Coro - the only real threads in perl


  use Coro;
  async {
     # some asynchronous thread of execution
     print "2\n";
     cede; # yield back to main
     print "4\n";
  print "1\n";
  cede; # yield to coro
  print "3\n";
  cede; # and again
  # use locking
  use Coro::Semaphore;
  my $lock = new Coro::Semaphore;
  my $locked;
  $locked = 1;


For a tutorial-style introduction, please read the Coro::Intro manpage. This manpage mainly contains reference information.

This module collection manages continuations in general, most often in the form of cooperative threads (also called coros, or simply "coro" in the documentation). They are similar to kernel threads but don't (in general) run in parallel at the same time even on SMP machines. The specific flavor of thread offered by this module also guarantees you that it will not switch between threads unless necessary, at easily-identified points in your program, so locking and parallel access are rarely an issue, making thread programming much safer and easier than using other thread models.

Unlike the so-called "Perl threads" (which are not actually real threads but only the windows process emulation (see section of same name for more details) ported to unix, and as such act as processes), Coro provides a full shared address space, which makes communication between threads very easy. And Coro's threads are fast, too: disabling the Windows process emulation code in your perl and using Coro can easily result in a two to four times speed increase for your programs. A parallel matrix multiplication benchmark runs over 300 times faster on a single core than perl's pseudo-threads on a quad core using all four cores.

Coro achieves that by supporting multiple running interpreters that share data, which is especially useful to code pseudo-parallel processes and for event-based programming, such as multiple HTTP-GET requests running concurrently. See Coro::AnyEvent to learn more on how to integrate Coro into an event-based environment.

In this module, a thread is defined as "callchain + lexical variables + some package variables + C stack), that is, a thread has its own callchain, its own set of lexicals and its own set of perls most important global variables (see Coro::State for more configuration and background info).

See also the SEE ALSO section at the end of this document - the Coro module family is quite large.



This variable stores the Coro object that represents the main program. While you cna ready it and do most other things you can do to coro, it is mainly useful to compare again $Coro::current, to see whether you are running in the main program or not.


The Coro object representing the current coro (the last coro that the Coro scheduler switched to). The initial value is $Coro::main (of course).

This variable is strictly read-only. You can take copies of the value stored in it and use it as any other Coro object, but you must not otherwise modify the variable itself.


This variable is mainly useful to integrate Coro into event loops. It is usually better to rely on Coro::AnyEvent or Coro::EV, as this is pretty low-level functionality.

This variable stores a Coro object that is put into the ready queue when there are no other ready threads (without invoking any ready hooks).

The default implementation dies with "FATAL: deadlock detected.", followed by a thread listing, because the program has no other way to continue.

This hook is overwritten by modules such as Coro::EV and Coro::AnyEvent to wait on an external event that hopefully wake up a coro so the scheduler can run it.

See Coro::EV or Coro::AnyEvent for examples of using this technique.


async { ... } [@args...]

Create a new coro and return its Coro object (usually unused). The coro will be put into the ready queue, so it will start running automatically on the next scheduler run.

The first argument is a codeblock/closure that should be executed in the coro. When it returns argument returns the coro is automatically terminated.

The remaining arguments are passed as arguments to the closure.

See the Coro::State::new constructor for info about the coro environment in which coro are executed.

Calling exit in a coro will do the same as calling exit outside the coro. Likewise, when the coro dies, the program will exit, just as it would in the main program.

If you do not want that, you can provide a default die handler, or simply avoid dieing (by use of eval).

Example: Create a new coro that just prints its arguments.

   async {
      print "@_\n";
   } 1,2,3,4;
async_pool { ... } [@args...]

Similar to async, but uses a coro pool, so you should not call terminate or join on it (although you are allowed to), and you get a coro that might have executed other code already (which can be good or bad :).

On the plus side, this function is about twice as fast as creating (and destroying) a completely new coro, so if you need a lot of generic coros in quick successsion, use async_pool, not async.

The code block is executed in an eval context and a warning will be issued in case of an exception instead of terminating the program, as async does. As the coro is being reused, stuff like on_destroy will not work in the expected way, unless you call terminate or cancel, which somehow defeats the purpose of pooling (but is fine in the exceptional case).

The priority will be reset to 0 after each run, tracing will be disabled, the description will be reset and the default output filehandle gets restored, so you can change all these. Otherwise the coro will be re-used "as-is": most notably if you change other per-coro global stuff such as $/ you must needs revert that change, which is most simply done by using local as in: local $/.

The idle pool size is limited to 8 idle coros (this can be adjusted by changing $Coro::POOL_SIZE), but there can be as many non-idle coros as required.

If you are concerned about pooled coros growing a lot because a single async_pool used a lot of stackspace you can e.g. async_pool { terminate } once per second or so to slowly replenish the pool. In addition to that, when the stacks used by a handler grows larger than 32kb (adjustable via $Coro::POOL_RSS) it will also be destroyed.


Static methods are actually functions that implicitly operate on the current coro.


Calls the scheduler. The scheduler will find the next coro that is to be run from the ready queue and switches to it. The next coro to be run is simply the one with the highest priority that is longest in its ready queue. If there is no coro ready, it will call the $Coro::idle hook.

Please note that the current coro will not be put into the ready queue, so calling this function usually means you will never be called again unless something else (e.g. an event handler) calls ->ready, thus waking you up.

This makes schedule the generic method to use to block the current coro and wait for events: first you remember the current coro in a variable, then arrange for some callback of yours to call ->ready on that once some event happens, and last you call schedule to put yourself to sleep. Note that a lot of things can wake your coro up, so you need to check whether the event indeed happened, e.g. by storing the status in a variable.

See HOW TO WAIT FOR A CALLBACK, below, for some ways to wait for callbacks.


"Cede" to other coros. This function puts the current coro into the ready queue and calls schedule, which has the effect of giving up the current "timeslice" to other coros of the same or higher priority. Once your coro gets its turn again it will automatically be resumed.

This function is often called yield in other languages.


Works like cede, but is not exported by default and will cede to any coro, regardless of priority. This is useful sometimes to ensure progress is made.

terminate [arg...]

Terminates the current coro with the given status values (see cancel).

Coro::on_enter BLOCK, Coro::on_leave BLOCK

These function install enter and leave winders in the current scope. The enter block will be executed when on_enter is called and whenever the current coro is re-entered by the scheduler, while the leave block is executed whenever the current coro is blocked by the scheduler, and also when the containing scope is exited (by whatever means, be it exit, die, last etc.).

Neither invoking the scheduler, nor exceptions, are allowed within those BLOCKs. That means: do not even think about calling die without an eval, and do not even think of entering the scheduler in any way.

Since both BLOCKs are tied to the current scope, they will automatically be removed when the current scope exits.

These functions implement the same concept as dynamic-wind in scheme does, and are useful when you want to localise some resource to a specific coro.

They slow down thread switching considerably for coros that use them (about 40% for a BLOCK with a single assignment, so thread switching is still reasonably fast if the handlers are fast).

These functions are best understood by an example: The following function will change the current timezone to "Antarctica/South_Pole", which requires a call to tzset, but by using on_enter and on_leave, which remember/change the current timezone and restore the previous value, respectively, the timezone is only changed for the coro that installed those handlers.

   use POSIX qw(tzset);

   async {
      my $old_tz; # store outside TZ value here

      Coro::on_enter {
         $old_tz = $ENV{TZ}; # remember the old value

         $ENV{TZ} = "Antarctica/South_Pole";
         tzset; # enable new value

      Coro::on_leave {
         $ENV{TZ} = $old_tz;
         tzset; # restore old value

      # at this place, the timezone is Antarctica/South_Pole,
      # without disturbing the TZ of any other coro.

This can be used to localise about any resource (locale, uid, current working directory etc.) to a block, despite the existance of other coros.

Another interesting example implements time-sliced multitasking using interval timers (this could obviously be optimised, but does the job):

   # "timeslice" the given block
   sub timeslice(&) {
      use Time::HiRes ();

      Coro::on_enter {
         # on entering the thread, we set an VTALRM handler to cede
         $SIG{VTALRM} = sub { cede };
         # and then start the interval timer
         Time::HiRes::setitimer &Time::HiRes::ITIMER_VIRTUAL, 0.01, 0.01;
      Coro::on_leave {
         # on leaving the thread, we stop the interval timer again
         Time::HiRes::setitimer &Time::HiRes::ITIMER_VIRTUAL, 0, 0;


   # use like this:
   timeslice {
      # The following is an endless loop that would normally
      # monopolise the process. Since it runs in a timesliced
      # environment, it will regularly cede to other threads.
      while () { }

Kills/terminates/cancels all coros except the currently running one.

Note that while this will try to free some of the main interpreter resources if the calling coro isn't the main coro, but one cannot free all of them, so if a coro that is not the main coro calls this function, there will be some one-time resource leak.


These are the methods you can call on coro objects (or to create them).

new Coro \&sub [, @args...]

Create a new coro and return it. When the sub returns, the coro automatically terminates as if terminate with the returned values were called. To make the coro run you must first put it into the ready queue by calling the ready method.

See async and Coro::State::new for additional info about the coro environment.

$success = $coro->ready

Put the given coro into the end of its ready queue (there is one queue for each priority) and return true. If the coro is already in the ready queue, do nothing and return false.

This ensures that the scheduler will resume this coro automatically once all the coro of higher priority and all coro of the same priority that were put into the ready queue earlier have been resumed.


Suspends the specified coro. A suspended coro works just like any other coro, except that the scheduler will not select a suspended coro for execution.

Suspending a coro can be useful when you want to keep the coro from running, but you don't want to destroy it, or when you want to temporarily freeze a coro (e.g. for debugging) to resume it later.

A scenario for the former would be to suspend all (other) coros after a fork and keep them alive, so their destructors aren't called, but new coros can be created.


If the specified coro was suspended, it will be resumed. Note that when the coro was in the ready queue when it was suspended, it might have been unreadied by the scheduler, so an activation might have been lost.

To avoid this, it is best to put a suspended coro into the ready queue unconditionally, as every synchronisation mechanism must protect itself against spurious wakeups, and the one in the Coro family certainly do that.

$is_ready = $coro->is_ready

Returns true iff the Coro object is in the ready queue. Unless the Coro object gets destroyed, it will eventually be scheduled by the scheduler.

$is_running = $coro->is_running

Returns true iff the Coro object is currently running. Only one Coro object can ever be in the running state (but it currently is possible to have multiple running Coro::States).

$is_suspended = $coro->is_suspended

Returns true iff this Coro object has been suspended. Suspended Coros will not ever be scheduled.

$coro->cancel (arg...)

Terminates the given Coro and makes it return the given arguments as status (default: the empty list). Never returns if the Coro is the current Coro.


Puts the current coro to sleep (like Coro::schedule), but instead of continuing with the next coro from the ready queue, always switch to the given coro object (regardless of priority etc.). The readyness state of that coro isn't changed.

This is an advanced method for special cases - I'd love to hear about any uses for this one.


Like schedule_to, but puts the current coro into the ready queue. This has the effect of temporarily switching to the given coro, and continuing some time later.

This is an advanced method for special cases - I'd love to hear about any uses for this one.

$coro->throw ([$scalar])

If $throw is specified and defined, it will be thrown as an exception inside the coro at the next convenient point in time. Otherwise clears the exception object.

Coro will check for the exception each time a schedule-like-function returns, i.e. after each schedule, cede, Coro::Semaphore->down, Coro::Handle->readable and so on. Most of these functions detect this case and return early in case an exception is pending.

The exception object will be thrown "as is" with the specified scalar in $@, i.e. if it is a string, no line number or newline will be appended (unlike with die).

This can be used as a softer means than cancel to ask a coro to end itself, although there is no guarantee that the exception will lead to termination, and if the exception isn't caught it might well end the whole program.

You might also think of throw as being the moral equivalent of killing a coro with a signal (in this case, a scalar).


Wait until the coro terminates and return any values given to the terminate or cancel functions. join can be called concurrently from multiple coro, and all will be resumed and given the status return once the $coro terminates.

$coro->on_destroy (\&cb)

Registers a callback that is called when this coro gets destroyed, but before it is joined. The callback gets passed the terminate arguments, if any, and must not die, under any circumstances.

$oldprio = $coro->prio ($newprio)

Sets (or gets, if the argument is missing) the priority of the coro. Higher priority coro get run before lower priority coro. Priorities are small signed integers (currently -4 .. +3), that you can refer to using PRIO_xxx constants (use the import tag :prio to get then):

       3    >     1     >      0      >    -1    >    -3     >    -4

   # set priority to HIGH
   current->prio (PRIO_HIGH);

The idle coro ($Coro::idle) always has a lower priority than any existing coro.

Changing the priority of the current coro will take effect immediately, but changing the priority of coro in the ready queue (but not running) will only take effect after the next schedule (of that coro). This is a bug that will be fixed in some future version.

$newprio = $coro->nice ($change)

Similar to prio, but subtract the given value from the priority (i.e. higher values mean lower priority, just as in unix).

$olddesc = $coro->desc ($newdesc)

Sets (or gets in case the argument is missing) the description for this coro. This is just a free-form string you can associate with a coro.

This method simply sets the $coro->{desc} member to the given string. You can modify this member directly if you wish.



Returns the number of coro that are currently in the ready state, i.e. that can be switched to by calling schedule directory or indirectly. The value 0 means that the only runnable coro is the currently running one, so cede would have no effect, and schedule would cause a deadlock unless there is an idle handler that wakes up some coro.

my $guard = Coro::guard { ... }

This function still exists, but is deprecated. Please use the Guard::guard function instead.

unblock_sub { ... }

This utility function takes a BLOCK or code reference and "unblocks" it, returning a new coderef. Unblocking means that calling the new coderef will return immediately without blocking, returning nothing, while the original code ref will be called (with parameters) from within another coro.

The reason this function exists is that many event libraries (such as the venerable Event module) are not thread-safe (a weaker form of reentrancy). This means you must not block within event callbacks, otherwise you might suffer from crashes or worse. The only event library currently known that is safe to use without unblock_sub is EV.

Coro will try to catch you when you block in the event loop ("FATAL:$Coro::IDLE blocked itself"), but this is just best effort and only works when you do not run your own event loop.

This function allows your callbacks to block by executing them in another coro where it is safe to block. One example where blocking is handy is when you use the Coro::AIO functions to save results to disk, for example.

In short: simply use unblock_sub { ... } instead of sub { ... } when creating event callbacks that want to block.

If your handler does not plan to block (e.g. simply sends a message to another coro, or puts some other coro into the ready queue), there is no reason to use unblock_sub.

Note that you also need to use unblock_sub for any other callbacks that are indirectly executed by any C-based event loop. For example, when you use a module that uses AnyEvent (and you use Coro::AnyEvent) and it provides callbacks that are the result of some event callback, then you must not block either, or use unblock_sub.

$cb = rouse_cb

Create and return a "rouse callback". That's a code reference that, when called, will remember a copy of its arguments and notify the owner coro of the callback.

See the next function.

@args = rouse_wait [$cb]

Wait for the specified rouse callback (or the last one that was created in this coro).

As soon as the callback is invoked (or when the callback was invoked before rouse_wait), it will return the arguments originally passed to the rouse callback. In scalar context, that means you get the last argument, just as if rouse_wait had a return ($a1, $a2, $a3...) statement at the end.

See the section HOW TO WAIT FOR A CALLBACK for an actual usage example.


It is very common for a coro to wait for some callback to be called. This occurs naturally when you use coro in an otherwise event-based program, or when you use event-based libraries.

These typically register a callback for some event, and call that callback when the event occured. In a coro, however, you typically want to just wait for the event, simplyifying things.

For example AnyEvent->child registers a callback to be called when a specific child has exited:

   my $child_watcher = AnyEvent->child (pid => $pid, cb => sub { ... });

But from within a coro, you often just want to write this:

   my $status = wait_for_child $pid;

Coro offers two functions specifically designed to make this easy, Coro::rouse_cb and Coro::rouse_wait.

The first function, rouse_cb, generates and returns a callback that, when invoked, will save its arguments and notify the coro that created the callback.

The second function, rouse_wait, waits for the callback to be called (by calling schedule to go to sleep) and returns the arguments originally passed to the callback.

Using these functions, it becomes easy to write the wait_for_child function mentioned above:

   sub wait_for_child($) {
      my ($pid) = @_;

      my $watcher = AnyEvent->child (pid => $pid, cb => Coro::rouse_cb);

      my ($rpid, $rstatus) = Coro::rouse_wait;

In the case where rouse_cb and rouse_wait are not flexible enough, you can roll your own, using schedule:

   sub wait_for_child($) {
      my ($pid) = @_;

      # store the current coro in $current,
      # and provide result variables for the closure passed to ->child
      my $current = $Coro::current;
      my ($done, $rstatus);

      # pass a closure to ->child
      my $watcher = AnyEvent->child (pid => $pid, cb => sub {
         $rstatus = $_[1]; # remember rstatus
         $done = 1; # mark $rstatus as valud

      # wait until the closure has been called
      schedule while !$done;



fork with pthread backend

When Coro is compiled using the pthread backend (which isn't recommended but required on many BSDs as their libcs are completely broken), then coro will not survive a fork. There is no known workaround except to fix your libc and use a saner backend.

perl process emulation ("threads")

This module is not perl-pseudo-thread-safe. You should only ever use this module from the first thread (this requirement might be removed in the future to allow per-thread schedulers, but Coro::State does not yet allow this). I recommend disabling thread support and using processes, as having the windows process emulation enabled under unix roughly halves perl performance, even when not used.

coro switching is not signal safe

You must not switch to another coro from within a signal handler (only relevant with %SIG - most event libraries provide safe signals), unless you are sure you are not interrupting a Coro function.

That means you MUST NOT call any function that might "block" the current coro - cede, schedule Coro::Semaphore->down or anything that calls those. Everything else, including calling ready, works.


A great many people seem to be confused about ithreads (for example, Chip Salzenberg called me unintelligent, incapable, stupid and gullible, while in the same mail making rather confused statements about perl ithreads (for example, that memory or files would be shared), showing his lack of understanding of this area - if it is hard to understand for Chip, it is probably not obvious to everybody).

What follows is an ultra-condensed version of my talk about threads in scripting languages given onthe perl workshop 2009:

The so-called "ithreads" were originally implemented for two reasons: first, to (badly) emulate unix processes on native win32 perls, and secondly, to replace the older, real thread model ("5.005-threads").

It does that by using threads instead of OS processes. The difference between processes and threads is that threads share memory (and other state, such as files) between threads within a single process, while processes do not share anything (at least not semantically). That means that modifications done by one thread are seen by others, while modifications by one process are not seen by other processes.

The "ithreads" work exactly like that: when creating a new ithreads process, all state is copied (memory is copied physically, files and code is copied logically). Afterwards, it isolates all modifications. On UNIX, the same behaviour can be achieved by using operating system processes, except that UNIX typically uses hardware built into the system to do this efficiently, while the windows process emulation emulates this hardware in software (rather efficiently, but of course it is still much slower than dedicated hardware).

As mentioned before, loading code, modifying code, modifying data structures and so on is only visible in the ithreads process doing the modification, not in other ithread processes within the same OS process.

This is why "ithreads" do not implement threads for perl at all, only processes. What makes it so bad is that on non-windows platforms, you can actually take advantage of custom hardware for this purpose (as evidenced by the forks module, which gives you the (i-) threads API, just much faster).

Sharing data is in the i-threads model is done by transfering data structures between threads using copying semantics, which is very slow - shared data simply does not exist. Benchmarks using i-threads which are communication-intensive show extremely bad behaviour with i-threads (in fact, so bad that Coro, which cannot take direct advantage of multiple CPUs, is often orders of magnitude faster because it shares data using real threads, refer to my talk for details).

As summary, i-threads *use* threads to implement processes, while the compatible forks module *uses* processes to emulate, uhm, processes. I-threads slow down every perl program when enabled, and outside of windows, serve no (or little) practical purpose, but disadvantages every single-threaded Perl program.

This is the reason that I try to avoid the name "ithreads", as it is misleading as it implies that it implements some kind of thread model for perl, and prefer the name "windows process emulation", which describes the actual use and behaviour of it much better.


Event-Loop integration: Coro::AnyEvent, Coro::EV, Coro::Event.

Debugging: Coro::Debug.

Support/Utility: Coro::Specific, Coro::Util.

Locking and IPC: Coro::Signal, Coro::Channel, Coro::Semaphore, Coro::SemaphoreSet, Coro::RWLock.

I/O and Timers: Coro::Timer, Coro::Handle, Coro::Socket, Coro::AIO.

Compatibility with other modules: Coro::LWP (but see also AnyEvent::HTTP for a better-working alternative), Coro::BDB, Coro::Storable, Coro::Select.

XS API: Coro::MakeMaker.

Low level Configuration, Thread Environment, Continuations: Coro::State.


 Marc Lehmann <>