NAME
    Coro - the only real threads in perl

SYNOPSIS
      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
      my $lock = new Coro::Semaphore;
      my $locked;
  
      $lock->down;
      $locked = 1;
      $lock->up;

DESCRIPTION
    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 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 (very communication-intensive) runs over 300
    times faster on a single core than perls 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.

CORO THREAD LIFE CYCLE
    During the long and exciting (or not) life of a coro thread, it goes
    through a number of states:

    1. Creation
        The first thing in the life of a coro thread is it's creation -
        obviously. The typical way to create a thread is to call the "async
        BLOCK" function:

           async {
              # thread code goes here
           };

        You can also pass arguments, which are put in @_:

           async {
              print $_[1]; # prints 2
           } 1, 2, 3;

        This creates a new coro thread and puts it into the ready queue,
        meaning it will run as soon as the CPU is free for it.

        "async" will return a Coro object - you can store this for future
        reference or ignore it - a thread that is running, ready to run or
        waiting for some event is alive on it's own.

        Another way to create a thread is to call the "new" constructor with
        a code-reference:

           new Coro sub {
              # thread code goes here
           }, @optional_arguments;

        This is quite similar to calling "async", but the important
        difference is that the new thread is not put into the ready queue,
        so the thread will not run until somebody puts it there. "async" is,
        therefore, identical to this sequence:

           my $coro = new Coro sub {
              # thread code goes here
           };
           $coro->ready;
           return $coro;

    2. Startup
        When a new coro thread is created, only a copy of the code reference
        and the arguments are stored, no extra memory for stacks and so on
        is allocated, keeping the coro thread in a low-memory state.

        Only when it actually starts executing will all the resources be
        finally allocated.

        The optional arguments specified at coro creation are available in
        @_, similar to function calls.

    3. Running / Blocking
        A lot can happen after the coro thread has started running. Quite
        usually, it will not run to the end in one go (because you could use
        a function instead), but it will give up the CPU regularly because
        it waits for external events.

        As long as a coro thread runs, its Coro object is available in the
        global variable $Coro::current.

        The low-level way to give up the CPU is to call the scheduler, which
        selects a new coro thread to run:

           Coro::schedule;

        Since running threads are not in the ready queue, calling the
        scheduler without doing anything else will block the coro thread
        forever - you need to arrange either for the coro to put woken up
        (readied) by some other event or some other thread, or you can put
        it into the ready queue before scheduling:

           # this is exactly what Coro::cede does
           $Coro::current->ready;
           Coro::schedule;

        All the higher-level synchronisation methods (Coro::Semaphore,
        Coro::rouse_*...) are actually implemented via "->ready" and
        "Coro::schedule".

        While the coro thread is running it also might get assigned a
        C-level thread, or the C-level thread might be unassigned from it,
        as the Coro runtime wishes. A C-level thread needs to be assigned
        when your perl thread calls into some C-level function and that
        function in turn calls perl and perl then wants to switch
        coroutines. This happens most often when you run an event loop and
        block in the callback, or when perl itself calls some function such
        as "AUTOLOAD" or methods via the "tie" mechanism.

    4. Termination
        Many threads actually terminate after some time. There are a number
        of ways to terminate a coro thread, the simplest is returning from
        the top-level code reference:

           async {
              # after returning from here, the coro thread is terminated
           };

           async {
              return if 0.5 <  rand; # terminate a little earlier, maybe
              print "got a chance to print this\n";
              # or here
           };

        Any values returned from the coroutine can be recovered using
        "->join":

           my $coro = async {
              "hello, world\n" # return a string
           };

           my $hello_world = $coro->join;

           print $hello_world;

        Another way to terminate is to call "Coro::terminate", which at any
        subroutine call nesting level:

           async {
              Coro::terminate "return value 1", "return value 2";
           };

        Yet another way is to "->cancel" (or "->safe_cancel") the coro
        thread from another thread:

           my $coro = async {
              exit 1;
           };

           $coro->cancel; # also accepts values for ->join to retrieve

        Cancellation *can* be dangerous - it's a bit like calling "exit"
        without actually exiting, and might leave C libraries and XS modules
        in a weird state. Unlike other thread implementations, however, Coro
        is exceptionally safe with regards to cancellation, as perl will
        always be in a consistent state, and for those cases where you want
        to do truly marvellous things with your coro while it is being
        cancelled - that is, make sure all cleanup code is executed from the
        thread being cancelled - there is even a "->safe_cancel" method.

        So, cancelling a thread that runs in an XS event loop might not be
        the best idea, but any other combination that deals with perl only
        (cancelling when a thread is in a "tie" method or an "AUTOLOAD" for
        example) is safe.

        Last not least, a coro thread object that isn't referenced is
        "->cancel"'ed automatically - just like other objects in Perl. This
        is not such a common case, however - a running thread is referencedy
        by $Coro::current, a thread ready to run is referenced by the ready
        queue, a thread waiting on a lock or semaphore is referenced by
        being in some wait list and so on. But a thread that isn't in any of
        those queues gets cancelled:

           async {
              schedule; # cede to other coros, don't go into the ready queue
           };

           cede;
           # now the async above is destroyed, as it is not referenced by anything.

        A slightly embellished example might make it clearer:

           async {
              my $guard = Guard::guard { print "destroyed\n" };
              schedule while 1;
           };

           cede;

        Superficially one might not expect any output - since the "async"
        implements an endless loop, the $guard will not be cleaned up.
        However, since the thread object returned by "async" is not stored
        anywhere, the thread is initially referenced because it is in the
        ready queue, when it runs it is referenced by $Coro::current, but
        when it calls "schedule", it gets "cancel"ed causing the guard
        object to be destroyed (see the next section), and printing it's
        message.

        If this seems a bit drastic, remember that this only happens when
        nothing references the thread anymore, which means there is no way
        to further execute it, ever. The only options at this point are
        leaking the thread, or cleaning it up, which brings us to...

    5. Cleanup
        Threads will allocate various resources. Most but not all will be
        returned when a thread terminates, during clean-up.

        Cleanup is quite similar to throwing an uncaught exception: perl
        will work it's way up through all subroutine calls and blocks. On
        it's way, it will release all "my" variables, undo all "local"'s and
        free any other resources truly local to the thread.

        So, a common way to free resources is to keep them referenced only
        by my variables:

           async {
              my $big_cache = new Cache ...;
           };

        If there are no other references, then the $big_cache object will be
        freed when the thread terminates, regardless of how it does so.

        What it does "NOT" do is unlock any Coro::Semaphores or similar
        resources, but that's where the "guard" methods come in handy:

           my $sem = new Coro::Semaphore;

           async {
              my $lock_guard = $sem->guard;
              # if we return, or die or get cancelled, here,
              # then the semaphore will be "up"ed.
           };

        The "Guard::guard" function comes in handy for any custom cleanup
        you might want to do (but you cannot switch to other coroutines from
        those code blocks):

           async {
              my $window = new Gtk2::Window "toplevel";
              # The window will not be cleaned up automatically, even when $window
              # gets freed, so use a guard to ensure it's destruction
              # in case of an error:
              my $window_guard = Guard::guard { $window->destroy };

              # we are safe here
           };

        Last not least, "local" can often be handy, too, e.g. when
        temporarily replacing the coro thread description:

           sub myfunction {
              local $Coro::current->{desc} = "inside myfunction(@_)";

              # if we return or die here, the description will be restored
           }

    6. Viva La Zombie Muerte
        Even after a thread has terminated and cleaned up its resources, the
        Coro object still is there and stores the return values of the
        thread.

        When there are no other references, it will simply be cleaned up and
        freed.

        If there areany references, the Coro object will stay around, and
        you can call "->join" as many times as you wish to retrieve the
        result values:

           async {
              print "hi\n";
              1
           };

           # run the async above, and free everything before returning
           # from Coro::cede:
           Coro::cede;

           {
              my $coro = async {
                 print "hi\n";
                 1
              };

              # run the async above, and clean up, but do not free the coro
              # object:
              Coro::cede;

              # optionally retrieve the result values
              my @results = $coro->join;

              # now $coro goes out of scope, and presumably gets freed
           };

GLOBAL VARIABLES
    $Coro::main
        This variable stores the Coro object that represents the main
        program. While you can "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.

    $Coro::current
        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.

    $Coro::idle
        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 wakes
        up a coro so the scheduler can run it.

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

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

    schedule
        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
        "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.

    Coro::cede_notself
        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). The values will not be copied, but referenced directly.

    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;
              }; 

              &{+shift};
           }  

           # 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 () { }
           };

    killall
        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.

CORO OBJECT METHODS
    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.

    $coro->suspend
        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.

    $coro->resume
        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.

    $state->is_new
        Returns true iff this Coro object is "new", i.e. has never been run
        yet. Those states basically consist of only the code reference to
        call and the arguments, but consumes very little other resources.
        New states will automatically get assigned a perl interpreter when
        they are transfered to.

    $state->is_zombie
        Returns true iff the Coro object has been cancelled, i.e. it's
        resources freed because they were "cancel"'ed, "terminate"'d,
        "safe_cancel"'ed or simply went out of scope.

        The name "zombie" stems from UNIX culture, where a process that has
        exited and only stores and exit status and no other resources is
        called a "zombie".

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

        This is a rather brutal way to free a coro, with some limitations -
        if the thread is inside a C callback that doesn't expect to be
        canceled, bad things can happen, or if the cancelled thread insists
        on running complicated cleanup handlers that rely on its thread
        context, things will not work.

        Any cleanup code being run (e.g. from "guard" blocks) will be run
        without a thread context, and is not allowed to switch to other
        threads. On the plus side, "->cancel" will always clean up the
        thread, no matter what. If your cleanup code is complex or you want
        to avoid cancelling a C-thread that doesn't know how to clean up
        itself, it can be better to "->throw" an exception, or use
        "->safe_cancel".

        The arguments to "->cancel" are not copied, but instead will be
        referenced directly (e.g. if you pass $var and after the call change
        that variable, then you might change the return values passed to
        e.g. "join", so don't do that).

        The resources of the Coro are usually freed (or destructed) before
        this call returns, but this can be delayed for an indefinite amount
        of time, as in some cases the manager thread has to run first to
        actually destruct the Coro object.

    $coro->safe_cancel ($arg...)
        Works mostly like "->cancel", but is inherently "safer", and
        consequently, can fail with an exception in cases the thread is not
        in a cancellable state.

        This method works a bit like throwing an exception that cannot be
        caught - specifically, it will clean up the thread from within
        itself, so all cleanup handlers (e.g. "guard" blocks) are run with
        full thread context and can block if they wish. The downside is that
        there is no guarantee that the thread can be cancelled when you call
        this method, and therefore, it might fail. It is also considerably
        slower than "cancel" or "terminate".

        A thread is in a safe-cancellable state if it either hasn't been run
        yet, or it has no C context attached and is inside an SLF function.

        The latter two basically mean that the thread isn't currently inside
        a perl callback called from some C function (usually via some XS
        modules) and isn't currently executing inside some C function itself
        (via Coro's XS API).

        This call returns true when it could cancel the thread, or croaks
        with an error otherwise (i.e. it either returns true or doesn't
        return at all).

        Why the weird interface? Well, there are two common models on how
        and when to cancel things. In the first, you have the expectation
        that your coro thread can be cancelled when you want to cancel it -
        if the thread isn't cancellable, this would be a bug somewhere, so
        "->safe_cancel" croaks to notify of the bug.

        In the second model you sometimes want to ask nicely to cancel a
        thread, but if it's not a good time, well, then don't cancel. This
        can be done relatively easy like this:

           if (! eval { $coro->safe_cancel }) {
              warn "unable to cancel thread: $@";
           }

        However, what you never should do is first try to cancel "safely"
        and if that fails, cancel the "hard" way with "->cancel". That makes
        no sense: either you rely on being able to execute cleanup code in
        your thread context, or you don't. If you do, then "->safe_cancel"
        is the only way, and if you don't, then "->cancel" is always faster
        and more direct.

    $coro->schedule_to
        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.

    $coro->cede_to
        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
        those functions (all that are part of Coro itself) 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 either "cancel" or
        "safe_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
        "kill"ing a coro with a signal (in this case, a scalar).

    $coro->join
        Wait until the coro terminates and return any values given to the
        "terminate" or "cancel" functions. "join" can be called concurrently
        from multiple threads, 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 thread gets
        destroyed, that is, after it's resources have been freed but before
        it is joined. The callback gets passed the terminate/cancel
        arguments, if any, and *must not* die, under any circumstances.

        There can be any number of "on_destroy" callbacks per coro, and
        there is no way currently to remove a callback once added.

    $oldprio = $coro->prio ($newprio)
        Sets (or gets, if the argument is missing) the priority of the coro
        thread. Higher priority coro get run before lower priority coros.
        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):

           PRIO_MAX > PRIO_HIGH > PRIO_NORMAL > PRIO_LOW > PRIO_IDLE > PRIO_MIN
               3    >     1     >      0      >    -1    >    -3     >    -4

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

        The idle coro thread ($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 a 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's nice
        command).

    $olddesc = $coro->desc ($newdesc)
        Sets (or gets in case the argument is missing) the description for
        this coro thread. 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, and in
        fact, this is often preferred to indicate major processing states
        that can then be seen for example in a Coro::Debug session:

           sub my_long_function {
              local $Coro::current->{desc} = "now in my_long_function";
              ...
              $Coro::current->{desc} = "my_long_function: phase 1";
              ...
              $Coro::current->{desc} = "my_long_function: phase 2";
              ...
           }

GLOBAL FUNCTIONS
    Coro::nready
        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 (but you might still run into deadlocks if all event loops are
        blocked).

        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.

HOW TO WAIT FOR A CALLBACK
    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,
    "rouse_cb" and "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 => rouse_cb);

          my ($rpid, $rstatus) = rouse_wait;
          $rstatus
       }

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

       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 valid
             $current->ready;  # wake up the waiting thread
          });

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

          $rstatus
       }

BUGS/LIMITATIONS
    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.

        Attempts to use threads created in another emulated process will
        crash ("cleanly", with a null pointer exception).

    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.

WINDOWS PROCESS EMULATION
    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 on the 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.

SEE ALSO
    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.

AUTHOR
     Marc Lehmann <schmorp@schmorp.de>
     http://home.schmorp.de/

