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Thread(3)
NAME
Thread - manipulate threads in Perl (for old code only)
CAVEAT
Perl has two thread models.
In Perl 5.005 the thread model was that all data is implicitly shared and
shared access to data has to be explicitly synchronized. This model is
called "5005threads".
In Perl 5.6 a new model was introduced in which all is was thread local and
shared access to data has to be explicitly declared. This model is called
"ithreads", for "interpreter threads".
In Perl 5.6 the ithreads model was not available as a public API, only as
an internal API that was available for extension writers, and to implement
fork() emulation on Win32 platforms.
In Perl 5.8 the ithreads model became available through the "threads"
module.
Neither model is configured by default into Perl (except, as mentioned
above, in Win32 ithreads are always available.) You can see your Perl's
threading configuration by running "perl -V" and looking for the
use...threads variables, or inside script by "use Config;" and testing for
$Config{use5005threads} and $Config{useithreads}.
For old code and interim backwards compatibility, the Thread module has
been reworked to function as a frontend for both 5005threads and ithreads.
Note that the compatibility is not complete: because the data sharing
models are directly opposed, anything to do with data sharing has to be
thought differently. With the ithreads you must explicitly share()
variables between the threads.
For new code the use of the "Thread" module is discouraged and the direct
use of the "threads" and "threads::shared" modules is encouraged instead.
Finally, note that there are many known serious problems with the
5005threads, one of the least of which is that regular expression match
variables like $1 are not threadsafe, that is, they easily get corrupted by
competing threads. Other problems include more insidious data corruption
and mysterious crashes. You are seriously urged to use ithreads instead.
SYNOPSIS
use Thread;
my $t = Thread->new(<!>start_sub, @start_args);
$result = $t->join;
$result = $t->eval;
$t->detach;
if ($t->done) {
$t->join;
}
if($t->equal($another_thread)) {
# ...
}
yield();
my $tid = Thread->self->tid;
lock($scalar);
lock(@array);
lock(%hash);
lock(<!>sub); # not available with ithreads
$flags = $t->flags; # not available with ithreads
my @list = Thread->list; # not available with ithreads
use Thread 'async';
DESCRIPTION
The "Thread" module provides multithreading support for perl.
FUNCTIONS
$thread = Thread->new(\!>start_sub)
$thread = Thread->new(\!>start_sub, LIST)
"new" starts a new thread of execution in the referenced
subroutine. The optional list is passed as parameters to the
subroutine. Execution continues in both the subroutine and the code
after the "new" call.
"Thread->new" returns a thread object representing the newly
created thread.
lock VARIABLE
"lock" places a lock on a variable until the lock goes out of
scope.
If the variable is locked by another thread, the "lock" call will
block until it's available. "lock" is recursive, so multiple calls
to "lock" are safe--the variable will remain locked until the
outermost lock on the variable goes out of scope.
Locks on variables only affect "lock" calls--they do not affect
normal access to a variable. (Locks on subs are different, and
covered in a bit.) If you really, really want locks to block
access, then go ahead and tie them to something and manage this
yourself. This is done on purpose. While managing access to
variables is a good thing, Perl doesn't force you out of its living
room...
If a container object, such as a hash or array, is locked, all the
elements of that container are not locked. For example, if a thread
does a "lock @a", any other thread doing a "lock($a[12])" won't
block.
With 5005threads you may also "lock" a sub, using "lock &sub". Any
calls to that sub from another thread will block until the lock is
released. This behaviour is not equivalent to declaring the sub
with the "locked" attribute. The "locked" attribute serializes
access to a subroutine, but allows different threads non-
simultaneous access. "lock &sub", on the other hand, will not allow
any other thread access for the duration of the lock.
Finally, "lock" will traverse up references exactly one level.
"lock(\$a)" is equivalent to "lock($a)", while "lock(\\$a)" is not.
async BLOCK;
"async" creates a thread to execute the block immediately following
it. This block is treated as an anonymous sub, and so must have a
semi-colon after the closing brace. Like "Thread->new", "async"
returns a thread object.
Thread->self
The "Thread->self" function returns a thread object that represents
the thread making the "Thread->self" call.
cond_wait VARIABLE
The "cond_wait" function takes a locked variable as a parameter,
unlocks the variable, and blocks until another thread does a
"cond_signal" or "cond_broadcast" for that same locked variable.
The variable that "cond_wait" blocked on is relocked after the
"cond_wait" is satisfied. If there are multiple threads
"cond_wait"ing on the same variable, all but one will reblock
waiting to reaquire the lock on the variable. (So if you're only
using "cond_wait" for synchronization, give up the lock as soon as
possible.)
cond_signal VARIABLE
The "cond_signal" function takes a locked variable as a parameter
and unblocks one thread that's "cond_wait"ing on that variable. If
more than one thread is blocked in a "cond_wait" on that variable,
only one (and which one is indeterminate) will be unblocked.
If there are no threads blocked in a "cond_wait" on the variable,
the signal is discarded.
cond_broadcast VARIABLE
The "cond_broadcast" function works similarly to "cond_signal".
"cond_broadcast", though, will unblock all the threads that are
blocked in a "cond_wait" on the locked variable, rather than only
one.
yield The "yield" function allows another thread to take control of the
CPU. The exact results are implementation-dependent.
METHODS
join "join" waits for a thread to end and returns any values the thread
exited with. "join" will block until the thread has ended, though
it won't block if the thread has already terminated.
If the thread being "join"ed "die"d, the error it died with will be
returned at this time. If you don't want the thread performing the
"join" to die as well, you should either wrap the "join" in an
"eval" or use the "eval" thread method instead of "join".
eval The "eval" method wraps an "eval" around a "join", and so waits for
a thread to exit, passing along any values the thread might have
returned. Errors, of course, get placed into $@. (Not available
with ithreads.)
detach "detach" tells a thread that it is never going to be joined i.e.
that all traces of its existence can be removed once it stops
running. Errors in detached threads will not be visible anywhere -
if you want to catch them, you should use $SIG{__DIE__} or
something like that.
equal "equal" tests whether two thread objects represent the same thread
and returns true if they do.
tid The "tid" method returns the tid of a thread. The tid is a
monotonically increasing integer assigned when a thread is created.
The main thread of a program will have a tid of zero, while
subsequent threads will have tids assigned starting with one.
flags The "flags" method returns the flags for the thread. This is the
integer value corresponding to the internal flags for the thread,
and the value may not be all that meaningful to you. (Not
available with ithreads.)
done The "done" method returns true if the thread you're checking has
finished, and false otherwise. (Not available with ithreads.)
LIMITATIONS
The sequence number used to assign tids is a simple integer, and no
checking is done to make sure the tid isn't currently in use. If a program
creates more than 2**32 - 1 threads in a single run, threads may be
assigned duplicate tids. This limitation may be lifted in a future version
of Perl.
SEE ALSO
threads::shared (not available with 5005threads)
attributes, Thread::Queue, Thread::Semaphore, Thread::Specific (not
available with ithreads)
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