment it by more than one? The answer is resource
711       availability.  Many resources that you want to manage access for can be
712       safely used by more than one thread at once.
713
714       For example, let's take a GUI driven program.  It has a semaphore that
715       it uses to synchronize access to the display, so only one thread is
716       ever drawing at once.  Handy, but of course you don't want any thread
717       to start drawing until things are properly set up.  In this case, you
718       can create a semaphore with a counter set to zero, and up it when
719       things are ready for drawing.
720
721       Semaphores with counters greater than one are also useful for
722       establishing quotas.  Say, for example, that you have a number of
723       threads that can do I/O at once.  You don't want all the threads
724       reading or writing at once though, since that can potentially swamp
725       your I/O channels, or deplete your process's quota of filehandles.  You
726       can use a semaphore initialized to the number of concurrent I/O
727       requests (or open files) that you want at any one time, and have your
728       threads quietly block and unblock themselves.
729
730       Larger increments or decrements are handy in those cases where a thread
731       needs to check out or return a number of resources at once.
732
733   Waiting for a Condition
734       The functions "cond_wait()" and "cond_signal()" can be used in
735       conjunction with locks to notify co-operating threads that a resource
736       has become available. They are very similar in use to the functions
737       found in "pthreads". However for most purposes, queues are simpler to
738       use and more intuitive. See threads::shared for more details.
739
740   Giving up control
741       There are times when you may find it useful to have a thread explicitly
742       give up the CPU to another thread.  You may be doing something
743       processor-intensive and want to make sure that the user-interface
744       thread gets called frequently.  Regardless, there are times that you
745       might want a thread to give up the processor.
746
747       Perl's threading package provides the "yield()" function that does
748       this. "yield()" is pretty straightforward, and works like this:
749
750           use threads;
751
752           sub loop {
753               my $thread = shift;
754               my $foo = 50;
755               while($foo--) { print("In thread $thread\n"); }
756               threads->yield();
757               $foo = 50;
758               while($foo--) { print("In thread $thread\n"); }
759           }
760
761           my $thr1 = threads->create(\&loop, 'first');
762           my $thr2 = threads->create(\&loop, 'second');
763           my $thr3 = threads->create(\&loop, 'third');
764
765       It is important to remember that "yield()" is only a hint to give up
766       the CPU, it depends on your hardware, OS and threading libraries what
767       actually happens.  On many operating systems, yield() is a no-op.
768       Therefore it is important to note that one should not build the
769       scheduling of the threads around "yield()" calls. It might work on your
770       platform but it won't work on another platform.
771

General Thread Utility Routines

773       We've covered the workhorse parts of Perl's threading package, and with
774       these tools you should be well on your way to writing threaded code and
775       packages.  There are a few useful little pieces that didn't really fit
776       in anyplace else.
777
778   What Thread Am I In?
779       The "threads->self()" class method provides your program with a way to
780       get an object representing the thread it's currently in.  You can use
781       this object in the same way as the ones returned from thread creation.
782
783   Thread IDs
784       "tid()" is a thread object method that returns the thread ID of the
785       thread the object represents.  Thread IDs are integers, with the main
786       thread in a program being 0.  Currently Perl assigns a unique TID to
787       every thread ever created in your program, assigning the first thread
788       to be created a TID of 1, and increasing the TID by 1 for each new
789       thread that's created.  When used as a class method, "threads->tid()"
790       can be used by a thread to get its own TID.
791
792   Are These Threads The Same?
793       The "equal()" method takes two thread objects and returns true if the
794       objects represent the same thread, and false if they don't.
795
796       Thread objects also have an overloaded "==" comparison so that you can
797       do comparison on them as you would with normal objects.
798
799   What Threads Are Running?
800       "threads->list()" returns a list of thread objects, one for each thread
801       that's currently running and not detached.  Handy for a number of
802       things, including cleaning up at the end of your program (from the main
803       Perl thread, of course):
804
805           # Loop through all the threads
806           foreach my $thr (threads->list()) {
807               $thr->join();
808           }
809
810       If some threads have not finished running when the main Perl thread
811       ends, Perl will warn you about it and die, since it is impossible for
812       Perl to clean up itself while other threads are running.
813
814       NOTE:  The main Perl thread (thread 0) is in a detached state, and so
815       does not appear in the list returned by "threads->list()".
816

A Complete Example

818       Confused yet? It's time for an example program to show some of the
819       things we've covered.  This program finds prime numbers using threads.
820
821          1 #!/usr/bin/perl
822          2 # prime-pthread, courtesy of Tom Christiansen
823          3
824          4 use strict;
825          5 use warnings;
826          6
827          7 use threads;
828          8 use Thread::Queue;
829          9
830         10 sub check_num {
831         11     my ($upstream, $cur_prime) = @_;
832         12     my $kid;
833         13     my $downstream = Thread::Queue->new();
834         14     while (my $num = $upstream->dequeue()) {
835         15         next unless ($num % $cur_prime);
836         16         if ($kid) {
837         17             $downstream->enqueue($num);
838         18         } else {
839         19             print("Found prime: $num\n");
840         20             $kid = threads->create(\&check_num, $downstream, $num);
841         21             if (! $kid) {
842         22                 warn("Sorry.  Ran out of threads.\n");
843         23                 last;
844         24             }
845         25         }
846         26     }
847         27     if ($kid) {
848         28         $downstream->enqueue(undef);
849         29         $kid->join();
850         30     }
851         31 }
852         32
853         33 my $stream = Thread::Queue->new(3..1000, undef);
854         34 check_num($stream, 2);
855
856       This program uses the pipeline model to generate prime numbers.  Each
857       thread in the pipeline has an input queue that feeds numbers to be
858       checked, a prime number that it's responsible for, and an output queue
859       into which it funnels numbers that have failed the check.  If the
860       thread has a number that's failed its check and there's no child
861       thread, then the thread must have found a new prime number.  In that
862       case, a new child thread is created for that prime and stuck on the end
863       of the pipeline.
864
865       This probably sounds a bit more confusing than it really is, so let's
866       go through this program piece by piece and see what it does.  (For
867       those of you who might be trying to remember exactly what a prime
868       number is, it's a number that's only evenly divisible by itself and 1.)
869
870       The bulk of the work is done by the "check_num()" subroutine, which
871       takes a reference to its input queue and a prime number that it's
872       responsible for.  After pulling in the input queue and the prime that
873       the subroutine is checking (line 11), we create a new queue (line 13)
874       and reserve a scalar for the thread that we're likely to create later
875       (line 12).
876
877       The while loop from line 14 to line 26 grabs a scalar off the input
878       queue and checks against the prime this thread is responsible for.
879       Line 15 checks to see if there's a remainder when we divide the number
880       to be checked by our prime.  If there is one, the number must not be
881       evenly divisible by our prime, so we need to either pass it on to the
882       next thread if we've created one (line 17) or create a new thread if we
883       haven't.
884
885       The new thread creation is line 20.  We pass on to it a reference to
886       the queue we've created, and the prime number we've found.  In lines 21
887       through 24, we check to make sure that our new thread got created, and
888       if not, we stop checking any remaining numbers in the queue.
889
890       Finally, once the loop terminates (because we got a 0 or "undef" in the
891       queue, which serves as a note to terminate), we pass on the notice to
892       our child, and wait for it to exit if we've created a child (lines 27
893       and 30).
894
895       Meanwhile, back in the main thread, we first create a queue (line 33)
896       and queue up all the numbers from 3 to 1000 for checking, plus a
897       termination notice.  Then all we have to do to get the ball rolling is
898       pass the queue and the first prime to the "check_num()" subroutine
899       (line 34).
900
901       That's how it works.  It's pretty simple; as with many Perl programs,
902       the explanation is much longer than the program.
903

Different implementations of threads

905       Some background on thread implementations from the operating system
906       viewpoint.  There are three basic categories of threads: user-mode
907       threads, kernel threads, and multiprocessor kernel threads.
908
909       User-mode threads are threads that live entirely within a program and
910       its libraries.  In this model, the OS knows nothing about threads.  As
911       far as it's concerned, your process is just a process.
912
913       This is the easiest way to implement threads, and the way most OSes
914       start.  The big disadvantage is that, since the OS knows nothing about
915       threads, if one thread blocks they all do.  Typical blocking activities
916       include most system calls, most I/O, and things like "sleep()".
917
918       Kernel threads are the next step in thread evolution.  The OS knows
919       about kernel threads, and makes allowances for them.  The main
920       difference between a kernel thread and a user-mode thread is blocking.
921       With kernel threads, things that block a single thread don't block
922       other threads.  This is not the case with user-mode threads, where the
923       kernel blocks at the process level and not the thread level.
924
925       This is a big step forward, and can give a threaded program quite a
926       performance boost over non-threaded programs.  Threads that block
927       performing I/O, for example, won't block threads that are doing other
928       things.  Each process still has only one thread running at once,
929       though, regardless of how many CPUs a system might have.
930
931       Since kernel threading can interrupt a thread at any time, they will
932       uncover some of the implicit locking assumptions you may make in your
933       program.  For example, something as simple as "$x = $x + 2" can behave
934       unpredictably with kernel threads if $x is visible to other threads, as
935       another thread may have changed $x between the time it was fetched on
936       the right hand side and the time the new value is stored.
937
938       Multiprocessor kernel threads are the final step in thread support.
939       With multiprocessor kernel threads on a machine with multiple CPUs, the
940       OS may schedule two or more threads to run simultaneously on different
941       CPUs.
942
943       This can give a serious performance boost to your threaded program,
944       since more than one thread will be executing at the same time.  As a
945       tradeoff, though, any of those nagging synchronization issues that
946       might not have shown with basic kernel threads will appear with a
947       vengeance.
948
949       In addition to the different levels of OS involvement in threads,
950       different OSes (and different thread implementations for a particular
951       OS) allocate CPU cycles to threads in different ways.
952
953       Cooperative multitasking systems have running threads give up control
954       if one of two things happen.  If a thread calls a yield function, it
955       gives up control.  It also gives up control if the thread does
956       something that would cause it to block, such as perform I/O.  In a
957       cooperative multitasking implementation, one thread can starve all the
958       others for CPU time if it so chooses.
959
960       Preemptive multitasking systems interrupt threads at regular intervals
961       while the system decides which thread should run next.  In a preemptive
962       multitasking system, one thread usually won't monopolize the CPU.
963
964       On some systems, there can be cooperative and preemptive threads
965       running simultaneously. (Threads running with realtime priorities often
966       behave cooperatively, for example, while threads running at normal
967       priorities behave preemptively.)
968
969       Most modern operating systems support preemptive multitasking nowadays.
970

Performance considerations

972       The main thing to bear in mind when comparing Perl's ithreads to other
973       threading models is the fact that for each new thread created, a
974       complete copy of all the variables and data of the parent thread has to
975       be taken. Thus, thread creation can be quite expensive, both in terms
976       of memory usage and time spent in creation. The ideal way to reduce
977       these costs is to have a relatively short number of long-lived threads,
978       all created fairly early on (before the base thread has accumulated too
979       much data). Of course, this may not always be possible, so compromises
980       have to be made. However, after a thread has been created, its
981       performance and extra memory usage should be little different than
982       ordinary code.
983
984       Also note that under the current implementation, shared variables use a
985       little more memory and are a little slower than ordinary variables.
986

Process-scope Changes

988       Note that while threads themselves are separate execution threads and
989       Perl data is thread-private unless explicitly shared, the threads can
990       affect process-scope state, affecting all the threads.
991
992       The most common example of this is changing the current working
993       directory using "chdir()".  One thread calls "chdir()", and the working
994       directory of all the threads changes.
995
996       Even more drastic example of a process-scope change is "chroot()": the
997       root directory of all the threads changes, and no thread can undo it
998       (as opposed to "chdir()").
999
1000       Further examples of process-scope changes include "umask()" and
1001       changing uids and gids.
1002
1003       Thinking of mixing "fork()" and threads?  Please lie down and wait
1004       until the feeling passes.  Be aware that the semantics of "fork()" vary
1005       between platforms.  For example, some Unix systems copy all the current
1006       threads into the child process, while others only copy the thread that
1007       called "fork()". You have been warned!
1008
1009       Similarly, mixing signals and threads may be problematic.
1010       Implementations are platform-dependent, and even the POSIX semantics
1011       may not be what you expect (and Perl doesn't even give you the full
1012       POSIX API).  For example, there is no way to guarantee that a signal
1013       sent to a multi-threaded Perl application will get intercepted by any
1014       particular thread.  (However, a recently added feature does provide the
1015       capability to send signals between threads.  See "THREAD SIGNALLING" in
1016       threads for more details.)
1017

Thread-Safety of System Libraries

1019       Whether various library calls are thread-safe is outside the control of
1020       Perl.  Calls often suffering from not being thread-safe include:
1021       "localtime()", "gmtime()",  functions fetching user, group and network
1022       information (such as "getgrent()", "gethostent()", "getnetent()" and so
1023       on), "readdir()", "rand()", and "srand()". In general, calls that
1024       depend on some global external state.
1025
1026       If the system Perl is compiled in has thread-safe variants of such
1027       calls, they will be used.  Beyond that, Perl is at the mercy of the
1028       thread-safety or -unsafety of the calls.  Please consult your C library
1029       call documentation.
1030
1031       On some platforms the thread-safe library interfaces may fail if the
1032       result buffer is too small (for example the user group databases may be
1033       rather large, and the reentrant interfaces may have to carry around a
1034       full snapshot of those databases).  Perl will start with a small
1035       buffer, but keep retrying and growing the result buffer until the
1036       result fits.  If this limitless growing sounds bad for security or
1037       memory consumption reasons you can recompile Perl with
1038       "PERL_REENTRANT_MAXSIZE" defined to the maximum number of bytes you
1039       will allow.
1040

Conclusion

1042       A complete thread tutorial could fill a book (and has, many times), but
1043       with what we've covered in this introduction, you should be well on
1044       your way to becoming a threaded Perl expert.
1045

SEE ALSO

1047       Annotated POD for threads:
1048       <https://web.archive.org/web/20171028020148/http://annocpan.org/?mode=search&field=Module&name=threads>
1049
1050       Latest version of threads on CPAN: <https://metacpan.org/pod/threads>
1051
1052       Annotated POD for threads::shared:
1053       <https://web.archive.org/web/20171028020148/http://annocpan.org/?mode=search&field=Module&name=threads%3A%3Ashared>
1054
1055       Latest version of threads::shared on CPAN:
1056       <https://metacpan.org/pod/threads::shared>
1057
1058       Perl threads mailing list: <https://lists.perl.org/list/ithreads.html>
1059

Bibliography

1061       Here's a short bibliography courtesy of Juergen Christoffel:
1062
1063   Introductory Texts
1064       Birrell, Andrew D. An Introduction to Programming with Threads. Digital
1065       Equipment Corporation, 1989, DEC-SRC Research Report #35 online as
1066       <https://www.hpl.hp.com/techreports/Compaq-DEC/SRC-RR-35.pdf> (highly
1067       recommended)
1068
1069       Robbins, Kay. A., and Steven Robbins. Practical Unix Programming: A
1070       Guide to Concurrency, Communication, and Multithreading. Prentice-Hall,
1071       1996.
1072
1073       Lewis, Bill, and Daniel J. Berg. Multithreaded Programming with
1074       Pthreads. Prentice Hall, 1997, ISBN 0-13-443698-9 (a well-written
1075       introduction to threads).
1076
1077       Nelson, Greg (editor). Systems Programming with Modula-3.  Prentice
1078       Hall, 1991, ISBN 0-13-590464-1.
1079
1080       Nichols, Bradford, Dick Buttlar, and Jacqueline Proulx Farrell.
1081       Pthreads Programming. O'Reilly & Associates, 1996, ISBN 156592-115-1
1082       (covers POSIX threads).
1083
1084   OS-Related References
1085       Boykin, Joseph, David Kirschen, Alan Langerman, and Susan LoVerso.
1086       Programming under Mach. Addison-Wesley, 1994, ISBN 0-201-52739-1.
1087
1088       Tanenbaum, Andrew S. Distributed Operating Systems. Prentice Hall,
1089       1995, ISBN 0-13-219908-4 (great textbook).
1090
1091       Silberschatz, Abraham, and Peter B. Galvin. Operating System Concepts,
1092       4th ed. Addison-Wesley, 1995, ISBN 0-201-59292-4
1093
1094   Other References
1095       Arnold, Ken and James Gosling. The Java Programming Language, 2nd ed.
1096       Addison-Wesley, 1998, ISBN 0-201-31006-6.
1097
1098       comp.programming.threads FAQ,
1099       <http://www.serpentine.com/~bos/threads-faq/>
1100
1101       Le Sergent, T. and B. Berthomieu. "Incremental MultiThreaded Garbage
1102       Collection on Virtually Shared Memory Architectures" in Memory
1103       Management: Proc. of the International Workshop IWMM 92, St. Malo,
1104       France, September 1992, Yves Bekkers and Jacques Cohen, eds. Springer,
1105       1992, ISBN 3540-55940-X (real-life thread applications).
1106
1107       Artur Bergman, "Where Wizards Fear To Tread", June 11, 2002,
1108       <http://www.perl.com/pub/a/2002/06/11/threads.html>
1109

Acknowledgements

1111       Thanks (in no particular order) to Chaim Frenkel, Steve Fink, Gurusamy
1112       Sarathy, Ilya Zakharevich, Benjamin Sugars, Juergen Christoffel, Joshua
1113       Pritikin, and Alan Burlison, for their help in reality-checking and
1114       polishing this article.  Big thanks to Tom Christiansen for his rewrite
1115       of the prime number generator.
1116

AUTHOR

1118       Dan Sugalski <dan@sidhe.org>
1119
1120       Slightly modified by Arthur Bergman to fit the new thread model/module.
1121
1122       Reworked slightly by Joerg Walter <jwalt@cpan.org> to be more concise
1123       about thread-safety of Perl code.
1124
1125       Rearranged slightly by Elizabeth Mattijsen <liz@dijkmat.nl> to put less
1126       emphasis on yield().
1127

Copyrights

1129       The original version of this article originally appeared in The Perl
1130       Journal #10, and is copyright 1998 The Perl Journal. It appears
1131       courtesy of Jon Orwant and The Perl Journal.  This document may be
1132       distributed under the same terms as Perl itself.
1133
1134
1135
1136perl v5.32.1                      2021-03-31                     PERLTHRTUT(1)
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