++ed by:

30 PAUSE users
32 non-PAUSE users.

Marc A. Lehmann


Coro::State - create and manage simple coroutines


 use Coro::State;

 $new = new Coro::State sub {
    print "in coroutine (called with @_), switching back\n";
    $new->transfer ($main);
    print "in coroutine again, switching back\n";
    $new->transfer ($main);
 }, 5;

 $main = new Coro::State;

 print "in main, switching to coroutine\n";
 $main->transfer ($new);
 print "back in main, switch to coroutine again\n";
 $main->transfer ($new);
 print "back in main\n";


This module implements coroutines. Coroutines, similar to continuations, allow you to run more than one "thread of execution" in parallel. Unlike threads, there is no parallelism and only voluntary switching is used so locking problems are greatly reduced.

This can be used to implement non-local jumps, exception handling, continuations and more.

This module provides only low-level functionality. See Coro and related modules for a higher level process abstraction including scheduling.


A newly created coroutine that has not been used only allocates a relatively small (a few hundred bytes) structure. Only on the first transfer will perl stacks (a few k) and optionally C stack (4-16k) be allocated. On systems supporting mmap a 128k stack is allocated, on the assumption that the OS has on-demand virtual memory. All this is very system-dependent. On my i686-pc-linux-gnu system this amounts to about 10k per coroutine, 5k when the experimental context sharing is enabled.


$coro = new [$coderef] [, @args...]

Create a new coroutine and return it. The first transfer call to this coroutine will start execution at the given coderef. If the subroutine returns it will be executed again.

If the coderef is omitted this function will create a new "empty" coroutine, i.e. a coroutine that cannot be transfered to but can be used to save the current coroutine in.

$prev->transfer ($next, $flags)

Save the state of the current subroutine in $prev and switch to the coroutine saved in $next.

The "state" of a subroutine includes the scope, i.e. lexical variables and the current execution state (subroutine, stack). The $flags value can be used to specify that additional state to be saved (and later restored), by oring the following constants together:

   Constant    Effect
   SAVE_DEFAV  save/restore @_
   SAVE_DEFSV  save/restore $_
   SAVE_ERRSV  save/restore $@
   SAVE_CCTXT  save/restore C-stack (you usually want this for coroutines)

These constants are not exported by default. If you don't need any extra additional state saved, use 0 as the flags value.

If you feel that something important is missing then tell me. Also remember that every function call that might call transfer (such as Coro::Channel::put) might clobber any global and/or special variables. Yes, this is by design ;) You can always create your own process abstraction model that saves these variables.

The easiest way to do this is to create your own scheduling primitive like this:

  sub schedule {
     local ($_, $@, ...);
     $old->transfer ($new);

IMPLEMENTORS NOTE: all Coro::State functions/methods expect either the usual Coro::State object or a hashref with a key named "_coro_state" that contains the real Coro::State object. That is, you can do:

  $obj->{_coro_state} = new Coro::State ...;
  Coro::State::transfer (..., $obj);

This exists mainly to ease subclassing (wether through @ISA or not).


This module has not yet been extensively tested, but works on most platforms. Expect segfaults and memleaks (but please don't be surprised if it works...)

This module is not thread-safe. You must only ever use this module from the same thread (this requirement might be loosened in the future).




 Marc Lehmann <schmorp@schmorp.de>