# cblas_dnrm2(_:_:_:) Computes the L2 norm (Euclidian length) of a vector (double precision). ``` func cblas_dnrm2(_ N: __LAPACK_int, _ X: UnsafePointer?, _ INCX: __LAPACK_int) -> Double ``` ## Parameters `N` Length of vector `X`. `X` Vector `X`. `INCX` Stride within `X`. For example, if `incX` is 7, every 7th element is used. ## Discussion > Important: > Apple provides the BLAS and LAPACK libraries under the Accelerate framework to be in line with LAPACK 3.9.1. Starting with iOS 26, iPadOS 26, macOS 26, tvOS 26, visionOS 26, and watchOS 26, the libraries are in line with LAPACK 3.12.0. These new interfaces provide additional functionality, as well as a new ILP64 interface. To use the new interfaces, define `ACCELERATE_NEW_LAPACK` before including the Accelerate or vecLib headers. For ILP64 interfaces, also define `ACCELERATE_LAPACK_ILP64`. For Swift projects, specify `ACCELERATE_NEW_LAPACK=1` and `ACCELERATE_LAPACK_ILP64=1` as preprocessor macros in Xcode build settings under Apple Clang - Preprocessing > Preprocessor Macros. --- 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)