| threads(3pm) | Perl Programmers Reference Guide | threads(3pm) |
threads - Perl interpreter-based threads
This document describes threads version 2.26
The "interpreter-based threads" provided by Perl are not the fast, lightweight system for multitasking that one might expect or hope for. Threads are implemented in a way that makes them easy to misuse. Few people know how to use them correctly or will be able to provide help.
The use of interpreter-based threads in perl is officially discouraged.
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 thread's object
$thr = threads->self();
$thr = threads->object($tid);
# Get a thread's 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 thread's context
my $wantarray = $thr->wantarray();
# Check thread's state
if ($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();
Since Perl 5.8, thread programming has been available using a model called interpreter threads which provides a new Perl interpreter for each thread, and, by default, results in no data or state information being shared between threads.
(Prior to Perl 5.8, 5005threads was available through the "Thread.pm" API. This threading model has been deprecated, and was removed as of Perl 5.10.0.)
As just mentioned, all variables are, by default, thread local. To use shared variables, you need to also load threads::shared:
use threads;
use threads::shared;
When loading threads::shared, you must "use threads" before you "use threads::shared". ("threads" will emit a warning if you do it the other way around.)
It is strongly recommended that you enable threads via "use threads" as early as possible in your script.
If needed, scripts can be written so as to run on both threaded and non-threaded Perls:
my $can_use_threads = eval 'use threads; 1';
if ($can_use_threads) {
# Do processing using threads
...
} else {
# Do it without using threads
...
}
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 $thr = threads->create(\&func, ...);
The "->new()" method is an alias for "->create()".
The context (void, scalar or list) for the return value(s) for "->join()" is determined at the time of thread creation.
# Create thread in list context (implicit)
my ($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.
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.
use threads qw(stringify);
my $thr = threads->create(...);
print("Thread $thr started\n"); # Prints: Thread 1 started
You may do "use threads qw(yield)", and then just use "yield()" in your code.
With a true argument (using "threads::running"), returns a list of all non-joined, non-detached threads objects that are still running.
With a false argument (using "threads::joinable"), returns a list of all non-joined, non-detached threads objects that have finished running (i.e., for which "->join()" will not block).
if ($thr1 == $thr2) {
print("Threads are the same\n");
}
# or
if ($thr1 != $thr2) {
print("Threads differ\n");
}
(Thread comparison is based on thread IDs.)
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.
The usual method for terminating a thread is to return() from the entry point function with the appropriate return value(s).
When called from the main thread, this behaves the same as exit(0).
When called from the main thread, this behaves the same as "exit(status)".
If "exit()" really is needed, then consider using the following:
threads->exit() if threads->can('exit'); # Thread friendly
exit(status);
Because of its global effect, this setting should not be used inside modules or the like.
The main thread is unaffected by this setting.
The main thread is unaffected by this call.
The main thread is unaffected by this call.
The following boolean methods are useful in determining the state of a thread.
As with subroutines, the type of value returned from a thread's entry point function may be determined by the thread's context: list, scalar or void. The thread's 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()".
Because thread creation and thread joining may occur in different contexts, it may be desirable to state the context explicitly to the thread's 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 thread's entry point function "foo" will be called in list (array) context such that the parent thread can receive a list (array) from the "->join()" call. ('array' is 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 = $thr->join();
If not explicitly stated, the thread's context is implied from the context of the "->create()" call:
# Create thread in list context
my ($thr) = threads->create(...);
# Create thread in scalar context
my $thr = threads->create(...);
# Create thread in void context
threads->create(...);
This returns the thread's context in the same manner as wantarray().
Class method to return the current thread's context. This returns the same value as running wantarray() inside the current thread's entry point function.
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, Perl's 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 application's needs, you may significantly reduce your application's 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.
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_size will 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.)
PERL5_ITHREADS_STACK_SIZE=1048576
export PERL5_ITHREADS_STACK_SIZE
perl -e'use threads; print(threads->get_stack_size(), "\n")'
This value overrides any "stack_size" parameter given to "use threads". Its primary purpose is to permit setting the per-thread stack size for legacy threaded applications.
my $thr = threads->create({'stack_size' => 32*4096},
\&foo, @args);
my $stack_size = $thr1->get_stack_size();
my $thr2 = threads->create({'stack_size' => $stack_size},
FUNCTION, ARGS);
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.
Returns the thread object to allow for method chaining:
$thr->kill('SIG...')->join();
Signal handlers need to be set up in the threads for the signals they are expected to act upon. Here's an example for 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 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/finished thread is ignored.
NOTE: If the main thread exits, then this warning cannot be suppressed using "no warnings 'threads';" as suggested below.
If needed, thread warnings can be suppressed by using:
no warnings 'threads';
in the appropriate scope.
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).
$thr->set_stack_size($size);
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.
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 "import" if 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 "require" and "->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 <https://rt.cpan.org/Public/> against the problematic module.
On MSWin32, each thread maintains its own the current working directory setting.
Each thread (except the main thread) is started using the C locale. The main thread is started like all other Perl programs; see "ENVIRONMENT" in perllocale. You can switch locales in any thread as often as you like.
If you want to inherit the parent thread's locale, you can, in the parent, set a variable like so:
$foo = POSIX::setlocale(LC_ALL, NULL);
and then pass to threads->create() a sub that closes over $foo. Then, in the child, you say
POSIX::setlocale(LC_ALL, $foo);
Or you can use the facilities in threads::shared to pass $foo; or if the environment hasn't changed, in the child, do
POSIX::setlocale(LC_ALL, "");
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.
This is especially true if trying to catch "SIGALRM" in a thread. To handle alarms in threads, set up a signal handler in the main thread, and then use "THREAD SIGNALLING" to relay the signal to the thread:
# Create thread with a task that may time out
my $thr = threads->create(sub {
threads->yield();
eval {
$SIG{ALRM} = sub { die("Timeout\n"); };
alarm(10);
... # Do work here
alarm(0);
};
if ($@ =~ /Timeout/) {
warn("Task in thread timed out\n");
}
};
# Set signal handler to relay SIGALRM to thread
$SIG{ALRM} = sub { $thr->kill('ALRM') };
... # Main thread continues working
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.
However, everything referenced by the returned value is a fresh copy in the joining thread, even if a returned object had in the child thread been a copy of something that previously existed in the parent thread. After joining, the parent will therefore have a duplicate of each such object. This sometimes matters, especially if the object gets mutated; this can especially matter for private data to which a returned subroutine provides access.
However, calling any threads methods in such an "END" block will most likely fail (e.g., the application may hang, or generate an error) due to mutexes that are needed to control functionality within the threads module.
For this reason, the use of "END" blocks in threads is strongly discouraged.
In prior perl versions, spawning threads with open directory handles would crash the interpreter. [perl #75154] <https://rt.perl.org/rt3/Public/Bug/Display.html?id=75154>
If you are using any code that requires the execution of the global destruction phase for clean up (e.g., removing temp files), then do not use detached threads, but rather join all threads before exiting the program.
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.
You can search for threads related bug reports at <https://rt.cpan.org/Public/>. If needed submit any new bugs, problems, patches, etc. to: <https://rt.cpan.org/Public/Dist/Display.html?Name=threads>
Perl 5.8.0 or later
threads on MetaCPAN: <https://metacpan.org/release/threads>
Code repository for CPAN distribution: <https://github.com/Dual-Life/threads>
threads::shared, perlthrtut
<https://www.perl.com/pub/a/2002/06/11/threads.html> and <https://www.perl.com/pub/a/2002/09/04/threads.html>
Perl threads mailing list: <https://lists.perl.org/list/ithreads.html>
Stack size discussion: <https://www.perlmonks.org/?node_id=532956>
Sample code in the examples directory of this distribution on CPAN.
Artur Bergman <sky AT crucially DOT net>
CPAN version produced by Jerry D. Hedden <jdhedden AT cpan DOT org>
threads is released under the same license as Perl.
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
| 2019-10-21 | perl v5.30.3 |