| 1 | =encoding utf8 |
| 2 | |
| 3 | =head1 NAME |
| 4 | |
| 5 | perlthrtut - Tutorial on threads in Perl |
| 6 | |
| 7 | =head1 DESCRIPTION |
| 8 | |
| 9 | This tutorial describes the use of Perl interpreter threads (sometimes |
| 10 | referred to as I<ithreads>). In this |
| 11 | model, each thread runs in its own Perl interpreter, and any data sharing |
| 12 | between threads must be explicit. The user-level interface for I<ithreads> |
| 13 | uses the L<threads> class. |
| 14 | |
| 15 | B<NOTE>: There was another older Perl threading flavor called the 5.005 model |
| 16 | that used the L<threads> class. This old model was known to have problems, is |
| 17 | deprecated, and was removed for release 5.10. You are |
| 18 | strongly encouraged to migrate any existing 5.005 threads code to the new |
| 19 | model as soon as possible. |
| 20 | |
| 21 | You can see which (or neither) threading flavour you have by |
| 22 | running C<perl -V> and looking at the C<Platform> section. |
| 23 | If you have C<useithreads=define> you have ithreads, if you |
| 24 | have C<use5005threads=define> you have 5.005 threads. |
| 25 | If you have neither, you don't have any thread support built in. |
| 26 | If you have both, you are in trouble. |
| 27 | |
| 28 | The L<threads> and L<threads::shared> modules are included in the core Perl |
| 29 | distribution. Additionally, they are maintained as a separate modules on |
| 30 | CPAN, so you can check there for any updates. |
| 31 | |
| 32 | =head1 What Is A Thread Anyway? |
| 33 | |
| 34 | A thread is a flow of control through a program with a single |
| 35 | execution point. |
| 36 | |
| 37 | Sounds an awful lot like a process, doesn't it? Well, it should. |
| 38 | Threads are one of the pieces of a process. Every process has at least |
| 39 | one thread and, up until now, every process running Perl had only one |
| 40 | thread. With 5.8, though, you can create extra threads. We're going |
| 41 | to show you how, when, and why. |
| 42 | |
| 43 | =head1 Threaded Program Models |
| 44 | |
| 45 | There are three basic ways that you can structure a threaded |
| 46 | program. Which model you choose depends on what you need your program |
| 47 | to do. For many non-trivial threaded programs, you'll need to choose |
| 48 | different models for different pieces of your program. |
| 49 | |
| 50 | =head2 Boss/Worker |
| 51 | |
| 52 | The boss/worker model usually has one I<boss> thread and one or more |
| 53 | I<worker> threads. The boss thread gathers or generates tasks that need |
| 54 | to be done, then parcels those tasks out to the appropriate worker |
| 55 | thread. |
| 56 | |
| 57 | This model is common in GUI and server programs, where a main thread |
| 58 | waits for some event and then passes that event to the appropriate |
| 59 | worker threads for processing. Once the event has been passed on, the |
| 60 | boss thread goes back to waiting for another event. |
| 61 | |
| 62 | The boss thread does relatively little work. While tasks aren't |
| 63 | necessarily performed faster than with any other method, it tends to |
| 64 | have the best user-response times. |
| 65 | |
| 66 | =head2 Work Crew |
| 67 | |
| 68 | In the work crew model, several threads are created that do |
| 69 | essentially the same thing to different pieces of data. It closely |
| 70 | mirrors classical parallel processing and vector processors, where a |
| 71 | large array of processors do the exact same thing to many pieces of |
| 72 | data. |
| 73 | |
| 74 | This model is particularly useful if the system running the program |
| 75 | will distribute multiple threads across different processors. It can |
| 76 | also be useful in ray tracing or rendering engines, where the |
| 77 | individual threads can pass on interim results to give the user visual |
| 78 | feedback. |
| 79 | |
| 80 | =head2 Pipeline |
| 81 | |
| 82 | The pipeline model divides up a task into a series of steps, and |
| 83 | passes the results of one step on to the thread processing the |
| 84 | next. Each thread does one thing to each piece of data and passes the |
| 85 | results to the next thread in line. |
| 86 | |
| 87 | This model makes the most sense if you have multiple processors so two |
| 88 | or more threads will be executing in parallel, though it can often |
| 89 | make sense in other contexts as well. It tends to keep the individual |
| 90 | tasks small and simple, as well as allowing some parts of the pipeline |
| 91 | to block (on I/O or system calls, for example) while other parts keep |
| 92 | going. If you're running different parts of the pipeline on different |
| 93 | processors you may also take advantage of the caches on each |
| 94 | processor. |
| 95 | |
| 96 | This model is also handy for a form of recursive programming where, |
| 97 | rather than having a subroutine call itself, it instead creates |
| 98 | another thread. Prime and Fibonacci generators both map well to this |
| 99 | form of the pipeline model. (A version of a prime number generator is |
| 100 | presented later on.) |
| 101 | |
| 102 | =head1 What kind of threads are Perl threads? |
| 103 | |
| 104 | If you have experience with other thread implementations, you might |
| 105 | find that things aren't quite what you expect. It's very important to |
| 106 | remember when dealing with Perl threads that I<Perl Threads Are Not X |
| 107 | Threads> for all values of X. They aren't POSIX threads, or |
| 108 | DecThreads, or Java's Green threads, or Win32 threads. There are |
| 109 | similarities, and the broad concepts are the same, but if you start |
| 110 | looking for implementation details you're going to be either |
| 111 | disappointed or confused. Possibly both. |
| 112 | |
| 113 | This is not to say that Perl threads are completely different from |
| 114 | everything that's ever come before. They're not. Perl's threading |
| 115 | model owes a lot to other thread models, especially POSIX. Just as |
| 116 | Perl is not C, though, Perl threads are not POSIX threads. So if you |
| 117 | find yourself looking for mutexes, or thread priorities, it's time to |
| 118 | step back a bit and think about what you want to do and how Perl can |
| 119 | do it. |
| 120 | |
| 121 | However, it is important to remember that Perl threads cannot magically |
| 122 | do things unless your operating system's threads allow it. So if your |
| 123 | system blocks the entire process on C<sleep()>, Perl usually will, as well. |
| 124 | |
| 125 | B<Perl Threads Are Different.> |
| 126 | |
| 127 | =head1 Thread-Safe Modules |
| 128 | |
| 129 | The addition of threads has changed Perl's internals |
| 130 | substantially. There are implications for people who write |
| 131 | modules with XS code or external libraries. However, since Perl data is |
| 132 | not shared among threads by default, Perl modules stand a high chance of |
| 133 | being thread-safe or can be made thread-safe easily. Modules that are not |
| 134 | tagged as thread-safe should be tested or code reviewed before being used |
| 135 | in production code. |
| 136 | |
| 137 | Not all modules that you might use are thread-safe, and you should |
| 138 | always assume a module is unsafe unless the documentation says |
| 139 | otherwise. This includes modules that are distributed as part of the |
| 140 | core. Threads are a relatively new feature, and even some of the standard |
| 141 | modules aren't thread-safe. |
| 142 | |
| 143 | Even if a module is thread-safe, it doesn't mean that the module is optimized |
| 144 | to work well with threads. A module could possibly be rewritten to utilize |
| 145 | the new features in threaded Perl to increase performance in a threaded |
| 146 | environment. |
| 147 | |
| 148 | If you're using a module that's not thread-safe for some reason, you |
| 149 | can protect yourself by using it from one, and only one thread at all. |
| 150 | If you need multiple threads to access such a module, you can use semaphores and |
| 151 | lots of programming discipline to control access to it. Semaphores |
| 152 | are covered in L</"Basic semaphores">. |
| 153 | |
| 154 | See also L</"Thread-Safety of System Libraries">. |
| 155 | |
| 156 | =head1 Thread Basics |
| 157 | |
| 158 | The L<threads> module provides the basic functions you need to write |
| 159 | threaded programs. In the following sections, we'll cover the basics, |
| 160 | showing you what you need to do to create a threaded program. After |
| 161 | that, we'll go over some of the features of the L<threads> module that |
| 162 | make threaded programming easier. |
| 163 | |
| 164 | =head2 Basic Thread Support |
| 165 | |
| 166 | Thread support is a Perl compile-time option. It's something that's |
| 167 | turned on or off when Perl is built at your site, rather than when |
| 168 | your programs are compiled. If your Perl wasn't compiled with thread |
| 169 | support enabled, then any attempt to use threads will fail. |
| 170 | |
| 171 | Your programs can use the Config module to check whether threads are |
| 172 | enabled. If your program can't run without them, you can say something |
| 173 | like: |
| 174 | |
| 175 | use Config; |
| 176 | $Config{useithreads} or |
| 177 | die('Recompile Perl with threads to run this program.'); |
| 178 | |
| 179 | A possibly-threaded program using a possibly-threaded module might |
| 180 | have code like this: |
| 181 | |
| 182 | use Config; |
| 183 | use MyMod; |
| 184 | |
| 185 | BEGIN { |
| 186 | if ($Config{useithreads}) { |
| 187 | # We have threads |
| 188 | require MyMod_threaded; |
| 189 | import MyMod_threaded; |
| 190 | } else { |
| 191 | require MyMod_unthreaded; |
| 192 | import MyMod_unthreaded; |
| 193 | } |
| 194 | } |
| 195 | |
| 196 | Since code that runs both with and without threads is usually pretty |
| 197 | messy, it's best to isolate the thread-specific code in its own |
| 198 | module. In our example above, that's what C<MyMod_threaded> is, and it's |
| 199 | only imported if we're running on a threaded Perl. |
| 200 | |
| 201 | =head2 A Note about the Examples |
| 202 | |
| 203 | In a real situation, care should be taken that all threads are finished |
| 204 | executing before the program exits. That care has B<not> been taken in these |
| 205 | examples in the interest of simplicity. Running these examples I<as is> will |
| 206 | produce error messages, usually caused by the fact that there are still |
| 207 | threads running when the program exits. You should not be alarmed by this. |
| 208 | |
| 209 | =head2 Creating Threads |
| 210 | |
| 211 | The L<threads> module provides the tools you need to create new |
| 212 | threads. Like any other module, you need to tell Perl that you want to use |
| 213 | it; C<use threads;> imports all the pieces you need to create basic |
| 214 | threads. |
| 215 | |
| 216 | The simplest, most straightforward way to create a thread is with C<create()>: |
| 217 | |
| 218 | use threads; |
| 219 | |
| 220 | my $thr = threads->create(\&sub1); |
| 221 | |
| 222 | sub sub1 { |
| 223 | print("In the thread\n"); |
| 224 | } |
| 225 | |
| 226 | The C<create()> method takes a reference to a subroutine and creates a new |
| 227 | thread that starts executing in the referenced subroutine. Control |
| 228 | then passes both to the subroutine and the caller. |
| 229 | |
| 230 | If you need to, your program can pass parameters to the subroutine as |
| 231 | part of the thread startup. Just include the list of parameters as |
| 232 | part of the C<threads-E<gt>create()> call, like this: |
| 233 | |
| 234 | use threads; |
| 235 | |
| 236 | my $Param3 = 'foo'; |
| 237 | my $thr1 = threads->create(\&sub1, 'Param 1', 'Param 2', $Param3); |
| 238 | my @ParamList = (42, 'Hello', 3.14); |
| 239 | my $thr2 = threads->create(\&sub1, @ParamList); |
| 240 | my $thr3 = threads->create(\&sub1, qw(Param1 Param2 Param3)); |
| 241 | |
| 242 | sub sub1 { |
| 243 | my @InboundParameters = @_; |
| 244 | print("In the thread\n"); |
| 245 | print('Got parameters >', join('<>',@InboundParameters), "<\n"); |
| 246 | } |
| 247 | |
| 248 | The last example illustrates another feature of threads. You can spawn |
| 249 | off several threads using the same subroutine. Each thread executes |
| 250 | the same subroutine, but in a separate thread with a separate |
| 251 | environment and potentially separate arguments. |
| 252 | |
| 253 | C<new()> is a synonym for C<create()>. |
| 254 | |
| 255 | =head2 Waiting For A Thread To Exit |
| 256 | |
| 257 | Since threads are also subroutines, they can return values. To wait |
| 258 | for a thread to exit and extract any values it might return, you can |
| 259 | use the C<join()> method: |
| 260 | |
| 261 | use threads; |
| 262 | |
| 263 | my ($thr) = threads->create(\&sub1); |
| 264 | |
| 265 | my @ReturnData = $thr->join(); |
| 266 | print('Thread returned ', join(', ', @ReturnData), "\n"); |
| 267 | |
| 268 | sub sub1 { return ('Fifty-six', 'foo', 2); } |
| 269 | |
| 270 | In the example above, the C<join()> method returns as soon as the thread |
| 271 | ends. In addition to waiting for a thread to finish and gathering up |
| 272 | any values that the thread might have returned, C<join()> also performs |
| 273 | any OS cleanup necessary for the thread. That cleanup might be |
| 274 | important, especially for long-running programs that spawn lots of |
| 275 | threads. If you don't want the return values and don't want to wait |
| 276 | for the thread to finish, you should call the C<detach()> method |
| 277 | instead, as described next. |
| 278 | |
| 279 | NOTE: In the example above, the thread returns a list, thus necessitating |
| 280 | that the thread creation call be made in list context (i.e., C<my ($thr)>). |
| 281 | See L<< threads/"$thr->join()" >> and L<threads/"THREAD CONTEXT"> for more |
| 282 | details on thread context and return values. |
| 283 | |
| 284 | =head2 Ignoring A Thread |
| 285 | |
| 286 | C<join()> does three things: it waits for a thread to exit, cleans up |
| 287 | after it, and returns any data the thread may have produced. But what |
| 288 | if you're not interested in the thread's return values, and you don't |
| 289 | really care when the thread finishes? All you want is for the thread |
| 290 | to get cleaned up after when it's done. |
| 291 | |
| 292 | In this case, you use the C<detach()> method. Once a thread is detached, |
| 293 | it'll run until it's finished; then Perl will clean up after it |
| 294 | automatically. |
| 295 | |
| 296 | use threads; |
| 297 | |
| 298 | my $thr = threads->create(\&sub1); # Spawn the thread |
| 299 | |
| 300 | $thr->detach(); # Now we officially don't care any more |
| 301 | |
| 302 | sleep(15); # Let thread run for awhile |
| 303 | |
| 304 | sub sub1 { |
| 305 | my $count = 0; |
| 306 | while (1) { |
| 307 | $count++; |
| 308 | print("\$count is $count\n"); |
| 309 | sleep(1); |
| 310 | } |
| 311 | } |
| 312 | |
| 313 | Once a thread is detached, it may not be joined, and any return data |
| 314 | that it might have produced (if it was done and waiting for a join) is |
| 315 | lost. |
| 316 | |
| 317 | C<detach()> can also be called as a class method to allow a thread to |
| 318 | detach itself: |
| 319 | |
| 320 | use threads; |
| 321 | |
| 322 | my $thr = threads->create(\&sub1); |
| 323 | |
| 324 | sub sub1 { |
| 325 | threads->detach(); |
| 326 | # Do more work |
| 327 | } |
| 328 | |
| 329 | =head2 Process and Thread Termination |
| 330 | |
| 331 | With threads one must be careful to make sure they all have a chance to |
| 332 | run to completion, assuming that is what you want. |
| 333 | |
| 334 | An action that terminates a process will terminate I<all> running |
| 335 | threads. die() and exit() have this property, |
| 336 | and perl does an exit when the main thread exits, |
| 337 | perhaps implicitly by falling off the end of your code, |
| 338 | even if that's not what you want. |
| 339 | |
| 340 | As an example of this case, this code prints the message |
| 341 | "Perl exited with active threads: 2 running and unjoined": |
| 342 | |
| 343 | use threads; |
| 344 | my $thr1 = threads->new(\&thrsub, "test1"); |
| 345 | my $thr2 = threads->new(\&thrsub, "test2"); |
| 346 | sub thrsub { |
| 347 | my ($message) = @_; |
| 348 | sleep 1; |
| 349 | print "thread $message\n"; |
| 350 | } |
| 351 | |
| 352 | But when the following lines are added at the end: |
| 353 | |
| 354 | $thr1->join(); |
| 355 | $thr2->join(); |
| 356 | |
| 357 | it prints two lines of output, a perhaps more useful outcome. |
| 358 | |
| 359 | =head1 Threads And Data |
| 360 | |
| 361 | Now that we've covered the basics of threads, it's time for our next |
| 362 | topic: Data. Threading introduces a couple of complications to data |
| 363 | access that non-threaded programs never need to worry about. |
| 364 | |
| 365 | =head2 Shared And Unshared Data |
| 366 | |
| 367 | The biggest difference between Perl I<ithreads> and the old 5.005 style |
| 368 | threading, or for that matter, to most other threading systems out there, |
| 369 | is that by default, no data is shared. When a new Perl thread is created, |
| 370 | all the data associated with the current thread is copied to the new |
| 371 | thread, and is subsequently private to that new thread! |
| 372 | This is similar in feel to what happens when a Unix process forks, |
| 373 | except that in this case, the data is just copied to a different part of |
| 374 | memory within the same process rather than a real fork taking place. |
| 375 | |
| 376 | To make use of threading, however, one usually wants the threads to share |
| 377 | at least some data between themselves. This is done with the |
| 378 | L<threads::shared> module and the C<:shared> attribute: |
| 379 | |
| 380 | use threads; |
| 381 | use threads::shared; |
| 382 | |
| 383 | my $foo :shared = 1; |
| 384 | my $bar = 1; |
| 385 | threads->create(sub { $foo++; $bar++; })->join(); |
| 386 | |
| 387 | print("$foo\n"); # Prints 2 since $foo is shared |
| 388 | print("$bar\n"); # Prints 1 since $bar is not shared |
| 389 | |
| 390 | In the case of a shared array, all the array's elements are shared, and for |
| 391 | a shared hash, all the keys and values are shared. This places |
| 392 | restrictions on what may be assigned to shared array and hash elements: only |
| 393 | simple values or references to shared variables are allowed - this is |
| 394 | so that a private variable can't accidentally become shared. A bad |
| 395 | assignment will cause the thread to die. For example: |
| 396 | |
| 397 | use threads; |
| 398 | use threads::shared; |
| 399 | |
| 400 | my $var = 1; |
| 401 | my $svar :shared = 2; |
| 402 | my %hash :shared; |
| 403 | |
| 404 | ... create some threads ... |
| 405 | |
| 406 | $hash{a} = 1; # All threads see exists($hash{a}) |
| 407 | # and $hash{a} == 1 |
| 408 | $hash{a} = $var; # okay - copy-by-value: same effect as previous |
| 409 | $hash{a} = $svar; # okay - copy-by-value: same effect as previous |
| 410 | $hash{a} = \$svar; # okay - a reference to a shared variable |
| 411 | $hash{a} = \$var; # This will die |
| 412 | delete($hash{a}); # okay - all threads will see !exists($hash{a}) |
| 413 | |
| 414 | Note that a shared variable guarantees that if two or more threads try to |
| 415 | modify it at the same time, the internal state of the variable will not |
| 416 | become corrupted. However, there are no guarantees beyond this, as |
| 417 | explained in the next section. |
| 418 | |
| 419 | =head2 Thread Pitfalls: Races |
| 420 | |
| 421 | While threads bring a new set of useful tools, they also bring a |
| 422 | number of pitfalls. One pitfall is the race condition: |
| 423 | |
| 424 | use threads; |
| 425 | use threads::shared; |
| 426 | |
| 427 | my $x :shared = 1; |
| 428 | my $thr1 = threads->create(\&sub1); |
| 429 | my $thr2 = threads->create(\&sub2); |
| 430 | |
| 431 | $thr1->join(); |
| 432 | $thr2->join(); |
| 433 | print("$x\n"); |
| 434 | |
| 435 | sub sub1 { my $foo = $x; $x = $foo + 1; } |
| 436 | sub sub2 { my $bar = $x; $x = $bar + 1; } |
| 437 | |
| 438 | What do you think C<$x> will be? The answer, unfortunately, is I<it |
| 439 | depends>. Both C<sub1()> and C<sub2()> access the global variable C<$x>, once |
| 440 | to read and once to write. Depending on factors ranging from your |
| 441 | thread implementation's scheduling algorithm to the phase of the moon, |
| 442 | C<$x> can be 2 or 3. |
| 443 | |
| 444 | Race conditions are caused by unsynchronized access to shared |
| 445 | data. Without explicit synchronization, there's no way to be sure that |
| 446 | nothing has happened to the shared data between the time you access it |
| 447 | and the time you update it. Even this simple code fragment has the |
| 448 | possibility of error: |
| 449 | |
| 450 | use threads; |
| 451 | my $x :shared = 2; |
| 452 | my $y :shared; |
| 453 | my $z :shared; |
| 454 | my $thr1 = threads->create(sub { $y = $x; $x = $y + 1; }); |
| 455 | my $thr2 = threads->create(sub { $z = $x; $x = $z + 1; }); |
| 456 | $thr1->join(); |
| 457 | $thr2->join(); |
| 458 | |
| 459 | Two threads both access C<$x>. Each thread can potentially be interrupted |
| 460 | at any point, or be executed in any order. At the end, C<$x> could be 3 |
| 461 | or 4, and both C<$y> and C<$z> could be 2 or 3. |
| 462 | |
| 463 | Even C<$x += 5> or C<$x++> are not guaranteed to be atomic. |
| 464 | |
| 465 | Whenever your program accesses data or resources that can be accessed |
| 466 | by other threads, you must take steps to coordinate access or risk |
| 467 | data inconsistency and race conditions. Note that Perl will protect its |
| 468 | internals from your race conditions, but it won't protect you from you. |
| 469 | |
| 470 | =head1 Synchronization and control |
| 471 | |
| 472 | Perl provides a number of mechanisms to coordinate the interactions |
| 473 | between themselves and their data, to avoid race conditions and the like. |
| 474 | Some of these are designed to resemble the common techniques used in thread |
| 475 | libraries such as C<pthreads>; others are Perl-specific. Often, the |
| 476 | standard techniques are clumsy and difficult to get right (such as |
| 477 | condition waits). Where possible, it is usually easier to use Perlish |
| 478 | techniques such as queues, which remove some of the hard work involved. |
| 479 | |
| 480 | =head2 Controlling access: lock() |
| 481 | |
| 482 | The C<lock()> function takes a shared variable and puts a lock on it. |
| 483 | No other thread may lock the variable until the variable is unlocked |
| 484 | by the thread holding the lock. Unlocking happens automatically |
| 485 | when the locking thread exits the block that contains the call to the |
| 486 | C<lock()> function. Using C<lock()> is straightforward: This example has |
| 487 | several threads doing some calculations in parallel, and occasionally |
| 488 | updating a running total: |
| 489 | |
| 490 | use threads; |
| 491 | use threads::shared; |
| 492 | |
| 493 | my $total :shared = 0; |
| 494 | |
| 495 | sub calc { |
| 496 | while (1) { |
| 497 | my $result; |
| 498 | # (... do some calculations and set $result ...) |
| 499 | { |
| 500 | lock($total); # Block until we obtain the lock |
| 501 | $total += $result; |
| 502 | } # Lock implicitly released at end of scope |
| 503 | last if $result == 0; |
| 504 | } |
| 505 | } |
| 506 | |
| 507 | my $thr1 = threads->create(\&calc); |
| 508 | my $thr2 = threads->create(\&calc); |
| 509 | my $thr3 = threads->create(\&calc); |
| 510 | $thr1->join(); |
| 511 | $thr2->join(); |
| 512 | $thr3->join(); |
| 513 | print("total=$total\n"); |
| 514 | |
| 515 | C<lock()> blocks the thread until the variable being locked is |
| 516 | available. When C<lock()> returns, your thread can be sure that no other |
| 517 | thread can lock that variable until the block containing the |
| 518 | lock exits. |
| 519 | |
| 520 | It's important to note that locks don't prevent access to the variable |
| 521 | in question, only lock attempts. This is in keeping with Perl's |
| 522 | longstanding tradition of courteous programming, and the advisory file |
| 523 | locking that C<flock()> gives you. |
| 524 | |
| 525 | You may lock arrays and hashes as well as scalars. Locking an array, |
| 526 | though, will not block subsequent locks on array elements, just lock |
| 527 | attempts on the array itself. |
| 528 | |
| 529 | Locks are recursive, which means it's okay for a thread to |
| 530 | lock a variable more than once. The lock will last until the outermost |
| 531 | C<lock()> on the variable goes out of scope. For example: |
| 532 | |
| 533 | my $x :shared; |
| 534 | doit(); |
| 535 | |
| 536 | sub doit { |
| 537 | { |
| 538 | { |
| 539 | lock($x); # Wait for lock |
| 540 | lock($x); # NOOP - we already have the lock |
| 541 | { |
| 542 | lock($x); # NOOP |
| 543 | { |
| 544 | lock($x); # NOOP |
| 545 | lockit_some_more(); |
| 546 | } |
| 547 | } |
| 548 | } # *** Implicit unlock here *** |
| 549 | } |
| 550 | } |
| 551 | |
| 552 | sub lockit_some_more { |
| 553 | lock($x); # NOOP |
| 554 | } # Nothing happens here |
| 555 | |
| 556 | Note that there is no C<unlock()> function - the only way to unlock a |
| 557 | variable is to allow it to go out of scope. |
| 558 | |
| 559 | A lock can either be used to guard the data contained within the variable |
| 560 | being locked, or it can be used to guard something else, like a section |
| 561 | of code. In this latter case, the variable in question does not hold any |
| 562 | useful data, and exists only for the purpose of being locked. In this |
| 563 | respect, the variable behaves like the mutexes and basic semaphores of |
| 564 | traditional thread libraries. |
| 565 | |
| 566 | =head2 A Thread Pitfall: Deadlocks |
| 567 | |
| 568 | Locks are a handy tool to synchronize access to data, and using them |
| 569 | properly is the key to safe shared data. Unfortunately, locks aren't |
| 570 | without their dangers, especially when multiple locks are involved. |
| 571 | Consider the following code: |
| 572 | |
| 573 | use threads; |
| 574 | |
| 575 | my $x :shared = 4; |
| 576 | my $y :shared = 'foo'; |
| 577 | my $thr1 = threads->create(sub { |
| 578 | lock($x); |
| 579 | sleep(20); |
| 580 | lock($y); |
| 581 | }); |
| 582 | my $thr2 = threads->create(sub { |
| 583 | lock($y); |
| 584 | sleep(20); |
| 585 | lock($x); |
| 586 | }); |
| 587 | |
| 588 | This program will probably hang until you kill it. The only way it |
| 589 | won't hang is if one of the two threads acquires both locks |
| 590 | first. A guaranteed-to-hang version is more complicated, but the |
| 591 | principle is the same. |
| 592 | |
| 593 | The first thread will grab a lock on C<$x>, then, after a pause during which |
| 594 | the second thread has probably had time to do some work, try to grab a |
| 595 | lock on C<$y>. Meanwhile, the second thread grabs a lock on C<$y>, then later |
| 596 | tries to grab a lock on C<$x>. The second lock attempt for both threads will |
| 597 | block, each waiting for the other to release its lock. |
| 598 | |
| 599 | This condition is called a deadlock, and it occurs whenever two or |
| 600 | more threads are trying to get locks on resources that the others |
| 601 | own. Each thread will block, waiting for the other to release a lock |
| 602 | on a resource. That never happens, though, since the thread with the |
| 603 | resource is itself waiting for a lock to be released. |
| 604 | |
| 605 | There are a number of ways to handle this sort of problem. The best |
| 606 | way is to always have all threads acquire locks in the exact same |
| 607 | order. If, for example, you lock variables C<$x>, C<$y>, and C<$z>, always lock |
| 608 | C<$x> before C<$y>, and C<$y> before C<$z>. It's also best to hold on to locks for |
| 609 | as short a period of time to minimize the risks of deadlock. |
| 610 | |
| 611 | The other synchronization primitives described below can suffer from |
| 612 | similar problems. |
| 613 | |
| 614 | =head2 Queues: Passing Data Around |
| 615 | |
| 616 | A queue is a special thread-safe object that lets you put data in one |
| 617 | end and take it out the other without having to worry about |
| 618 | synchronization issues. They're pretty straightforward, and look like |
| 619 | this: |
| 620 | |
| 621 | use threads; |
| 622 | use Thread::Queue; |
| 623 | |
| 624 | my $DataQueue = Thread::Queue->new(); |
| 625 | my $thr = threads->create(sub { |
| 626 | while (my $DataElement = $DataQueue->dequeue()) { |
| 627 | print("Popped $DataElement off the queue\n"); |
| 628 | } |
| 629 | }); |
| 630 | |
| 631 | $DataQueue->enqueue(12); |
| 632 | $DataQueue->enqueue("A", "B", "C"); |
| 633 | sleep(10); |
| 634 | $DataQueue->enqueue(undef); |
| 635 | $thr->join(); |
| 636 | |
| 637 | You create the queue with C<Thread::Queue-E<gt>new()>. Then you can |
| 638 | add lists of scalars onto the end with C<enqueue()>, and pop scalars off |
| 639 | the front of it with C<dequeue()>. A queue has no fixed size, and can grow |
| 640 | as needed to hold everything pushed on to it. |
| 641 | |
| 642 | If a queue is empty, C<dequeue()> blocks until another thread enqueues |
| 643 | something. This makes queues ideal for event loops and other |
| 644 | communications between threads. |
| 645 | |
| 646 | =head2 Semaphores: Synchronizing Data Access |
| 647 | |
| 648 | Semaphores are a kind of generic locking mechanism. In their most basic |
| 649 | form, they behave very much like lockable scalars, except that they |
| 650 | can't hold data, and that they must be explicitly unlocked. In their |
| 651 | advanced form, they act like a kind of counter, and can allow multiple |
| 652 | threads to have the I<lock> at any one time. |
| 653 | |
| 654 | =head2 Basic semaphores |
| 655 | |
| 656 | Semaphores have two methods, C<down()> and C<up()>: C<down()> decrements the resource |
| 657 | count, while C<up()> increments it. Calls to C<down()> will block if the |
| 658 | semaphore's current count would decrement below zero. This program |
| 659 | gives a quick demonstration: |
| 660 | |
| 661 | use threads; |
| 662 | use Thread::Semaphore; |
| 663 | |
| 664 | my $semaphore = Thread::Semaphore->new(); |
| 665 | my $GlobalVariable :shared = 0; |
| 666 | |
| 667 | $thr1 = threads->create(\&sample_sub, 1); |
| 668 | $thr2 = threads->create(\&sample_sub, 2); |
| 669 | $thr3 = threads->create(\&sample_sub, 3); |
| 670 | |
| 671 | sub sample_sub { |
| 672 | my $SubNumber = shift(@_); |
| 673 | my $TryCount = 10; |
| 674 | my $LocalCopy; |
| 675 | sleep(1); |
| 676 | while ($TryCount--) { |
| 677 | $semaphore->down(); |
| 678 | $LocalCopy = $GlobalVariable; |
| 679 | print("$TryCount tries left for sub $SubNumber " |
| 680 | ."(\$GlobalVariable is $GlobalVariable)\n"); |
| 681 | sleep(2); |
| 682 | $LocalCopy++; |
| 683 | $GlobalVariable = $LocalCopy; |
| 684 | $semaphore->up(); |
| 685 | } |
| 686 | } |
| 687 | |
| 688 | $thr1->join(); |
| 689 | $thr2->join(); |
| 690 | $thr3->join(); |
| 691 | |
| 692 | The three invocations of the subroutine all operate in sync. The |
| 693 | semaphore, though, makes sure that only one thread is accessing the |
| 694 | global variable at once. |
| 695 | |
| 696 | =head2 Advanced Semaphores |
| 697 | |
| 698 | By default, semaphores behave like locks, letting only one thread |
| 699 | C<down()> them at a time. However, there are other uses for semaphores. |
| 700 | |
| 701 | Each semaphore has a counter attached to it. By default, semaphores are |
| 702 | created with the counter set to one, C<down()> decrements the counter by |
| 703 | one, and C<up()> increments by one. However, we can override any or all |
| 704 | of these defaults simply by passing in different values: |
| 705 | |
| 706 | use threads; |
| 707 | use Thread::Semaphore; |
| 708 | |
| 709 | my $semaphore = Thread::Semaphore->new(5); |
| 710 | # Creates a semaphore with the counter set to five |
| 711 | |
| 712 | my $thr1 = threads->create(\&sub1); |
| 713 | my $thr2 = threads->create(\&sub1); |
| 714 | |
| 715 | sub sub1 { |
| 716 | $semaphore->down(5); # Decrements the counter by five |
| 717 | # Do stuff here |
| 718 | $semaphore->up(5); # Increment the counter by five |
| 719 | } |
| 720 | |
| 721 | $thr1->detach(); |
| 722 | $thr2->detach(); |
| 723 | |
| 724 | If C<down()> attempts to decrement the counter below zero, it blocks until |
| 725 | the counter is large enough. Note that while a semaphore can be created |
| 726 | with a starting count of zero, any C<up()> or C<down()> always changes the |
| 727 | counter by at least one, and so C<< $semaphore->down(0) >> is the same as |
| 728 | C<< $semaphore->down(1) >>. |
| 729 | |
| 730 | The question, of course, is why would you do something like this? Why |
| 731 | create a semaphore with a starting count that's not one, or why |
| 732 | decrement or increment it by more than one? The answer is resource |
| 733 | availability. Many resources that you want to manage access for can be |
| 734 | safely used by more than one thread at once. |
| 735 | |
| 736 | For example, let's take a GUI driven program. It has a semaphore that |
| 737 | it uses to synchronize access to the display, so only one thread is |
| 738 | ever drawing at once. Handy, but of course you don't want any thread |
| 739 | to start drawing until things are properly set up. In this case, you |
| 740 | can create a semaphore with a counter set to zero, and up it when |
| 741 | things are ready for drawing. |
| 742 | |
| 743 | Semaphores with counters greater than one are also useful for |
| 744 | establishing quotas. Say, for example, that you have a number of |
| 745 | threads that can do I/O at once. You don't want all the threads |
| 746 | reading or writing at once though, since that can potentially swamp |
| 747 | your I/O channels, or deplete your process's quota of filehandles. You |
| 748 | can use a semaphore initialized to the number of concurrent I/O |
| 749 | requests (or open files) that you want at any one time, and have your |
| 750 | threads quietly block and unblock themselves. |
| 751 | |
| 752 | Larger increments or decrements are handy in those cases where a |
| 753 | thread needs to check out or return a number of resources at once. |
| 754 | |
| 755 | =head2 Waiting for a Condition |
| 756 | |
| 757 | The functions C<cond_wait()> and C<cond_signal()> |
| 758 | can be used in conjunction with locks to notify |
| 759 | co-operating threads that a resource has become available. They are |
| 760 | very similar in use to the functions found in C<pthreads>. However |
| 761 | for most purposes, queues are simpler to use and more intuitive. See |
| 762 | L<threads::shared> for more details. |
| 763 | |
| 764 | =head2 Giving up control |
| 765 | |
| 766 | There are times when you may find it useful to have a thread |
| 767 | explicitly give up the CPU to another thread. You may be doing something |
| 768 | processor-intensive and want to make sure that the user-interface thread |
| 769 | gets called frequently. Regardless, there are times that you might want |
| 770 | a thread to give up the processor. |
| 771 | |
| 772 | Perl's threading package provides the C<yield()> function that does |
| 773 | this. C<yield()> is pretty straightforward, and works like this: |
| 774 | |
| 775 | use threads; |
| 776 | |
| 777 | sub loop { |
| 778 | my $thread = shift; |
| 779 | my $foo = 50; |
| 780 | while($foo--) { print("In thread $thread\n"); } |
| 781 | threads->yield(); |
| 782 | $foo = 50; |
| 783 | while($foo--) { print("In thread $thread\n"); } |
| 784 | } |
| 785 | |
| 786 | my $thr1 = threads->create(\&loop, 'first'); |
| 787 | my $thr2 = threads->create(\&loop, 'second'); |
| 788 | my $thr3 = threads->create(\&loop, 'third'); |
| 789 | |
| 790 | It is important to remember that C<yield()> is only a hint to give up the CPU, |
| 791 | it depends on your hardware, OS and threading libraries what actually happens. |
| 792 | B<On many operating systems, yield() is a no-op.> Therefore it is important |
| 793 | to note that one should not build the scheduling of the threads around |
| 794 | C<yield()> calls. It might work on your platform but it won't work on another |
| 795 | platform. |
| 796 | |
| 797 | =head1 General Thread Utility Routines |
| 798 | |
| 799 | We've covered the workhorse parts of Perl's threading package, and |
| 800 | with these tools you should be well on your way to writing threaded |
| 801 | code and packages. There are a few useful little pieces that didn't |
| 802 | really fit in anyplace else. |
| 803 | |
| 804 | =head2 What Thread Am I In? |
| 805 | |
| 806 | The C<threads-E<gt>self()> class method provides your program with a way to |
| 807 | get an object representing the thread it's currently in. You can use this |
| 808 | object in the same way as the ones returned from thread creation. |
| 809 | |
| 810 | =head2 Thread IDs |
| 811 | |
| 812 | C<tid()> is a thread object method that returns the thread ID of the |
| 813 | thread the object represents. Thread IDs are integers, with the main |
| 814 | thread in a program being 0. Currently Perl assigns a unique TID to |
| 815 | every thread ever created in your program, assigning the first thread |
| 816 | to be created a TID of 1, and increasing the TID by 1 for each new |
| 817 | thread that's created. When used as a class method, C<threads-E<gt>tid()> |
| 818 | can be used by a thread to get its own TID. |
| 819 | |
| 820 | =head2 Are These Threads The Same? |
| 821 | |
| 822 | The C<equal()> method takes two thread objects and returns true |
| 823 | if the objects represent the same thread, and false if they don't. |
| 824 | |
| 825 | Thread objects also have an overloaded C<==> comparison so that you can do |
| 826 | comparison on them as you would with normal objects. |
| 827 | |
| 828 | =head2 What Threads Are Running? |
| 829 | |
| 830 | C<threads-E<gt>list()> returns a list of thread objects, one for each thread |
| 831 | that's currently running and not detached. Handy for a number of things, |
| 832 | including cleaning up at the end of your program (from the main Perl thread, |
| 833 | of course): |
| 834 | |
| 835 | # Loop through all the threads |
| 836 | foreach my $thr (threads->list()) { |
| 837 | $thr->join(); |
| 838 | } |
| 839 | |
| 840 | If some threads have not finished running when the main Perl thread |
| 841 | ends, Perl will warn you about it and die, since it is impossible for Perl |
| 842 | to clean up itself while other threads are running. |
| 843 | |
| 844 | NOTE: The main Perl thread (thread 0) is in a I<detached> state, and so |
| 845 | does not appear in the list returned by C<threads-E<gt>list()>. |
| 846 | |
| 847 | =head1 A Complete Example |
| 848 | |
| 849 | Confused yet? It's time for an example program to show some of the |
| 850 | things we've covered. This program finds prime numbers using threads. |
| 851 | |
| 852 | 1 #!/usr/bin/perl |
| 853 | 2 # prime-pthread, courtesy of Tom Christiansen |
| 854 | 3 |
| 855 | 4 use strict; |
| 856 | 5 use warnings; |
| 857 | 6 |
| 858 | 7 use threads; |
| 859 | 8 use Thread::Queue; |
| 860 | 9 |
| 861 | 10 sub check_num { |
| 862 | 11 my ($upstream, $cur_prime) = @_; |
| 863 | 12 my $kid; |
| 864 | 13 my $downstream = Thread::Queue->new(); |
| 865 | 14 while (my $num = $upstream->dequeue()) { |
| 866 | 15 next unless ($num % $cur_prime); |
| 867 | 16 if ($kid) { |
| 868 | 17 $downstream->enqueue($num); |
| 869 | 18 } else { |
| 870 | 19 print("Found prime: $num\n"); |
| 871 | 20 $kid = threads->create(\&check_num, $downstream, $num); |
| 872 | 21 if (! $kid) { |
| 873 | 22 warn("Sorry. Ran out of threads.\n"); |
| 874 | 23 last; |
| 875 | 24 } |
| 876 | 25 } |
| 877 | 26 } |
| 878 | 27 if ($kid) { |
| 879 | 28 $downstream->enqueue(undef); |
| 880 | 29 $kid->join(); |
| 881 | 30 } |
| 882 | 31 } |
| 883 | 32 |
| 884 | 33 my $stream = Thread::Queue->new(3..1000, undef); |
| 885 | 34 check_num($stream, 2); |
| 886 | |
| 887 | This program uses the pipeline model to generate prime numbers. Each |
| 888 | thread in the pipeline has an input queue that feeds numbers to be |
| 889 | checked, a prime number that it's responsible for, and an output queue |
| 890 | into which it funnels numbers that have failed the check. If the thread |
| 891 | has a number that's failed its check and there's no child thread, then |
| 892 | the thread must have found a new prime number. In that case, a new |
| 893 | child thread is created for that prime and stuck on the end of the |
| 894 | pipeline. |
| 895 | |
| 896 | This probably sounds a bit more confusing than it really is, so let's |
| 897 | go through this program piece by piece and see what it does. (For |
| 898 | those of you who might be trying to remember exactly what a prime |
| 899 | number is, it's a number that's only evenly divisible by itself and 1.) |
| 900 | |
| 901 | The bulk of the work is done by the C<check_num()> subroutine, which |
| 902 | takes a reference to its input queue and a prime number that it's |
| 903 | responsible for. After pulling in the input queue and the prime that |
| 904 | the subroutine is checking (line 11), we create a new queue (line 13) |
| 905 | and reserve a scalar for the thread that we're likely to create later |
| 906 | (line 12). |
| 907 | |
| 908 | The while loop from line 14 to line 26 grabs a scalar off the input |
| 909 | queue and checks against the prime this thread is responsible |
| 910 | for. Line 15 checks to see if there's a remainder when we divide the |
| 911 | number to be checked by our prime. If there is one, the number |
| 912 | must not be evenly divisible by our prime, so we need to either pass |
| 913 | it on to the next thread if we've created one (line 17) or create a |
| 914 | new thread if we haven't. |
| 915 | |
| 916 | The new thread creation is line 20. We pass on to it a reference to |
| 917 | the queue we've created, and the prime number we've found. In lines 21 |
| 918 | through 24, we check to make sure that our new thread got created, and |
| 919 | if not, we stop checking any remaining numbers in the queue. |
| 920 | |
| 921 | Finally, once the loop terminates (because we got a 0 or C<undef> in the |
| 922 | queue, which serves as a note to terminate), we pass on the notice to our |
| 923 | child, and wait for it to exit if we've created a child (lines 27 and |
| 924 | 30). |
| 925 | |
| 926 | Meanwhile, back in the main thread, we first create a queue (line 33) and |
| 927 | queue up all the numbers from 3 to 1000 for checking, plus a termination |
| 928 | notice. Then all we have to do to get the ball rolling is pass the queue |
| 929 | and the first prime to the C<check_num()> subroutine (line 34). |
| 930 | |
| 931 | That's how it works. It's pretty simple; as with many Perl programs, |
| 932 | the explanation is much longer than the program. |
| 933 | |
| 934 | =head1 Different implementations of threads |
| 935 | |
| 936 | Some background on thread implementations from the operating system |
| 937 | viewpoint. There are three basic categories of threads: user-mode threads, |
| 938 | kernel threads, and multiprocessor kernel threads. |
| 939 | |
| 940 | User-mode threads are threads that live entirely within a program and |
| 941 | its libraries. In this model, the OS knows nothing about threads. As |
| 942 | far as it's concerned, your process is just a process. |
| 943 | |
| 944 | This is the easiest way to implement threads, and the way most OSes |
| 945 | start. The big disadvantage is that, since the OS knows nothing about |
| 946 | threads, if one thread blocks they all do. Typical blocking activities |
| 947 | include most system calls, most I/O, and things like C<sleep()>. |
| 948 | |
| 949 | Kernel threads are the next step in thread evolution. The OS knows |
| 950 | about kernel threads, and makes allowances for them. The main |
| 951 | difference between a kernel thread and a user-mode thread is |
| 952 | blocking. With kernel threads, things that block a single thread don't |
| 953 | block other threads. This is not the case with user-mode threads, |
| 954 | where the kernel blocks at the process level and not the thread level. |
| 955 | |
| 956 | This is a big step forward, and can give a threaded program quite a |
| 957 | performance boost over non-threaded programs. Threads that block |
| 958 | performing I/O, for example, won't block threads that are doing other |
| 959 | things. Each process still has only one thread running at once, |
| 960 | though, regardless of how many CPUs a system might have. |
| 961 | |
| 962 | Since kernel threading can interrupt a thread at any time, they will |
| 963 | uncover some of the implicit locking assumptions you may make in your |
| 964 | program. For example, something as simple as C<$x = $x + 2> can behave |
| 965 | unpredictably with kernel threads if C<$x> is visible to other |
| 966 | threads, as another thread may have changed C<$x> between the time it |
| 967 | was fetched on the right hand side and the time the new value is |
| 968 | stored. |
| 969 | |
| 970 | Multiprocessor kernel threads are the final step in thread |
| 971 | support. With multiprocessor kernel threads on a machine with multiple |
| 972 | CPUs, the OS may schedule two or more threads to run simultaneously on |
| 973 | different CPUs. |
| 974 | |
| 975 | This can give a serious performance boost to your threaded program, |
| 976 | since more than one thread will be executing at the same time. As a |
| 977 | tradeoff, though, any of those nagging synchronization issues that |
| 978 | might not have shown with basic kernel threads will appear with a |
| 979 | vengeance. |
| 980 | |
| 981 | In addition to the different levels of OS involvement in threads, |
| 982 | different OSes (and different thread implementations for a particular |
| 983 | OS) allocate CPU cycles to threads in different ways. |
| 984 | |
| 985 | Cooperative multitasking systems have running threads give up control |
| 986 | if one of two things happen. If a thread calls a yield function, it |
| 987 | gives up control. It also gives up control if the thread does |
| 988 | something that would cause it to block, such as perform I/O. In a |
| 989 | cooperative multitasking implementation, one thread can starve all the |
| 990 | others for CPU time if it so chooses. |
| 991 | |
| 992 | Preemptive multitasking systems interrupt threads at regular intervals |
| 993 | while the system decides which thread should run next. In a preemptive |
| 994 | multitasking system, one thread usually won't monopolize the CPU. |
| 995 | |
| 996 | On some systems, there can be cooperative and preemptive threads |
| 997 | running simultaneously. (Threads running with realtime priorities |
| 998 | often behave cooperatively, for example, while threads running at |
| 999 | normal priorities behave preemptively.) |
| 1000 | |
| 1001 | Most modern operating systems support preemptive multitasking nowadays. |
| 1002 | |
| 1003 | =head1 Performance considerations |
| 1004 | |
| 1005 | The main thing to bear in mind when comparing Perl's I<ithreads> to other threading |
| 1006 | models is the fact that for each new thread created, a complete copy of |
| 1007 | all the variables and data of the parent thread has to be taken. Thus, |
| 1008 | thread creation can be quite expensive, both in terms of memory usage and |
| 1009 | time spent in creation. The ideal way to reduce these costs is to have a |
| 1010 | relatively short number of long-lived threads, all created fairly early |
| 1011 | on (before the base thread has accumulated too much data). Of course, this |
| 1012 | may not always be possible, so compromises have to be made. However, after |
| 1013 | a thread has been created, its performance and extra memory usage should |
| 1014 | be little different than ordinary code. |
| 1015 | |
| 1016 | Also note that under the current implementation, shared variables |
| 1017 | use a little more memory and are a little slower than ordinary variables. |
| 1018 | |
| 1019 | =head1 Process-scope Changes |
| 1020 | |
| 1021 | Note that while threads themselves are separate execution threads and |
| 1022 | Perl data is thread-private unless explicitly shared, the threads can |
| 1023 | affect process-scope state, affecting all the threads. |
| 1024 | |
| 1025 | The most common example of this is changing the current working |
| 1026 | directory using C<chdir()>. One thread calls C<chdir()>, and the working |
| 1027 | directory of all the threads changes. |
| 1028 | |
| 1029 | Even more drastic example of a process-scope change is C<chroot()>: |
| 1030 | the root directory of all the threads changes, and no thread can |
| 1031 | undo it (as opposed to C<chdir()>). |
| 1032 | |
| 1033 | Further examples of process-scope changes include C<umask()> and |
| 1034 | changing uids and gids. |
| 1035 | |
| 1036 | Thinking of mixing C<fork()> and threads? Please lie down and wait |
| 1037 | until the feeling passes. Be aware that the semantics of C<fork()> vary |
| 1038 | between platforms. For example, some Unix systems copy all the current |
| 1039 | threads into the child process, while others only copy the thread that |
| 1040 | called C<fork()>. You have been warned! |
| 1041 | |
| 1042 | Similarly, mixing signals and threads may be problematic. |
| 1043 | Implementations are platform-dependent, and even the POSIX |
| 1044 | semantics may not be what you expect (and Perl doesn't even |
| 1045 | give you the full POSIX API). For example, there is no way to |
| 1046 | guarantee that a signal sent to a multi-threaded Perl application |
| 1047 | will get intercepted by any particular thread. (However, a recently |
| 1048 | added feature does provide the capability to send signals between |
| 1049 | threads. See L<threads/THREAD SIGNALLING> for more details.) |
| 1050 | |
| 1051 | =head1 Thread-Safety of System Libraries |
| 1052 | |
| 1053 | Whether various library calls are thread-safe is outside the control |
| 1054 | of Perl. Calls often suffering from not being thread-safe include: |
| 1055 | C<localtime()>, C<gmtime()>, functions fetching user, group and |
| 1056 | network information (such as C<getgrent()>, C<gethostent()>, |
| 1057 | C<getnetent()> and so on), C<readdir()>, C<rand()>, and C<srand()>. In |
| 1058 | general, calls that depend on some global external state. |
| 1059 | |
| 1060 | If the system Perl is compiled in has thread-safe variants of such |
| 1061 | calls, they will be used. Beyond that, Perl is at the mercy of |
| 1062 | the thread-safety or -unsafety of the calls. Please consult your |
| 1063 | C library call documentation. |
| 1064 | |
| 1065 | On some platforms the thread-safe library interfaces may fail if the |
| 1066 | result buffer is too small (for example the user group databases may |
| 1067 | be rather large, and the reentrant interfaces may have to carry around |
| 1068 | a full snapshot of those databases). Perl will start with a small |
| 1069 | buffer, but keep retrying and growing the result buffer |
| 1070 | until the result fits. If this limitless growing sounds bad for |
| 1071 | security or memory consumption reasons you can recompile Perl with |
| 1072 | C<PERL_REENTRANT_MAXSIZE> defined to the maximum number of bytes you will |
| 1073 | allow. |
| 1074 | |
| 1075 | =head1 Conclusion |
| 1076 | |
| 1077 | A complete thread tutorial could fill a book (and has, many times), |
| 1078 | but with what we've covered in this introduction, you should be well |
| 1079 | on your way to becoming a threaded Perl expert. |
| 1080 | |
| 1081 | =head1 SEE ALSO |
| 1082 | |
| 1083 | Annotated POD for L<threads>: |
| 1084 | L<http://annocpan.org/?mode=search&field=Module&name=threads> |
| 1085 | |
| 1086 | Latest version of L<threads> on CPAN: |
| 1087 | L<http://search.cpan.org/search?module=threads> |
| 1088 | |
| 1089 | Annotated POD for L<threads::shared>: |
| 1090 | L<http://annocpan.org/?mode=search&field=Module&name=threads%3A%3Ashared> |
| 1091 | |
| 1092 | Latest version of L<threads::shared> on CPAN: |
| 1093 | L<http://search.cpan.org/search?module=threads%3A%3Ashared> |
| 1094 | |
| 1095 | Perl threads mailing list: |
| 1096 | L<http://lists.perl.org/list/ithreads.html> |
| 1097 | |
| 1098 | =head1 Bibliography |
| 1099 | |
| 1100 | Here's a short bibliography courtesy of Jürgen Christoffel: |
| 1101 | |
| 1102 | =head2 Introductory Texts |
| 1103 | |
| 1104 | Birrell, Andrew D. An Introduction to Programming with |
| 1105 | Threads. Digital Equipment Corporation, 1989, DEC-SRC Research Report |
| 1106 | #35 online as |
| 1107 | L<ftp://ftp.dec.com/pub/DEC/SRC/research-reports/SRC-035.pdf> |
| 1108 | (highly recommended) |
| 1109 | |
| 1110 | Robbins, Kay. A., and Steven Robbins. Practical Unix Programming: A |
| 1111 | Guide to Concurrency, Communication, and |
| 1112 | Multithreading. Prentice-Hall, 1996. |
| 1113 | |
| 1114 | Lewis, Bill, and Daniel J. Berg. Multithreaded Programming with |
| 1115 | Pthreads. Prentice Hall, 1997, ISBN 0-13-443698-9 (a well-written |
| 1116 | introduction to threads). |
| 1117 | |
| 1118 | Nelson, Greg (editor). Systems Programming with Modula-3. Prentice |
| 1119 | Hall, 1991, ISBN 0-13-590464-1. |
| 1120 | |
| 1121 | Nichols, Bradford, Dick Buttlar, and Jacqueline Proulx Farrell. |
| 1122 | Pthreads Programming. O'Reilly & Associates, 1996, ISBN 156592-115-1 |
| 1123 | (covers POSIX threads). |
| 1124 | |
| 1125 | =head2 OS-Related References |
| 1126 | |
| 1127 | Boykin, Joseph, David Kirschen, Alan Langerman, and Susan |
| 1128 | LoVerso. Programming under Mach. Addison-Wesley, 1994, ISBN |
| 1129 | 0-201-52739-1. |
| 1130 | |
| 1131 | Tanenbaum, Andrew S. Distributed Operating Systems. Prentice Hall, |
| 1132 | 1995, ISBN 0-13-219908-4 (great textbook). |
| 1133 | |
| 1134 | Silberschatz, Abraham, and Peter B. Galvin. Operating System Concepts, |
| 1135 | 4th ed. Addison-Wesley, 1995, ISBN 0-201-59292-4 |
| 1136 | |
| 1137 | =head2 Other References |
| 1138 | |
| 1139 | Arnold, Ken and James Gosling. The Java Programming Language, 2nd |
| 1140 | ed. Addison-Wesley, 1998, ISBN 0-201-31006-6. |
| 1141 | |
| 1142 | comp.programming.threads FAQ, |
| 1143 | L<http://www.serpentine.com/~bos/threads-faq/> |
| 1144 | |
| 1145 | Le Sergent, T. and B. Berthomieu. "Incremental MultiThreaded Garbage |
| 1146 | Collection on Virtually Shared Memory Architectures" in Memory |
| 1147 | Management: Proc. of the International Workshop IWMM 92, St. Malo, |
| 1148 | France, September 1992, Yves Bekkers and Jacques Cohen, eds. Springer, |
| 1149 | 1992, ISBN 3540-55940-X (real-life thread applications). |
| 1150 | |
| 1151 | Artur Bergman, "Where Wizards Fear To Tread", June 11, 2002, |
| 1152 | L<http://www.perl.com/pub/a/2002/06/11/threads.html> |
| 1153 | |
| 1154 | =head1 Acknowledgements |
| 1155 | |
| 1156 | Thanks (in no particular order) to Chaim Frenkel, Steve Fink, Gurusamy |
| 1157 | Sarathy, Ilya Zakharevich, Benjamin Sugars, Jürgen Christoffel, Joshua |
| 1158 | Pritikin, and Alan Burlison, for their help in reality-checking and |
| 1159 | polishing this article. Big thanks to Tom Christiansen for his rewrite |
| 1160 | of the prime number generator. |
| 1161 | |
| 1162 | =head1 AUTHOR |
| 1163 | |
| 1164 | Dan Sugalski E<lt>dan@sidhe.org<gt> |
| 1165 | |
| 1166 | Slightly modified by Arthur Bergman to fit the new thread model/module. |
| 1167 | |
| 1168 | Reworked slightly by Jörg Walter E<lt>jwalt@cpan.org<gt> to be more concise |
| 1169 | about thread-safety of Perl code. |
| 1170 | |
| 1171 | Rearranged slightly by Elizabeth Mattijsen E<lt>liz@dijkmat.nl<gt> to put |
| 1172 | less emphasis on yield(). |
| 1173 | |
| 1174 | =head1 Copyrights |
| 1175 | |
| 1176 | The original version of this article originally appeared in The Perl |
| 1177 | Journal #10, and is copyright 1998 The Perl Journal. It appears courtesy |
| 1178 | of Jon Orwant and The Perl Journal. This document may be distributed |
| 1179 | under the same terms as Perl itself. |
| 1180 | |
| 1181 | =cut |