threads::shared(3pm)

1threads::shared(3pm)   Perl Programmers Reference Guide   threads::shared(3pm)
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NAME

6       threads::shared - Perl extension for sharing data structures between
7       threads
8

VERSION

10       This document describes threads::shared version 1.29
11

SYNOPSIS

13         use threads;
14         use threads::shared;
15
16         my $var :shared;
17         my %hsh :shared;
18         my @ary :shared;
19
20         my ($scalar, @array, %hash);
21         share($scalar);
22         share(@array);
23         share(%hash);
24
25         $var = $scalar_value;
26         $var = $shared_ref_value;
27         $var = shared_clone($non_shared_ref_value);
28         $var = shared_clone({'foo' => [qw/foo bar baz/]});
29
30         $hsh{'foo'} = $scalar_value;
31         $hsh{'bar'} = $shared_ref_value;
32         $hsh{'baz'} = shared_clone($non_shared_ref_value);
33         $hsh{'quz'} = shared_clone([1..3]);
34
35         $ary[0] = $scalar_value;
36         $ary[1] = $shared_ref_value;
37         $ary[2] = shared_clone($non_shared_ref_value);
38         $ary[3] = shared_clone([ {}, [] ]);
39
40         { lock(%hash); ...  }
41
42         cond_wait($scalar);
43         cond_timedwait($scalar, time() + 30);
44         cond_broadcast(@array);
45         cond_signal(%hash);
46
47         my $lockvar :shared;
48         # condition var != lock var
49         cond_wait($var, $lockvar);
50         cond_timedwait($var, time()+30, $lockvar);
51

DESCRIPTION

53       By default, variables are private to each thread, and each newly
54       created thread gets a private copy of each existing variable.  This
55       module allows you to share variables across different threads (and
56       pseudo-forks on Win32).  It is used together with the threads module.
57
58       This module supports the sharing of the following data types only:
59       scalars and scalar refs, arrays and array refs, and hashes and hash
60       refs.
61

EXPORT

63       The following functions are exported by this module: "share",
64       "shared_clone", "is_shared", "cond_wait", "cond_timedwait",
65       "cond_signal" and "cond_broadcast"
66
67       Note that if this module is imported when threads has not yet been
68       loaded, then these functions all become no-ops.  This makes it possible
69       to write modules that will work in both threaded and non-threaded
70       environments.
71

FUNCTIONS

73       share VARIABLE
74           "share" takes a variable and marks it as shared:
75
76             my ($scalar, @array, %hash);
77             share($scalar);
78             share(@array);
79             share(%hash);
80
81           "share" will return the shared rvalue, but always as a reference.
82
83           Variables can also be marked as shared at compile time by using the
84           ":shared" attribute:
85
86             my ($var, %hash, @array) :shared;
87
88           Shared variables can only store scalars, refs of shared variables,
89           or refs of shared data (discussed in next section):
90
91             my ($var, %hash, @array) :shared;
92             my $bork;
93
94             # Storing scalars
95             $var = 1;
96             $hash{'foo'} = 'bar';
97             $array[0] = 1.5;
98
99             # Storing shared refs
100             $var = \%hash;
101             $hash{'ary'} = \@array;
102             $array[1] = \$var;
103
104             # The following are errors:
105             #   $var = \$bork;                    # ref of non-shared variable
106             #   $hash{'bork'} = [];               # non-shared array ref
107             #   push(@array, { 'x' => 1 });       # non-shared hash ref
108
109       shared_clone REF
110           "shared_clone" takes a reference, and returns a shared version of
111           its argument, performing a deep copy on any non-shared elements.
112           Any shared elements in the argument are used as is (i.e., they are
113           not cloned).
114
115             my $cpy = shared_clone({'foo' => [qw/foo bar baz/]});
116
117           Object status (i.e., the class an object is blessed into) is also
118           cloned.
119
120             my $obj = {'foo' => [qw/foo bar baz/]};
121             bless($obj, 'Foo');
122             my $cpy = shared_clone($obj);
123             print(ref($cpy), "\n");         # Outputs 'Foo'
124
125           For cloning empty array or hash refs, the following may also be
126           used:
127
128             $var = &share([]);   # Same as $var = shared_clone([]);
129             $var = &share({});   # Same as $var = shared_clone({});
130
131       is_shared VARIABLE
132           "is_shared" checks if the specified variable is shared or not.  If
133           shared, returns the variable's internal ID (similar to refaddr()).
134           Otherwise, returns "undef".
135
136             if (is_shared($var)) {
137                 print("\$var is shared\n");
138             } else {
139                 print("\$var is not shared\n");
140             }
141
142           When used on an element of an array or hash, "is_shared" checks if
143           the specified element belongs to a shared array or hash.  (It does
144           not check the contents of that element.)
145
146             my %hash :shared;
147             if (is_shared(%hash)) {
148                 print("\%hash is shared\n");
149             }
150
151             $hash{'elem'} = 1;
152             if (is_shared($hash{'elem'})) {
153                 print("\$hash{'elem'} is in a shared hash\n");
154             }
155
156       lock VARIABLE
157           "lock" places a advisory lock on a variable until the lock goes out
158           of scope.  If the variable is locked by another thread, the "lock"
159           call will block until it's available.  Multiple calls to "lock" by
160           the same thread from within dynamically nested scopes are safe --
161           the variable will remain locked until the outermost lock on the
162           variable goes out of scope.
163
164           "lock" follows references exactly one level:
165
166             my %hash :shared;
167             my $ref = \%hash;
168             lock($ref);           # This is equivalent to lock(%hash)
169
170           Note that you cannot explicitly unlock a variable; you can only
171           wait for the lock to go out of scope.  This is most easily
172           accomplished by locking the variable inside a block.
173
174             my $var :shared;
175             {
176                 lock($var);
177                 # $var is locked from here to the end of the block
178                 ...
179             }
180             # $var is now unlocked
181
182           As locks are advisory, they do not prevent data access or
183           modification by another thread that does not itself attempt to
184           obtain a lock on the variable.
185
186           You cannot lock the individual elements of a container variable:
187
188             my %hash :shared;
189             $hash{'foo'} = 'bar';
190             #lock($hash{'foo'});          # Error
191             lock(%hash);                  # Works
192
193           If you need more fine-grained control over shared variable access,
194           see Thread::Semaphore.
195
196       cond_wait VARIABLE
197       cond_wait CONDVAR, LOCKVAR
198           The "cond_wait" function takes a locked variable as a parameter,
199           unlocks the variable, and blocks until another thread does a
200           "cond_signal" or "cond_broadcast" for that same locked variable.
201           The variable that "cond_wait" blocked on is relocked after the
202           "cond_wait" is satisfied.  If there are multiple threads
203           "cond_wait"ing on the same variable, all but one will re-block
204           waiting to reacquire the lock on the variable. (So if you're only
205           using "cond_wait" for synchronisation, give up the lock as soon as
206           possible).  The two actions of unlocking the variable and entering
207           the blocked wait state are atomic, the two actions of exiting from
208           the blocked wait state and re-locking the variable are not.
209
210           In its second form, "cond_wait" takes a shared, unlocked variable
211           followed by a shared, locked variable.  The second variable is
212           unlocked and thread execution suspended until another thread
213           signals the first variable.
214
215           It is important to note that the variable can be notified even if
216           no thread "cond_signal" or "cond_broadcast" on the variable.  It is
217           therefore important to check the value of the variable and go back
218           to waiting if the requirement is not fulfilled.  For example, to
219           pause until a shared counter drops to zero:
220
221             { lock($counter); cond_wait($counter) until $counter == 0; }
222
223       cond_timedwait VARIABLE, ABS_TIMEOUT
224       cond_timedwait CONDVAR, ABS_TIMEOUT, LOCKVAR
225           In its two-argument form, "cond_timedwait" takes a locked variable
226           and an absolute timeout as parameters, unlocks the variable, and
227           blocks until the timeout is reached or another thread signals the
228           variable.  A false value is returned if the timeout is reached, and
229           a true value otherwise.  In either case, the variable is re-locked
230           upon return.
231
232           Like "cond_wait", this function may take a shared, locked variable
233           as an additional parameter; in this case the first parameter is an
234           unlocked condition variable protected by a distinct lock variable.
235
236           Again like "cond_wait", waking up and reacquiring the lock are not
237           atomic, and you should always check your desired condition after
238           this function returns.  Since the timeout is an absolute value,
239           however, it does not have to be recalculated with each pass:
240
241             lock($var);
242             my $abs = time() + 15;
243             until ($ok = desired_condition($var)) {
244                 last if !cond_timedwait($var, $abs);
245             }
246             # we got it if $ok, otherwise we timed out!
247
248       cond_signal VARIABLE
249           The "cond_signal" function takes a locked variable as a parameter
250           and unblocks one thread that's "cond_wait"ing on that variable. If
251           more than one thread is blocked in a "cond_wait" on that variable,
252           only one (and which one is indeterminate) will be unblocked.
253
254           If there are no threads blocked in a "cond_wait" on the variable,
255           the signal is discarded. By always locking before signaling, you
256           can (with care), avoid signaling before another thread has entered
257           cond_wait().
258
259           "cond_signal" will normally generate a warning if you attempt to
260           use it on an unlocked variable. On the rare occasions where doing
261           this may be sensible, you can suppress the warning with:
262
263             { no warnings 'threads'; cond_signal($foo); }
264
265       cond_broadcast VARIABLE
266           The "cond_broadcast" function works similarly to "cond_signal".
267           "cond_broadcast", though, will unblock all the threads that are
268           blocked in a "cond_wait" on the locked variable, rather than only
269           one.
270

OBJECTS

272       threads::shared exports a version of bless() that works on shared
273       objects such that blessings propagate across threads.
274
275         # Create a shared 'Foo' object
276         my $foo :shared = shared_clone({});
277         bless($foo, 'Foo');
278
279         # Create a shared 'Bar' object
280         my $bar :shared = shared_clone({});
281         bless($bar, 'Bar');
282
283         # Put 'bar' inside 'foo'
284         $foo->{'bar'} = $bar;
285
286         # Rebless the objects via a thread
287         threads->create(sub {
288             # Rebless the outer object
289             bless($foo, 'Yin');
290
291             # Cannot directly rebless the inner object
292             #bless($foo->{'bar'}, 'Yang');
293
294             # Retrieve and rebless the inner object
295             my $obj = $foo->{'bar'};
296             bless($obj, 'Yang');
297             $foo->{'bar'} = $obj;
298
299         })->join();
300
301         print(ref($foo),          "\n");    # Prints 'Yin'
302         print(ref($foo->{'bar'}), "\n");    # Prints 'Yang'
303         print(ref($bar),          "\n");    # Also prints 'Yang'
304

NOTES

306       threads::shared is designed to disable itself silently if threads are
307       not available.  This allows you to write modules and packages that can
308       be used in both threaded and non-threaded applications.
309
310       If you want access to threads, you must "use threads" before you "use
311       threads::shared".  threads will emit a warning if you use it after
312       threads::shared.
313

BUGS AND LIMITATIONS

315       When "share" is used on arrays, hashes, array refs or hash refs, any
316       data they contain will be lost.
317
318         my @arr = qw(foo bar baz);
319         share(@arr);
320         # @arr is now empty (i.e., == ());
321
322         # Create a 'foo' object
323         my $foo = { 'data' => 99 };
324         bless($foo, 'foo');
325
326         # Share the object
327         share($foo);        # Contents are now wiped out
328         print("ERROR: \$foo is empty\n")
329             if (! exists($foo->{'data'}));
330
331       Therefore, populate such variables after declaring them as shared.
332       (Scalar and scalar refs are not affected by this problem.)
333
334       It is often not wise to share an object unless the class itself has
335       been written to support sharing.  For example, an object's destructor
336       may get called multiple times, once for each thread's scope exit.
337       Another danger is that the contents of hash-based objects will be lost
338       due to the above mentioned limitation.  See examples/class.pl (in the
339       CPAN distribution of this module) for how to create a class that
340       supports object sharing.
341
342       Does not support "splice" on arrays!
343
344       Taking references to the elements of shared arrays and hashes does not
345       autovivify the elements, and neither does slicing a shared array/hash
346       over non-existent indices/keys autovivify the elements.
347
348       "share()" allows you to "share($hashref->{key})" and
349       "share($arrayref->[idx])" without giving any error message.  But the
350       "$hashref->{key}" or "$arrayref->[idx]" is not shared, causing the
351       error "lock can only be used on shared values" to occur when you
352       attempt to "lock($hasref->{key})" or "lock($arrayref->[idx])" in
353       another thread.
354
355       Using refaddr()) is unreliable for testing whether or not two shared
356       references are equivalent (e.g., when testing for circular references).
357       Use "is_shared VARIABLE" in is_shared(), instead:
358
359           use threads;
360           use threads::shared;
361           use Scalar::Util qw(refaddr);
362
363           # If ref is shared, use threads::shared's internal ID.
364           # Otherwise, use refaddr().
365           my $addr1 = is_shared($ref1) || refaddr($ref1);
366           my $addr2 = is_shared($ref2) || refaddr($ref2);
367
368           if ($addr1 == $addr2) {
369               # The refs are equivalent
370           }
371
372       each() does not work properly on shared references embedded in shared
373       structures.  For example:
374
375           my %foo :shared;
376           $foo{'bar'} = shared_clone({'a'=>'x', 'b'=>'y', 'c'=>'z'});
377
378           while (my ($key, $val) = each(%{$foo{'bar'}})) {
379               ...
380           }
381
382       Either of the following will work instead:
383
384           my $ref = $foo{'bar'};
385           while (my ($key, $val) = each(%{$ref})) {
386               ...
387           }
388
389           foreach my $key (keys(%{$foo{'bar'}})) {
390               my $val = $foo{'bar'}{$key};
391               ...
392           }
393
394       View existing bug reports at, and submit any new bugs, problems,
395       patches, etc.  to:
396       <http://rt.cpan.org/Public/Dist/Display.html?Name=threads-shared>
397

AUTHOR

416       Artur Bergman <sky AT crucially DOT net>
417
418       Documentation borrowed from the old Thread.pm.
419
420       CPAN version produced by Jerry D. Hedden <jdhedden AT cpan DOT org>.
421

LICENSE

423       threads::shared is released under the same license as Perl.
424
425
426
427perl v5.10.1                      2009-06-23              threads::shared(3pm)