NAME¶
Thread - Manipulate threads in Perl (for old code only)
DEPRECATED¶
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.
HISTORY¶
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.
SYNOPSIS¶
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;
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.
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.
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".
- 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.
DEFUNCT¶
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.
SEE ALSO¶
threads, threads::shared, Thread::Queue, Thread::Semaphore