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 intialize a sempahore 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 [inital count]
- Creates a new sempahore 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¶
Marc Lehmann <schmorp@schmorp.de>
http://home.schmorp.de/