threads.pm

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=item $thr->kill('SIG...');

Sends the specified signal to the thread.  Signal names and (positive) signal
numbers are the same as those supported by
L<kill()|perlfunc/"kill SIGNAL, LIST">.  For example, 'SIGTERM', 'TERM' and
(depending on the OS) 15 are all valid arguments to C<-E<gt>kill()>.

Returns the thread object to allow for method chaining:

    $thr->kill('SIG...')->join();

=back

Signal handlers need to be set up in the threads for the signals they are
expected to act upon.  Here's an example for I<cancelling> a thread:

    use threads;

    sub thr_func
    {
        # Thread 'cancellation' signal handler
        $SIG{'KILL'} = sub { threads->exit(); };

        ...
    }

    # Create a thread
    my $thr = threads->create('thr_func');

    ...

    # Signal the thread to terminate, and then detach
    # it so that it will get cleaned up automatically
    $thr->kill('KILL')->detach();

Here's another simplistic example that illustrates the use of thread
signalling in conjunction with a semaphore to provide rudimentary I<suspend>
and I<resume> capabilities:

    use threads;
    use Thread::Semaphore;

    sub thr_func
    {
        my $sema = shift;

        # Thread 'suspend/resume' signal handler
        $SIG{'STOP'} = sub {
            $sema->down();      # Thread suspended
            $sema->up();        # Thread resumes
        };

        ...
    }

    # Create a semaphore and pass it to a thread
    my $sema = Thread::Semaphore->new();
    my $thr = threads->create('thr_func', $sema);

    # Suspend the thread
    $sema->down();
    $thr->kill('STOP');

    ...

    # Allow the thread to continue
    $sema->up();

CAVEAT:  The thread signalling capability provided by this module does not
actually send signals via the OS.  It I<emulates> signals at the Perl-level
such that signal handlers are called in the appropriate thread.  For example,
sending C<$thr-E<gt>kill('STOP')> does not actually suspend a thread (or the
whole process), but does cause a C<$SIG{'STOP'}> handler to be called in that
thread (as illustrated above).

As such, signals that would normally not be appropriate to use in the
C<kill()> command (e.g., C<kill('KILL', $$)>) are okay to use with the
C<-E<gt>kill()> method (again, as illustrated above).

Correspondingly, sending a signal to a thread does not disrupt the operation
the thread is currently working on:  The signal will be acted upon after the
current operation has completed.  For instance, if the thread is I<stuck> on
an I/O call, sending it a signal will not cause the I/O call to be interrupted
such that the signal is acted up immediately.

Sending a signal to a terminated thread is ignored.

=head1 WARNINGS

=over 4

=item Perl exited with active threads:

If the program exits without all threads having either been joined or
detached, then this warning will be issued.

NOTE:  If the I<main> thread exits, then this warning cannot be suppressed
using C<no warnings 'threads';> as suggested below.

=item Thread creation failed: pthread_create returned #

See the appropriate I<man> page for C<pthread_create> to determine the actual
cause for the failure.

=item Thread # terminated abnormally: ...

A thread terminated in some manner other than just returning from its entry
point function, or by using C<threads-E<gt>exit()>.  For example, the thread
may have terminated because of an error, or by using C<die>.

=item Using minimum thread stack size of #

Some platforms have a minimum thread stack size.  Trying to set the stack size
below this value will result in the above warning, and the stack size will be
set to the minimum.

=item Thread creation failed: pthread_attr_setstacksize(I<SIZE>) returned 22

The specified I<SIZE> exceeds the system's maximum stack size.  Use a smaller
value for the stack size.

=back

If needed, thread warnings can be suppressed by using:

    no warnings 'threads';

in the appropriate scope.

=head1 ERRORS

=over 4

=item This Perl not built to support threads

The particular copy of Perl that you're trying to use was not built using the
C<useithreads> configuration option.

Having threads support requires all of Perl and all of the XS modules in the
Perl installation to be rebuilt; it is not just a question of adding the
L<threads> module (i.e., threaded and non-threaded Perls are binary
incompatible.)

=item Cannot change stack size of an existing thread

The stack size of currently extant threads cannot be changed, therefore, the
following results in the above error:

    $thr->set_stack_size($size);

=item Cannot signal threads without safe signals

Safe signals must be in effect to use the C<-E<gt>kill()> signalling method.
See L</"Unsafe signals"> for more details.

=item Unrecognized signal name: ...

The particular copy of Perl that you're trying to use does not support the
specified signal being used in a C<-E<gt>kill()> call.

=back

=head1 BUGS AND LIMITATIONS

Before you consider posting a bug report, please consult, and possibly post a
message to the discussion forum to see if what you've encountered is a known
problem.

=over

=item Thread-safe modules

See L<perlmod/"Making your module threadsafe"> when creating modules that may
be used in threaded applications, especially if those modules use non-Perl
data, or XS code.

=item Using non-thread-safe modules

Unfortunately, you may encounter Perl modules that are not I<thread-safe>.
For example, they may crash the Perl interpreter during execution, or may dump
core on termination.  Depending on the module and the requirements of your
application, it may be possible to work around such difficulties.

If the module will only be used inside a thread, you can try loading the
module from inside the thread entry point function using C<require> (and
C<import> if needed):

    sub thr_func
    {
        require Unsafe::Module
        # Unsafe::Module->import(...);

        ....
    }

If the module is needed inside the I<main> thread, try modifying your
application so that the module is loaded (again using C<require> and
C<-E<gt>import()>) after any threads are started, and in such a way that no
other threads are started afterwards.

If the above does not work, or is not adequate for your application, then file
a bug report on L<http://rt.cpan.org/Public/> against the problematic module.

=item Current working directory

On all platforms except MSWin32, the setting for the current working directory
is shared among all threads such that changing it in one thread (e.g., using
C<chdir()>) will affect all the threads in the application.

On MSWin32, each thread maintains its own the current working directory
setting.

=item Environment variables

Currently, on all platforms except MSWin32, all I<system> calls (e.g., using
C<system()> or back-ticks) made from threads use the environment variable
settings from the I<main> thread.  In other words, changes made to C<%ENV> in
a thread will not be visible in I<system> calls made by that thread.

To work around this, set environment variables as part of the I<system> call.
For example:

    my $msg = 'hello';
    system("FOO=$msg; echo \$FOO");   # Outputs 'hello' to STDOUT

On MSWin32, each thread maintains its own set of environment variables.

=item Parent-child threads

On some platforms, it might not be possible to destroy I<parent> threads while
there are still existing I<child> threads.

=item Creating threads inside special blocks

Creating threads inside C<BEGIN>, C<CHECK> or C<INIT> blocks should not be
relied upon.  Depending on the Perl version and the application code, results
may range from success, to (apparently harmless) warnings of leaked scalar, or
all the way up to crashing of the Perl interpreter.

=item Unsafe signals

Since Perl 5.8.0, signals have been made safer in Perl by postponing their
handling until the interpreter is in a I<safe> state.  See
L<perl58delta/"Safe Signals"> and L<perlipc/"Deferred Signals (Safe Signals)">
for more details.

Safe signals is the default behavior, and the old, immediate, unsafe
signalling behavior is only in effect in the following situations:

=over 4

=item * Perl has been built with C<PERL_OLD_SIGNALS> (see C<perl -V>).

=item * The environment variable C<PERL_SIGNALS> is set to C<unsafe> (see L<perlrun/"PERL_SIGNALS">).

=item * The module L<Perl::Unsafe::Signals> is used.

=back

If unsafe signals is in effect, then signal handling is not thread-safe, and
the C<-E<gt>kill()> signalling method cannot be used.

=item Returning closures from threads

Returning closures from threads should not be relied upon.  Depending of the
Perl version and the application code, results may range from success, to
(apparently harmless) warnings of leaked scalar, or all the way up to crashing
of the Perl interpreter.

=item Returning objects from threads

Returning objects from threads does not work.  Depending on the classes
involved, you may be able to work around this by returning a serialized
version of the object (e.g., using L<Data::Dumper> or L<Storable>), and then
reconstituting it in the joining thread.

=item Perl Bugs and the CPAN Version of L<threads>

Support for threads extends beyond the code in this module (i.e.,
F<threads.pm> and F<threads.xs>), and into the Perl interpreter itself.  Older
versions of Perl contain bugs that may manifest themselves despite using the
latest version of L<threads> from CPAN.  There is no workaround for this other
than upgrading to the latest version of Perl.

Even with the latest version of Perl, it is known that certain constructs
with threads may result in warning messages concerning leaked scalars or
unreferenced scalars.  However, such warnings are harmless, and may safely be
ignored.

=back

=head1 REQUIREMENTS

Perl 5.8.0 or later

=head1 SEE ALSO

L<threads> Discussion Forum on CPAN:
L<http://www.cpanforum.com/dist/threads>

Annotated POD for L<threads>:
L<http://annocpan.org/~JDHEDDEN/threads-1.67/threads.pm>

Source repository:
L<http://code.google.com/p/threads-shared/>

L<threads::shared>, L<perlthrtut>

L<http://www.perl.com/pub/a/2002/06/11/threads.html> and
L<http://www.perl.com/pub/a/2002/09/04/threads.html>

Perl threads mailing list:
L<http://lists.cpan.org/showlist.cgi?name=iThreads>

Stack size discussion:
L<http://www.perlmonks.org/?node_id=532956>

=head1 AUTHOR

Artur Bergman E<lt>sky AT crucially DOT netE<gt>

threads is released under the same license as Perl.

CPAN version produced by Jerry D. Hedden <jdhedden AT cpan DOT org>

=head1 ACKNOWLEDGEMENTS

Richard Soderberg E<lt>perl AT crystalflame DOT netE<gt> -
Helping me out tons, trying to find reasons for races and other weird bugs!

Simon Cozens E<lt>simon AT brecon DOT co DOT ukE<gt> -
Being there to answer zillions of annoying questions

Rocco Caputo E<lt>troc AT netrus DOT netE<gt>

Vipul Ved Prakash E<lt>mail AT vipul DOT netE<gt> -
Helping with debugging

Dean Arnold E<lt>darnold AT presicient DOT comE<gt> -
Stack size API

=cut