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[perl5.git] / ext / threads / shared / shared.pm
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b050c948 1package threads::shared;
73e09c8f 2
c46325ea 3use 5.008;
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4use strict;
5use warnings;
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6BEGIN {
7 require Exporter;
8 our @ISA = qw(Exporter);
a0e036c1 9 our @EXPORT = qw(share cond_wait cond_timedwait cond_broadcast cond_signal);
4438fa18 10 our $VERSION = '0.94_01';
73e09c8f 11
5c360ac5 12 if ($threads::threads) {
6f942b98 13 *cond_wait = \&cond_wait_enabled;
a0e036c1 14 *cond_timedwait = \&cond_timedwait_enabled;
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15 *cond_signal = \&cond_signal_enabled;
16 *cond_broadcast = \&cond_broadcast_enabled;
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17 require XSLoader;
18 XSLoader::load('threads::shared',$VERSION);
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19 push @EXPORT,'bless';
20 }
21 else {
b050c948 22
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23# String eval is generally evil, but we don't want these subs to exist at all
24# if threads are loaded successfully. Vivifying them conditionally this way
25# saves on average about 4K of memory per thread.
b050c948 26
df5c998e 27 eval <<'EOD';
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28sub cond_wait (\[$@%];\[$@%]) { undef }
29sub cond_timedwait (\[$@%]$;\[$@%]) { undef }
30sub cond_signal (\[$@%]) { undef }
31sub cond_broadcast (\[$@%]) { undef }
32sub share (\[$@%]) { return $_[0] }
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33EOD
34 }
35}
b050c948 36
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37$threads::shared::threads_shared = 1;
38
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39sub threads::shared::tie::SPLICE
40{
41 die "Splice not implemented for shared arrays";
42}
43
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44__END__
45
46=head1 NAME
47
48threads::shared - Perl extension for sharing data structures between threads
49
50=head1 SYNOPSIS
51
73e09c8f 52 use threads;
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53 use threads::shared;
54
38875929 55 my $var : shared;
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56 $var = $scalar_value;
57 $var = $shared_ref_value;
58 $var = &share($simple_unshared_ref_value);
59 $var = &share(new Foo);
38875929 60
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61 my($scalar, @array, %hash);
62 share($scalar);
63 share(@array);
aaf3876d 64 share(%hash);
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65 my $bar = &share([]);
66 $hash{bar} = &share({});
b050c948 67
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68 { lock(%hash); ... }
69
b050c948 70 cond_wait($scalar);
a0e036c1 71 cond_timedwait($scalar, time() + 30);
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72 cond_broadcast(@array);
73 cond_signal(%hash);
b050c948 74
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75 my $lockvar : shared;
76 # condition var != lock var
77 cond_wait($var, $lockvar);
78 cond_timedwait($var, time()+30, $lockvar);
79
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80=head1 DESCRIPTION
81
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82By default, variables are private to each thread, and each newly created
83thread gets a private copy of each existing variable. This module allows
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84you to share variables across different threads (and pseudoforks on Win32).
85It is used together with the threads module.
b050c948 86
515f0976 87=head1 EXPORT
b050c948 88
a0e036c1 89C<share>, C<cond_wait>, C<cond_timedwait>, C<cond_signal>, C<cond_broadcast>
515f0976 90
e67b86b3 91Note that if this module is imported when C<threads> has not yet been
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92loaded, then these functions all become no-ops. This makes it possible
93to write modules that will work in both threaded and non-threaded
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94environments.
95
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96=head1 FUNCTIONS
97
98=over 4
99
100=item share VARIABLE
101
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102C<share> takes a value and marks it as shared. You can share a scalar,
103array, hash, scalar ref, array ref or hash ref. C<share> will return
0a9af0ff 104the shared rvalue but always as a reference.
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105
106C<share> will traverse up references exactly I<one> level.
107C<share(\$a)> is equivalent to C<share($a)>, while C<share(\\$a)> is not.
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108This means that you must create nested shared data structures by first
109creating individual shared leaf notes, then adding them to a shared hash
110or array.
515f0976 111
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112A variable can also be marked as shared at compile time by using the
113C<shared> attribute: C<my $var : shared>.
114
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115If you want to share a newly created reference unfortunately you
116need to use C<&share([])> and C<&share({})> syntax due to problems
117with Perl's prototyping.
caf25f3b 118
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119The only values that can be assigned to a shared scalar are other scalar
120values, or shared refs, eg
121
122 my $var : shared;
123 $var = 1; # ok
124 $var = &share([]); # ok
125 $var = []; # error
126 $var = A->new; # error
127 $var = &share(A->new); # ok as long as the A object is not nested
128
129Note that it is often not wise to share an object unless the class itself
130has been written to support sharing; for example, an object's destructor
131may get called multiple times, one for each thread's scope exit.
132
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133=item lock VARIABLE
134
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135C<lock> places a lock on a variable until the lock goes out of scope.
136If the variable is locked by another thread, the C<lock> call will
137block until it's available. C<lock> is recursive, so multiple calls
138to C<lock> are safe -- the variable will remain locked until the
139outermost lock on the variable goes out of scope.
515f0976 140
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141If a container object, such as a hash or array, is locked, all the
142elements of that container are not locked. For example, if a thread
143does a C<lock @a>, any other thread doing a C<lock($a[12])> won't block.
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144
145C<lock> will traverse up references exactly I<one> level.
146C<lock(\$a)> is equivalent to C<lock($a)>, while C<lock(\\$a)> is not.
147
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148Note that you cannot explicitly unlock a variable; you can only wait
149for the lock to go out of scope. If you need more fine-grained
83272a45 150control, see L<Thread::Semaphore>.
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151
152=item cond_wait VARIABLE
153
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154=item cond_wait CONDVAR, LOCKVAR
155
515f0976 156The C<cond_wait> function takes a B<locked> variable as a parameter,
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157unlocks the variable, and blocks until another thread does a
158C<cond_signal> or C<cond_broadcast> for that same locked variable.
159The variable that C<cond_wait> blocked on is relocked after the
160C<cond_wait> is satisfied. If there are multiple threads
161C<cond_wait>ing on the same variable, all but one will reblock waiting
162to reacquire the lock on the variable. (So if you're only using
163C<cond_wait> for synchronisation, give up the lock as soon as
164possible). The two actions of unlocking the variable and entering the
a0e036c1 165blocked wait state are atomic, the two actions of exiting from the
38875929 166blocked wait state and relocking the variable are not.
515f0976 167
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168In its second form, C<cond_wait> takes a shared, B<unlocked> variable
169followed by a shared, B<locked> variable. The second variable is
170unlocked and thread execution suspended until another thread signals
171the first variable.
172
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173It is important to note that the variable can be notified even if
174no thread C<cond_signal> or C<cond_broadcast> on the variable.
175It is therefore important to check the value of the variable and
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176go back to waiting if the requirement is not fulfilled. For example,
177to pause until a shared counter drops to zero:
178
179 { lock($counter); cond_wait($count) until $counter == 0; }
180
181=item cond_timedwait VARIABLE, ABS_TIMEOUT
182
183=item cond_timedwait CONDVAR, ABS_TIMEOUT, LOCKVAR
184
185In its two-argument form, C<cond_timedwait> takes a B<locked> variable
186and an absolute timeout as parameters, unlocks the variable, and blocks
187until the timeout is reached or another thread signals the variable. A
188false value is returned if the timeout is reached, and a true value
189otherwise. In either case, the variable is re-locked upon return.
190
191Like C<cond_wait>, this function may take a shared, B<locked> variable
192as an additional parameter; in this case the first parameter is an
193B<unlocked> condition variable protected by a distinct lock variable.
194
195Again like C<cond_wait>, waking up and reacquiring the lock are not
196atomic, and you should always check your desired condition after this
197function returns. Since the timeout is an absolute value, however, it
198does not have to be recalculated with each pass:
199
200 lock($var);
201 my $abs = time() + 15;
202 until ($ok = desired_condition($var)) {
203 last if !cond_timedwait($var, $abs);
204 }
205 # we got it if $ok, otherwise we timed out!
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206
207=item cond_signal VARIABLE
208
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209The C<cond_signal> function takes a B<locked> variable as a parameter
210and unblocks one thread that's C<cond_wait>ing on that variable. If
211more than one thread is blocked in a C<cond_wait> on that variable,
212only one (and which one is indeterminate) will be unblocked.
515f0976 213
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214If there are no threads blocked in a C<cond_wait> on the variable,
215the signal is discarded. By always locking before signaling, you can
216(with care), avoid signaling before another thread has entered cond_wait().
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217
218C<cond_signal> will normally generate a warning if you attempt to use it
219on an unlocked variable. On the rare occasions where doing this may be
220sensible, you can skip the warning with
221
222 { no warnings 'threads'; cond_signal($foo) }
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223
224=item cond_broadcast VARIABLE
225
226The C<cond_broadcast> function works similarly to C<cond_signal>.
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227C<cond_broadcast>, though, will unblock B<all> the threads that are
228blocked in a C<cond_wait> on the locked variable, rather than only one.
b050c948 229
4cab98c0 230=back
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231
232=head1 NOTES
233
8c5dce87 234threads::shared is designed to disable itself silently if threads are
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235not available. If you want access to threads, you must C<use threads>
236before you C<use threads::shared>. threads will emit a warning if you
8c5dce87 237use it after threads::shared.
dab065ea 238
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239=head1 BUGS
240
4cab98c0 241C<bless> is not supported on shared references. In the current version,
515f0976 242C<bless> will only bless the thread local reference and the blessing
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243will not propagate to the other threads. This is expected to be
244implemented in a future version of Perl.
515f0976 245
b050c948 246Does not support splice on arrays!
b050c948 247
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248Taking references to the elements of shared arrays and hashes does not
249autovivify the elements, and neither does slicing a shared array/hash
250over non-existent indices/keys autovivify the elements.
251
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252share() allows you to C<< share $hashref->{key} >> without giving any error
253message. But the C<< $hashref->{key} >> is B<not> shared, causing the error
3d32476b 254"locking can only be used on shared values" to occur when you attempt to
72ac79b3 255C<< lock $hasref->{key} >>.
3d32476b 256
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257=head1 AUTHOR
258
aaf3876d 259Arthur Bergman E<lt>arthur at contiller.seE<gt>
b050c948 260
aaf3876d 261threads::shared is released under the same license as Perl
b050c948 262
5e549d84 263Documentation borrowed from the old Thread.pm
515f0976 264
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265=head1 SEE ALSO
266
5e549d84 267L<threads>, L<perlthrtut>, L<http://www.perl.com/pub/a/2002/06/11/threads.html>
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268
269=cut