Thread(3) Perl Programmers Reference Guide Thread(3)NAMEThread - manipulate threads in Perl (for old code only)
CAVEAT
Perl has two thread models.
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.
Neither model is configured by default into Perl (except, as mentioned
above, in Win32 ithreads are always available.) You can see your
Perl's threading configuration by running "perl -V" and looking for the
use...threads variables, or inside script by "use Config;" and testing
for $Config{use5005threads} and $Config{useithreads}.
For old code and interim backwards compatibility, the Thread module has
been reworked to function as a frontend for both 5005threads and
ithreads.
Note that the compatibility is not complete: because the data sharing
models are directly opposed, anything to do with data sharing has to be
thought differently. With the ithreads you must explicitly share()
variables between the threads.
For new code the use of the "Thread" module is discouraged and the
direct use of the "threads" and "threads::shared" modules is encouraged
instead.
Finally, note that there are many known serious problems with the
5005threads, one of the least of which is that regular expression match
variables like $1 are not threadsafe, that is, they easily get cor‐
rupted by competing threads. Other problems include more insidious
data corruption and mysterious crashes. You are seriously urged to use
ithreads instead.
SYNOPSIS
use Thread;
my $t = Thread->new(\&start_sub, @start_args);
$result = $t->join;
$result = $t->eval;
$t->detach;
if ($t->done) {
$t->join;
}
if($t->equal($another_thread)) {
# ...
}
yield();
my $tid = Thread->self->tid;
lock($scalar);
lock(@array);
lock(%hash);
lock(\&sub); # not available with ithreads
$flags = $t->flags; # not available with ithreads
my @list = Thread->list; # not available with ithreads
use Thread 'async';
DESCRIPTION
The "Thread" module provides multithreading support for perl.
FUNCTIONS
$thread = Thread->new(\&start_sub)
$thread = Thread->new(\&start_sub, LIST)
"new" starts a new thread of execution in the referenced sub‐
routine. The optional list is passed as parameters to the sub‐
routine. Execution continues in both the subroutine and the
code after the "new" call.
"Thread->new" returns a thread object representing the newly
created thread.
lock VARIABLE
"lock" places a lock on a variable until the lock goes out of
scope.
If the variable is locked by another thread, the "lock" call
will block until it's available. "lock" is recursive, so mul‐
tiple calls to "lock" are safe--the variable will remain locked
until the outermost lock on the variable goes out of scope.
Locks on variables only affect "lock" calls--they do not affect
normal access to a variable. (Locks on subs are different, and
covered in a bit.) If you really, really want locks to block
access, then go ahead and tie them to something and manage this
yourself. This is done on purpose. While managing access to
variables is a good thing, Perl doesn't force you out of its
living room...
If a container object, such as a hash or array, is locked, all
the elements of that container are not locked. For example, if
a thread does a "lock @a", any other thread doing a
"lock($a[12])" won't block.
With 5005threads you may also "lock" a sub, using "lock &sub".
Any calls to that sub from another thread will block until the
lock is released. This behaviour is not equivalent to declaring
the sub with the "locked" attribute. The "locked" attribute
serializes access to a subroutine, but allows different threads
non-simultaneous access. "lock &sub", on the other hand, will
not allow any other thread access for the duration of the lock.
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 fol‐
lowing 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 repre‐
sents the thread making the "Thread->self" call.
cond_wait VARIABLE
The "cond_wait" function takes a locked variable as a parame‐
ter, unlocks the variable, and blocks until another thread does
a "cond_signal" or "cond_broadcast" for that same locked vari‐
able. 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 reaquire 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 parame‐
ter and unblocks one thread that's "cond_wait"ing on that vari‐
able. 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 vari‐
able, 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 termi‐
nated.
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".
eval The "eval" method wraps an "eval" around a "join", and so waits
for a thread to exit, passing along any values the thread might
have returned. Errors, of course, get placed into $@. (Not
available with ithreads.)
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 cre‐
ated. The main thread of a program will have a tid of zero,
while subsequent threads will have tids assigned starting with
one.
flags The "flags" method returns the flags for the thread. This is
the integer value corresponding to the internal flags for the
thread, and the value may not be all that meaningful to you.
(Not available with ithreads.)
done The "done" method returns true if the thread you're checking
has finished, and false otherwise. (Not available with
ithreads.)
LIMITATIONS
The sequence number used to assign tids is a simple integer, and no
checking is done to make sure the tid isn't currently in use. If a
program creates more than 2**32 - 1 threads in a single run, threads
may be assigned duplicate tids. This limitation may be lifted in a
future version of Perl.
SEE ALSO
threads::shared (not available with 5005threads)
attributes, Thread::Queue, Thread::Semaphore, Thread::Specific (not
available with ithreads)
perl v5.8.8 2004-05-07 Thread(3)
