Thread(3pm) Perl Programmers Reference Guide Thread(3pm)
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
perl v5.26.3 2018-03-01 Thread(3pm)
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