Thread - Manipulate threads in Perl (for old code only)
The Thread module served as the frontend to the old-style thread model,
called 5005threads, that was introduced in release 5.005. That model was
deprecated, and has been removed in version 5.10.
For old code and interim backwards compatibility, the Thread module has
been reworked to function as a frontend for the new interpreter threads
(ithreads) model. However, some previous functionality is not available.
Further, the data sharing models between the two thread models are completely
different, and anything to do with data sharing has to be thought differently.
With ithreads, you must explicitly share() variables between the
threads.
You are strongly encouraged to migrate any existing threaded code to the new
model (i.e., use the threads and threads::shared modules) as soon as
possible.
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, and the 5005threads model was deprecated.
In Perl 5.10, the 5005threads model was removed from the Perl interpreter.
use Thread qw(:DEFAULT async yield);
my $t = Thread->new(\&start_sub, @start_args);
$result = $t->join;
$t->detach;
if ($t->done) {
$t->join;
}
if($t->equal($another_thread)) {
# ...
}
yield();
my $tid = Thread->self->tid;
lock($scalar);
lock(@array);
lock(%hash);
my @list = Thread->list;
The Thread module provides multithreading support for Perl.
- $thread = Thread->new(\&start_sub)
-
- $thread = Thread->new(\&start_sub, LIST)$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($a[12]) won't
block.
Finally, lock will traverse up references exactly one level.
lock(\$a) is equivalent to lock($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.
- Thread->list
-
Returns a list of all non-joined, non-detached Thread objects.
- 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 re-acquire 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.
- 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 .
- 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.
- done
-
The
done method returns true if the thread you're checking has
finished, and false otherwise.
The following were implemented with 5005threads, but are no longer
available with ithreads.
lock(\&sub) lock(\&sub)
-
With 5005threads, you could also
lock a sub such that any calls to that sub
from another thread would block until the lock was released.
Also, subroutines could be declared with the :locked attribute which would
serialize access to the subroutine, but allowed different threads
non-simultaneous access.
- eval
-
The
eval method wrapped an eval around a join , and so waited for a
thread to exit, passing along any values the thread might have returned and
placing any errors into $@ .
- flags
-
The
flags method returned the flags for the thread - an integer value
corresponding to the internal flags for the thread.
threads, the threads::shared manpage, the Thread::Queue manpage, the Thread::Semaphore manpage
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