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
 my $lock = new Coro::Semaphore;
 my $locked;

 $locked = 1;


This module collection manages coroutines. Coroutines are similar to threads but don't 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 than threads programming.

(Perl, however, does not natively support real threads but instead does a very slow and memory-intensive emulation of processes using threads. This is a performance win on Windows machines, and a loss everywhere else).

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 coroutine represents the main program.

$current (or as function: current)

The current coroutine (the last coroutine switched to). The initial value is $main (of course).

This variable is strictly read-only. It is provided for performance reasons. If performance is not essential you are encouraged to use the Coro::current function instead.


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::Event to wait on an external event that hopefully wake up a coroutine so the scheduler can run it.

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


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

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

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

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

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.

   # 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 (although you are allowed to), and you get a coroutine that might have executed other code already (which can be good or bad :).

Also, the 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.

The priority will be reset to 0 after each job, tracing will be disabled, the description will be reset and the default output filehandle gets restored, so you can change alkl these. Otherwise the coroutine will be re-used "as-is": most notably if you change other per-coroutine global stuff such as $/ you need 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 with $Coro::POOL_RSS) it will also exit.


Calls the scheduler. 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.

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.


Works like cede, but is not exported by default and will cede to any coroutine, regardless of priority, once.

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.

# dynamic methods


These are the methods you can call on coroutine objects.

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 ready queue (according to it's priority) and return true. If the coroutine is already in the ready queue, do nothing and return false.

$is_ready = $coroutine->is_ready

Return wether 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.

$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.

$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. 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 the new coderef. This means that the new coderef will return immediately without blocking, returning nothing, while the original code ref will be called (with parameters) from within its own 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.

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.

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


 - you must make very sure that no coro is still active on global
   destruction. very bad things might happen otherwise (usually segfaults).

 - this module is not thread-safe. You should only ever use this module
   from the same thread (this requirement might be loosened in the future
   to allow per-thread schedulers, but Coro::State does not yet allow


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

Debugging: Coro::Debug.

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

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

Event/IO: Coro::Timer, Coro::Event, Coro::Handle, Coro::Socket.

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

Embedding: Coro::MakeMaker.


 Marc Lehmann <>