The ithreads code has been available since Perl 5.6.0, and is considered
stable. The user-level interface to ithreads (the L<threads> classes)
-appeared in the 5.8.0 release, and as of this time is considered stable,
-although as with all new features, should be treated with caution.
+appeared in the 5.8.0 release, and as of this time is considered stable
+although it should be treated with caution as with all new features.
=head1 What Is A Thread Anyway?
Perl Threads Are Different.
-=head1 Threadsafe Modules
+=head1 Thread-Safe Modules
-The addition of threads has changed Perl's internals
+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 the threads
-do not share data, pure Perl modules that don't interact with external
-systems should be safe. Modules that are not tagged as thread-safe should
-be tested or code reviewed before being used in production code.
+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
core. Threads are a new feature, and even some of the standard
modules aren't thread-safe.
-Even if a module is threadsafe, it doesn't mean that the module is optimized
+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 semaphores and lots of programming
-discipline to control access to the module. Semaphores are covered
-later in the article.
+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 L</"Basic semaphores">.
-See also L</"Threadsafety of System Libraries">.
+See also L</"Thread-Safety of System Libraries">.
=head1 Thread Basics
A possibly-threaded program using a possibly-threaded module might
have code like this:
- use Config;
- use MyMod;
+ use Config;
+ use MyMod;
BEGIN {
- if ($Config{useithreads}) {
- # We have threads
- require MyMod_threaded;
- import MyMod_threaded;
- } else {
- require MyMod_unthreaded;
- import MyMod_unthreaded;
+ if ($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.
+=head2 A Note about the Examples
+
+Although thread support is considered to be stable, there are still a number
+of quirks that may startle you when you try out any of the examples below.
+In a real situation, care should be taken that all threads are finished
+executing before the program exits. That care has B<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.
+Future versions of Perl may fix this problem.
+
=head2 Creating Threads
The L<threads> package provides the tools you need to create new
-threads. Like any other module, you need to tell Perl you want to use
+threads. Like any other module, you need to tell Perl that you want to use
it; C<use threads> imports all the pieces you need to create basic
threads.
-The simplest, straightforward way to create a thread is with new():
+The simplest, most straightforward way to create a thread is with new():
use threads;
$Param3 = "foo";
$thr = threads->new(\&sub1, "Param 1", "Param 2", $Param3);
$thr = threads->new(\&sub1, @ParamList);
- $thr = threads->new(\&sub1, qw(Param1 Param2 $Param3));
+ $thr = threads->new(\&sub1, qw(Param1 Param2 Param3));
sub sub1 {
my @InboundParameters = @_;
There are times when you may find it useful to have a thread
explicitly give up the CPU to another thread. Your threading package
might not support preemptive multitasking for threads, for example, or
-you may be doing something compute-intensive and want to make sure
+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.
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();
+ threads->yield;
$foo = 50;
while($foo--) { print "in thread $thread\n" }
}
my $thread1 = threads->new(\&loop, 'first');
my $thread2 = threads->new(\&loop, 'second');
my $thread3 = threads->new(\&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.
Therefore it is important to note that one should not build the scheduling of
$thr->detach; # Now we officially don't care any more
- sub sub1 {
+ sub sub1 {
$a = 0;
while (1) {
$a++;
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 want the threads to share
+To make use of threading however, one usually wants the threads to share
at least some data between themselves. This is done with the
L<threads::shared> module and the C< : shared> attribute:
my $foo : shared = 1;
my $bar = 1;
threads->new(sub { $foo++; $bar++ })->join;
-
+
print "$foo\n"; #prints 2 since $foo is shared
print "$bar\n"; #prints 1 since $bar is not 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 affect as previous
- $hash{a} = $svar # okay - copy-by-value: same affect as previous
+ $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})
my $c : shared;
my $thr1 = threads->create(sub { $b = $a; $a = $b + 1; });
my $thr2 = threads->create(sub { $c = $a; $a = $c + 1; });
- $thr1->join();
- $thr2->join();
+ $thr1->join;
+ $thr2->join;
Two threads both access $a. Each thread can potentially be interrupted
at any point, or be executed in any order. At the end, $a could be 3
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 C<pthreads>; others are Perl-specific. Often, the
-standard techniques are clumsily and difficult to get right (such as
+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.
=head2 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 the variable is unlocked
+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 exists the outermost block that contains
+when the locking thread exits the outermost block that contains
C<lock()> function. Using lock() is straightforward: this example has
several threads doing some calculations in parallel, and occasionally
updating a running total:
# (... do some calculations and set $result ...)
{
lock($total); # block until we obtain the lock
- $total += $result
+ $total += $result;
} # lock implicitly released at end of scope
last if $result == 0;
}
{
{
lock($x); # wait for lock
- lock($x): # NOOP - we already have the lock
+ lock($x); # NOOP - we already have the lock
{
lock($x); # NOOP
{
this:
use threads;
- use threads::shared::queue;
+ use Thread::Queue;
- my $DataQueue = threads::shared::queue->new();
+ my $DataQueue = Thread::Queue->new;
$thr = threads->new(sub {
while ($DataElement = $DataQueue->dequeue) {
print "Popped $DataElement off the queue\n";
$DataQueue->enqueue(\$thr);
sleep 10;
$DataQueue->enqueue(undef);
- $thr->join();
+ $thr->join;
-You create the queue with C<new threads::shared::queue>. Then you can
+You create the queue with C<new Thread::Queue>. 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.
gives a quick demonstration:
use threads qw(yield);
- use threads::shared::semaphore;
+ use Thread::Semaphore;
- my $semaphore = new threads::shared::semaphore;
+ my $semaphore = new Thread::Semaphore;
my $GlobalVariable : shared = 0;
$thr1 = new threads \&sample_sub, 1;
}
}
- $thr1->join();
- $thr2->join();
- $thr3->join();
+ $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
of these defaults simply by passing in different values:
use threads;
- use threads::shared::semaphore;
- my $semaphore = threads::shared::semaphore->new(5);
+ use Thread::Semaphore;
+ my $semaphore = Thread::Semaphore->new(5);
# Creates a semaphore with the counter set to five
$thr1 = threads->new(\&sub1);
$semaphore->up(5); # Increment the counter by five
}
- $thr1->detach();
- $thr2->detach();
+ $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
4 use strict;
5
6 use threads;
- 7 use threads::shared::queue;
+ 7 use Thread::Queue;
8
- 9 my $stream = new threads::shared::queue;
+ 9 my $stream = new Thread::Queue;
10 my $kid = new threads(\&check_num, $stream, 2);
11
12 for my $i ( 3 .. 1000 ) {
14 }
15
16 $stream->enqueue(undef);
- 17 $kid->join();
+ 17 $kid->join;
18
19 sub check_num {
20 my ($upstream, $cur_prime) = @_;
21 my $kid;
- 22 my $downstream = new threads::shared::queue;
+ 22 my $downstream = new Thread::Queue;
23 while (my $num = $upstream->dequeue) {
24 next unless $num % $cur_prime;
25 if ($kid) {
30 }
31 }
32 $downstream->enqueue(undef) if $kid;
- 33 $kid->join() if $kid;
+ 33 $kid->join if $kid;
34 }
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
-that into which it funnels numbers that have failed the check. If the thread
+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
Also note that under the current implementation, shared variables
use a little more memory and are a little slower than ordinary variables.
-=head1 Threadsafety of System Libraries
+=head1 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/gids.
+
+Thinking of mixing fork() and threads? Please lie down and wait
+until the feeling passes-- but in case you really want to know,
+the semantics is that fork() duplicates all the threads.
+(In UNIX, at least, other platforms will do something different.)
-Whether various library calls are threadsafe is outside the control
-of Perl. Calls often suffering from not being threadsafe include
+Similarly, mixing signals and threads should not be attempted.
+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).
+
+=head1 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(), get{gr,host,net,proto,serv,pw}*(), readdir(),
-rand(), srand(). If the system Perl is compiled in has threadsafe
-variants of these calls, they will be used, but besides that, Perl is
-at the mercy of the thread safety or unsafety of the calls. Please
-consult your C library call documentation.
+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.
=head1 Conclusion
Arnold, Ken and James Gosling. The Java Programming Language, 2nd
ed. Addison-Wesley, 1998, ISBN 0-201-31006-6.
+comp.programming.threads FAQ,
+L<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,
+L<http://www.perl.com/pub/a/2002/06/11/threads.html>
+
=head1 Acknowledgements
Thanks (in no particular order) to Chaim Frenkel, Steve Fink, Gurusamy
Slightly modified by Arthur Bergman to fit the new thread model/module.
+Reworked slightly by Jörg Walter E<lt>jwalt@cpan.org<gt> to be more concise
+about thread-safety of perl code.
+
=head1 Copyrights
The original version of this article originally appeared in The Perl
https://perl5.git.perl.org / perl5.git / blobdiff