# Cancel Stops the queue from dequeueing any further tasks, and notifies the specified handler when all in-flight tasks finish. ``` kern_return_t Cancel(IODispatchQueueCancelHandler handler); ``` ## Parameters `handler` The block to execute when the queue finishes all in-flight work. ## Return Value [`kIOReturnSuccess`](/documentation/DriverKit/kIOReturnSuccess) on success, or another value if an error occurs. For a list of error codes, see [Error Codes](/documentation/DriverKit/error-codes). ## Discussion This method asynchronously tells the dispatch queue to stop the execution of queued tasks, and returns immediately. If a task is already executing, that task runs to completion. After all tasks finish, the dispatch queue executes the block in the `handler` parameter. --- 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)