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PERLTHRTUT(1)	       Perl Programmers	Reference Guide		 PERLTHRTUT(1)

NAME
       perlthrtut - Tutorial on	threads	in Perl

DESCRIPTION
       This tutorial describes the use of Perl interpreter threads (sometimes
       referred	to as ithreads).  In this model, each thread runs in its own
       Perl interpreter, and any data sharing between threads must be
       explicit.  The user-level interface for ithreads	uses the threads
       class.

       NOTE: There was another older Perl threading flavor called the 5.005
       model that used the threads class.  This	old model was known to have
       problems, is deprecated,	and was	removed	for release 5.10.  You are
       strongly	encouraged to migrate any existing 5.005 threads code to the
       new model as soon as possible.

       You can see which (or neither) threading	flavour	you have by running
       "perl -V" and looking at	the "Platform" section.	 If you	have
       "useithreads=define" you	have ithreads, if you have
       "use5005threads=define" you have	5.005 threads.	If you have neither,
       you don't have any thread support built in.  If you have	both, you are
       in trouble.

       The threads and threads::shared modules are included in the core	Perl
       distribution.  Additionally, they are maintained	as a separate modules
       on CPAN,	so you can check there for any updates.

What Is	A Thread Anyway?
       A thread	is a flow of control through a program with a single execution
       point.

       Sounds an awful lot like	a process, doesn't it? Well, it	should.
       Threads are one of the pieces of	a process.  Every process has at least
       one thread and, up until	now, every process running Perl	had only one
       thread.	With 5.8, though, you can create extra threads.	 We're going
       to show you how,	when, and why.

Threaded Program Models
       There are three basic ways that you can structure a threaded program.
       Which model you choose depends on what you need your program to do.
       For many	non-trivial threaded programs, you'll need to choose different
       models for different pieces of your program.

   Boss/Worker
       The boss/worker model usually has one boss thread and one or more
       worker threads.	The boss thread	gathers	or generates tasks that	need
       to be done, then	parcels	those tasks out	to the appropriate worker
       thread.

       This model is common in GUI and server programs,	where a	main thread
       waits for some event and	then passes that event to the appropriate
       worker threads for processing.  Once the	event has been passed on, the
       boss thread goes	back to	waiting	for another event.

       The boss	thread does relatively little work.  While tasks aren't
       necessarily performed faster than with any other	method,	it tends to
       have the	best user-response times.

   Work	Crew
       In the work crew	model, several threads are created that	do essentially
       the same	thing to different pieces of data.  It closely mirrors
       classical parallel processing and vector	processors, where a large
       array of	processors do the exact	same thing to many pieces of data.

       This model is particularly useful if the	system running the program
       will distribute multiple	threads	across different processors.  It can
       also be useful in ray tracing or	rendering engines, where the
       individual threads can pass on interim results to give the user visual
       feedback.

   Pipeline
       The pipeline model divides up a task into a series of steps, and	passes
       the results of one step on to the thread	processing the next.  Each
       thread does one thing to	each piece of data and passes the results to
       the next	thread in line.

       This model makes	the most sense if you have multiple processors so two
       or more threads will be executing in parallel, though it	can often make
       sense in	other contexts as well.	 It tends to keep the individual tasks
       small and simple, as well as allowing some parts	of the pipeline	to
       block (on I/O or	system calls, for example) while other parts keep
       going.  If you're running different parts of the	pipeline on different
       processors you may also take advantage of the caches on each processor.

       This model is also handy	for a form of recursive	programming where,
       rather than having a subroutine call itself, it instead creates another
       thread.	Prime and Fibonacci generators both map	well to	this form of
       the pipeline model. (A version of a prime number	generator is presented
       later on.)

What kind of threads are Perl threads?
       If you have experience with other thread	implementations, you might
       find that things	aren't quite what you expect.  It's very important to
       remember	when dealing with Perl threads that Perl Threads Are Not X
       Threads for all values of X.  They aren't POSIX threads,	or DecThreads,
       or Java's Green threads,	or Win32 threads.  There are similarities, and
       the broad concepts are the same,	but if you start looking for
       implementation details you're going to be either	disappointed or
       confused.  Possibly both.

       This is not to say that Perl threads are	completely different from
       everything that's ever come before. They're not.	 Perl's	threading
       model owes a lot	to other thread	models,	especially POSIX.  Just	as
       Perl is not C, though, Perl threads are not POSIX threads.  So if you
       find yourself looking for mutexes, or thread priorities,	it's time to
       step back a bit and think about what you	want to	do and how Perl	can do
       it.

       However,	it is important	to remember that Perl threads cannot magically
       do things unless	your operating system's	threads	allow it. So if	your
       system blocks the entire	process	on "sleep()", Perl usually will, as
       well.

       Perl Threads Are	Different.

Thread-Safe Modules
       The addition of threads has changed Perl's internals substantially.
       There are implications for people who write modules with	XS code	or
       external	libraries. However, since Perl data is not shared among
       threads by default, Perl	modules	stand a	high chance of being thread-
       safe or can be made thread-safe easily.	Modules	that are not tagged as
       thread-safe should be tested or code reviewed before being used in
       production code.

       Not all modules that you	might use are thread-safe, and you should
       always assume a module is unsafe	unless the documentation says
       otherwise.  This	includes modules that are distributed as part of the
       core.  Threads are a relatively new feature, and	even some of the
       standard	modules	aren't thread-safe.

       Even if a module	is thread-safe,	it doesn't mean	that the module	is
       optimized to work well with threads. A module could possibly be
       rewritten to utilize the	new features in	threaded Perl to increase
       performance in a	threaded environment.

       If you're using a module	that's not thread-safe for some	reason,	you
       can protect yourself by using it	from one, and only one thread at all.
       If you need multiple threads to access such a module, you can use
       semaphores and lots of programming discipline to	control	access to it.
       Semaphores are covered in "Basic	semaphores".

       See also	"Thread-Safety of System Libraries".

Thread Basics
       The threads module provides the basic functions you need	to write
       threaded	programs.  In the following sections, we'll cover the basics,
       showing you what	you need to do to create a threaded program.   After
       that, we'll go over some	of the features	of the threads module that
       make threaded programming easier.

   Basic Thread	Support
       Thread support is a Perl	compile-time option. It's something that's
       turned on or off	when Perl is built at your site, rather	than when your
       programs	are compiled. If your Perl wasn't compiled with	thread support
       enabled,	then any attempt to use	threads	will fail.

       Your programs can use the Config	module to check	whether	threads	are
       enabled.	If your	program	can't run without them,	you can	say something
       like:

	   use Config;
	   $Config{useithreads}	or
	       die('Recompile Perl with	threads	to run this program.');

       A possibly-threaded program using a possibly-threaded module might have
       code like this:

	   use Config;
	   use MyMod;

	   BEGIN {
	       if ($Config{useithreads}) {
		   # We	have threads
		   require MyMod_threaded;
		   import MyMod_threaded;
	       } else {
		   require MyMod_unthreaded;
		   import MyMod_unthreaded;
	       }
	   }

       Since code that runs both with and without threads is usually pretty
       messy, it's best	to isolate the thread-specific code in its own module.
       In our example above, that's what "MyMod_threaded" is, and it's only
       imported	if we're running on a threaded Perl.

   A Note about	the Examples
       In a real situation, care should	be taken that all threads are finished
       executing before	the program exits.  That care has not been taken in
       these examples in the interest of simplicity.  Running these examples
       as is will produce error	messages, usually caused by the	fact that
       there are still threads running when the	program	exits.	You should not
       be alarmed by this.

   Creating Threads
       The threads module provides the tools you need to create	new threads.
       Like any	other module, you need to tell Perl that you want to use it;
       "use threads;" imports all the pieces you need to create	basic threads.

       The simplest, most straightforward way to create	a thread is with
       "create()":

	   use threads;

	   my $thr = threads->create(\&sub1);

	   sub sub1 {
	       print("In the thread\n");
	   }

       The "create()" method takes a reference to a subroutine and creates a
       new thread that starts executing	in the referenced subroutine.  Control
       then passes both	to the subroutine and the caller.

       If you need to, your program can	pass parameters	to the subroutine as
       part of the thread startup.  Just include the list of parameters	as
       part of the "threads->create()" call, like this:

	   use threads;

	   my $Param3 =	'foo';
	   my $thr1 = threads->create(\&sub1, 'Param 1', 'Param	2', $Param3);
	   my @ParamList = (42,	'Hello', 3.14);
	   my $thr2 = threads->create(\&sub1, @ParamList);
	   my $thr3 = threads->create(\&sub1, qw(Param1	Param2 Param3));

	   sub sub1 {
	       my @InboundParameters = @_;
	       print("In the thread\n");
	       print('Got parameters >', join('<>',@InboundParameters),	"<\n");
	   }

       The last	example	illustrates another feature of threads.	 You can spawn
       off several threads using the same subroutine.  Each thread executes
       the same	subroutine, but	in a separate thread with a separate
       environment and potentially separate arguments.

       "new()" is a synonym for	"create()".

   Waiting For A Thread	To Exit
       Since threads are also subroutines, they	can return values.  To wait
       for a thread to exit and	extract	any values it might return, you	can
       use the "join()"	method:

	   use threads;

	   my ($thr) = threads->create(\&sub1);

	   my @ReturnData = $thr->join();
	   print('Thread returned ', join(', ',	@ReturnData), "\n");

	   sub sub1 { return ('Fifty-six', 'foo', 2); }

       In the example above, the "join()" method returns as soon as the	thread
       ends.  In addition to waiting for a thread to finish and	gathering up
       any values that the thread might	have returned, "join()"	also performs
       any OS cleanup necessary	for the	thread.	 That cleanup might be
       important, especially for long-running programs that spawn lots of
       threads.	 If you	don't want the return values and don't want to wait
       for the thread to finish, you should call the "detach()"	method
       instead,	as described next.

       NOTE: In	the example above, the thread returns a	list, thus
       necessitating that the thread creation call be made in list context
       (i.e., "my ($thr)").  See "$thr->join()"	in threads and "THREAD
       CONTEXT"	in threads for more details on thread context and return
       values.

   Ignoring A Thread
       "join()"	does three things: it waits for	a thread to exit, cleans up
       after it, and returns any data the thread may have produced.  But what
       if you're not interested	in the thread's	return values, and you don't
       really care when	the thread finishes? All you want is for the thread to
       get cleaned up after when it's done.

       In this case, you use the "detach()" method.  Once a thread is
       detached, it'll run until it's finished;	then Perl will clean up	after
       it automatically.

	   use threads;

	   my $thr = threads->create(\&sub1);	# Spawn	the thread

	   $thr->detach();   # Now we officially don't care any	more

	   sleep(15);	     # Let thread run for awhile

	   sub sub1 {
	       my $count = 0;
	       while (1) {
		   $count++;
		   print("\$count is $count\n");
		   sleep(1);
	       }
	   }

       Once a thread is	detached, it may not be	joined,	and any	return data
       that it might have produced (if it was done and waiting for a join) is
       lost.

       "detach()" can also be called as	a class	method to allow	a thread to
       detach itself:

	   use threads;

	   my $thr = threads->create(\&sub1);

	   sub sub1 {
	       threads->detach();
	       # Do more work
	   }

   Process and Thread Termination
       With threads one	must be	careful	to make	sure they all have a chance to
       run to completion, assuming that	is what	you want.

       An action that terminates a process will	terminate all running threads.
       die() and exit()	have this property, and	perl does an exit when the
       main thread exits, perhaps implicitly by	falling	off the	end of your
       code, even if that's not	what you want.

       As an example of	this case, this	code prints the	message	"Perl exited
       with active threads: 2 running and unjoined":

	   use threads;
	   my $thr1 = threads->new(\&thrsub, "test1");
	   my $thr2 = threads->new(\&thrsub, "test2");
	   sub thrsub {
	      my ($message) = @_;
	      sleep 1;
	      print "thread $message\n";
	   }

       But when	the following lines are	added at the end:

	   $thr1->join();
	   $thr2->join();

       it prints two lines of output, a	perhaps	more useful outcome.

Threads	And Data
       Now that	we've covered the basics of threads, it's time for our next
       topic: Data.  Threading introduces a couple of complications to data
       access that non-threaded	programs never need to worry about.

   Shared And Unshared Data
       The biggest difference between Perl ithreads and	the old	5.005 style
       threading, or for that matter, to most other threading systems out
       there, is that by default, no data is shared. When a new	Perl thread is
       created,	all the	data associated	with the current thread	is copied to
       the new thread, and is subsequently private to that new thread!	This
       is similar in feel to what happens when a Unix process forks, except
       that in this case, the data is just copied to a different part of
       memory within the same process rather than a real fork taking place.

       To make use of threading, however, one usually wants the	threads	to
       share at	least some data	between	themselves. This is done with the
       threads::shared module and the ":shared"	attribute:

	   use threads;
	   use threads::shared;

	   my $foo :shared = 1;
	   my $bar = 1;
	   threads->create(sub { $foo++; $bar++; })->join();

	   print("$foo\n");  # Prints 2	since $foo is shared
	   print("$bar\n");  # Prints 1	since $bar is not shared

       In the case of a	shared array, all the array's elements are shared, and
       for a shared hash, all the keys and values are shared. This places
       restrictions on what may	be assigned to shared array and	hash elements:
       only simple values or references	to shared variables are	allowed	- this
       is so that a private variable can't accidentally	become shared. A bad
       assignment will cause the thread	to die.	For example:

	   use threads;
	   use threads::shared;

	   my $var	    = 1;
	   my $svar :shared = 2;
	   my %hash :shared;

	   ... create some threads ...

	   $hash{a} = 1;       # All threads see exists($hash{a})
			       # and $hash{a} == 1
	   $hash{a} = $var;    # okay -	copy-by-value: same effect as previous
	   $hash{a} = $svar;   # okay -	copy-by-value: same effect as previous
	   $hash{a} = \$svar;  # okay -	a reference to a shared	variable
	   $hash{a} = \$var;   # This will die
	   delete($hash{a});   # okay -	all threads will see !exists($hash{a})

       Note that a shared variable guarantees that if two or more threads try
       to modify it at the same	time, the internal state of the	variable will
       not become corrupted. However, there are	no guarantees beyond this, as
       explained in the	next section.

   Thread Pitfalls: Races
       While threads bring a new set of	useful tools, they also	bring a	number
       of pitfalls.  One pitfall is the	race condition:

	   use threads;
	   use threads::shared;

	   my $x :shared = 1;
	   my $thr1 = threads->create(\&sub1);
	   my $thr2 = threads->create(\&sub2);

	   $thr1->join();
	   $thr2->join();
	   print("$x\n");

	   sub sub1 { my $foo =	$x; $x = $foo +	1; }
	   sub sub2 { my $bar =	$x; $x = $bar +	1; }

       What do you think $x will be? The answer, unfortunately,	is it depends.
       Both "sub1()" and "sub2()" access the global variable $x, once to read
       and once	to write.  Depending on	factors	ranging	from your thread
       implementation's	scheduling algorithm to	the phase of the moon, $x can
       be 2 or 3.

       Race conditions are caused by unsynchronized access to shared data.
       Without explicit	synchronization, there's no way	to be sure that
       nothing has happened to the shared data between the time	you access it
       and the time you	update it.  Even this simple code fragment has the
       possibility of error:

	   use threads;
	   my $x :shared = 2;
	   my $y :shared;
	   my $z :shared;
	   my $thr1 = threads->create(sub { $y = $x; $x	= $y + 1; });
	   my $thr2 = threads->create(sub { $z = $x; $x	= $z + 1; });
	   $thr1->join();
	   $thr2->join();

       Two threads both	access $x.  Each thread	can potentially	be interrupted
       at any point, or	be executed in any order.  At the end, $x could	be 3
       or 4, and both $y and $z	could be 2 or 3.

       Even "$x	+= 5" or "$x++"	are not	guaranteed to be atomic.

       Whenever	your program accesses data or resources	that can be accessed
       by other	threads, you must take steps to	coordinate access or risk data
       inconsistency and race conditions. Note that Perl will protect its
       internals from your race	conditions, but	it won't protect you from you.

Synchronization	and control
       Perl provides a number of mechanisms to coordinate the interactions
       between themselves and their data, to avoid race	conditions and the
       like.  Some of these are	designed to resemble the common	techniques
       used in thread libraries	such as	"pthreads"; others are Perl-specific.
       Often, the standard techniques are clumsy and difficult to get right
       (such as	condition waits). Where	possible, it is	usually	easier to use
       Perlish techniques such as queues, which	remove some of the hard	work
       involved.

   Controlling access: lock()
       The "lock()" function takes a shared variable and puts a	lock on	it.
       No other	thread may lock	the variable until the variable	is unlocked by
       the thread holding the lock. Unlocking happens automatically when the
       locking thread exits the	block that contains the	call to	the "lock()"
       function.  Using	"lock()" is straightforward: This example has several
       threads doing some calculations in parallel, and	occasionally updating
       a running total:

	   use threads;
	   use threads::shared;

	   my $total :shared = 0;

	   sub calc {
	       while (1) {
		   my $result;
		   # (... do some calculations and set $result ...)
		   {
		       lock($total);  #	Block until we obtain the lock
		       $total += $result;
		   } # Lock implicitly released	at end of scope
		   last	if $result == 0;
	       }
	   }

	   my $thr1 = threads->create(\&calc);
	   my $thr2 = threads->create(\&calc);
	   my $thr3 = threads->create(\&calc);
	   $thr1->join();
	   $thr2->join();
	   $thr3->join();
	   print("total=$total\n");

       "lock()"	blocks the thread until	the variable being locked is
       available.  When	"lock()" returns, your thread can be sure that no
       other thread can	lock that variable until the block containing the lock
       exits.

       It's important to note that locks don't prevent access to the variable
       in question, only lock attempts.	 This is in keeping with Perl's
       longstanding tradition of courteous programming,	and the	advisory file
       locking that "flock()" gives you.

       You may lock arrays and hashes as well as scalars.  Locking an array,
       though, will not	block subsequent locks on array	elements, just lock
       attempts	on the array itself.

       Locks are recursive, which means	it's okay for a	thread to lock a
       variable	more than once.	 The lock will last until the outermost
       "lock()"	on the variable	goes out of scope. For example:

	   my $x :shared;
	   doit();

	   sub doit {
	       {
		   {
		       lock($x); # Wait	for lock
		       lock($x); # NOOP	- we already have the lock
		       {
			   lock($x); # NOOP
			   {
			       lock($x); # NOOP
			       lockit_some_more();
			   }
		       }
		   } # *** Implicit unlock here	***
	       }
	   }

	   sub lockit_some_more	{
	       lock($x); # NOOP
	   } # Nothing happens here

       Note that there is no "unlock()"	function - the only way	to unlock a
       variable	is to allow it to go out of scope.

       A lock can either be used to guard the data contained within the
       variable	being locked, or it can	be used	to guard something else, like
       a section of code. In this latter case, the variable in question	does
       not hold	any useful data, and exists only for the purpose of being
       locked. In this respect,	the variable behaves like the mutexes and
       basic semaphores	of traditional thread libraries.

   A Thread Pitfall: Deadlocks
       Locks are a handy tool to synchronize access to data, and using them
       properly	is the key to safe shared data.	 Unfortunately,	locks aren't
       without their dangers, especially when multiple locks are involved.
       Consider	the following code:

	   use threads;

	   my $x :shared = 4;
	   my $y :shared = 'foo';
	   my $thr1 = threads->create(sub {
	       lock($x);
	       sleep(20);
	       lock($y);
	   });
	   my $thr2 = threads->create(sub {
	       lock($y);
	       sleep(20);
	       lock($x);
	   });

       This program will probably hang until you kill it.  The only way	it
       won't hang is if	one of the two threads acquires	both locks first.  A
       guaranteed-to-hang version is more complicated, but the principle is
       the same.

       The first thread	will grab a lock on $x,	then, after a pause during
       which the second	thread has probably had	time to	do some	work, try to
       grab a lock on $y.  Meanwhile, the second thread	grabs a	lock on	$y,
       then later tries	to grab	a lock on $x.  The second lock attempt for
       both threads will block,	each waiting for the other to release its
       lock.

       This condition is called	a deadlock, and	it occurs whenever two or more
       threads are trying to get locks on resources that the others own.  Each
       thread will block, waiting for the other	to release a lock on a
       resource.  That never happens, though, since the	thread with the
       resource	is itself waiting for a	lock to	be released.

       There are a number of ways to handle this sort of problem.  The best
       way is to always	have all threads acquire locks in the exact same
       order.  If, for example,	you lock variables $x, $y, and $z, always lock
       $x before $y, and $y before $z.	It's also best to hold on to locks for
       as short	a period of time to minimize the risks of deadlock.

       The other synchronization primitives described below can	suffer from
       similar problems.

   Queues: Passing Data	Around
       A queue is a special thread-safe	object that lets you put data in one
       end and take it out the other without having to worry about
       synchronization issues.	They're	pretty straightforward,	and look like
       this:

	   use threads;
	   use Thread::Queue;

	   my $DataQueue = Thread::Queue->new();
	   my $thr = threads->create(sub {
	       while (my $DataElement =	$DataQueue->dequeue()) {
		   print("Popped $DataElement off the queue\n");
	       }
	   });

	   $DataQueue->enqueue(12);
	   $DataQueue->enqueue("A", "B", "C");
	   sleep(10);
	   $DataQueue->enqueue(undef);
	   $thr->join();

       You create the queue with "Thread::Queue->new()".  Then you can add
       lists of	scalars	onto the end with "enqueue()", and pop scalars off the
       front of	it with	"dequeue()".  A	queue has no fixed size, and can grow
       as needed to hold everything pushed on to it.

       If a queue is empty, "dequeue()"	blocks until another thread enqueues
       something.  This	makes queues ideal for event loops and other
       communications between threads.

   Semaphores: Synchronizing Data Access
       Semaphores are a	kind of	generic	locking	mechanism. In their most basic
       form, they behave very much like	lockable scalars, except that they
       can't hold data,	and that they must be explicitly unlocked. In their
       advanced	form, they act like a kind of counter, and can allow multiple
       threads to have the lock	at any one time.

   Basic semaphores
       Semaphores have two methods, "down()" and "up()": "down()" decrements
       the resource count, while "up()"	increments it. Calls to	"down()" will
       block if	the semaphore's	current	count would decrement below zero.
       This program gives a quick demonstration:

	   use threads;
	   use Thread::Semaphore;

	   my $semaphore = Thread::Semaphore->new();
	   my $GlobalVariable :shared =	0;

	   $thr1 = threads->create(\&sample_sub, 1);
	   $thr2 = threads->create(\&sample_sub, 2);
	   $thr3 = threads->create(\&sample_sub, 3);

	   sub sample_sub {
	       my $SubNumber = shift(@_);
	       my $TryCount = 10;
	       my $LocalCopy;
	       sleep(1);
	       while ($TryCount--) {
		   $semaphore->down();
		   $LocalCopy =	$GlobalVariable;
		   print("$TryCount tries left for sub $SubNumber "
			."(\$GlobalVariable is $GlobalVariable)\n");
		   sleep(2);
		   $LocalCopy++;
		   $GlobalVariable = $LocalCopy;
		   $semaphore->up();
	       }
	   }

	   $thr1->join();
	   $thr2->join();
	   $thr3->join();

       The three invocations of	the subroutine all operate in sync.  The
       semaphore, though, makes	sure that only one thread is accessing the
       global variable at once.

   Advanced Semaphores
       By default, semaphores behave like locks, letting only one thread
       "down()"	them at	a time.	 However, there	are other uses for semaphores.

       Each semaphore has a counter attached to	it. By default,	semaphores are
       created with the	counter	set to one, "down()" decrements	the counter by
       one, and	"up()" increments by one. However, we can override any or all
       of these	defaults simply	by passing in different	values:

	   use threads;
	   use Thread::Semaphore;

	   my $semaphore = Thread::Semaphore->new(5);
			   # Creates a semaphore with the counter set to five

	   my $thr1 = threads->create(\&sub1);
	   my $thr2 = threads->create(\&sub1);

	   sub sub1 {
	       $semaphore->down(5); # Decrements the counter by	five
	       # Do stuff here
	       $semaphore->up(5); # Increment the counter by five
	   }

	   $thr1->detach();
	   $thr2->detach();

       If "down()" attempts to decrement the counter below zero, it blocks
       until the counter is large enough.  Note	that while a semaphore can be
       created with a starting count of	zero, any "up()" or "down()" always
       changes the counter by at least one, and	so "$semaphore->down(0)" is
       the same	as "$semaphore->down(1)".

       The question, of	course,	is why would you do something like this? Why
       create a	semaphore with a starting count	that's not one,	or why
       decrement or increment it by more than one? The answer is resource
       availability.  Many resources that you want to manage access for	can be
       safely used by more than	one thread at once.

       For example, let's take a GUI driven program.  It has a semaphore that
       it uses to synchronize access to	the display, so	only one thread	is
       ever drawing at once.  Handy, but of course you don't want any thread
       to start	drawing	until things are properly set up.  In this case, you
       can create a semaphore with a counter set to zero, and up it when
       things are ready	for drawing.

       Semaphores with counters	greater	than one are also useful for
       establishing quotas.  Say, for example, that you	have a number of
       threads that can	do I/O at once.	 You don't want	all the	threads
       reading or writing at once though, since	that can potentially swamp
       your I/O	channels, or deplete your process's quota of filehandles.  You
       can use a semaphore initialized to the number of	concurrent I/O
       requests	(or open files)	that you want at any one time, and have	your
       threads quietly block and unblock themselves.

       Larger increments or decrements are handy in those cases	where a	thread
       needs to	check out or return a number of	resources at once.

   Waiting for a Condition
       The functions "cond_wait()" and "cond_signal()" can be used in
       conjunction with	locks to notify	co-operating threads that a resource
       has become available. They are very similar in use to the functions
       found in	"pthreads". However for	most purposes, queues are simpler to
       use and more intuitive. See threads::shared for more details.

   Giving up control
       There are times when you	may find it useful to have a thread explicitly
       give up the CPU to another thread.  You may be doing something
       processor-intensive and want to make sure that the user-interface
       thread gets called frequently.  Regardless, there are times that	you
       might want a thread to give up the processor.

       Perl's threading	package	provides the "yield()" function	that does
       this. "yield()" is pretty straightforward, and works like this:

	   use threads;

	   sub loop {
	       my $thread = shift;
	       my $foo = 50;
	       while($foo--) { print("In thread	$thread\n"); }
	       threads->yield();
	       $foo = 50;
	       while($foo--) { print("In thread	$thread\n"); }
	   }

	   my $thr1 = threads->create(\&loop, 'first');
	   my $thr2 = threads->create(\&loop, 'second');
	   my $thr3 = threads->create(\&loop, 'third');

       It is important to remember that	"yield()" is only a hint to give up
       the CPU,	it depends on your hardware, OS	and threading libraries	what
       actually	happens.  On many operating systems, yield() is	a no-op.
       Therefore it is important to note that one should not build the
       scheduling of the threads around	"yield()" calls. It might work on your
       platform	but it won't work on another platform.

General	Thread Utility Routines
       We've covered the workhorse parts of Perl's threading package, and with
       these tools you should be well on your way to writing threaded code and
       packages.  There	are a few useful little	pieces that didn't really fit
       in anyplace else.

   What	Thread Am I In?
       The "threads->self()" class method provides your	program	with a way to
       get an object representing the thread it's currently in.	 You can use
       this object in the same way as the ones returned	from thread creation.

   Thread IDs
       "tid()" is a thread object method that returns the thread ID of the
       thread the object represents.  Thread IDs are integers, with the	main
       thread in a program being 0.  Currently Perl assigns a unique TID to
       every thread ever created in your program, assigning the	first thread
       to be created a TID of 1, and increasing	the TID	by 1 for each new
       thread that's created.  When used as a class method, "threads->tid()"
       can be used by a	thread to get its own TID.

   Are These Threads The Same?
       The "equal()" method takes two thread objects and returns true if the
       objects represent the same thread, and false if they don't.

       Thread objects also have	an overloaded "==" comparison so that you can
       do comparison on	them as	you would with normal objects.

   What	Threads	Are Running?
       "threads->list()" returns a list	of thread objects, one for each	thread
       that's currently	running	and not	detached.  Handy for a number of
       things, including cleaning up at	the end	of your	program	(from the main
       Perl thread, of course):

	   # Loop through all the threads
	   foreach my $thr (threads->list()) {
	       $thr->join();
	   }

       If some threads have not	finished running when the main Perl thread
       ends, Perl will warn you	about it and die, since	it is impossible for
       Perl to clean up	itself while other threads are running.

       NOTE:  The main Perl thread (thread 0) is in a detached state, and so
       does not	appear in the list returned by "threads->list()".

A Complete Example
       Confused	yet? It's time for an example program to show some of the
       things we've covered.  This program finds prime numbers using threads.

	  1 #!/usr/bin/perl
	  2 # prime-pthread, courtesy of Tom Christiansen
	  3
	  4 use	strict;
	  5 use	warnings;
	  6
	  7 use	threads;
	  8 use	Thread::Queue;
	  9
	 10 sub	check_num {
	 11	my ($upstream, $cur_prime) = @_;
	 12	my $kid;
	 13	my $downstream = Thread::Queue->new();
	 14	while (my $num = $upstream->dequeue()) {
	 15	    next unless	($num %	$cur_prime);
	 16	    if ($kid) {
	 17		$downstream->enqueue($num);
	 18	    } else {
	 19		print("Found prime: $num\n");
	 20		$kid = threads->create(\&check_num, $downstream, $num);
	 21		if (! $kid) {
	 22		    warn("Sorry.  Ran out of threads.\n");
	 23		    last;
	 24		}
	 25	    }
	 26	}
	 27	if ($kid) {
	 28	    $downstream->enqueue(undef);
	 29	    $kid->join();
	 30	}
	 31 }
	 32
	 33 my $stream = Thread::Queue->new(3..1000, undef);
	 34 check_num($stream, 2);

       This program uses the pipeline model to generate	prime numbers.	Each
       thread in the pipeline has an input queue that feeds numbers to be
       checked,	a prime	number that it's responsible for, and an output	queue
       into which it funnels numbers that have failed the check.  If the
       thread has a number that's failed its check and there's no child
       thread, then the	thread must have found a new prime number.  In that
       case, a new child thread	is created for that prime and stuck on the end
       of the pipeline.

       This probably sounds a bit more confusing than it really	is, so let's
       go through this program piece by	piece and see what it does.  (For
       those of	you who	might be trying	to remember exactly what a prime
       number is, it's a number	that's only evenly divisible by	itself and 1.)

       The bulk	of the work is done by the "check_num()" subroutine, which
       takes a reference to its	input queue and	a prime	number that it's
       responsible for.	 After pulling in the input queue and the prime	that
       the subroutine is checking (line	11), we	create a new queue (line 13)
       and reserve a scalar for	the thread that	we're likely to	create later
       (line 12).

       The while loop from line	14 to line 26 grabs a scalar off the input
       queue and checks	against	the prime this thread is responsible for.
       Line 15 checks to see if	there's	a remainder when we divide the number
       to be checked by	our prime.  If there is	one, the number	must not be
       evenly divisible	by our prime, so we need to either pass	it on to the
       next thread if we've created one	(line 17) or create a new thread if we
       haven't.

       The new thread creation is line 20.  We pass on to it a reference to
       the queue we've created,	and the	prime number we've found.  In lines 21
       through 24, we check to make sure that our new thread got created, and
       if not, we stop checking	any remaining numbers in the queue.

       Finally,	once the loop terminates (because we got a 0 or	"undef"	in the
       queue, which serves as a	note to	terminate), we pass on the notice to
       our child, and wait for it to exit if we've created a child (lines 27
       and 30).

       Meanwhile, back in the main thread, we first create a queue (line 33)
       and queue up all	the numbers from 3 to 1000 for checking, plus a
       termination notice.  Then all we	have to	do to get the ball rolling is
       pass the	queue and the first prime to the "check_num()" subroutine
       (line 34).

       That's how it works.  It's pretty simple; as with many Perl programs,
       the explanation is much longer than the program.

Different implementations of threads
       Some background on thread implementations from the operating system
       viewpoint.  There are three basic categories of threads:	user-mode
       threads,	kernel threads,	and multiprocessor kernel threads.

       User-mode threads are threads that live entirely	within a program and
       its libraries.  In this model, the OS knows nothing about threads.  As
       far as it's concerned, your process is just a process.

       This is the easiest way to implement threads, and the way most OSes
       start.  The big disadvantage is that, since the OS knows	nothing	about
       threads,	if one thread blocks they all do.  Typical blocking activities
       include most system calls, most I/O, and	things like "sleep()".

       Kernel threads are the next step	in thread evolution.  The OS knows
       about kernel threads, and makes allowances for them.  The main
       difference between a kernel thread and a	user-mode thread is blocking.
       With kernel threads, things that	block a	single thread don't block
       other threads.  This is not the case with user-mode threads, where the
       kernel blocks at	the process level and not the thread level.

       This is a big step forward, and can give	a threaded program quite a
       performance boost over non-threaded programs.  Threads that block
       performing I/O, for example, won't block	threads	that are doing other
       things.	Each process still has only one	thread running at once,
       though, regardless of how many CPUs a system might have.

       Since kernel threading can interrupt a thread at	any time, they will
       uncover some of the implicit locking assumptions	you may	make in	your
       program.	 For example, something	as simple as "$x = $x +	2" can behave
       unpredictably with kernel threads if $x is visible to other threads, as
       another thread may have changed $x between the time it was fetched on
       the right hand side and the time	the new	value is stored.

       Multiprocessor kernel threads are the final step	in thread support.
       With multiprocessor kernel threads on a machine with multiple CPUs, the
       OS may schedule two or more threads to run simultaneously on different
       CPUs.

       This can	give a serious performance boost to your threaded program,
       since more than one thread will be executing at the same	time.  As a
       tradeoff, though, any of	those nagging synchronization issues that
       might not have shown with basic kernel threads will appear with a
       vengeance.

       In addition to the different levels of OS involvement in	threads,
       different OSes (and different thread implementations for	a particular
       OS) allocate CPU	cycles to threads in different ways.

       Cooperative multitasking	systems	have running threads give up control
       if one of two things happen.  If	a thread calls a yield function, it
       gives up	control.  It also gives	up control if the thread does
       something that would cause it to	block, such as perform I/O.  In	a
       cooperative multitasking	implementation,	one thread can starve all the
       others for CPU time if it so chooses.

       Preemptive multitasking systems interrupt threads at regular intervals
       while the system	decides	which thread should run	next.  In a preemptive
       multitasking system, one	thread usually won't monopolize	the CPU.

       On some systems,	there can be cooperative and preemptive	threads
       running simultaneously. (Threads	running	with realtime priorities often
       behave cooperatively, for example, while	threads	running	at normal
       priorities behave preemptively.)

       Most modern operating systems support preemptive	multitasking nowadays.

Performance considerations
       The main	thing to bear in mind when comparing Perl's ithreads to	other
       threading models	is the fact that for each new thread created, a
       complete	copy of	all the	variables and data of the parent thread	has to
       be taken. Thus, thread creation can be quite expensive, both in terms
       of memory usage and time	spent in creation. The ideal way to reduce
       these costs is to have a	relatively short number	of long-lived threads,
       all created fairly early	on (before the base thread has accumulated too
       much data). Of course, this may not always be possible, so compromises
       have to be made.	However, after a thread	has been created, its
       performance and extra memory usage should be little different than
       ordinary	code.

       Also note that under the	current	implementation,	shared variables use a
       little more memory and are a little slower than ordinary	variables.

Process-scope Changes
       Note that while threads themselves are separate execution threads and
       Perl data is thread-private unless explicitly shared, the threads can
       affect process-scope state, affecting all the threads.

       The most	common example of this is changing the current working
       directory using "chdir()".  One thread calls "chdir()", and the working
       directory of all	the threads changes.

       Even more drastic example of a process-scope change is "chroot()": the
       root directory of all the threads changes, and no thread	can undo it
       (as opposed to "chdir()").

       Further examples	of process-scope changes include "umask()" and
       changing	uids and gids.

       Thinking	of mixing "fork()" and threads?	 Please	lie down and wait
       until the feeling passes.  Be aware that	the semantics of "fork()" vary
       between platforms.  For example,	some Unix systems copy all the current
       threads into the	child process, while others only copy the thread that
       called "fork()".	You have been warned!

       Similarly, mixing signals and threads may be problematic.
       Implementations are platform-dependent, and even	the POSIX semantics
       may not be what you expect (and Perl doesn't even give you the full
       POSIX API).  For	example, there is no way to guarantee that a signal
       sent to a multi-threaded	Perl application will get intercepted by any
       particular thread.  (However, a recently	added feature does provide the
       capability to send signals between threads.  See	"THREAD	SIGNALLING" in
       threads for more	details.)

Thread-Safety of System	Libraries
       Whether various library calls are thread-safe is	outside	the control of
       Perl.  Calls often suffering from not being thread-safe include:
       "localtime()", "gmtime()",  functions fetching user, group and network
       information (such as "getgrent()", "gethostent()", "getnetent()"	and so
       on), "readdir()", "rand()", and "srand()". In general, calls that
       depend on some global external state.

       If the system Perl is compiled in has thread-safe variants of such
       calls, they will	be used.  Beyond that, Perl is at the mercy of the
       thread-safety or	-unsafety of the calls.	 Please	consult	your C library
       call documentation.

       On some platforms the thread-safe library interfaces may	fail if	the
       result buffer is	too small (for example the user	group databases	may be
       rather large, and the reentrant interfaces may have to carry around a
       full snapshot of	those databases).  Perl	will start with	a small
       buffer, but keep	retrying and growing the result	buffer until the
       result fits.  If	this limitless growing sounds bad for security or
       memory consumption reasons you can recompile Perl with
       "PERL_REENTRANT_MAXSIZE"	defined	to the maximum number of bytes you
       will allow.

Conclusion
       A complete thread tutorial could	fill a book (and has, many times), but
       with what we've covered in this introduction, you should	be well	on
       your way	to becoming a threaded Perl expert.

SEE ALSO
       Annotated POD for threads:
       <https://web.archive.org/web/20171028020148/http://annocpan.org/?mode=search&field=Module&name=threads>

       Latest version of threads on CPAN: <https://metacpan.org/pod/threads>

       Annotated POD for threads::shared:
       <https://web.archive.org/web/20171028020148/http://annocpan.org/?mode=search&field=Module&name=threads%3A%3Ashared>

       Latest version of threads::shared on CPAN:
       <https://metacpan.org/pod/threads::shared>

       Perl threads mailing list: <https://lists.perl.org/list/ithreads.html>

Bibliography
       Here's a	short bibliography courtesy of Juergen Christoffel:

   Introductory	Texts
       Birrell,	Andrew D. An Introduction to Programming with Threads. Digital
       Equipment Corporation, 1989, DEC-SRC Research Report #35	online as
       <https://www.hpl.hp.com/techreports/Compaq-DEC/SRC-RR-35.pdf> (highly
       recommended)

       Robbins,	Kay. A., and Steven Robbins. Practical Unix Programming: A
       Guide to	Concurrency, Communication, and	Multithreading.	Prentice-Hall,
       1996.

       Lewis, Bill, and	Daniel J. Berg.	Multithreaded Programming with
       Pthreads. Prentice Hall,	1997, ISBN 0-13-443698-9 (a well-written
       introduction to threads).

       Nelson, Greg (editor). Systems Programming with Modula-3.  Prentice
       Hall, 1991, ISBN	0-13-590464-1.

       Nichols,	Bradford, Dick Buttlar,	and Jacqueline Proulx Farrell.
       Pthreads	Programming. O'Reilly &	Associates, 1996, ISBN 156592-115-1
       (covers POSIX threads).

   OS-Related References
       Boykin, Joseph, David Kirschen, Alan Langerman, and Susan LoVerso.
       Programming under Mach. Addison-Wesley, 1994, ISBN 0-201-52739-1.

       Tanenbaum, Andrew S. Distributed	Operating Systems. Prentice Hall,
       1995, ISBN 0-13-219908-4	(great textbook).

       Silberschatz, Abraham, and Peter	B. Galvin. Operating System Concepts,
       4th ed. Addison-Wesley, 1995, ISBN 0-201-59292-4

   Other References
       Arnold, Ken and James Gosling. The Java Programming Language, 2nd ed.
       Addison-Wesley, 1998, ISBN 0-201-31006-6.

       comp.programming.threads	FAQ,
       <http://www.serpentine.com/~bos/threads-faq/>

       Le Sergent, T. and B. Berthomieu. "Incremental MultiThreaded Garbage
       Collection on Virtually Shared Memory Architectures" in Memory
       Management: Proc. of the	International Workshop IWMM 92,	St. Malo,
       France, September 1992, Yves Bekkers and	Jacques	Cohen, eds. Springer,
       1992, ISBN 3540-55940-X (real-life thread applications).

       Artur Bergman, "Where Wizards Fear To Tread", June 11, 2002,
       <http://www.perl.com/pub/a/2002/06/11/threads.html>

Acknowledgements
       Thanks (in no particular	order) to Chaim	Frenkel, Steve Fink, Gurusamy
       Sarathy,	Ilya Zakharevich, Benjamin Sugars, Juergen Christoffel,	Joshua
       Pritikin, and Alan Burlison, for	their help in reality-checking and
       polishing this article.	Big thanks to Tom Christiansen for his rewrite
       of the prime number generator.

AUTHOR
       Dan Sugalski <dan@sidhe.org>

       Slightly	modified by Arthur Bergman to fit the new thread model/module.

       Reworked	slightly by Joerg Walter <jwalt@cpan.org> to be	more concise
       about thread-safety of Perl code.

       Rearranged slightly by Elizabeth	Mattijsen <liz@dijkmat.nl> to put less
       emphasis	on yield().

Copyrights
       The original version of this article originally appeared	in The Perl
       Journal #10, and	is copyright 1998 The Perl Journal. It appears
       courtesy	of Jon Orwant and The Perl Journal.  This document may be
       distributed under the same terms	as Perl	itself.

perl v5.32.0			  2020-06-14			 PERLTHRTUT(1)

NAME | DESCRIPTION | What Is A Thread Anyway? | Threaded Program Models | What kind of threads are Perl threads? | Thread-Safe Modules | Thread Basics | Threads And Data | Synchronization and control | General Thread Utility Routines | A Complete Example | Different implementations of threads | Performance considerations | Process-scope Changes | Thread-Safety of System Libraries | Conclusion | SEE ALSO | Bibliography | Acknowledgements | AUTHOR | Copyrights

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