Thread(3) Perl Programmers Reference Guide Thread(3)NAMEThread - manipulate threads in Perl (EXPERIMENTAL, subject
to change)
CAVEAT
The Thread extension requires Perl to be built in a par
ticular way to enable the older 5.005 threading model.
Just to confuse matters, there is an alternate threading
model known as "ithreads" that does NOT support this
extension. If you are using a binary distribution such as
ActivePerl that is built with ithreads support, this
extension CANNOT be used.
SYNOPSIS
use Thread;
my $t = new Thread \&start_sub, @start_args;
$result = $t->join;
$result = $t->eval;
$t->detach;
if($t->equal($another_thread)) {
# ...
}
my $tid = Thread->self->tid;
my $tlist = Thread->list;
lock($scalar);
yield();
use Thread 'async';
DESCRIPTION
WARNING: Threading is an experimental feature. Both the interface
and implementation are subject to change drastically. In fact, this
documentation describes the flavor of threads that was in version
5.005. Perl 5.6.0 and later have the beginnings of support for
interpreter threads, which (when finished) is expected to be
significantly different from what is described here. The information
contained here may therefore soon be obsolete. Use at your own risk!
The "Thread" module provides multithreading support for
perl.
FUNCTIONS
new \&start_sub
new \&start_sub, LIST
"new" starts a new thread of execution in the ref
erenced subroutine. The optional list is passed as
parameters to the subroutine. Execution continues
in both the subroutine and the code after the
"new" call.
"new Thread" 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 man
aging 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($a[12])" won't
block.
You may also "lock" a sub, using "lock &sub". Any
calls to that sub from another thread will block
until the lock is released. This behaviour is not
equivalent to declaring the sub with the "locked"
attribute. The "locked" attribute serializes
access to a subroutine, but allows different
threads non-simultaneous access. "lock &sub", on
the other hand, will not allow any other thread
access for the duration of the lock.
Finally, "lock" will traverse up references
exactly one level. "lock(\$a)" is equivalent to
"lock($a)", while "lock(\\$a)" is not.
async BLOCK;
"async" creates a thread to execute the block
immediately following it. This block is treated as
an anonymous sub, and so must have a semi-colon
after the closing brace. Like "new Thread",
"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
"Thread->list" returns a list of thread objects
for all running and finished but un-"join"ed
threads.
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
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 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 vari
able, 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 imple
mentation-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".
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 "$@".
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.
LIMITATIONS
The sequence number used to assign tids is a simple inte
ger, 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
the attributes manpage, the Thread::Queue manpage, the
Thread::Semaphore manpage, the Thread::Specific manpage.
2001-03-18 perl v5.6.1 Thread(3)
