[ViewVC] Annotation of: cvs/Coro/README

NAME
    Coro - coroutine process abstraction

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

DESCRIPTION
    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).

    $Coro::main
        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.

    $Coro::current
        The coroutine object representing the current coroutine (the last
        coroutine 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 coroutine 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 L"Coro::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.

  SIMPLE COROUTINE CREATION
    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 about twice as fast as 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* revert that
        change, which is most simply done by using local as in: "local $/".

        The idle pool size is limited to 8 idle coroutines (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 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
    Static methods are actually functions that operate on the current
    coroutine.

    schedule
        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.

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

    cede
        "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.

    Coro::cede_notself
        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).

    killall
        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.

  COROUTINE METHODS
    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.

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

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

    $coroutine->cede_to
        Like "schedule_to", but puts the current coroutine into the ready
        queue. This has the effect of temporarily switching to the given
        coroutine, and continuing some time later.

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

    $coroutine->throw ([$scalar])
        If $throw is specified and defined, it will be thrown as an
        exception inside the coroutine 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 coroutine
        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 coroutine with a signal (in this case, a scalar).

    $coroutine->join
        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):

           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 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.

  GLOBAL FUNCTIONS
    Coro::nready
        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".

    $cb = Coro::rouse_cb
        Create and return a "rouse callback". That's a code reference that,
        when called, will save its arguments and notify the owner coroutine
        of the callback.

        See the next function.

    @args = Coro::rouse_wait [$cb]
        Wait for the specified rouse callback (or the last one tht was
        created in this coroutine).

        As soon as the callback is invoked (or when the calback was invoked
        before "rouse_wait"), it will return a copy of the arguments
        originally passed to the rouse callback.

        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 coroutine to wait for some callback to be
    called. This occurs naturally when you use coroutines 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 coroutine, 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 withina coroutine, 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 it's arguments and notify the coroutine 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;
          $rstatus
       }

    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 coroutine 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;

          $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 coroutines 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 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 having the windows process emulation enabled under
        unix roughly halves perl performance, even when not used.

    coroutine switching not signal safe
        You must not switch to another coroutine from within a signal
        handler (only relevant with %SIG - most event libraries provide safe
        signals).

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

SEE ALSO
    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.

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

Revision:	1.18
Committed:	Thu Nov 20 09:37:21 2008 UTC (15 years, 6 months ago) by root
Branch:	MAIN
CVS Tags:	rel-5_0
Changes since 1.17:	+134 -28 lines
Log Message:	5.0