Thread(3p) Perl Programmers Reference Guide Thread(3p)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 impli-
citly 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 exten-
sion 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 reg-
ular expression match variables like $1 are not threadsafe,
that is, they easily get corrupted by competing threads.
Other problems include more insidious data corruption and
mysterious crashes. You are seriously urged to use ithreads
instead.
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Thread(3p) Perl Programmers Reference Guide Thread(3p)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)
$thread = Thread->new(\&start_sub, LIST)
"new" starts a new thread of execution in the refer-
enced subroutine. The optional list is passed as
parameters to the subroutine. Execution continues in
both the subroutine and the code after the "new"
call.
"Thread->new" returns a thread object represent-
ing the newly created thread.
lock VARIABLE
"lock" places a lock on a variable until the lock
goes out of scope.
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If the variable is locked by another thread, the
"lock" call will block until it's available. "lock"
is recursive, so multiple calls to "lock" are
safe--the variable will remain locked until the
outermost lock on the variable goes out of scope.
Locks on variables only affect "lock" calls--they do
not affect normal access to a variable. (Locks on
subs are different, and covered in a bit.) If you
really, really want locks to block access, then go
ahead and tie them to something and manage this
yourself. This is done on purpose. While managing
access to variables is a good thing, Perl doesn't
force you out of its living room...
If a container object, such as a hash or array, is
locked, all the elements of that container are not
locked. For example, if a thread does a "lock @a",
any other thread doing a "lock($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 dif-
ferent 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
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after the "cond_wait" is satisfied. If there are
multiple threads "cond_wait"ing on the same vari-
able, 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 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".
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
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to catch them, you should use $SIG{__DIE__} or some-
thing 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 pro-
gram 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)
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