# setCancelHandler(handler:) Sets the cancellation handler block for the dispatch source. ``` func setCancelHandler(handler: DispatchWorkItem) ``` ## Parameters `handler` The event handler block to submit to the source’s target queue. ## Discussion The cancellation handler (if specified) is submitted to the source’s target queue in response to a call to a call to the [`cancel()`](/documentation/Dispatch/DispatchSourceProtocol/cancel()) method once the system has released all references to the source’s underlying handle and the source’s event handler block has returned. To safely close a file descriptor or destroy a Mach port, a cancellation handler is required for that descriptor or port. Closing the descriptor or port before the cancellation handler runs can result in a race condition. If a new descriptor is allocated with the same value as the recently closed descriptor while the source’s event handler is still running, the event handler may read/write data using the wrong descriptor. --- 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)