# CMSimpleQueue A simple, lockless FIFO queue of elements. ## Discussion Simple queues are Core Foundation-based objects that implement a simple, lockless FIFO queue of (`void *`) elements. The elements in the queue can be pointers or simple pointer-sized numeric values (`NULL` or `0` elements aren’t allowed). If the elements are pointers to allocated memory buffers, handle lifetime management externally. A simple queue can safely handle one enqueueing thread and one dequeueing thread. Simple queues are lockless, so enqueues and dequeues can occur on the Core Audio `ioProc` thread, where the system forbids locking and blocking. You can query the state of a simple queue to get the current number of elements and the maximum capacity of the queue. You can also determine the queue’s fullness, which the system provides as a percentage of its capacity. You can reset a simple queue, which returns it to its newly created state, with no elements in the queue (but with the maximum capacity unchanged). ## Topics ### Creating a Queue [`func CMSimpleQueueCreate(allocator: CFAllocator?, capacity: Int32, queueOut: UnsafeMutablePointer) -> OSStatus`](/documentation/CoreMedia/CMSimpleQueueCreate(allocator:capacity:queueOut:)) Creates a queue that has the specified capacity. ### Managing Queues [`func CMSimpleQueueEnqueue(CMSimpleQueue, element: UnsafeRawPointer) -> OSStatus`](/documentation/CoreMedia/CMSimpleQueueEnqueue(_:element:)) Enqueues an element in the queue. [`func CMSimpleQueueDequeue(CMSimpleQueue) -> UnsafeRawPointer?`](/documentation/CoreMedia/CMSimpleQueueDequeue(_:)) Dequeues an element from the queue. [`func CMSimpleQueueReset(CMSimpleQueue) -> OSStatus`](/documentation/CoreMedia/CMSimpleQueueReset(_:)) Resets the queue. ### Inspecting Queues [`func CMSimpleQueueGetHead(CMSimpleQueue) -> UnsafeRawPointer?`](/documentation/CoreMedia/CMSimpleQueueGetHead(_:)) Returns the element at the head of the queue. [`func CMSimpleQueueGetCapacity(CMSimpleQueue) -> Int32`](/documentation/CoreMedia/CMSimpleQueueGetCapacity(_:)) Returns the number of elements that the queue can hold. [`func CMSimpleQueueGetCount(CMSimpleQueue) -> Int32`](/documentation/CoreMedia/CMSimpleQueueGetCount(_:)) Returns the number of elements currently in the queue. [`#define CMSimpleQueueGetFullness(queue)`](/documentation/CoreMedia/CMSimpleQueueGetFullness) A convenience macro that returns a queue’s fullness as a percentage of its capacity. ### Accessing the Type Identifier [`func CMSimpleQueueGetTypeID() -> CFTypeID`](/documentation/CoreMedia/CMSimpleQueueGetTypeID()) Returns the type identifier of sample buffer objects. ### Data Types [`class CMSimpleQueue`](/documentation/CoreMedia/CMSimpleQueue) A reference to an instance that provides a simple lockless queue of elements. ### Errors [Simple Queue Error Codes](/documentation/CoreMedia/simple-queue-errors) Error codes that simple queue operations generate. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)