# sync(execute:) Submits a block object for execution and returns after that block finishes executing. ``` func sync(execute block: () -> Void) ``` ## Parameters `block` The block that contains the work to perform. This block has no return value and no parameters. This parameter cannot be `NULL`. ## Discussion This function submits a block to the specified dispatch queue for synchronous execution. Unlike [`dispatch_async`](/documentation/Dispatch/dispatch_async), this function does not return until the block has finished. Calling this function and targeting the current queue results in deadlock. Unlike with [`dispatch_async`](/documentation/Dispatch/dispatch_async), no retain is performed on the target queue. Because calls to this function are synchronous, it “borrows” the reference of the caller. Moreover, no `Block_copy` is performed on the block. As a performance optimization, this function executes blocks on the current thread whenever possible, with one exception: Blocks submitted to the main dispatch queue always run on the main thread. --- 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)