NAME

       Coro::Semaphore - counting semaphores

SYNOPSIS

        use Coro;

        $sig = new Coro::Semaphore [initial value];

        $sig->down; # wait for signal

        # ... some other "thread"

        $sig->up;

DESCRIPTION

       This module implements counting semaphores. You can initialize a mutex with any level of
       parallel users, that is, you can initialize a semaphore that can be "down"ed more than
       once until it blocks. There is no owner associated with semaphores, so one thread can
       "down" it while another can "up" it (or vice versa), "up" can be called before "down" and
       so on: the semaphore is really just an integer counter that optionally blocks when it is
       0.

       Counting semaphores are typically used to coordinate access to resources, with the
       semaphore count initialized to the number of free resources. Threads then increment the
       count when resources are added and decrement the count when resources are removed.

       You don't have to load "Coro::Semaphore" manually, it will be loaded automatically when
       you "use Coro" and call the "new" constructor.

       new [initial count]
           Creates a new semaphore object with the given initial lock count. The default lock
           count is 1, which means it is unlocked by default. Zero (or negative values) are also
           allowed, in which case the semaphore is locked by default.

       $sem->count
           Returns the current semaphore count. The semaphore can be down'ed without blocking
           when the count is strictly higher than 0.

       $sem->adjust ($diff)
           Atomically adds the amount given to the current semaphore count. If the count becomes
           positive, wakes up any waiters. Does not block if the count becomes negative, however.

       $sem->down
           Decrement the counter, therefore "locking" the semaphore. This method waits until the
           semaphore is available if the counter is zero or less.

       $sem->wait
           Similar to "down", but does not actually decrement the counter. Instead, when this
           function returns, a following call to "down" or "try" is guaranteed to succeed without
           blocking, until the next thread switch ("cede" etc.).

           Note that using "wait" is much less efficient than using "down", so try to prefer
           "down" whenever possible.

       $sem->wait ($callback)
           If you pass a callback argument to "wait", it will not wait, but immediately return.
           The callback will be called as soon as the semaphore becomes available (which might be
           instantly), and gets passed the semaphore as first argument.

           The callback might "down" the semaphore exactly once, might wake up other threads, but
           is NOT allowed to block (switch to other threads).

       $sem->up
           Unlock the semaphore again.

       $sem->try
           Try to "down" the semaphore. Returns true when this was possible, otherwise return
           false and leave the semaphore unchanged.

       $sem->waiters
           In scalar context, returns the number of threads waiting for this semaphore. Might
           accidentally cause WW3 if called in other contexts, so don't use these.

       $guard = $sem->guard
           This method calls "down" and then creates a guard object. When the guard object is
           destroyed it automatically calls "up".

AUTHOR/SUPPORT/CONTACT

          Marc A. Lehmann <schmorp@schmorp.de>
          http://software.schmorp.de/pkg/Coro.html