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Thread(3) Perl Programmers Reference Guide Thread(3)
NAME
Thread - 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 explic-
itly declared. This model is called "ithreads", for
"interpreter threads".
In Perl 5.6 the ithreads model was not available as a pub-
lic 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 avail-
able.) You can see your Perl's threading configuration by
running "perl -V" and looking for the use...threads vari-
ables, 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 thread-
safe, that is, they easily get corrupted by competing
threads. Other problems include more insidious data cor-
ruption 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)
$thread = Thread->new(\&start_sub, LIST)
"new" starts a new thread of execution in the ref-
erenced subroutine. The optional list is passed as
parameters to the subroutine. Execution continues
in both the subroutine and the code after the
"new" call.
"Thread->new" returns a thread object repre-
senting the newly created thread.
lock VARIABLE
"lock" places a lock on a variable until the lock
goes out of scope.
If the variable is locked by another thread, the
"lock" call will block until it's available.
"lock" is recursive, so multiple calls to "lock"
are safe--the variable will remain locked until
the outermost lock on the variable goes out of
scope.
Locks on variables only affect "lock" calls--they
do not affect normal access to a variable. (Locks
on subs are different, and covered in a bit.) If
you really, really want locks to block access,
then go ahead and tie them to something and manage
this yourself. This is done on purpose. While
managing access to variables is a good thing, Perl
doesn't force you out of its living room...
If a container object, such as a hash or array, is
locked, all the elements of that container are not
locked. For example, if a thread does a "lock @a",
any other thread doing a "lock($a[12])" won't
block.
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 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.
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 $@. (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 created. 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 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 dupli-
cate tids. This limitation may be lifted in a future ver-
sion of Perl.
SEE ALSO
threads::shared (not available with 5005threads)
attributes, Thread::Queue, Thread::Semaphore, Thread::Spe-
cific (not available with ithreads)
perl v5.8.8 2001-09-21 Thread(3)