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Thread(3perl) Perl Programmers Reference Guide Thread(3perl)
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 v5.34.0 2023-11-23 Thread(3perl)
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