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Thread(3) |
Perl Programmers Reference Guide |
Thread(3) |
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)
- $thread = Thread->new(\&start_sub, LIST)
- "new" starts a new thread of execution
in the referenced subroutine. The optional list is passed as parameters to
the subroutine. Execution continues in both the subroutine and the code
after the "new" call.
"Thread->new" returns a
thread object representing the newly created thread.
- lock VARIABLE
- "lock" places a lock on a variable until
the lock goes out of scope.
If the variable is locked by another thread, the
"lock" call will block until it's
available. "lock" is recursive, so
multiple calls to "lock" are safe--the
variable will remain locked until the outermost lock on the variable
goes out of scope.
Locks on variables only affect
"lock" calls--they do not
affect normal access to a variable. (Locks on subs are different, and
covered in a bit.) If you really, really want locks to block
access, then go ahead and tie them to something and manage this
yourself. This is done on purpose. While managing access to variables is
a good thing, Perl doesn't force you out of its living room...
If a container object, such as a hash or array, is locked, all
the elements of that container are not locked. For example, if a thread
does a "lock @a", any other thread
doing a "lock($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)
- 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, threads::shared, Thread::Queue, Thread::Semaphore
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