# blockmapFile(_:offset:length:flags:operationID:packer:replyHandler:) Maps a file’s disk space into extents, allowing the kernel to perform I/O with that space. ``` func blockmapFile(_ file: FSItem, offset: off_t, length: Int, flags: FSBlockmapFlags, operationID: FSOperationID, packer: FSExtentPacker, replyHandler reply: @escaping @Sendable ((any Error)?) -> Void) ``` ## Parameters `file` The file for which to map disk space. `offset` The starting logical offset of the range to be mapped (in bytes). `length` The length of the range to be mapped (in bytes). `flags` Flags that affect the behavior of the blockmap operation. `operationID` A unique identifier of the blockmap call. Any value other than `0` (Objective-C) or [`unspecified`](/documentation/FSKit/FSOperationID/unspecified) (Swift) indicates the beginning of an I/O operation. A value of `0` or [`unspecified`](/documentation/FSKit/FSOperationID/unspecified) indicates the kernel maps the file without performing I/O. In this case, FSKit doesn’t perform a corresponding call to `completeIO(for:offset:length:status:flags:operationID:)`. `packer` An extent packer you use to pack the requested range of the file’s allocated disk space. FSKit sends all of the packed extents to the kernel when it invokes `reply`. `reply` A block or closure to indicate success or failure. If mapping fails, pass an error as the one parameter to the reply handler. If mapping succeeds, pass `nil`. For an `async` Swift implementation, there’s no reply handler; simply throw an error or return normally. ## Discussion FSKit calls this method when the kernel needs to get a mapping of logical-to-physical offsets of the file’s data. This call may occur as part of an I/O operation on the file, or just to get the mapping as part of an `fcntl(F_LOG2PHYS)` system call. In the case of an I/O operation on the file, `operationID` has a nonzero value; a future call to `completeIO(for:offset:length:status:flags:operationID:)` uses the same `operationID` to indicate which operation it completes. In the case of an `fcntl(F_LOG2PHYS)` system call, the `operationID` parameter is `0` (Objective-C) or [`unspecified`](/documentation/FSKit/FSOperationID/unspecified) (Swift). In both cases the kernel retains the mapping, and it may perform I/O to this range (or a part of it) at any time. If satisfying a blockmap request requires more extents than `packer` can handle, FSKit makes additional calls to this method with the same operation ID to collect the remainder. --- 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)