threads
threads(3) Perl Programmers Reference Guide threads(3)
NAME
threads - Perl extension allowing use of interpreter based threads
from perl
SYNOPSIS
use threads;
sub start_thread {
print "Thread started\n";
}
my $thread = threads->create("start_thread","argument");
my $thread2 = $thread->create(sub { print "I am a thread"},"argument");
my $thread3 = async { foreach (@files) { ... } };
$thread->join();
$thread->detach();
$thread = threads->self();
$thread = threads->object( $tid );
$thread->tid();
threads->tid();
threads->self->tid();
threads->yield();
threads->list();
DESCRIPTION
Perl 5.6 introduced something called interpreter threads. Interpreter
threads are different from "5005threads" (the thread model of Perl
5.005) by creating a new perl interpreter per thread and not sharing
any data or state between threads by default.
Prior to perl 5.8 this has only been available to people embedding
perl and for emulating fork() on windows.
The threads API is loosely based on the old Thread.pm API. It is very
important to note that variables are not shared between threads, all
variables are per default thread local. To use shared variables one
must use threads::shared.
It is also important to note that you must enable threads by doing
"use threads" as early as possible in the script itself and that it is
not possible to enable threading inside an "eval """, "do", "require",
or "use". In particular, if you are intending to share variables with
threads::shared, you must "use threads" before you "use
threads::shared" and "threads" will emit a warning if you do it the
other way around.
$thread = threads->create(function, LIST)
This will create a new thread with the entry point function and
give it LIST as parameters. It will return the corresponding
threads object, or "undef" if thread creation failed. The new()
method is an alias for create().
$thread->join
This will wait for the corresponding thread to join. When the
thread finishes, join() will return the return values of the entry
point function. If the thread has been detached, an error will be
thrown.
The context (void, scalar or list) of the thread creation is also
the context for join(). This means that if you intend to return
an array from a thread, you must use "my ($thread) =
threads-"new(...)>, and that if you intend to return a scalar, you
must use "my $thread = ...".
If the program exits without all other threads having been either
joined or detached, then a warning will be issued. (A program
exits either because one of its threads explicitly calls exit(),
or in the case of the main thread, reaches the end of the main
program file.)
$thread->detach
Will make the thread unjoinable, and cause any eventual return
value to be discarded.
threads->self
This will return the thread object for the current thread.
$thread->tid
This will return the id of the thread. 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, assign-
ing the first thread to be created a tid of 1, and increasing the
tid by 1 for each new thread that’s created.
NB the class method "threads->tid()" is a quick way to get the
current thread id if you don’t have your thread object handy.
threads->object( tid )
This will return the thread object for the thread associated with
the specified tid. Returns undef if there is no thread associated
with the tid or no tid is specified or the specified tid is undef.
threads->yield();
This is a suggestion to the OS to let this thread yield CPU time
to other threads. What actually happens is highly dependent upon
the underlying thread implementation.
You may do "use threads qw(yield)" then use just a bare "yield" in
your code.
threads->list();
This will return a list of all non joined, non detached threads.
async BLOCK;
"async" creates a thread to execute the block immediately follow-
ing it. This block is treated as an anonymous sub, and so must
have a semi-colon after the closing brace. Like "threads->new",
"async" returns a thread object.
WARNINGS
A thread exited while %d other threads were still running
A thread (not necessarily the main thread) exited while there were
still other threads running. Usually it’s a good idea to first
collect the return values of the created threads by joining them,
and only then exit from the main thread.
TODO
The current implementation of threads has been an attempt to get a
correct threading system working that could be built on, and opti-
mized, in newer versions of perl.
Currently the overhead of creating a thread is rather large, also the
cost of returning values can be large. These are areas were there most
likely will be work done to optimize what data that needs to be
cloned.
BUGS
Parent-Child threads.
On some platforms it might not be possible to destroy "parent"
threads while there are still existing child "threads".
This will possibly be fixed in later versions of perl.
tid is I32
The thread id is a 32 bit integer, it can potentially overflow.
This might be fixed in a later version of perl.
Returning objects
When you return an object the entire stash that the object is
blessed as well. This will lead to a large memory usage. The
ideal situation would be to detect the original stash if it
existed.
Creating threads inside BEGIN blocks
Creating threads inside BEGIN blocks (or during the compilation
phase in general) does not work. (In Windows, trying to use
fork() inside BEGIN blocks is an equally losing proposition, since
it has been implemented in very much the same way as threads.)
PERL_OLD_SIGNALS are not threadsafe, will not be.
If your Perl has been built with PERL_OLD_SIGNALS (one has to
explicitly add that symbol to ccflags, see "perl -V"), signal han-
dling is not threadsafe.
AUTHOR and COPYRIGHT
Arthur Bergman <sky at nanisky.com>
threads is released under the same license as Perl.
Thanks to
Richard Soderberg <perl at crystalflame.net> Helping me out tons, try-
ing to find reasons for races and other weird bugs!
Simon Cozens <simon at brecon.co.uk> Being there to answer zillions of
annoying questions
Rocco Caputo <troc at netrus.net>
Vipul Ved Prakash <mail at vipul.net> Helping with debugging.
please join perl-ithreads@perl.org for more information
SEE ALSO
threads::shared, perlthrtut,
<http://www.perl.com/pub/a/2002/06/11/threads.html>, perlcall, perlem-
bed, perlguts
perl v5.8.8 2001-09-21 threads(3)
