1threads::shared(3)    User Contributed Perl Documentation   threads::shared(3)
2
3
4

NAME

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

VERSION

10       This document describes threads::shared version 1.60
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           Not all Perl data types can be cloned (e.g., globs, code refs).  By
132           default, "shared_clone" will croak if it encounters such items.  To
133           change this behaviour to a warning, then set the following:
134
135             $threads::shared::clone_warn = 1;
136
137           In this case, "undef" will be substituted for the item to be
138           cloned.  If set to zero:
139
140             $threads::shared::clone_warn = 0;
141
142           then the "undef" substitution will be performed silently.
143
144       is_shared VARIABLE
145           "is_shared" checks if the specified variable is shared or not.  If
146           shared, returns the variable's internal ID (similar to "refaddr()"
147           (see Scalar::Util).  Otherwise, returns "undef".
148
149             if (is_shared($var)) {
150                 print("\$var is shared\n");
151             } else {
152                 print("\$var is not shared\n");
153             }
154
155           When used on an element of an array or hash, "is_shared" checks if
156           the specified element belongs to a shared array or hash.  (It does
157           not check the contents of that element.)
158
159             my %hash :shared;
160             if (is_shared(%hash)) {
161                 print("\%hash is shared\n");
162             }
163
164             $hash{'elem'} = 1;
165             if (is_shared($hash{'elem'})) {
166                 print("\$hash{'elem'} is in a shared hash\n");
167             }
168
169       lock VARIABLE
170           "lock" places a advisory lock on a variable until the lock goes out
171           of scope.  If the variable is locked by another thread, the "lock"
172           call will block until it's available.  Multiple calls to "lock" by
173           the same thread from within dynamically nested scopes are safe --
174           the variable will remain locked until the outermost lock on the
175           variable goes out of scope.
176
177           "lock" follows references exactly one level:
178
179             my %hash :shared;
180             my $ref = \%hash;
181             lock($ref);           # This is equivalent to lock(%hash)
182
183           Note that you cannot explicitly unlock a variable; you can only
184           wait for the lock to go out of scope.  This is most easily
185           accomplished by locking the variable inside a block.
186
187             my $var :shared;
188             {
189                 lock($var);
190                 # $var is locked from here to the end of the block
191                 ...
192             }
193             # $var is now unlocked
194
195           As locks are advisory, they do not prevent data access or
196           modification by another thread that does not itself attempt to
197           obtain a lock on the variable.
198
199           You cannot lock the individual elements of a container variable:
200
201             my %hash :shared;
202             $hash{'foo'} = 'bar';
203             #lock($hash{'foo'});          # Error
204             lock(%hash);                  # Works
205
206           If you need more fine-grained control over shared variable access,
207           see Thread::Semaphore.
208
209       cond_wait VARIABLE
210       cond_wait CONDVAR, LOCKVAR
211           The "cond_wait" function takes a locked variable as a parameter,
212           unlocks the variable, and blocks until another thread does a
213           "cond_signal" or "cond_broadcast" for that same locked variable.
214           The variable that "cond_wait" blocked on is re-locked after the
215           "cond_wait" is satisfied.  If there are multiple threads
216           "cond_wait"ing on the same variable, all but one will re-block
217           waiting to reacquire the lock on the variable.  (So if you're only
218           using "cond_wait" for synchronization, give up the lock as soon as
219           possible).  The two actions of unlocking the variable and entering
220           the blocked wait state are atomic, the two actions of exiting from
221           the blocked wait state and re-locking the variable are not.
222
223           In its second form, "cond_wait" takes a shared, unlocked variable
224           followed by a shared, locked variable.  The second variable is
225           unlocked and thread execution suspended until another thread
226           signals the first variable.
227
228           It is important to note that the variable can be notified even if
229           no thread "cond_signal" or "cond_broadcast" on the variable.  It is
230           therefore important to check the value of the variable and go back
231           to waiting if the requirement is not fulfilled.  For example, to
232           pause until a shared counter drops to zero:
233
234             { lock($counter); cond_wait($counter) until $counter == 0; }
235
236       cond_timedwait VARIABLE, ABS_TIMEOUT
237       cond_timedwait CONDVAR, ABS_TIMEOUT, LOCKVAR
238           In its two-argument form, "cond_timedwait" takes a locked variable
239           and an absolute timeout in epoch seconds (see time() in perlfunc
240           for more) as parameters, unlocks the variable, and blocks until the
241           timeout is reached or another thread signals the variable.  A false
242           value is returned if the timeout is reached, and a true value
243           otherwise.  In either case, the variable is re-locked upon return.
244
245           Like "cond_wait", this function may take a shared, locked variable
246           as an additional parameter; in this case the first parameter is an
247           unlocked condition variable protected by a distinct lock variable.
248
249           Again like "cond_wait", waking up and reacquiring the lock are not
250           atomic, and you should always check your desired condition after
251           this function returns.  Since the timeout is an absolute value,
252           however, it does not have to be recalculated with each pass:
253
254             lock($var);
255             my $abs = time() + 15;
256             until ($ok = desired_condition($var)) {
257                 last if !cond_timedwait($var, $abs);
258             }
259             # we got it if $ok, otherwise we timed out!
260
261       cond_signal VARIABLE
262           The "cond_signal" function takes a locked variable as a parameter
263           and unblocks one thread that's "cond_wait"ing on that variable.  If
264           more than one thread is blocked in a "cond_wait" on that variable,
265           only one (and which one is indeterminate) will be unblocked.
266
267           If there are no threads blocked in a "cond_wait" on the variable,
268           the signal is discarded.  By always locking before signaling, you
269           can (with care), avoid signaling before another thread has entered
270           cond_wait().
271
272           "cond_signal" will normally generate a warning if you attempt to
273           use it on an unlocked variable.  On the rare occasions where doing
274           this may be sensible, you can suppress the warning with:
275
276             { no warnings 'threads'; cond_signal($foo); }
277
278       cond_broadcast VARIABLE
279           The "cond_broadcast" function works similarly to "cond_signal".
280           "cond_broadcast", though, will unblock all the threads that are
281           blocked in a "cond_wait" on the locked variable, rather than only
282           one.
283

OBJECTS

285       threads::shared exports a version of bless() that works on shared
286       objects such that blessings propagate across threads.
287
288         # Create a shared 'Foo' object
289         my $foo :shared = shared_clone({});
290         bless($foo, 'Foo');
291
292         # Create a shared 'Bar' object
293         my $bar :shared = shared_clone({});
294         bless($bar, 'Bar');
295
296         # Put 'bar' inside 'foo'
297         $foo->{'bar'} = $bar;
298
299         # Rebless the objects via a thread
300         threads->create(sub {
301             # Rebless the outer object
302             bless($foo, 'Yin');
303
304             # Cannot directly rebless the inner object
305             #bless($foo->{'bar'}, 'Yang');
306
307             # Retrieve and rebless the inner object
308             my $obj = $foo->{'bar'};
309             bless($obj, 'Yang');
310             $foo->{'bar'} = $obj;
311
312         })->join();
313
314         print(ref($foo),          "\n");    # Prints 'Yin'
315         print(ref($foo->{'bar'}), "\n");    # Prints 'Yang'
316         print(ref($bar),          "\n");    # Also prints 'Yang'
317

NOTES

319       threads::shared is designed to disable itself silently if threads are
320       not available.  This allows you to write modules and packages that can
321       be used in both threaded and non-threaded applications.
322
323       If you want access to threads, you must "use threads" before you "use
324       threads::shared".  threads will emit a warning if you use it after
325       threads::shared.
326

WARNINGS

328       cond_broadcast() called on unlocked variable
329       cond_signal() called on unlocked variable
330           See "cond_signal VARIABLE", above.
331

BUGS AND LIMITATIONS

333       When "share" is used on arrays, hashes, array refs or hash refs, any
334       data they contain will be lost.
335
336         my @arr = qw(foo bar baz);
337         share(@arr);
338         # @arr is now empty (i.e., == ());
339
340         # Create a 'foo' object
341         my $foo = { 'data' => 99 };
342         bless($foo, 'foo');
343
344         # Share the object
345         share($foo);        # Contents are now wiped out
346         print("ERROR: \$foo is empty\n")
347             if (! exists($foo->{'data'}));
348
349       Therefore, populate such variables after declaring them as shared.
350       (Scalar and scalar refs are not affected by this problem.)
351
352       Blessing a shared item after it has been nested in another shared item
353       does not propagate the blessing to the shared reference:
354
355         my $foo = &share({});
356         my $bar = &share({});
357         $bar->{foo} = $foo;
358         bless($foo, 'baz');   # $foo is now of class 'baz',
359                               # but $bar->{foo} is unblessed.
360
361       Therefore, you should bless objects before sharing them.
362
363       It is often not wise to share an object unless the class itself has
364       been written to support sharing.  For example, a shared object's
365       destructor may get called multiple times, once for each thread's scope
366       exit, or may not get called at all if it is embedded inside another
367       shared object.  Another issue is that the contents of hash-based
368       objects will be lost due to the above mentioned limitation.  See
369       examples/class.pl (in the CPAN distribution of this module) for how to
370       create a class that supports object sharing.
371
372       Destructors may not be called on objects if those objects still exist
373       at global destruction time.  If the destructors must be called, make
374       sure there are no circular references and that nothing is referencing
375       the objects before the program ends.
376
377       Does not support "splice" on arrays.  Does not support explicitly
378       changing array lengths via $#array -- use "push" and "pop" instead.
379
380       Taking references to the elements of shared arrays and hashes does not
381       autovivify the elements, and neither does slicing a shared array/hash
382       over non-existent indices/keys autovivify the elements.
383
384       "share()" allows you to "share($hashref->{key})" and
385       "share($arrayref->[idx])" without giving any error message.  But the
386       "$hashref->{key}" or "$arrayref->[idx]" is not shared, causing the
387       error "lock can only be used on shared values" to occur when you
388       attempt to "lock($hashref->{key})" or "lock($arrayref->[idx])" in
389       another thread.
390
391       Using "refaddr()" is unreliable for testing whether or not two shared
392       references are equivalent (e.g., when testing for circular references).
393       Use is_shared(), instead:
394
395           use threads;
396           use threads::shared;
397           use Scalar::Util qw(refaddr);
398
399           # If ref is shared, use threads::shared's internal ID.
400           # Otherwise, use refaddr().
401           my $addr1 = is_shared($ref1) || refaddr($ref1);
402           my $addr2 = is_shared($ref2) || refaddr($ref2);
403
404           if ($addr1 == $addr2) {
405               # The refs are equivalent
406           }
407
408       each() does not work properly on shared references embedded in shared
409       structures.  For example:
410
411           my %foo :shared;
412           $foo{'bar'} = shared_clone({'a'=>'x', 'b'=>'y', 'c'=>'z'});
413
414           while (my ($key, $val) = each(%{$foo{'bar'}})) {
415               ...
416           }
417
418       Either of the following will work instead:
419
420           my $ref = $foo{'bar'};
421           while (my ($key, $val) = each(%{$ref})) {
422               ...
423           }
424
425           foreach my $key (keys(%{$foo{'bar'}})) {
426               my $val = $foo{'bar'}{$key};
427               ...
428           }
429
430       This module supports dual-valued variables created using "dualvar()"
431       from Scalar::Util.  However, while $! acts like a dualvar, it is
432       implemented as a tied SV.  To propagate its value, use the follow
433       construct, if needed:
434
435           my $errno :shared = dualvar($!,$!);
436
437       View existing bug reports at, and submit any new bugs, problems,
438       patches, etc.  to:
439       <http://rt.cpan.org/Public/Dist/Display.html?Name=threads-shared>
440

SEE ALSO

442       threads::shared on MetaCPAN:
443       <https://metacpan.org/release/threads-shared>
444
445       Code repository for CPAN distribution:
446       <https://github.com/Dual-Life/threads-shared>
447
448       threads, perlthrtut
449
450       <http://www.perl.com/pub/a/2002/06/11/threads.html> and
451       <http://www.perl.com/pub/a/2002/09/04/threads.html>
452
453       Perl threads mailing list: <http://lists.perl.org/list/ithreads.html>
454
455       Sample code in the examples directory of this distribution on CPAN.
456

AUTHOR

458       Artur Bergman <sky AT crucially DOT net>
459
460       Documentation borrowed from the old Thread.pm.
461
462       CPAN version produced by Jerry D. Hedden <jdhedden AT cpan DOT org>.
463

LICENSE

465       threads::shared is released under the same license as Perl.
466
467
468
469perl v5.30.1                      2020-01-30                threads::shared(3)
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