NAME
threads - Perl interpreter-based threads
VERSION
This document describes threads version 1.67
SYNOPSIS
use threads (yield,
stack_size => 64*4096,
exit => threads_only,
stringify);
sub start_thread {
my @args = @_;
print(Thread started: , join( , @args), "\n");
}
my $thr = threads->create(start_thread, argument);
$thr->join();
threads->create(sub { print("I am a thread\n"); })->join();
my $thr2 = async { foreach (@files) { ... } };
$thr2->join();
if (my $err = $thr2->error()) {
warn("Thread error: $err\n");
}
# Invoke thread in list context (implicit) so it can return a list
my ($thr) = threads->create(sub { return (qw/a b c/); });
# or specify list context explicitly
my $thr = threads->create({context => list},
sub { return (qw/a b c/); });
my @results = $thr->join();
$thr->detach();
# Get a threads object
$thr = threads->self();
$thr = threads->object($tid);
# Get a threads ID
$tid = threads->tid();
$tid = $thr->tid();
$tid = "$thr";
# Give other threads a chance to run
threads->yield();
yield();
# Lists of non-detached threads
my @threads = threads->list();
my $thread_count = threads->list();
my @running = threads->list(threads::running);
my @joinable = threads->list(threads::joinable);
# Test thread objects
if ($thr1 == $thr2) {
...
}
# Manage thread stack size
$stack_size = threads->get_stack_size();
$old_size = threads->set_stack_size(32*4096);
# Create a thread with a specific context and stack size
my $thr = threads->create({ context => list,
stack_size => 32*4096,
exit => thread_only },
\&foo);
# Get threads context
my $wantarray = $thr->wantarray();
# Check threads state
if ($thr->is_running()) {
sleep(1);
}
if ($thr->is_joinable()) {
$thr->join();
}
# Send a signal to a thread
$thr->kill(SIGUSR1);
# Exit a thread
threads->exit();
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 by default thread local. To use shared variables one must also use threads::shared:
use threads;
use threads::shared;
It is also important to note that you must enable threads by doing
use threadsas 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 threadsbefore you
use threads::shared. (
threadswill emit a warning if you do it the other way around.)
$thr= threads->create(FUNCTION, ARGS) |
This will create a new thread that will begin execution with the specified
entry point function, and give it the ARGS list as parameters. It will
return the corresponding threads object, or undefif thread creation failed. FUNCTION may either be the name of a function, an anonymous subroutine, or a code ref.
my $thr = threads->create(func_name, ...);
# or
my $thr = threads->create(sub { ... }, ...);
# or
my $thr = threads->create(\&func, ...);
The ->new()method is an alias for ->create(). |
$thr->join() |
This will wait for the corresponding thread to complete its execution. When
the thread finishes, ->join()will return the return value(s) of the entry point function. The context (void, scalar or list) for the return value(s) for ->join()is determined at the time of thread creation.
# Create thread in list context (implicit)
my ($thr1) = threads->create(sub {
my @results = qw(a b c);
return (@results);
});
# or (explicit)
my $thr1 = threads->create({context => list},
sub {
my @results = qw(a b c);
return (@results);
});
# Retrieve list results from thread
my @res1 = $thr1->join();
# Create thread in scalar context (implicit)
my $thr2 = threads->create(sub {
my $result = 42;
return ($result);
});
# Retrieve scalar result from thread
my $res2 = $thr2->join();
# Create a thread in void context (explicit)
my $thr3 = threads->create({void => 1},
sub { print("Hello, world\n"); });
# Join the thread in void context (i.e., no return value)
$thr3->join();
See THREAD CONTEXT for more details. If the program exits without all threads having either been joined or detached, then a warning will be issued. Calling ->join()or ->detach()on an already joined thread will cause an error to be thrown. |
$thr->detach() |
Makes the thread unjoinable, and causes any eventual return value to be
discarded. When the program exits, any detached threads that are still
running are silently terminated.
If the program exits without all threads having either been joined or detached, then a warning will be issued. Calling ->join()or ->detach()on an already detached thread will cause an error to be thrown. |
threads->detach() | Class method that allows a thread to detach itself. |
threads->self() | Class method that allows a thread to obtain its own threads object. |
$thr->tid() | Returns the ID of the thread. Thread IDs are unique integers with the main thread in a program being 0, and incrementing by 1 for every thread created. |
threads->tid() | Class method that allows a thread to obtain its own ID. |
$thr |
If you add the stringifyimport option to your use threadsdeclaration, then using a threads object in a string or a string context (e.g., as a hash key) will cause its ID to be used as the value:
use threads qw(stringify);
my $thr = threads->create(...);
print("Thread $thr started...\n"); # Prints out: Thread 1 started...
|
threads->object($tid) |
This will return the threads object for the active thread associated
with the specified thread ID. Returns undefif there is no thread associated with the TID, if the thread is joined or detached, if no TID is specified or if 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), and then just use yield()in your code. |
threads->list() | |
threads->list(threads::all) | |
threads->list(threads::running) | |
threads->list(threads::joinable) |
With no arguments (or using threads::all) and in a list context, returns a list of all non-joined, non-detached threads objects. In a scalar context, returns a count of the same. With a true argument (using threads::running), returns a list of all non-joined, non-detached threads objects that are still running. With a false argument (using threads::joinable), returns a list of all non-joined, non-detached threads objects that have finished running (i.e., for which ->join()will not block). |
$thr1->equal($thr2) |
Tests if two threads objects are the same thread or not. This is overloaded
to the more natural forms:
if ($thr1 == $thr2) {
print("Threads are the same\n");
}
# or
if ($thr1 != $thr2) {
print("Threads differ\n");
}
(Thread comparison is based on thread IDs.) |
async BLOCK; |
asynccreates a thread to execute the block immediately following it. This block is treated as an anonymous subroutine, and so must have a semicolon after the closing brace. Like threads->create(), asyncreturns a threads object. |
$thr->error() |
Threads are executed in an evalcontext. This method will return undefif the thread terminates normally. Otherwise, it returns the value of $@associated with the threads execution status in its evalcontext. |
$thr->_handle() |
This private method returns the memory location of the internal thread
structure associated with a threads object. For Win32, this is a pointer to
the HANDLEvalue returned by CreateThread(i.e., HANDLE *); for other platforms, it is a pointer to the pthread_tstructure used in the pthread_createcall (i.e., pthread_t *). This method is of no use for general Perl threads programming. Its intent is to provide other (XS-based) thread modules with the capability to access, and possibly manipulate, the underlying thread structure associated with a Perl thread. |
threads->_handle() | Class method that allows a thread to obtain its own handle. |
EXITING A THREAD
The usual method for terminating a thread is to return() from the entry point function with the appropriate return value(s).
threads->exit() |
If needed, a thread can be exited at any time by calling
threads->exit(). This will cause the thread to return undefin a scalar context, or the empty list in a list context. When called from the main thread, this behaves the same as exit(0). |
threads->exit(status) |
When called from a thread, this behaves like threads->exit()(i.e., the exit status code is ignored). When called from the main thread, this behaves the same as exit(status). |
die() |
Calling die()in a thread indicates an abnormal exit for the thread. Any $SIG{__DIE__}handler in the thread will be called first, and then the thread will exit with a warning message that will contain any arguments passed in the die()call. |
exit(status) |
Calling exit() inside a thread causes the whole
application to terminate. Because of this, the use of exit()inside threaded code, or in modules that might be used in threaded applications, is strongly discouraged. If exit()really is needed, then consider using the following:
threads->exit() if threads->can(exit); # Thread friendly
exit(status);
|
use threads exit => threads_only |
This globally overrides the default behavior of calling exit()inside a thread, and effectively causes such calls to behave the same as threads->exit(). In other words, with this setting, calling exit()causes only the thread to terminate. Because of its global effect, this setting should not be used inside modules or the like. The main thread is unaffected by this setting. |
threads->create({exit => thread_only}, ...) |
This overrides the default behavior of exit()inside the newly created thread only. |
$thr->set_thread_exit_only(boolean) |
This can be used to change the exit thread only behavior for a thread after
it has been created. With a true argument, exit()will cause only the thread to exit. With a false argument, exit()will terminate the application. The main thread is unaffected by this call. |
threads->set_thread_exit_only(boolean) |
Class method for use inside a thread to change its own behavior for exit(). The main thread is unaffected by this call. |
THREAD STATE
The following boolean methods are useful in determining the state of a thread.
$thr->is_running() | Returns true if a thread is still running (i.e., if its entry point function has not yet finished or exited). |
$thr->is_joinable() |
Returns true if the thread has finished running, is not detached and has not
yet been joined. In other words, the thread is ready to be joined, and a call
to $thr->join()will not block. |
$thr->is_detached() | Returns true if the thread has been detached. |
threads->is_detached() | Class method that allows a thread to determine whether or not it is detached. |
THREAD CONTEXT
As with subroutines, the type of value returned from a threads entry point function may be determined by the threads context: list, scalar or void. The threads context is determined at thread creation. This is necessary so that the context is available to the entry point function via wantarray(). The thread may then specify a value of the appropriate type to be returned from
->join().
Explicit context
Because thread creation and thread joining may occur in different contexts, it may be desirable to state the context explicitly to the threads entry point function. This may be done by calling
->create()with a hash reference as the first argument:
my $thr = threads->create({context => list}, \&foo);
...
my @results = $thr->join();
In the above, the threads object is returned to the parent thread in scalar context, and the threads entry point function
foowill be called in list (array) context such that the parent thread can receive a list (array) from the
->join()call. (
arrayis synonymous with
list.)
Similarly, if you need the threads object, but your thread will not be returning a value (i.e., void context), you would do the following:
my $thr = threads->create({context => void}, \&foo);
...
$thr->join();
The context type may also be used as the key in the hash reference followed by a true value:
threads->create({scalar => 1}, \&foo);
...
my ($thr) = threads->list();
my $result = $thr->join();
Implicit context
If not explicitly stated, the threads context is implied from the context of the
->create()call:
# Create thread in list context
my ($thr) = threads->create(...);
# Create thread in scalar context
my $thr = threads->create(...);
# Create thread in void context
threads->create(...);
\f(CW$thr\fP\->\fIwantarray()\fP
This returns the threads context in the same manner as wantarray().
threads\->\fIwantarray()\fP
Class method to return the current threads context. This returns the same value as running wantarray() inside the current threads entry point function.
THREAD STACK SIZE
The default per-thread stack size for different platforms varies significantly, and is almost always far more than is needed for most applications. On Win32, Perls makefile explicitly sets the default stack to 16 MB; on most other platforms, the system default is used, which again may be much larger than is needed.
By tuning the stack size to more accurately reflect your applications needs, you may significantly reduce your applications memory usage, and increase the number of simultaneously running threads.
Note that on Windows, address space allocation granularity is 64 KB, therefore, setting the stack smaller than that on Win32 Perl will not save any more memory.
threads->get_stack_size(); | Returns the current default per-thread stack size. The default is zero, which means the system default stack size is currently in use. |
$size= $thr->get_stack_size(); | Returns the stack size for a particular thread. A return value of zero indicates the system default stack size was used for the thread. |
$old_size= threads->set_stack_size($new_size); |
Sets a new default per-thread stack size, and returns the previous setting.
Some platforms have a minimum thread stack size. Trying to set the stack size below this value will result in a warning, and the minimum stack size will be used. Some Linux platforms have a maximum stack size. Setting too large of a stack size will cause thread creation to fail. If needed, $new_sizewill be rounded up to the next multiple of the memory page size (usually 4096 or 8192). Threads created after the stack size is set will then either call pthread_attr_setstacksize()(for pthreads platforms), or supply the stack size to CreateThread()(for Win32 Perl). (Obviously, this call does not affect any currently extant threads.) |
use threads (stack_size => VALUE); | This sets the default per-thread stack size at the start of the application. |
$ENV{PERL5_ITHREADS_STACK_SIZE} |
The default per-thread stack size may be set at the start of the application
through the use of the environment variable PERL5_ITHREADS_STACK_SIZE:
PERL5_ITHREADS_STACK_SIZE=1048576
export PERL5_ITHREADS_STACK_SIZE
perl -euse threads; print(threads->get_stack_size(), "\n")
This value overrides any stack_sizeparameter given to use threads. Its primary purpose is to permit setting the per-thread stack size for legacy threaded applications. |
threads->create({stack_size => VALUE}, FUNCTION, ARGS) |
To specify a particular stack size for any individual thread, call
->create()with a hash reference as the first argument:
my $thr = threads->create({stack_size => 32*4096}, \&foo, @args);
|
$thr2= $thr1->create(FUNCTION, ARGS) |
This creates a new thread ($thr2) that inherits the stack size from an existing thread ( $thr1). This is shorthand for the following:
my $stack_size = $thr1->get_stack_size();
my $thr2 = threads->create({stack_size => $stack_size}, FUNCTION, ARGS);
|
THREAD SIGNALLING
When safe signals is in effect (the default behavior - see Unsafe signals for more details), then signals may be sent and acted upon by individual threads.
$thr->kill(SIG...); |
Sends the specified signal to the thread. Signal names and (positive) signal
numbers are the same as those supported by
kill(). For example, SIGTERM, TERM and
(depending on the OS) 15 are all valid arguments to ->kill(). Returns the thread object to allow for method chaining:
$thr->kill(SIG...)->join();
|
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();
Heres another simplistic example that illustrates the use of thread signalling in conjunction with a semaphore to provide rudimentary suspend and 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 emulates signals at the Perl-level such that signal handlers are called in the appropriate thread. For example, sending
$thr->kill(STOP)does not actually suspend a thread (or the whole process), but does cause a
$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
kill()command (e.g.,
kill(KILL, $$)) are okay to use with the
->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 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.
WARNINGS
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 main thread exits, then this warning cannot be suppressed using no warnings threads;as suggested below. |
Thread creation failed: pthread_create returned # |
See the appropriate man page for pthread_createto determine the actual cause for the failure. |
Thread # terminated abnormally: ... |
A thread terminated in some manner other than just returning from its entry
point function, or by using threads->exit(). For example, the thread may have terminated because of an error, or by using die. |
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. |
Thread creation failed: pthread_attr_setstacksize(SIZE) returned 22 | The specified SIZE exceeds the systems maximum stack size. Use a smaller value for the stack size. |
no warnings threads;
in the appropriate scope.
ERRORS
This Perl not built to support threads |
The particular copy of Perl that youre trying to use was not built using the
useithreadsconfiguration 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 threads module (i.e., threaded and non-threaded Perls are binary incompatible.) |
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);
|
Cannot signal threads without safe signals |
Safe signals must be in effect to use the ->kill()signalling method. See Unsafe signals for more details. |
Unrecognized signal name: ... |
The particular copy of Perl that youre trying to use does not support the
specified signal being used in a ->kill()call. |
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 youve encountered is a known problem.
Thread-safe modules | See Making your module threadsafe in perlmod when creating modules that may be used in threaded applications, especially if those modules use non-Perl data, or XS code. | ||||||
Using non-thread-safe modules |
Unfortunately, you may encounter Perl modules that are not 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 require(and importif needed):
sub thr_func
{
require Unsafe::Module
# Unsafe::Module->import(...);
....
}
If the module is needed inside the main thread, try modifying your application so that the module is loaded (again using requireand ->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 <http://rt.cpan.org/Public/> against the problematic module. | ||||||
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
chdir()) will affect all the threads in the application. On MSWin32, each thread maintains its own the current working directory setting. | ||||||
Environment variables |
Currently, on all platforms except MSWin32, all system calls (e.g., using
system()or back-ticks) made from threads use the environment variable settings from the main thread. In other words, changes made to %ENVin a thread will not be visible in system calls made by that thread. To work around this, set environment variables as part of the 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. | ||||||
Parent-child threads | On some platforms, it might not be possible to destroy parent threads while there are still existing child threads. | ||||||
Creating threads inside special blocks |
Creating threads inside BEGIN, CHECKor INITblocks 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. | ||||||
Unsafe signals |
Since Perl 5.8.0, signals have been made safer in Perl by postponing their
handling until the interpreter is in a safe state. See
Safe Signals in perl58delta and Deferred Signals (Safe Signals) in perlipc
for more details.
Safe signals is the default behavior, and the old, immediate, unsafe signalling behavior is only in effect in the following situations:
If unsafe signals is in effect, then signal handling is not thread-safe, and the ->kill()signalling method cannot be used. | ||||||
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. | ||||||
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 Data::Dumper or Storable), and then reconstituting it in the joining thread. | ||||||
Perl Bugs and the CPAN Version of threads |
Support for threads extends beyond the code in this module (i.e.,
threads.pm and threads.xs), and into the Perl interpreter itself. Older
versions of Perl contain bugs that may manifest themselves despite using the
latest version of 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. | ||||||
REQUIREMENTS
Perl 5.8.0 or later
SEE ALSO
threads Discussion Forum on CPAN: <http://www.cpanforum.com/dist/threads>
Annotated POD for threads: <http://annocpan.org/~JDHEDDEN/threads-1.67/threads.pm>
Source repository: <http://code.google.com/p/threads-shared/>
threads::shared, perlthrtut
<http://www.perl.com/pub/a/2002/06/11/threads.html> and <http://www.perl.com/pub/a/2002/09/04/threads.html>
Perl threads mailing list: <http://lists.cpan.org/showlist.cgi?name=iThreads>
Stack size discussion: <http://www.perlmonks.org/?node_id=532956>
AUTHOR
Artur Bergman <sky AT crucially DOT net>
threads is released under the same license as Perl.
CPAN version produced by Jerry D. Hedden <jdhedden AT cpan DOT org>
ACKNOWLEDGEMENTS
Richard Soderberg <perl AT crystalflame DOT net> - Helping me out tons, trying to find reasons for races and other weird bugs!
Simon Cozens <simon AT brecon DOT co DOT uk> - Being there to answer zillions of annoying questions
Rocco Caputo <troc AT netrus DOT net>
Vipul Ved Prakash <mail AT vipul DOT net> - Helping with debugging
Dean Arnold <darnold AT presicient DOT com> - Stack size API