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Marc A. Lehmann


Coro - coroutine process abstraction


  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 coroutine
  print "3\n";
  cede; # and again
  # use locking
  use Coro::Semaphore;
  my $lock = new Coro::Semaphore;
  my $locked;
  $locked = 1;


This module collection manages coroutines. Coroutines are similar to threads but don't (in general) run in parallel at the same time even on SMP machines. The specific flavor of coroutine used in this module also guarantees you that it will not switch between coroutines unless necessary, at easily-identified points in your program, so locking and parallel access are rarely an issue, making coroutine programming much safer and easier than threads programming.

Unlike a normal perl program, however, coroutines allow you to have 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.

Coroutines are also useful because Perl has no support for threads (the so called "threads" that perl offers are nothing more than the (bad) process emulation coming from the Windows platform: On standard operating systems they serve no purpose whatsoever, except by making your programs slow and making them use a lot of memory. Best disable them when building perl, or aks your software vendor/distributor to do it for you).

In this module, coroutines are defined as "callchain + lexical variables + @_ + $_ + $@ + $/ + C stack), that is, a coroutine 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).


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


The coroutine object representing the current coroutine (the last coroutine that the Coro scheduler switched to). The initial value is $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 coroutine 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 LCoro::EV, as this is pretty low-level functionality.

This variable stores a callback that is called whenever the scheduler finds no ready coroutines to run. The default implementation prints "FATAL: deadlock detected" and exits, because the program has no other way to continue.

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

Note that the callback must not, under any circumstances, block the current coroutine. Normally, this is achieved by having an "idle coroutine" that calls the event loop and then blocks again, and then readying that coroutine in the idle handler.

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

Please note that if your callback recursively invokes perl (e.g. for event handlers), then it must be prepared to be called recursively itself.


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

Create a new coroutine and return it's coroutine object (usually unused). The coroutine 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 coroutine. When it returns argument returns the coroutine is automatically terminated.

The remaining arguments are passed as arguments to the closure.

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

Calling exit in a coroutine will do the same as calling exit outside the coroutine. Likewise, when the coroutine 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 coroutine that just prints its arguments.

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

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

On the plus side, this function is faster than creating (and destroying) a completely new coroutine, so if you need a lot of generic coroutines 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 coroutine 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 coroutine will be re-used "as-is": most notably if you change other per-coroutine global stuff such as $/ you must needs to revert that change, which is most simply done by using local as in: local $/ .

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

If you are concerned about pooled coroutines 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 16kb (adjustable via $Coro::POOL_RSS) it will also be destroyed.


Static methods are actually functions that operate on the current coroutine.


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

Please note that the current coroutine 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 coroutine and wait for events: first you remember the current coroutine 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 coroutine up, so you need to check whether the event indeed happened, e.g. by storing the status in a variable.

The canonical way to wait on external events is this:

      # remember current coroutine
      my $current = $Coro::current;

      # register a hypothetical event handler
      on_event_invoke sub {
         # wake up sleeping coroutine
         undef $current;

      # call schedule until event occurred.
      # in case we are woken up for other reasons
      # (current still defined), loop.
      Coro::schedule while $current;

"Cede" to other coroutines. This function puts the current coroutine into the ready queue and calls schedule, which has the effect of giving up the current "timeslice" to other coroutines of the same or higher priority. Once your coroutine 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 coroutine, regardless of priority. This is useful sometimes to ensure progress is made.

terminate [arg...]

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


Kills/terminates/cancels all coroutines except the currently running one. This is useful after a fork, either in the child or the parent, as usually only one of them should inherit the running coroutines.

Note that while this will try to free some of the main programs resources, you cannot free all of them, so if a coroutine that is not the main program calls this function, there will be some one-time resource leak.


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

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

Create a new coroutine and return it. When the sub returns, the coroutine automatically terminates as if terminate with the returned values were called. To make the coroutine 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 coroutine environment.

$success = $coroutine->ready

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

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

$is_ready = $coroutine->is_ready

Return whether the coroutine is currently the ready queue or not,

$coroutine->cancel (arg...)

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


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

$coroutine->on_destroy (\&cb)

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

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

Sets (or gets, if the argument is missing) the priority of the coroutine. Higher priority coroutines get run before lower priority coroutines. 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

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

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

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

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

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

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

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

$coroutine->throw ([$scalar])

If $throw is specified and defined, it will be thrown as an exception inside the coroutine at the next convinient point in time (usually after it gains control at the next schedule/transfer/cede). Otherwise clears the exception object.

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 coroutine to end itself, although there is no guarentee that the exception will lead to termination, and if the exception isn't caught it might well end the whole program.



Returns the number of coroutines 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 coroutine 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 coroutines.

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

This creates and returns a guard object. Nothing happens until the object gets destroyed, in which case the codeblock given as argument will be executed. This is useful to free locks or other resources in case of a runtime error or when the coroutine gets canceled, as in both cases the guard block will be executed. The guard object supports only one method, ->cancel, which will keep the codeblock from being executed.

Example: set some flag and clear it again when the coroutine gets canceled or the function returns:

   sub do_something {
      my $guard = Coro::guard { $busy = 0 };
      $busy = 1;

      # do something that requires $busy to be true
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 coroutine.

The reason this function exists is that many event libraries (such as the venerable Event module) are not coroutine-safe (a weaker form of thread-safety). 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.

This function allows your callbacks to block by executing them in another coroutine 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 coroutine, or puts some other coroutine 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.


This module is not perl-pseudo-thread-safe. You should only ever use this module from the same 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 this is much faster and uses less memory.


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

Debugging: Coro::Debug.

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

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

IO/Timers: Coro::Timer, Coro::Handle, Coro::Socket, Coro::AIO.

Compatibility: Coro::LWP, Coro::BDB, Coro::Storable, Coro::Select.

XS API: Coro::MakeMaker.

Low level Configuration, Coroutine Environment: Coro::State.


 Marc Lehmann <schmorp@schmorp.de>