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threads(3) - threads - Perl extension allowing use of interpreter based threads from perl - man 3 threads

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threads(3)             Perl Programmers Reference Guide             threads(3)

       threads - Perl extension allowing use of interpreter based threads from

           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 = threads->self();
           $thread = threads->object( $tid );




       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 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(1,8) the script itself and that it is not
       possible to enable threading inside an "eval """, "do", "require", or
       "use".  In particular, if(3,n) 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(3,n) 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(3,n) thread creation failed. The new()
           method is an alias for create().

           This will wait for the corresponding thread to join. When the
           thread finishes, join(1,n)() will return the return values of the entry
           point function. If the thread has been detached, an error(8,n) will be

           The context (scalar or list) of the thread creation is also the
           context for join(1,n)().  This means that if(3,n) you intend to return an
           array from a thread, you must use "my ($thread) =
           threads-"new(...)>, and that if(3,n) 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(3,n,1 builtins)(), or in(1,8)
           the case of the main thread, reaches the end of the main program

           Will make the thread unjoinable, and cause any eventual return
           value to be discarded.

           This will return the thread object for the current thread.

           This will return the id of the thread.  Thread IDs are integers,
           with the main thread in(1,8) a program being 0.  Currently Perl assigns
           a unique tid to every thread ever created in(1,8) 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 cur-
           rent thread id if(3,n) 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(3,n) there is no thread associated
           with the tid or no tid is specified or the specified tid is undef.

           This is a suggestion to the OS to let this thread yield CPU time(1,2,n) 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(1,8)
           your code.

           This will return a list of all non joined, non detached threads.

       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 "threads->new", "async"
           returns a thread object.

       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(3,n,1 builtins) from the main thread.

       The current implementation of threads has been an attempt to get a cor-
       rect threading system working that could be built on, and optimized, in(1,8)
       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.

       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(1,8) later versions of perl.

       tid is I32
           The thread id is a 32 bit integer, it can potentially overflow.
           This might be fixed in(1,8) a later version(1,3,5) 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(3,n) it

       Creating threads inside BEGIN blocks
           Creating threads inside BEGIN blocks (or during the compilation
           phase in(1,8) general) does not work.  (In Windows, trying to use fork()
           inside BEGIN blocks is an equally losing proposition, since it has
           been implemented in(1,8) 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(2,7) han-
           dling is not threadsafe.

       Arthur Bergman <sky at>

       threads is released under the same license as Perl.

       Thanks to

       Richard Soderberg <perl at> Helping me out tons, try-
       ing to find reasons for races and other weird bugs!

       Simon Cozens <simon at> Being there to answer zillions of
       annoying questions

       Rocco Caputo <troc at>

       Vipul Ved Prakash <mail(1,8) at> Helping with debugging.

       please join(1,n) for more information

       threads::shared, perlthrtut,
       <>, perlcall, perlem-
       bed, perlguts

perl v5.8.5                       2001-09-21                        threads(3)

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