Safe Haskell	None
Language	Haskell2010

System semaphores

data Semaphore Source

A system semaphore (POSIX or Win32).

newtype SemaphoreName Source

createSemaphore Source

Create a new semaphore with the given name and initial amount of available resources.

Throws an error if a semaphore by this name already exists.

freshSemaphore Source

Create a fresh semaphore with the given amount of tokens.

Its name will start with the given prefix, but will have a random suffix appended to it.

openSemaphore :: SemaphoreName -> IO Semaphore Source

Open a semaphore with the given name.

If no such semaphore exists, throws an error.

waitOnSemaphore :: Semaphore -> IO () Source

Indefinitely wait on a semaphore.

If you want to be able to cancel a wait operation, use forkWaitOnSemaphoreInterruptible instead.

tryWaitOnSemaphore :: Semaphore -> IO Bool Source

Try to obtain a token from the semaphore, without blocking.

Immediately returns False if no resources are available.

data WaitId Source

WaitId stores the information we need to cancel a thread which is waiting on a semaphore.

See forkWaitOnSemaphoreInterruptible and interruptWaitOnSemaphore.

forkWaitOnSemaphoreInterruptible Source

Spawn a thread that waits on the given semaphore.

In this thread, asynchronous exceptions will be masked.

The waiting operation can be interrupted using the interruptWaitOnSemaphore function.

This implements a similar pattern to the forkFinally function.

interruptWaitOnSemaphore :: WaitId -> IO () Source

Interrupt a semaphore wait operation initiated by forkWaitOnSemaphoreInterruptible.

getSemaphoreValue :: Semaphore -> IO Int Source

Query the current semaphore value (how many tokens it has available).

This is mainly for debugging use, as it is easy to introduce race conditions when nontrivial program logic depends on the value returned by this function.

releaseSemaphore :: Semaphore -> Int -> IO () Source

Release a semaphore: add n to its internal counter.

No-op when `n <= 0`.

destroySemaphore :: Semaphore -> IO () Source

Destroy the given semaphore.

Abstract semaphores

data AbstractSem Source

Abstraction over the operations of a semaphore.

withAbstractSem :: AbstractSem -> IO b -> IO b Source