# Crate parking_lot_core [−] [src]

This library exposes a low-level API for creating your own efficient synchronization primitives.

# The parking lot

To keep synchronization primitives small, all thread queuing and suspending functionality is offloaded to the parking lot. The idea behind this is based on the Webkit `WTF::ParkingLot` class, which essentially consists of a hash table mapping of lock addresses to queues of parked (sleeping) threads. The Webkit parking lot was itself inspired by Linux futexes, but it is more powerful since it allows invoking callbacks while holding a queue lock.

There are two main operations that can be performed on the parking lot:

- Parking refers to suspending the thread while simultaneously enqueuing it on a queue keyed by some address.
- Unparking refers to dequeuing a thread from a queue keyed by some address and resuming it.

See the documentation of the individual functions for more details.

# Building custom synchronization primitives

Building custom synchronization primitives is very simple since the parking lot takes care of all the hard parts for you. A simple example for a custom primitive would be to integrate a `Mutex` inside another data type. Since a mutex only requires 2 bits, it can share space with other data. For example, one could create an `ArcMutex` type that combines the atomic reference count and the two mutex bits in the same atomic word.

## Modules

deadlock | [Experimental] Deadlock detection

## Structs

ParkToken | A value associated with a parked thread which can be used by `unpark_filter`.
SpinWait | A counter used to perform exponential backoff in spin loops.
UnparkResult | Result of an unpark operation.
UnparkToken | A value which is passed from an unparker to a parked thread.

## Enums

FilterOp | Operation that `unpark_filter` should perform for each thread.
ParkResult | Result of a park operation.
RequeueOp | Operation that `unpark_requeue` should perform.

## Constants

DEFAULT_PARK_TOKEN | A default park token to use.
DEFAULT_UNPARK_TOKEN | A default unpark token to use.

## Functions

park ⚠ | Parks the current thread in the queue associated with the given key.
unpark_all ⚠ | Unparks all threads in the queue associated with the given key.
unpark_filter ⚠ | Unparks a number of threads from the front of the queue associated with `key` depending on the results of a filter function which inspects the `ParkToken` associated with each thread.
unpark_one ⚠ | Unparks one thread from the queue associated with the given key.
unpark_requeue ⚠ | Removes all threads from the queue associated with `key_from`, optionally unparks the first one and requeues the rest onto the queue associated with `key_to`.