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| 128 | .rm #[ #] #H #V #F C |
| 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> |