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1=head1 NAME
2
3perlthrtut - tutorial on threads in Perl
4
5=head1 DESCRIPTION
6
53d7eaa8 7B<NOTE>: this tutorial describes the new Perl threading flavour
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8introduced in Perl 5.6.0 called interpreter threads, or B<ithreads>
9for short. In this model each thread runs in its own Perl interpreter,
10and any data sharing between threads must be explicit.
11
12There is another older Perl threading flavour called the 5.005 model,
13unsurprisingly for 5.005 versions of Perl. The old model is known to
14have problems, deprecated, and will probably be removed around release
155.10. You are strongly encouraged to migrate any existing 5.005
16threads code to the new model as soon as possible.
2a4bf773 17
53d7eaa8 18You can see which (or neither) threading flavour you have by
6eded8f3 19running C<perl -V> and looking at the C<Platform> section.
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20If you have C<useithreads=define> you have ithreads, if you
21have C<use5005threads=define> you have 5.005 threads.
22If you have neither, you don't have any thread support built in.
23If you have both, you are in trouble.
2605996a 24
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25The user-level interface to the 5.005 threads was via the L<Threads>
26class, while ithreads uses the L<threads> class. Note the change in case.
27
28=head1 Status
29
30The ithreads code has been available since Perl 5.6.0, and is considered
31stable. The user-level interface to ithreads (the L<threads> classes)
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32appeared in the 5.8.0 release, and as of this time is considered stable
33although it should be treated with caution as with all new features.
2605996a 34
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35=head1 What Is A Thread Anyway?
36
37A thread is a flow of control through a program with a single
38execution point.
39
40Sounds an awful lot like a process, doesn't it? Well, it should.
41Threads are one of the pieces of a process. Every process has at least
42one thread and, up until now, every process running Perl had only one
43thread. With 5.8, though, you can create extra threads. We're going
44to show you how, when, and why.
45
46=head1 Threaded Program Models
47
48There are three basic ways that you can structure a threaded
49program. Which model you choose depends on what you need your program
50to do. For many non-trivial threaded programs you'll need to choose
51different models for different pieces of your program.
52
53=head2 Boss/Worker
54
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55The boss/worker model usually has one "boss" thread and one or more
56"worker" threads. The boss thread gathers or generates tasks that need
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57to be done, then parcels those tasks out to the appropriate worker
58thread.
59
60This model is common in GUI and server programs, where a main thread
61waits for some event and then passes that event to the appropriate
62worker threads for processing. Once the event has been passed on, the
63boss thread goes back to waiting for another event.
64
65The boss thread does relatively little work. While tasks aren't
66necessarily performed faster than with any other method, it tends to
67have the best user-response times.
68
69=head2 Work Crew
70
71In the work crew model, several threads are created that do
72essentially the same thing to different pieces of data. It closely
73mirrors classical parallel processing and vector processors, where a
74large array of processors do the exact same thing to many pieces of
75data.
76
77This model is particularly useful if the system running the program
78will distribute multiple threads across different processors. It can
79also be useful in ray tracing or rendering engines, where the
80individual threads can pass on interim results to give the user visual
81feedback.
82
83=head2 Pipeline
84
85The pipeline model divides up a task into a series of steps, and
86passes the results of one step on to the thread processing the
87next. Each thread does one thing to each piece of data and passes the
88results to the next thread in line.
89
90This model makes the most sense if you have multiple processors so two
91or more threads will be executing in parallel, though it can often
92make sense in other contexts as well. It tends to keep the individual
93tasks small and simple, as well as allowing some parts of the pipeline
94to block (on I/O or system calls, for example) while other parts keep
95going. If you're running different parts of the pipeline on different
96processors you may also take advantage of the caches on each
97processor.
98
99This model is also handy for a form of recursive programming where,
100rather than having a subroutine call itself, it instead creates
101another thread. Prime and Fibonacci generators both map well to this
102form of the pipeline model. (A version of a prime number generator is
103presented later on.)
104
bfce6503 105=head1 What kind of threads are Perl threads?
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106
107If you have experience with other thread implementations, you might
108find that things aren't quite what you expect. It's very important to
109remember when dealing with Perl threads that Perl Threads Are Not X
110Threads, for all values of X. They aren't POSIX threads, or
111DecThreads, or Java's Green threads, or Win32 threads. There are
112similarities, and the broad concepts are the same, but if you start
113looking for implementation details you're going to be either
114disappointed or confused. Possibly both.
115
116This is not to say that Perl threads are completely different from
117everything that's ever come before--they're not. Perl's threading
118model owes a lot to other thread models, especially POSIX. Just as
119Perl is not C, though, Perl threads are not POSIX threads. So if you
120find yourself looking for mutexes, or thread priorities, it's time to
121step back a bit and think about what you want to do and how Perl can
122do it.
123
6eded8f3 124However it is important to remember that Perl threads cannot magically
c975c451 125do things unless your operating systems threads allows it. So if your
bfce6503 126system blocks the entire process on sleep(), Perl usually will as well.
c975c451 127
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128Perl Threads Are Different.
129
cf5baa48 130=head1 Thread-Safe Modules
c975c451 131
cf5baa48 132The addition of threads has changed Perl's internals
c975c451 133substantially. There are implications for people who write
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134modules with XS code or external libraries. However, since perl data is
135not shared among threads by default, Perl modules stand a high chance of
136being thread-safe or can be made thread-safe easily. Modules that are not
137tagged as thread-safe should be tested or code reviewed before being used
138in production code.
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139
140Not all modules that you might use are thread-safe, and you should
141always assume a module is unsafe unless the documentation says
142otherwise. This includes modules that are distributed as part of the
143core. Threads are a new feature, and even some of the standard
bfce6503 144modules aren't thread-safe.
c975c451 145
cf5baa48 146Even if a module is thread-safe, it doesn't mean that the module is optimized
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147to work well with threads. A module could possibly be rewritten to utilize
148the new features in threaded Perl to increase performance in a threaded
149environment.
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150
151If you're using a module that's not thread-safe for some reason, you
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152can protect yourself by using it from one, and only one thread at all.
153If you need multiple threads to access such a module, you can use semaphores and
154lots of programming discipline to control access to it. Semaphores
155are covered in L</"Basic semaphores">.
9316ed2f 156
cf5baa48 157See also L</"Thread-Safety of System Libraries">.
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158
159=head1 Thread Basics
160
161The core L<threads> module provides the basic functions you need to write
162threaded programs. In the following sections we'll cover the basics,
163showing you what you need to do to create a threaded program. After
164that, we'll go over some of the features of the L<threads> module that
165make threaded programming easier.
166
167=head2 Basic Thread Support
168
6eded8f3 169Thread support is a Perl compile-time option - it's something that's
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170turned on or off when Perl is built at your site, rather than when
171your programs are compiled. If your Perl wasn't compiled with thread
172support enabled, then any attempt to use threads will fail.
173
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174Your programs can use the Config module to check whether threads are
175enabled. If your program can't run without them, you can say something
176like:
177
9316ed2f 178 $Config{useithreads} or die "Recompile Perl with threads to run this program.";
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179
180A possibly-threaded program using a possibly-threaded module might
181have code like this:
182
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183 use Config;
184 use MyMod;
c975c451 185
9316ed2f 186 BEGIN {
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187 if ($Config{useithreads}) {
188 # We have threads
189 require MyMod_threaded;
190 import MyMod_threaded;
191 } else {
192 require MyMod_unthreaded;
193 import MyMod_unthreaded;
9316ed2f 194 }
cf5baa48 195 }
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196
197Since code that runs both with and without threads is usually pretty
198messy, it's best to isolate the thread-specific code in its own
199module. In our example above, that's what MyMod_threaded is, and it's
200only imported if we're running on a threaded Perl.
201
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202=head2 A Note about the Examples
203
204Although thread support is considered to be stable, there are still a number
205of quirks that may startle you when you try out any of the examples below.
206In a real situation, care should be taken that all threads are finished
207executing before the program exits. That care has B<not> been taken in these
208examples in the interest of simplicity. Running these examples "as is" will
209produce error messages, usually caused by the fact that there are still
210threads running when the program exits. You should not be alarmed by this.
211Future versions of Perl may fix this problem.
212
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213=head2 Creating Threads
214
215The L<threads> package provides the tools you need to create new
9e75ef81 216threads. Like any other module, you need to tell Perl that you want to use
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217it; C<use threads> imports all the pieces you need to create basic
218threads.
219
9e75ef81 220The simplest, most straightforward way to create a thread is with new():
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221
222 use threads;
223
224 $thr = threads->new(\&sub1);
225
226 sub sub1 {
227 print "In the thread\n";
228 }
229
230The new() method takes a reference to a subroutine and creates a new
231thread, which starts executing in the referenced subroutine. Control
232then passes both to the subroutine and the caller.
233
234If you need to, your program can pass parameters to the subroutine as
235part of the thread startup. Just include the list of parameters as
236part of the C<threads::new> call, like this:
237
238 use threads;
bfce6503 239
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240 $Param3 = "foo";
241 $thr = threads->new(\&sub1, "Param 1", "Param 2", $Param3);
242 $thr = threads->new(\&sub1, @ParamList);
8f95bfb9 243 $thr = threads->new(\&sub1, qw(Param1 Param2 Param3));
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244
245 sub sub1 {
246 my @InboundParameters = @_;
247 print "In the thread\n";
248 print "got parameters >", join("<>", @InboundParameters), "<\n";
249 }
250
251
252The last example illustrates another feature of threads. You can spawn
253off several threads using the same subroutine. Each thread executes
254the same subroutine, but in a separate thread with a separate
255environment and potentially separate arguments.
256
9316ed2f 257C<create()> is a synonym for C<new()>.
bfce6503 258
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259=head2 Waiting For A Thread To Exit
260
261Since threads are also subroutines, they can return values. To wait
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262for a thread to exit and extract any values it might return, you can
263use the join() method:
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264
265 use threads;
bfce6503 266
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267 $thr = threads->new(\&sub1);
268
269 @ReturnData = $thr->join;
270 print "Thread returned @ReturnData";
271
272 sub sub1 { return "Fifty-six", "foo", 2; }
273
274In the example above, the join() method returns as soon as the thread
275ends. In addition to waiting for a thread to finish and gathering up
276any values that the thread might have returned, join() also performs
277any OS cleanup necessary for the thread. That cleanup might be
278important, especially for long-running programs that spawn lots of
279threads. If you don't want the return values and don't want to wait
280for the thread to finish, you should call the detach() method
bfce6503 281instead, as described next.
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282
283=head2 Ignoring A Thread
284
285join() does three things: it waits for a thread to exit, cleans up
286after it, and returns any data the thread may have produced. But what
287if you're not interested in the thread's return values, and you don't
288really care when the thread finishes? All you want is for the thread
289to get cleaned up after when it's done.
290
291In this case, you use the detach() method. Once a thread is detached,
292it'll run until it's finished, then Perl will clean up after it
293automatically.
294
295 use threads;
bfce6503 296
6eded8f3 297 $thr = threads->new(\&sub1); # Spawn the thread
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298
299 $thr->detach; # Now we officially don't care any more
300
cf5baa48 301 sub sub1 {
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302 $a = 0;
303 while (1) {
304 $a++;
305 print "\$a is $a\n";
306 sleep 1;
307 }
308 }
309
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310Once a thread is detached, it may not be joined, and any return data
311that it might have produced (if it was done and waiting for a join) is
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312lost.
313
314=head1 Threads And Data
315
316Now that we've covered the basics of threads, it's time for our next
317topic: data. Threading introduces a couple of complications to data
318access that non-threaded programs never need to worry about.
319
320=head2 Shared And Unshared Data
321
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322The biggest difference between Perl ithreads and the old 5.005 style
323threading, or for that matter, to most other threading systems out there,
324is that by default, no data is shared. When a new perl thread is created,
325all the data associated with the current thread is copied to the new
326thread, and is subsequently private to that new thread!
327This is similar in feel to what happens when a UNIX process forks,
328except that in this case, the data is just copied to a different part of
329memory within the same process rather than a real fork taking place.
c975c451 330
9e75ef81 331To make use of threading however, one usually wants the threads to share
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332at least some data between themselves. This is done with the
333L<threads::shared> module and the C< : shared> attribute:
334
335 use threads;
336 use threads::shared;
337
338 my $foo : shared = 1;
339 my $bar = 1;
340 threads->new(sub { $foo++; $bar++ })->join;
818c4caa 341
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342 print "$foo\n"; #prints 2 since $foo is shared
343 print "$bar\n"; #prints 1 since $bar is not shared
344
345In the case of a shared array, all the array's elements are shared, and for
346a shared hash, all the keys and values are shared. This places
347restrictions on what may be assigned to shared array and hash elements: only
348simple values or references to shared variables are allowed - this is
f3278b06 349so that a private variable can't accidentally become shared. A bad
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350assignment will cause the thread to die. For example:
351
352 use threads;
353 use threads::shared;
354
355 my $var = 1;
356 my $svar : shared = 2;
357 my %hash : shared;
358
359 ... create some threads ...
360
361 $hash{a} = 1; # all threads see exists($hash{a}) and $hash{a} == 1
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362 $hash{a} = $var # okay - copy-by-value: same effect as previous
363 $hash{a} = $svar # okay - copy-by-value: same effect as previous
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364 $hash{a} = \$svar # okay - a reference to a shared variable
365 $hash{a} = \$var # This will die
366 delete $hash{a} # okay - all threads will see !exists($hash{a})
367
368Note that a shared variable guarantees that if two or more threads try to
369modify it at the same time, the internal state of the variable will not
370become corrupted. However, there are no guarantees beyond this, as
371explained in the next section.
c975c451 372
6eded8f3 373=head2 Thread Pitfalls: Races
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374
375While threads bring a new set of useful tools, they also bring a
376number of pitfalls. One pitfall is the race condition:
377
378 use threads;
379 use threads::shared;
bfce6503 380
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381 my $a : shared = 1;
382 $thr1 = threads->new(\&sub1);
383 $thr2 = threads->new(\&sub2);
384
385 $thr1->join;
386 $thr2->join;
387 print "$a\n";
388
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389 sub sub1 { my $foo = $a; $a = $foo + 1; }
390 sub sub2 { my $bar = $a; $a = $bar + 1; }
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391
392What do you think $a will be? The answer, unfortunately, is "it
393depends." Both sub1() and sub2() access the global variable $a, once
394to read and once to write. Depending on factors ranging from your
395thread implementation's scheduling algorithm to the phase of the moon,
396$a can be 2 or 3.
397
398Race conditions are caused by unsynchronized access to shared
399data. Without explicit synchronization, there's no way to be sure that
400nothing has happened to the shared data between the time you access it
401and the time you update it. Even this simple code fragment has the
402possibility of error:
403
404 use threads;
405 my $a : shared = 2;
406 my $b : shared;
407 my $c : shared;
408 my $thr1 = threads->create(sub { $b = $a; $a = $b + 1; });
409 my $thr2 = threads->create(sub { $c = $a; $a = $c + 1; });
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410 $thr1->join;
411 $thr2->join;
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412
413Two threads both access $a. Each thread can potentially be interrupted
414at any point, or be executed in any order. At the end, $a could be 3
415or 4, and both $b and $c could be 2 or 3.
416
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417Even C<$a += 5> or C<$a++> are not guaranteed to be atomic.
418
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419Whenever your program accesses data or resources that can be accessed
420by other threads, you must take steps to coordinate access or risk
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421data inconsistency and race conditions. Note that Perl will protect its
422internals from your race conditions, but it won't protect you from you.
423
f3278b06 424=head1 Synchronization and control
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425
426Perl provides a number of mechanisms to coordinate the interactions
427between themselves and their data, to avoid race conditions and the like.
428Some of these are designed to resemble the common techniques used in thread
429libraries such as C<pthreads>; others are Perl-specific. Often, the
9e75ef81 430standard techniques are clumsy and difficult to get right (such as
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431condition waits). Where possible, it is usually easier to use Perlish
432techniques such as queues, which remove some of the hard work involved.
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433
434=head2 Controlling access: lock()
435
436The lock() function takes a shared variable and puts a lock on it.
a6d05634 437No other thread may lock the variable until the variable is unlocked
bfce6503 438by the thread holding the lock. Unlocking happens automatically
8f95bfb9 439when the locking thread exits the outermost block that contains
bfce6503 440C<lock()> function. Using lock() is straightforward: this example has
f3278b06 441several threads doing some calculations in parallel, and occasionally
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442updating a running total:
443
444 use threads;
445 use threads::shared;
446
447 my $total : shared = 0;
448
449 sub calc {
450 for (;;) {
451 my $result;
452 # (... do some calculations and set $result ...)
453 {
454 lock($total); # block until we obtain the lock
8f95bfb9 455 $total += $result;
f3278b06 456 } # lock implicitly released at end of scope
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457 last if $result == 0;
458 }
459 }
460
461 my $thr1 = threads->new(\&calc);
462 my $thr2 = threads->new(\&calc);
463 my $thr3 = threads->new(\&calc);
464 $thr1->join;
465 $thr2->join;
466 $thr3->join;
467 print "total=$total\n";
c975c451 468
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469
470lock() blocks the thread until the variable being locked is
471available. When lock() returns, your thread can be sure that no other
bfce6503 472thread can lock that variable until the outermost block containing the
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473lock exits.
474
475It's important to note that locks don't prevent access to the variable
476in question, only lock attempts. This is in keeping with Perl's
477longstanding tradition of courteous programming, and the advisory file
478locking that flock() gives you.
479
480You may lock arrays and hashes as well as scalars. Locking an array,
481though, will not block subsequent locks on array elements, just lock
482attempts on the array itself.
483
bfce6503 484Locks are recursive, which means it's okay for a thread to
c975c451 485lock a variable more than once. The lock will last until the outermost
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486lock() on the variable goes out of scope. For example:
487
488 my $x : shared;
489 doit();
490
491 sub doit {
492 {
493 {
494 lock($x); # wait for lock
8f95bfb9 495 lock($x); # NOOP - we already have the lock
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496 {
497 lock($x); # NOOP
498 {
499 lock($x); # NOOP
500 lockit_some_more();
501 }
502 }
503 } # *** implicit unlock here ***
504 }
505 }
506
507 sub lockit_some_more {
508 lock($x); # NOOP
509 } # nothing happens here
510
511Note that there is no unlock() function - the only way to unlock a
512variable is to allow it to go out of scope.
513
514A lock can either be used to guard the data contained within the variable
515being locked, or it can be used to guard something else, like a section
516of code. In this latter case, the variable in question does not hold any
517useful data, and exists only for the purpose of being locked. In this
518respect, the variable behaves like the mutexes and basic semaphores of
519traditional thread libraries.
c975c451 520
bfce6503 521=head2 A Thread Pitfall: Deadlocks
c975c451 522
bfce6503 523Locks are a handy tool to synchronize access to data, and using them
c975c451 524properly is the key to safe shared data. Unfortunately, locks aren't
f3278b06 525without their dangers, especially when multiple locks are involved.
bfce6503 526Consider the following code:
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527
528 use threads;
bfce6503 529
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530 my $a : shared = 4;
531 my $b : shared = "foo";
532 my $thr1 = threads->new(sub {
533 lock($a);
c975c451 534 sleep 20;
bfce6503 535 lock($b);
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536 });
537 my $thr2 = threads->new(sub {
538 lock($b);
c975c451 539 sleep 20;
bfce6503 540 lock($a);
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541 });
542
543This program will probably hang until you kill it. The only way it
bfce6503 544won't hang is if one of the two threads acquires both locks
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545first. A guaranteed-to-hang version is more complicated, but the
546principle is the same.
547
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548The first thread will grab a lock on $a, then, after a pause during which
549the second thread has probably had time to do some work, try to grab a
550lock on $b. Meanwhile, the second thread grabs a lock on $b, then later
551tries to grab a lock on $a. The second lock attempt for both threads will
552block, each waiting for the other to release its lock.
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553
554This condition is called a deadlock, and it occurs whenever two or
555more threads are trying to get locks on resources that the others
556own. Each thread will block, waiting for the other to release a lock
557on a resource. That never happens, though, since the thread with the
558resource is itself waiting for a lock to be released.
559
560There are a number of ways to handle this sort of problem. The best
561way is to always have all threads acquire locks in the exact same
562order. If, for example, you lock variables $a, $b, and $c, always lock
563$a before $b, and $b before $c. It's also best to hold on to locks for
564as short a period of time to minimize the risks of deadlock.
565
48b96218 566The other synchronization primitives described below can suffer from
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567similar problems.
568
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569=head2 Queues: Passing Data Around
570
571A queue is a special thread-safe object that lets you put data in one
572end and take it out the other without having to worry about
573synchronization issues. They're pretty straightforward, and look like
574this:
575
576 use threads;
83272a45 577 use Thread::Queue;
c975c451 578
83272a45 579 my $DataQueue = Thread::Queue->new;
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580 $thr = threads->new(sub {
581 while ($DataElement = $DataQueue->dequeue) {
582 print "Popped $DataElement off the queue\n";
583 }
584 });
585
586 $DataQueue->enqueue(12);
587 $DataQueue->enqueue("A", "B", "C");
588 $DataQueue->enqueue(\$thr);
589 sleep 10;
590 $DataQueue->enqueue(undef);
8f95bfb9 591 $thr->join;
c975c451 592
83272a45 593You create the queue with C<new Thread::Queue>. Then you can
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594add lists of scalars onto the end with enqueue(), and pop scalars off
595the front of it with dequeue(). A queue has no fixed size, and can grow
596as needed to hold everything pushed on to it.
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597
598If a queue is empty, dequeue() blocks until another thread enqueues
599something. This makes queues ideal for event loops and other
600communications between threads.
601
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602=head2 Semaphores: Synchronizing Data Access
603
bfce6503 604Semaphores are a kind of generic locking mechanism. In their most basic
fa11829f 605form, they behave very much like lockable scalars, except that they
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606can't hold data, and that they must be explicitly unlocked. In their
607advanced form, they act like a kind of counter, and can allow multiple
608threads to have the 'lock' at any one time.
2605996a 609
bfce6503 610=head2 Basic semaphores
2605996a 611
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612Semaphores have two methods, down() and up(): down() decrements the resource
613count, while up increments it. Calls to down() will block if the
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614semaphore's current count would decrement below zero. This program
615gives a quick demonstration:
616
536bca94 617 use threads;
83272a45 618 use Thread::Semaphore;
bfce6503 619
83272a45 620 my $semaphore = new Thread::Semaphore;
bfce6503 621 my $GlobalVariable : shared = 0;
2605996a 622
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623 $thr1 = new threads \&sample_sub, 1;
624 $thr2 = new threads \&sample_sub, 2;
625 $thr3 = new threads \&sample_sub, 3;
2605996a 626
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627 sub sample_sub {
628 my $SubNumber = shift @_;
629 my $TryCount = 10;
630 my $LocalCopy;
631 sleep 1;
632 while ($TryCount--) {
633 $semaphore->down;
634 $LocalCopy = $GlobalVariable;
635 print "$TryCount tries left for sub $SubNumber (\$GlobalVariable is $GlobalVariable)\n";
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636 sleep 2;
637 $LocalCopy++;
638 $GlobalVariable = $LocalCopy;
639 $semaphore->up;
640 }
641 }
6eded8f3 642
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643 $thr1->join;
644 $thr2->join;
645 $thr3->join;
2605996a 646
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647The three invocations of the subroutine all operate in sync. The
648semaphore, though, makes sure that only one thread is accessing the
649global variable at once.
2605996a 650
bfce6503 651=head2 Advanced Semaphores
2605996a 652
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653By default, semaphores behave like locks, letting only one thread
654down() them at a time. However, there are other uses for semaphores.
2605996a 655
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656Each semaphore has a counter attached to it. By default, semaphores are
657created with the counter set to one, down() decrements the counter by
658one, and up() increments by one. However, we can override any or all
659of these defaults simply by passing in different values:
660
661 use threads;
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662 use Thread::Semaphore;
663 my $semaphore = Thread::Semaphore->new(5);
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664 # Creates a semaphore with the counter set to five
665
666 $thr1 = threads->new(\&sub1);
667 $thr2 = threads->new(\&sub1);
668
669 sub sub1 {
670 $semaphore->down(5); # Decrements the counter by five
671 # Do stuff here
672 $semaphore->up(5); # Increment the counter by five
673 }
674
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675 $thr1->detach;
676 $thr2->detach;
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677
678If down() attempts to decrement the counter below zero, it blocks until
679the counter is large enough. Note that while a semaphore can be created
680with a starting count of zero, any up() or down() always changes the
681counter by at least one, and so $semaphore->down(0) is the same as
682$semaphore->down(1).
2605996a 683
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684The question, of course, is why would you do something like this? Why
685create a semaphore with a starting count that's not one, or why
686decrement/increment it by more than one? The answer is resource
687availability. Many resources that you want to manage access for can be
688safely used by more than one thread at once.
2605996a 689
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690For example, let's take a GUI driven program. It has a semaphore that
691it uses to synchronize access to the display, so only one thread is
692ever drawing at once. Handy, but of course you don't want any thread
693to start drawing until things are properly set up. In this case, you
694can create a semaphore with a counter set to zero, and up it when
695things are ready for drawing.
2605996a 696
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697Semaphores with counters greater than one are also useful for
698establishing quotas. Say, for example, that you have a number of
699threads that can do I/O at once. You don't want all the threads
700reading or writing at once though, since that can potentially swamp
701your I/O channels, or deplete your process' quota of filehandles. You
702can use a semaphore initialized to the number of concurrent I/O
703requests (or open files) that you want at any one time, and have your
704threads quietly block and unblock themselves.
2605996a 705
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706Larger increments or decrements are handy in those cases where a
707thread needs to check out or return a number of resources at once.
2605996a 708
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709=head2 cond_wait() and cond_signal()
710
711These two functions can be used in conjunction with locks to notify
712co-operating threads that a resource has become available. They are
713very similar in use to the functions found in C<pthreads>. However
714for most purposes, queues are simpler to use and more intuitive. See
715L<threads::shared> for more details.
2605996a 716
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717=head2 Giving up control
718
719There are times when you may find it useful to have a thread
720explicitly give up the CPU to another thread. You may be doing something
721processor-intensive and want to make sure that the user-interface thread
722gets called frequently. Regardless, there are times that you might want
723a thread to give up the processor.
724
725Perl's threading package provides the yield() function that does
726this. yield() is pretty straightforward, and works like this:
727
728 use threads;
729
730 sub loop {
731 my $thread = shift;
732 my $foo = 50;
733 while($foo--) { print "in thread $thread\n" }
734 threads->yield;
735 $foo = 50;
736 while($foo--) { print "in thread $thread\n" }
737 }
738
739 my $thread1 = threads->new(\&loop, 'first');
740 my $thread2 = threads->new(\&loop, 'second');
741 my $thread3 = threads->new(\&loop, 'third');
742
743It is important to remember that yield() is only a hint to give up the CPU,
744it depends on your hardware, OS and threading libraries what actually happens.
745B<On many operating systems, yield() is a no-op.> Therefore it is important
746to note that one should not build the scheduling of the threads around
747yield() calls. It might work on your platform but it won't work on another
748platform.
749
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750=head1 General Thread Utility Routines
751
752We've covered the workhorse parts of Perl's threading package, and
753with these tools you should be well on your way to writing threaded
754code and packages. There are a few useful little pieces that didn't
755really fit in anyplace else.
756
757=head2 What Thread Am I In?
758
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759The C<< threads->self >> class method provides your program with a way to
760get an object representing the thread it's currently in. You can use this
6eded8f3 761object in the same way as the ones returned from thread creation.
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762
763=head2 Thread IDs
764
765tid() is a thread object method that returns the thread ID of the
766thread the object represents. Thread IDs are integers, with the main
767thread in a program being 0. Currently Perl assigns a unique tid to
768every thread ever created in your program, assigning the first thread
769to be created a tid of 1, and increasing the tid by 1 for each new
770thread that's created.
771
772=head2 Are These Threads The Same?
773
774The equal() method takes two thread objects and returns true
775if the objects represent the same thread, and false if they don't.
776
777Thread objects also have an overloaded == comparison so that you can do
778comparison on them as you would with normal objects.
779
780=head2 What Threads Are Running?
781
bfce6503 782C<< threads->list >> returns a list of thread objects, one for each thread
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783that's currently running and not detached. Handy for a number of things,
784including cleaning up at the end of your program:
785
786 # Loop through all the threads
787 foreach $thr (threads->list) {
788 # Don't join the main thread or ourselves
789 if ($thr->tid && !threads::equal($thr, threads->self)) {
790 $thr->join;
791 }
792 }
793
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794If some threads have not finished running when the main Perl thread
795ends, Perl will warn you about it and die, since it is impossible for Perl
6eded8f3 796to clean up itself while other threads are running
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797
798=head1 A Complete Example
799
800Confused yet? It's time for an example program to show some of the
801things we've covered. This program finds prime numbers using threads.
802
803 1 #!/usr/bin/perl -w
804 2 # prime-pthread, courtesy of Tom Christiansen
805 3
806 4 use strict;
807 5
808 6 use threads;
83272a45 809 7 use Thread::Queue;
c975c451 810 8
83272a45 811 9 my $stream = new Thread::Queue;
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812 10 my $kid = new threads(\&check_num, $stream, 2);
813 11
814 12 for my $i ( 3 .. 1000 ) {
815 13 $stream->enqueue($i);
816 14 }
817 15
818 16 $stream->enqueue(undef);
8f95bfb9 819 17 $kid->join;
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820 18
821 19 sub check_num {
822 20 my ($upstream, $cur_prime) = @_;
823 21 my $kid;
83272a45 824 22 my $downstream = new Thread::Queue;
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825 23 while (my $num = $upstream->dequeue) {
826 24 next unless $num % $cur_prime;
827 25 if ($kid) {
828 26 $downstream->enqueue($num);
829 27 } else {
830 28 print "Found prime $num\n";
831 29 $kid = new threads(\&check_num, $downstream, $num);
832 30 }
833 31 }
834 32 $downstream->enqueue(undef) if $kid;
8f95bfb9 835 33 $kid->join if $kid;
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836 34 }
837
838This program uses the pipeline model to generate prime numbers. Each
839thread in the pipeline has an input queue that feeds numbers to be
840checked, a prime number that it's responsible for, and an output queue
9e75ef81 841into which it funnels numbers that have failed the check. If the thread
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842has a number that's failed its check and there's no child thread, then
843the thread must have found a new prime number. In that case, a new
844child thread is created for that prime and stuck on the end of the
845pipeline.
846
6eded8f3 847This probably sounds a bit more confusing than it really is, so let's
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848go through this program piece by piece and see what it does. (For
849those of you who might be trying to remember exactly what a prime
850number is, it's a number that's only evenly divisible by itself and 1)
851
852The bulk of the work is done by the check_num() subroutine, which
853takes a reference to its input queue and a prime number that it's
854responsible for. After pulling in the input queue and the prime that
855the subroutine's checking (line 20), we create a new queue (line 22)
856and reserve a scalar for the thread that we're likely to create later
857(line 21).
858
859The while loop from lines 23 to line 31 grabs a scalar off the input
860queue and checks against the prime this thread is responsible
861for. Line 24 checks to see if there's a remainder when we modulo the
862number to be checked against our prime. If there is one, the number
863must not be evenly divisible by our prime, so we need to either pass
864it on to the next thread if we've created one (line 26) or create a
865new thread if we haven't.
866
867The new thread creation is line 29. We pass on to it a reference to
868the queue we've created, and the prime number we've found.
869
870Finally, once the loop terminates (because we got a 0 or undef in the
871queue, which serves as a note to die), we pass on the notice to our
6eded8f3 872child and wait for it to exit if we've created a child (lines 32 and
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87337).
874
875Meanwhile, back in the main thread, we create a queue (line 9) and the
876initial child thread (line 10), and pre-seed it with the first prime:
8772. Then we queue all the numbers from 3 to 1000 for checking (lines
87812-14), then queue a die notice (line 16) and wait for the first child
879thread to terminate (line 17). Because a child won't die until its
880child has died, we know that we're done once we return from the join.
881
882That's how it works. It's pretty simple; as with many Perl programs,
883the explanation is much longer than the program.
884
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885=head1 Different implementations of threads
886
887Some background on thread implementations from the operating system
888viewpoint. There are three basic categories of threads: user-mode threads,
889kernel threads, and multiprocessor kernel threads.
890
891User-mode threads are threads that live entirely within a program and
892its libraries. In this model, the OS knows nothing about threads. As
893far as it's concerned, your process is just a process.
894
895This is the easiest way to implement threads, and the way most OSes
896start. The big disadvantage is that, since the OS knows nothing about
897threads, if one thread blocks they all do. Typical blocking activities
898include most system calls, most I/O, and things like sleep().
899
900Kernel threads are the next step in thread evolution. The OS knows
901about kernel threads, and makes allowances for them. The main
902difference between a kernel thread and a user-mode thread is
903blocking. With kernel threads, things that block a single thread don't
904block other threads. This is not the case with user-mode threads,
905where the kernel blocks at the process level and not the thread level.
906
907This is a big step forward, and can give a threaded program quite a
908performance boost over non-threaded programs. Threads that block
909performing I/O, for example, won't block threads that are doing other
910things. Each process still has only one thread running at once,
911though, regardless of how many CPUs a system might have.
912
913Since kernel threading can interrupt a thread at any time, they will
914uncover some of the implicit locking assumptions you may make in your
915program. For example, something as simple as C<$a = $a + 2> can behave
916unpredictably with kernel threads if $a is visible to other
917threads, as another thread may have changed $a between the time it
918was fetched on the right hand side and the time the new value is
919stored.
920
921Multiprocessor kernel threads are the final step in thread
922support. With multiprocessor kernel threads on a machine with multiple
923CPUs, the OS may schedule two or more threads to run simultaneously on
924different CPUs.
925
926This can give a serious performance boost to your threaded program,
927since more than one thread will be executing at the same time. As a
928tradeoff, though, any of those nagging synchronization issues that
929might not have shown with basic kernel threads will appear with a
930vengeance.
931
932In addition to the different levels of OS involvement in threads,
933different OSes (and different thread implementations for a particular
934OS) allocate CPU cycles to threads in different ways.
935
936Cooperative multitasking systems have running threads give up control
937if one of two things happen. If a thread calls a yield function, it
938gives up control. It also gives up control if the thread does
939something that would cause it to block, such as perform I/O. In a
940cooperative multitasking implementation, one thread can starve all the
941others for CPU time if it so chooses.
942
943Preemptive multitasking systems interrupt threads at regular intervals
944while the system decides which thread should run next. In a preemptive
945multitasking system, one thread usually won't monopolize the CPU.
946
947On some systems, there can be cooperative and preemptive threads
948running simultaneously. (Threads running with realtime priorities
949often behave cooperatively, for example, while threads running at
950normal priorities behave preemptively.)
951
952Most modern operating systems support preemptive multitasking nowadays.
953
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954=head1 Performance considerations
955
956The main thing to bear in mind when comparing ithreads to other threading
957models is the fact that for each new thread created, a complete copy of
958all the variables and data of the parent thread has to be taken. Thus
959thread creation can be quite expensive, both in terms of memory usage and
960time spent in creation. The ideal way to reduce these costs is to have a
961relatively short number of long-lived threads, all created fairly early
962on - before the base thread has accumulated too much data. Of course, this
963may not always be possible, so compromises have to be made. However, after
964a thread has been created, its performance and extra memory usage should
965be little different than ordinary code.
966
967Also note that under the current implementation, shared variables
968use a little more memory and are a little slower than ordinary variables.
969
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970=head1 Process-scope Changes
971
972Note that while threads themselves are separate execution threads and
973Perl data is thread-private unless explicitly shared, the threads can
974affect process-scope state, affecting all the threads.
975
976The most common example of this is changing the current working
977directory using chdir(). One thread calls chdir(), and the working
978directory of all the threads changes.
bdcfa4c7 979
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980Even more drastic example of a process-scope change is chroot():
981the root directory of all the threads changes, and no thread can
982undo it (as opposed to chdir()).
983
984Further examples of process-scope changes include umask() and
985changing uids/gids.
986
987Thinking of mixing fork() and threads? Please lie down and wait
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988until the feeling passes. Be aware that the semantics of fork() vary
989between platforms. For example, some UNIX systems copy all the current
990threads into the child process, while others only copy the thread that
991called fork(). You have been warned!
cf5baa48 992
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993Similarly, mixing signals and threads should not be attempted.
994Implementations are platform-dependent, and even the POSIX
995semantics may not be what you expect (and Perl doesn't even
996give you the full POSIX API).
997
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998=head1 Thread-Safety of System Libraries
999
1000Whether various library calls are thread-safe is outside the control
1001of Perl. Calls often suffering from not being thread-safe include:
bdcfa4c7 1002localtime(), gmtime(), get{gr,host,net,proto,serv,pw}*(), readdir(),
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1003rand(), and srand() -- in general, calls that depend on some global
1004external state.
80bbcbc4 1005
cf5baa48 1006If the system Perl is compiled in has thread-safe variants of such
80bbcbc4 1007calls, they will be used. Beyond that, Perl is at the mercy of
cf5baa48 1008the thread-safety or -unsafety of the calls. Please consult your
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1009C library call documentation.
1010
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1011On some platforms the thread-safe library interfaces may fail if the
1012result buffer is too small (for example the user group databases may
1013be rather large, and the reentrant interfaces may have to carry around
1014a full snapshot of those databases). Perl will start with a small
1015buffer, but keep retrying and growing the result buffer
1016until the result fits. If this limitless growing sounds bad for
1017security or memory consumption reasons you can recompile Perl with
1018PERL_REENTRANT_MAXSIZE defined to the maximum number of bytes you will
1019allow.
bdcfa4c7 1020
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1021=head1 Conclusion
1022
1023A complete thread tutorial could fill a book (and has, many times),
6eded8f3
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1024but with what we've covered in this introduction, you should be well
1025on your way to becoming a threaded Perl expert.
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1026
1027=head1 Bibliography
1028
1029Here's a short bibliography courtesy of Jürgen Christoffel:
1030
1031=head2 Introductory Texts
1032
1033Birrell, Andrew D. An Introduction to Programming with
1034Threads. Digital Equipment Corporation, 1989, DEC-SRC Research Report
1035#35 online as
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1036http://gatekeeper.dec.com/pub/DEC/SRC/research-reports/abstracts/src-rr-035.html
1037(highly recommended)
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1038
1039Robbins, Kay. A., and Steven Robbins. Practical Unix Programming: A
1040Guide to Concurrency, Communication, and
1041Multithreading. Prentice-Hall, 1996.
1042
1043Lewis, Bill, and Daniel J. Berg. Multithreaded Programming with
1044Pthreads. Prentice Hall, 1997, ISBN 0-13-443698-9 (a well-written
1045introduction to threads).
1046
1047Nelson, Greg (editor). Systems Programming with Modula-3. Prentice
1048Hall, 1991, ISBN 0-13-590464-1.
1049
1050Nichols, Bradford, Dick Buttlar, and Jacqueline Proulx Farrell.
1051Pthreads Programming. O'Reilly & Associates, 1996, ISBN 156592-115-1
1052(covers POSIX threads).
1053
1054=head2 OS-Related References
1055
1056Boykin, Joseph, David Kirschen, Alan Langerman, and Susan
1057LoVerso. Programming under Mach. Addison-Wesley, 1994, ISBN
10580-201-52739-1.
1059
1060Tanenbaum, Andrew S. Distributed Operating Systems. Prentice Hall,
10611995, ISBN 0-13-219908-4 (great textbook).
1062
1063Silberschatz, Abraham, and Peter B. Galvin. Operating System Concepts,
10644th ed. Addison-Wesley, 1995, ISBN 0-201-59292-4
1065
1066=head2 Other References
1067
1068Arnold, Ken and James Gosling. The Java Programming Language, 2nd
1069ed. Addison-Wesley, 1998, ISBN 0-201-31006-6.
1070
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1071comp.programming.threads FAQ,
1072L<http://www.serpentine.com/~bos/threads-faq/>
1073
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1074Le Sergent, T. and B. Berthomieu. "Incremental MultiThreaded Garbage
1075Collection on Virtually Shared Memory Architectures" in Memory
1076Management: Proc. of the International Workshop IWMM 92, St. Malo,
1077France, September 1992, Yves Bekkers and Jacques Cohen, eds. Springer,
10781992, ISBN 3540-55940-X (real-life thread applications).
1079
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1080Artur Bergman, "Where Wizards Fear To Tread", June 11, 2002,
1081L<http://www.perl.com/pub/a/2002/06/11/threads.html>
1082
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1083=head1 Acknowledgements
1084
1085Thanks (in no particular order) to Chaim Frenkel, Steve Fink, Gurusamy
1086Sarathy, Ilya Zakharevich, Benjamin Sugars, Jürgen Christoffel, Joshua
1087Pritikin, and Alan Burlison, for their help in reality-checking and
1088polishing this article. Big thanks to Tom Christiansen for his rewrite
1089of the prime number generator.
1090
1091=head1 AUTHOR
1092
9316ed2f 1093Dan Sugalski E<lt>dan@sidhe.org<gt>
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1094
1095Slightly modified by Arthur Bergman to fit the new thread model/module.
1096
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1097Reworked slightly by Jörg Walter E<lt>jwalt@cpan.org<gt> to be more concise
1098about thread-safety of perl code.
1099
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1100Rearranged slightly by Elizabeth Mattijsen E<lt>liz@dijkmat.nl<gt> to put
1101less emphasis on yield().
1102
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1103=head1 Copyrights
1104
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1105The original version of this article originally appeared in The Perl
1106Journal #10, and is copyright 1998 The Perl Journal. It appears courtesy
1107of Jon Orwant and The Perl Journal. This document may be distributed
1108under the same terms as Perl itself.
2605996a 1109
53d7eaa8 1110For more information please see L<threads> and L<threads::shared>.