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129 | .\" ======================================================================== | |
130 | .\" | |
131 | .IX Title "threads::shared 3" | |
132 | .TH threads::shared 3 "2001-09-21" "perl v5.8.8" "Perl Programmers Reference Guide" | |
133 | .SH "NAME" | |
134 | threads::shared \- Perl extension for sharing data structures between threads | |
135 | .SH "SYNOPSIS" | |
136 | .IX Header "SYNOPSIS" | |
137 | .Vb 2 | |
138 | \& use threads; | |
139 | \& use threads::shared; | |
140 | .Ve | |
141 | .PP | |
142 | .Vb 5 | |
143 | \& my $var : shared; | |
144 | \& $var = $scalar_value; | |
145 | \& $var = $shared_ref_value; | |
146 | \& $var = &share($simple_unshared_ref_value); | |
147 | \& $var = &share(new Foo); | |
148 | .Ve | |
149 | .PP | |
150 | .Vb 6 | |
151 | \& my($scalar, @array, %hash); | |
152 | \& share($scalar); | |
153 | \& share(@array); | |
154 | \& share(%hash); | |
155 | \& my $bar = &share([]); | |
156 | \& $hash{bar} = &share({}); | |
157 | .Ve | |
158 | .PP | |
159 | .Vb 1 | |
160 | \& { lock(%hash); ... } | |
161 | .Ve | |
162 | .PP | |
163 | .Vb 4 | |
164 | \& cond_wait($scalar); | |
165 | \& cond_timedwait($scalar, time() + 30); | |
166 | \& cond_broadcast(@array); | |
167 | \& cond_signal(%hash); | |
168 | .Ve | |
169 | .PP | |
170 | .Vb 4 | |
171 | \& my $lockvar : shared; | |
172 | \& # condition var != lock var | |
173 | \& cond_wait($var, $lockvar); | |
174 | \& cond_timedwait($var, time()+30, $lockvar); | |
175 | .Ve | |
176 | .SH "DESCRIPTION" | |
177 | .IX Header "DESCRIPTION" | |
178 | By default, variables are private to each thread, and each newly created | |
179 | thread gets a private copy of each existing variable. This module allows | |
180 | you to share variables across different threads (and pseudoforks on Win32). | |
181 | It is used together with the threads module. | |
182 | .SH "EXPORT" | |
183 | .IX Header "EXPORT" | |
184 | \&\f(CW\*(C`share\*(C'\fR, \f(CW\*(C`cond_wait\*(C'\fR, \f(CW\*(C`cond_timedwait\*(C'\fR, \f(CW\*(C`cond_signal\*(C'\fR, \f(CW\*(C`cond_broadcast\*(C'\fR | |
185 | .PP | |
186 | Note that if this module is imported when \f(CW\*(C`threads\*(C'\fR has not yet been | |
187 | loaded, then these functions all become no\-ops. This makes it possible | |
188 | to write modules that will work in both threaded and non-threaded | |
189 | environments. | |
190 | .SH "FUNCTIONS" | |
191 | .IX Header "FUNCTIONS" | |
192 | .IP "share \s-1VARIABLE\s0" 4 | |
193 | .IX Item "share VARIABLE" | |
194 | \&\f(CW\*(C`share\*(C'\fR takes a value and marks it as shared. You can share a scalar, | |
195 | array, hash, scalar ref, array ref or hash ref. \f(CW\*(C`share\*(C'\fR will return | |
196 | the shared rvalue but always as a reference. | |
197 | .Sp | |
198 | \&\f(CW\*(C`share\*(C'\fR will traverse up references exactly \fIone\fR level. | |
199 | \&\f(CW\*(C`share(\e$a)\*(C'\fR is equivalent to \f(CW\*(C`share($a)\*(C'\fR, while \f(CW\*(C`share(\e\e$a)\*(C'\fR is not. | |
200 | This means that you must create nested shared data structures by first | |
201 | creating individual shared leaf notes, then adding them to a shared hash | |
202 | or array. | |
203 | .Sp | |
204 | A variable can also be marked as shared at compile time by using the | |
205 | \&\f(CW\*(C`shared\*(C'\fR attribute: \f(CW\*(C`my $var : shared\*(C'\fR. | |
206 | .Sp | |
207 | If you want to share a newly created reference unfortunately you | |
208 | need to use \f(CW\*(C`&share([])\*(C'\fR and \f(CW\*(C`&share({})\*(C'\fR syntax due to problems | |
209 | with Perl's prototyping. | |
210 | .Sp | |
211 | The only values that can be assigned to a shared scalar are other scalar | |
212 | values, or shared refs, eg | |
213 | .Sp | |
214 | .Vb 6 | |
215 | \& my $var : shared; | |
216 | \& $var = 1; # ok | |
217 | \& $var = &share([]); # ok | |
218 | \& $var = []; # error | |
219 | \& $var = A->new; # error | |
220 | \& $var = &share(A->new); # ok as long as the A object is not nested | |
221 | .Ve | |
222 | .Sp | |
223 | Note that it is often not wise to share an object unless the class itself | |
224 | has been written to support sharing; for example, an object's destructor | |
225 | may get called multiple times, one for each thread's scope exit. | |
226 | .IP "lock \s-1VARIABLE\s0" 4 | |
227 | .IX Item "lock VARIABLE" | |
228 | \&\f(CW\*(C`lock\*(C'\fR places a lock on a variable until the lock goes out of scope. | |
229 | If the variable is locked by another thread, the \f(CW\*(C`lock\*(C'\fR call will | |
230 | block until it's available. \f(CW\*(C`lock\*(C'\fR is recursive, so multiple calls | |
231 | to \f(CW\*(C`lock\*(C'\fR are safe \*(-- the variable will remain locked until the | |
232 | outermost lock on the variable goes out of scope. | |
233 | .Sp | |
234 | If a container object, such as a hash or array, is locked, all the | |
235 | elements of that container are not locked. For example, if a thread | |
236 | does a \f(CW\*(C`lock @a\*(C'\fR, any other thread doing a \f(CW\*(C`lock($a[12])\*(C'\fR won't block. | |
237 | .Sp | |
238 | \&\f(CW\*(C`lock\*(C'\fR will traverse up references exactly \fIone\fR level. | |
239 | \&\f(CW\*(C`lock(\e$a)\*(C'\fR is equivalent to \f(CW\*(C`lock($a)\*(C'\fR, while \f(CW\*(C`lock(\e\e$a)\*(C'\fR is not. | |
240 | .Sp | |
241 | Note that you cannot explicitly unlock a variable; you can only wait | |
242 | for the lock to go out of scope. If you need more fine-grained | |
243 | control, see Thread::Semaphore. | |
244 | .IP "cond_wait \s-1VARIABLE\s0" 4 | |
245 | .IX Item "cond_wait VARIABLE" | |
246 | .PD 0 | |
247 | .IP "cond_wait \s-1CONDVAR\s0, \s-1LOCKVAR\s0" 4 | |
248 | .IX Item "cond_wait CONDVAR, LOCKVAR" | |
249 | .PD | |
250 | The \f(CW\*(C`cond_wait\*(C'\fR function takes a \fBlocked\fR variable as a parameter, | |
251 | unlocks the variable, and blocks until another thread does a | |
252 | \&\f(CW\*(C`cond_signal\*(C'\fR or \f(CW\*(C`cond_broadcast\*(C'\fR for that same locked variable. | |
253 | The variable that \f(CW\*(C`cond_wait\*(C'\fR blocked on is relocked after the | |
254 | \&\f(CW\*(C`cond_wait\*(C'\fR is satisfied. If there are multiple threads | |
255 | \&\f(CW\*(C`cond_wait\*(C'\fRing on the same variable, all but one will reblock waiting | |
256 | to reacquire the lock on the variable. (So if you're only using | |
257 | \&\f(CW\*(C`cond_wait\*(C'\fR for synchronisation, give up the lock as soon as | |
258 | possible). The two actions of unlocking the variable and entering the | |
259 | blocked wait state are atomic, the two actions of exiting from the | |
260 | blocked wait state and relocking the variable are not. | |
261 | .Sp | |
262 | In its second form, \f(CW\*(C`cond_wait\*(C'\fR takes a shared, \fBunlocked\fR variable | |
263 | followed by a shared, \fBlocked\fR variable. The second variable is | |
264 | unlocked and thread execution suspended until another thread signals | |
265 | the first variable. | |
266 | .Sp | |
267 | It is important to note that the variable can be notified even if | |
268 | no thread \f(CW\*(C`cond_signal\*(C'\fR or \f(CW\*(C`cond_broadcast\*(C'\fR on the variable. | |
269 | It is therefore important to check the value of the variable and | |
270 | go back to waiting if the requirement is not fulfilled. For example, | |
271 | to pause until a shared counter drops to zero: | |
272 | .Sp | |
273 | .Vb 1 | |
274 | \& { lock($counter); cond_wait($count) until $counter == 0; } | |
275 | .Ve | |
276 | .IP "cond_timedwait \s-1VARIABLE\s0, \s-1ABS_TIMEOUT\s0" 4 | |
277 | .IX Item "cond_timedwait VARIABLE, ABS_TIMEOUT" | |
278 | .PD 0 | |
279 | .IP "cond_timedwait \s-1CONDVAR\s0, \s-1ABS_TIMEOUT\s0, \s-1LOCKVAR\s0" 4 | |
280 | .IX Item "cond_timedwait CONDVAR, ABS_TIMEOUT, LOCKVAR" | |
281 | .PD | |
282 | In its two-argument form, \f(CW\*(C`cond_timedwait\*(C'\fR takes a \fBlocked\fR variable | |
283 | and an absolute timeout as parameters, unlocks the variable, and blocks | |
284 | until the timeout is reached or another thread signals the variable. A | |
285 | false value is returned if the timeout is reached, and a true value | |
286 | otherwise. In either case, the variable is re-locked upon return. | |
287 | .Sp | |
288 | Like \f(CW\*(C`cond_wait\*(C'\fR, this function may take a shared, \fBlocked\fR variable | |
289 | as an additional parameter; in this case the first parameter is an | |
290 | \&\fBunlocked\fR condition variable protected by a distinct lock variable. | |
291 | .Sp | |
292 | Again like \f(CW\*(C`cond_wait\*(C'\fR, waking up and reacquiring the lock are not | |
293 | atomic, and you should always check your desired condition after this | |
294 | function returns. Since the timeout is an absolute value, however, it | |
295 | does not have to be recalculated with each pass: | |
296 | .Sp | |
297 | .Vb 6 | |
298 | \& lock($var); | |
299 | \& my $abs = time() + 15; | |
300 | \& until ($ok = desired_condition($var)) { | |
301 | \& last if !cond_timedwait($var, $abs); | |
302 | \& } | |
303 | \& # we got it if $ok, otherwise we timed out! | |
304 | .Ve | |
305 | .IP "cond_signal \s-1VARIABLE\s0" 4 | |
306 | .IX Item "cond_signal VARIABLE" | |
307 | The \f(CW\*(C`cond_signal\*(C'\fR function takes a \fBlocked\fR variable as a parameter | |
308 | and unblocks one thread that's \f(CW\*(C`cond_wait\*(C'\fRing on that variable. If | |
309 | more than one thread is blocked in a \f(CW\*(C`cond_wait\*(C'\fR on that variable, | |
310 | only one (and which one is indeterminate) will be unblocked. | |
311 | .Sp | |
312 | If there are no threads blocked in a \f(CW\*(C`cond_wait\*(C'\fR on the variable, | |
313 | the signal is discarded. By always locking before signaling, you can | |
314 | (with care), avoid signaling before another thread has entered \fIcond_wait()\fR. | |
315 | .Sp | |
316 | \&\f(CW\*(C`cond_signal\*(C'\fR will normally generate a warning if you attempt to use it | |
317 | on an unlocked variable. On the rare occasions where doing this may be | |
318 | sensible, you can skip the warning with | |
319 | .Sp | |
320 | .Vb 1 | |
321 | \& { no warnings 'threads'; cond_signal($foo) } | |
322 | .Ve | |
323 | .IP "cond_broadcast \s-1VARIABLE\s0" 4 | |
324 | .IX Item "cond_broadcast VARIABLE" | |
325 | The \f(CW\*(C`cond_broadcast\*(C'\fR function works similarly to \f(CW\*(C`cond_signal\*(C'\fR. | |
326 | \&\f(CW\*(C`cond_broadcast\*(C'\fR, though, will unblock \fBall\fR the threads that are | |
327 | blocked in a \f(CW\*(C`cond_wait\*(C'\fR on the locked variable, rather than only one. | |
328 | .SH "NOTES" | |
329 | .IX Header "NOTES" | |
330 | threads::shared is designed to disable itself silently if threads are | |
331 | not available. If you want access to threads, you must \f(CW\*(C`use threads\*(C'\fR | |
332 | before you \f(CW\*(C`use threads::shared\*(C'\fR. threads will emit a warning if you | |
333 | use it after threads::shared. | |
334 | .SH "BUGS" | |
335 | .IX Header "BUGS" | |
336 | \&\f(CW\*(C`bless\*(C'\fR is not supported on shared references. In the current version, | |
337 | \&\f(CW\*(C`bless\*(C'\fR will only bless the thread local reference and the blessing | |
338 | will not propagate to the other threads. This is expected to be | |
339 | implemented in a future version of Perl. | |
340 | .PP | |
341 | Does not support splice on arrays! | |
342 | .PP | |
343 | Taking references to the elements of shared arrays and hashes does not | |
344 | autovivify the elements, and neither does slicing a shared array/hash | |
345 | over non-existent indices/keys autovivify the elements. | |
346 | .PP | |
347 | \&\fIshare()\fR allows you to \f(CW\*(C`share $hashref\->{key}\*(C'\fR without giving any error | |
348 | message. But the \f(CW\*(C`$hashref\->{key}\*(C'\fR is \fBnot\fR shared, causing the error | |
349 | \&\*(L"locking can only be used on shared values\*(R" to occur when you attempt to | |
350 | \&\f(CW\*(C`lock $hasref\->{key}\*(C'\fR. | |
351 | .SH "AUTHOR" | |
352 | .IX Header "AUTHOR" | |
353 | Arthur Bergman <arthur at contiller.se> | |
354 | .PP | |
355 | threads::shared is released under the same license as Perl | |
356 | .PP | |
357 | Documentation borrowed from the old Thread.pm | |
358 | .SH "SEE ALSO" | |
359 | .IX Header "SEE ALSO" | |
360 | threads, perlthrtut, <http://www.perl.com/pub/a/2002/06/11/threads.html> |