1threads::shared(3pm) Perl Programmers Reference Guide threads::shared(3pm)
2
6 threads::shared - Perl extension for sharing data structures between
7 threads
8
10 use threads;
11 use threads::shared;
12 my $var : shared;
14 $var = $scalar_value;
15 $var = $shared_ref_value;
16 $var = &share($simple_unshared_ref_value);
17 $var = &share(new Foo);
18 my($scalar, @array, %hash);
20 share($scalar);
21 share(@array);
22 share(%hash);
23 my $bar = &share([]);
24 $hash{bar} = &share({});
25 { lock(%hash); ... }
27 cond_wait($scalar);
29 cond_timedwait($scalar, time() + 30);
30 cond_broadcast(@array);
31 cond_signal(%hash);
32 my $lockvar : shared;
34 # condition var != lock var
35 cond_wait($var, $lockvar);
36 cond_timedwait($var, time()+30, $lockvar);
37
39 By default, variables are private to each thread, and each newly cre‐
40 ated thread gets a private copy of each existing variable. This module
41 allows you to share variables across different threads (and pseudoforks
42 on Win32). It is used together with the threads module.
43
45 "share", "cond_wait", "cond_timedwait", "cond_signal", "cond_broadcast"
46 Note that if this module is imported when "threads" has not yet been
48 loaded, then these functions all become no-ops. This makes it possible
49 to write modules that will work in both threaded and non-threaded envi‐
50 ronments.
51
53 share VARIABLE
54 "share" takes a value and marks it as shared. You can share a
55 scalar, array, hash, scalar ref, array ref or hash ref. "share"
56 will return the shared rvalue but always as a reference.
57 "share" will traverse up references exactly one level.
59 "share(\$a)" is equivalent to "share($a)", while "share(\\$a)" is
60 not. This means that you must create nested shared data structures
61 by first creating individual shared leaf notes, then adding them to
62 a shared hash or array.
63 A variable can also be marked as shared at compile time by using
65 the "shared" attribute: "my $var : shared".
66 If you want to share a newly created reference unfortunately you
68 need to use "&share([])" and "&share({})" syntax due to problems
69 with Perl's prototyping.
70 The only values that can be assigned to a shared scalar are other
72 scalar values, or shared refs, eg
73 my $var : shared;
75 $var = 1; # ok
76 $var = &share([]); # ok
77 $var = []; # error
78 $var = A->new; # error
79 $var = &share(A->new); # ok as long as the A object is not nested
80 Note that it is often not wise to share an object unless the class
82 itself has been written to support sharing; for example, an
83 object's destructor may get called multiple times, one for each
84 thread's scope exit.
85 lock VARIABLE
87 "lock" places a lock on a variable until the lock goes out of
88 scope. If the variable is locked by another thread, the "lock"
89 call will block until it's available. "lock" is recursive, so mul‐
90 tiple calls to "lock" are safe -- the variable will remain locked
91 until the outermost lock on the variable goes out of scope.
92 If a container object, such as a hash or array, is locked, all the
94 elements of that container are not locked. For example, if a thread
95 does a "lock @a", any other thread doing a "lock($a[12])" won't
96 block.
97 "lock" will traverse up references exactly one level. "lock(\$a)"
99 is equivalent to "lock($a)", while "lock(\\$a)" is not.
100 Note that you cannot explicitly unlock a variable; you can only
102 wait for the lock to go out of scope. If you need more fine-grained
103 control, see Thread::Semaphore.
104 cond_wait VARIABLE
106 cond_wait CONDVAR, LOCKVAR
107 The "cond_wait" function takes a locked variable as a parameter,
108 unlocks the variable, and blocks until another thread does a
109 "cond_signal" or "cond_broadcast" for that same locked variable.
110 The variable that "cond_wait" blocked on is relocked after the
111 "cond_wait" is satisfied. If there are multiple threads
112 "cond_wait"ing on the same variable, all but one will reblock wait‐
113 ing to reacquire the lock on the variable. (So if you're only using
114 "cond_wait" for synchronisation, give up the lock as soon as possi‐
115 ble). The two actions of unlocking the variable and entering the
116 blocked wait state are atomic, the two actions of exiting from the
117 blocked wait state and relocking the variable are not.
118 In its second form, "cond_wait" takes a shared, unlocked variable
120 followed by a shared, locked variable. The second variable is
121 unlocked and thread execution suspended until another thread sig‐
122 nals the first variable.
123 It is important to note that the variable can be notified even if
125 no thread "cond_signal" or "cond_broadcast" on the variable. It is
126 therefore important to check the value of the variable and go back
127 to waiting if the requirement is not fulfilled. For example, to
128 pause until a shared counter drops to zero:
129 { lock($counter); cond_wait($count) until $counter == 0; }
131 cond_timedwait VARIABLE, ABS_TIMEOUT
133 cond_timedwait CONDVAR, ABS_TIMEOUT, LOCKVAR
134 In its two-argument form, "cond_timedwait" takes a locked variable
135 and an absolute timeout as parameters, unlocks the variable, and
136 blocks until the timeout is reached or another thread signals the
137 variable. A false value is returned if the timeout is reached, and
138 a true value otherwise. In either case, the variable is re-locked
139 upon return.
140 Like "cond_wait", this function may take a shared, locked variable
142 as an additional parameter; in this case the first parameter is an
143 unlocked condition variable protected by a distinct lock variable.
144 Again like "cond_wait", waking up and reacquiring the lock are not
146 atomic, and you should always check your desired condition after
147 this function returns. Since the timeout is an absolute value,
148 however, it does not have to be recalculated with each pass:
149 lock($var);
151 my $abs = time() + 15;
152 until ($ok = desired_condition($var)) {
153 last if !cond_timedwait($var, $abs);
154 }
155 # we got it if $ok, otherwise we timed out!
156 cond_signal VARIABLE
158 The "cond_signal" function takes a locked variable as a parameter
159 and unblocks one thread that's "cond_wait"ing on that variable. If
160 more than one thread is blocked in a "cond_wait" on that variable,
161 only one (and which one is indeterminate) will be unblocked.
162 If there are no threads blocked in a "cond_wait" on the variable,
164 the signal is discarded. By always locking before signaling, you
165 can (with care), avoid signaling before another thread has entered
166 cond_wait().
167 "cond_signal" will normally generate a warning if you attempt to
169 use it on an unlocked variable. On the rare occasions where doing
170 this may be sensible, you can skip the warning with
171 { no warnings 'threads'; cond_signal($foo) }
173 cond_broadcast VARIABLE
175 The "cond_broadcast" function works similarly to "cond_signal".
176 "cond_broadcast", though, will unblock all the threads that are
177 blocked in a "cond_wait" on the locked variable, rather than only
178 one.
179
181 threads::shared is designed to disable itself silently if threads are
182 not available. If you want access to threads, you must "use threads"
183 before you "use threads::shared". threads will emit a warning if you
184 use it after threads::shared.
185
187 "bless" is not supported on shared references. In the current version,
188 "bless" will only bless the thread local reference and the blessing
189 will not propagate to the other threads. This is expected to be imple‐
190 mented in a future version of Perl.
191 Does not support splice on arrays!
193 Taking references to the elements of shared arrays and hashes does not
195 autovivify the elements, and neither does slicing a shared array/hash
196 over non-existent indices/keys autovivify the elements.
197 share() allows you to "share $hashref->{key}" without giving any error
199 message. But the "$hashref->{key}" is not shared, causing the error
200 "locking can only be used on shared values" to occur when you attempt
201 to "lock $hasref->{key}".
202
204 Arthur Bergman <arthur at contiller.se>
205 threads::shared is released under the same license as Perl
207 Documentation borrowed from the old Thread.pm
209
211 threads, perlthrtut,
212 <http://www.perl.com/pub/a/2002/06/11/threads.html>
213perl v5.8.8 2001-09-21 threads::shared(3pm)