dlpark

A pure Rust implementation of dmlc/dlpack.

This implementation focuses on transferring tensors between Rust and Python, and between Rust tensor/array libraries, without copying.

Installation

dlpark ships no default features — enable the interop backends you need:

cargo add dlpark --features "ndarray half"          # Rust-only
cargo add dlpark --features "pyo3 image"            # Python extension
cargo add dlpark --features "cudarc"                # CUDA (needs a CUDA toolchain)

The cpu-all feature group enables every CPU-testable backend (candle, half, image, ndarray, pyo3) in one go. The crate targets Rust edition 2024.

Mental model

A producer wraps its data into a [Builder], which holds the owning context plus scalar tensor fields. Building the builder produces a [Local<M>] — an RAII handle over a raw DLPack managed tensor pointer that calls the DLPack deleter on drop. M selects the ABI:

• [legacy::Dlpack] = Local<DLManagedTensor> — the pre-v0.8 "dltensor" capsule.
• [versioned::Dlpack] = Local<DLManagedTensorVersioned> — the current "dltensor_versioned" capsule, carrying version and flags.

A consumer receives a Local (from Python, another Rust library, or a raw pointer) and reads its metadata and data through the accessors below. The flow is always: owning value → Builder → Local → borrowed views/slices, never the reverse.

What is DLPack?

DLPack is a common in-memory tensor structure that enables sharing tensor data between different deep learning frameworks. It provides a standardized way to exchange tensor data without copying, making it efficient for framework interoperability.

Key features of DLPack:

• Zero-copy tensor sharing between frameworks
• Support for various data types and devices (CPU, GPU, etc.)
• Memory management through deleter functions
• Versioned ABI for compatibility

Implementation Details

Versioning

The library implements both legacy and versioned DLPack structures:

• legacy::Dlpack: Legacy managed tensor capsule support
• versioned::Dlpack: Versioned managed tensor capsule support with:
  • Major version: 1
  • Minor version: 3
  • Additional flags for tensor properties (read-only, copied, sub-byte type padding)

Safe Abstractions

The library provides a Rust ownership wrapper over the C-style DLPack structures, Local<M>:

• RAII wrapper around a raw managed DLPack tensor pointer
• Automatic cleanup through the DLPack deleter function on drop
• legacy::Dlpack and versioned::Dlpack are convenience aliases for Local<DLManagedTensor> and Local<DLManagedTensorVersioned> — the two concrete forms you'll actually use

Other key features:

• Memory safety through Rust's ownership system
• Support for image buffers, ndarray, and candle tensors, plus raw DLPack tensor layouts
• Python interoperability through PyO3
• Optional DLPack 1.3 C Exchange API fast path when a producer type exposes __dlpack_c_exchange_api__

Choosing Builder Metadata

Builder<C, L> uses the metadata type L to select its allocation strategy:

• CopiedArray<S, T, N> — fixed rank known at compile time. Shape and strides are copied into the managed tensor allocation. build and build_raw are infallible.
• CopiedSlice<S, T> — dynamic rank. The containers may be borrowed slices or owned values such as Vec<i64>. Use try_build or try_build_raw.
• GenericArray<S, T, A, B, N> — fixed-rank metadata with elements that implement TryInto<i64>. Values are converted directly into the managed tensor allocation; use try_build or try_build_raw.
• GenericSlice<S, T, A, B> — dynamic-rank metadata with elements that implement TryInto<i64>. It avoids allocating temporary Vec<i64> values.
• BorrowedArray<N> — fixed-rank, zero-copy metadata. Its build methods are unsafe because the arrays must outlive the managed tensor.
• BorrowedSlice — dynamic-rank, zero-copy metadata. Its try_build methods are unsafe for the same lifetime reason.

Copied metadata and the managed tensor share one allocation. Borrowed metadata only allocates the managed tensor header.

Use GenericArray or GenericSlice when a framework exposes dimensions or strides as an integer type other than i64:

use dlpark::{Builder, legacy};
use dlpark::metadata::GenericArray;

let shape = [2u32, 3];
let strides = [3i16, 1];
let mut data = vec![0f32; 6];
let data_ptr = data.as_mut_ptr().cast();
let builder = Builder::new(Box::new(data), GenericArray::new(&shape, &strides));
// SAFETY: the boxed Vec owns six initialized f32 values at data_ptr.
let dlpack: legacy::Dlpack = unsafe { builder.data(data_ptr) }
    .try_build()
    .unwrap();

The generic path converts each value directly into the trailing i64 metadata storage. In the included length-64 microbenchmark, direct conversion takes approximately 11.7 ns, while allocating temporary Vec<i64> storage and then calling copy_nonoverlapping takes approximately 54.9 ns on the development machine. Reproduce it with:

cargo bench --bench builder -- generic_metadata_copy

The ndarray exporter uses the same direct path for its usize shape and isize strides, so exporting an owned array does not allocate temporary Vec<i64> metadata.

Python Exchange Paths

The pyo3 feature supports the standard Python DLPack capsule protocol:

• legacy::Dlpack consumes or produces legacy "dltensor" capsules.
• versioned::Dlpack consumes or produces "dltensor_versioned" capsules.
• python::dlpack_device(obj) calls and validates obj.__dlpack_device__(), returning a Rust DLDevice.
• When extracting a versioned tensor from a Python object, dlpark first checks the object's type for a __dlpack_c_exchange_api__ PyCapsule named "dlpack_exchange_api". If present, it uses the DLPack C Exchange API no-sync function table. Otherwise it calls obj.__dlpack__(max_version=(1, 3)). Producers that only implement the legacy no-argument protocol must be extracted as legacy::Dlpack, because they return the incompatible "dltensor" capsule ABI.
• Consumers can call versioned::Dlpack::extract_with_options(obj, stream, copy) to pass optional stream and tri-state copy requests to __dlpack__; extract_with_stream is the typed convenience path for GPU consumers. The cudarc feature implements stream mapping for CudaStream; other backends can implement the unsafe python::DlpackStream trait.

The C Exchange API is intended for extension/library use where the consumer can borrow tensors and coordinate work on the producer's current stream. It is not a replacement for the normal __dlpack__ ingestion path.

Reading tensor data

Once you hold a Local, its consumer-side accessors read metadata and CPU data without unsafe:

use dlpark::DlpackElement;

let shape = dlpack.shape()?;                  // &[i64]
let strides = dlpack.strides()?;             // Option<&[i64]> (None = compact)
let n = dlpack.num_elements()?;
let bytes = dlpack.num_bytes()?;              // sub-byte-packing aware
let data = unsafe { dlpack.tensor().cpu_slice::<f32>()? }; // compact CPU data, dtype-checked

cpu_slice validates device (CPU only), dtype match, and compact row-major layout before forming the slice. Read-only data access lives on raw DLTensor and is unsafe, because DLPack metadata cannot prove that its public pointers are readable or within an allocation. Low-level consumers may use DLTensor::offset_data_ptr / offset_bytes_ptr to obtain a device-agnostic pointer with byte_offset applied; data_ptr returns the original unadjusted pointer. Mutable access lives on Local, where versioned flags can enforce READ_ONLY and IS_COPIED.

Mutable access and the IS_COPIED flag. Writing into a DLPack tensor is gated by two versioned flags, because exclusive ownership cannot be proven from a &mut Local alone — the producer may hold aliases:

• DlpackFlags::IS_COPIED asserts the export owns an unaliased copy. cpu_slice_mut requires it and needs no unsafe.
• DlpackFlags::READ_ONLY forbids mutation; both mut accessors reject it.

Without IS_COPIED, use the unsafe ..._mut_unchecked accessors and prove exclusivity yourself. Legacy DLManagedTensor has no flags field, so it can never satisfy IS_COPIED — mutation of a legacy local tensor always goes through the _unchecked path. Foreign interop uses the unsafe TryFromDlpack trait because descriptor pointers and aliasing cannot be proven by validation; mutable conversions require the caller to establish exclusivity rather than trusting foreign flags.

Local::flags() / version() read the versioned fields; flags_mut is unsafe because setting IS_COPIED or clearing READ_ONLY asserts the corresponding ownership/mutability guarantee.

Features

No features are enabled by default — enable the backends you need (see Installation).

Feature	Description	Status
pyo3	Python interop via pyo3 (capsule protocol + DLPack C Exchange API fast path)	✅
image	Zero-copy conversion with image buffers	✅
ndarray	Zero-copy conversion with ndarray arrays/views	✅
half	f16/bf16 element type support (via half)	✅
candle	Conversion with candle Tensor — CPU only; candle's CUDA backend needs separate integration work	✅
cudarc	Zero-copy conversion with cudarc CudaSlice<T> — no automated tests here, needs a CUDA-capable device to exercise	✅

Quick Start

Two runnable examples:

• examples/dlparkimg — a Python extension module (via pyo3) transferring image::RgbImage to/from Python (e.g. torch.Tensor).
• examples/ndarray-candle — a plain binary round-tripping data through DLPack: ndarray::Array2 → versioned::Dlpack → candle::Tensor → versioned::Dlpack → ndarray view, run with cargo run -p ndarray-candle.

Usage Examples

Converting between Rust and Python

use dlpark::{Builder, TryFromDlpack, versioned};
use pyo3::prelude::*;

// Rust to Python
#[pyfunction]
fn read_image(filename: &str) -> versioned::Dlpack {
    let img = image::open(filename).unwrap().to_rgb8();
    Builder::from(Box::new(img)).build()
}

// Python to Rust
#[pyfunction]
fn write_image(filename: &str, tensor: versioned::Dlpack) {
    let img: image::RgbImage = (&tensor).try_into().unwrap();
    img.save(filename).unwrap();
}

Image Processing

use dlpark::{Builder, versioned};
use image::{ImageBuffer, Rgb};

let img = ImageBuffer::<Rgb<u8>, _>::from_vec(100, 100, vec![0; 100 * 100 * 3])?;
let tensor: versioned::Dlpack = Builder::from(Box::new(img)).build();
let tensor = tensor.into_foreign();
let img2 = unsafe { ImageBuffer::<Rgb<u8>, _>::try_from_dlpack(&tensor)? };

ndarray

use dlpark::{Builder, TryFromDlpack, versioned};
use ndarray::{ArrayD, ArrayViewD, arr2};

let array = arr2(&[[1i32, 2, 3], [4, 5, 6]]);
let tensor: versioned::Dlpack = Builder::from(Box::new(array)).try_build()?;
let tensor = tensor.into_foreign();
let view = unsafe { ArrayViewD::<i32>::try_from_dlpack(&tensor)? };

assert_eq!(view[[1, 2]], 6);

let dynamic: ArrayD<i32> = arr2(&[[1, 2], [3, 4]]).into_dyn();
let dynamic_tensor: versioned::Dlpack = Builder::from(Box::new(dynamic)).try_build()?;

candle

Zero-copy from candle::Tensor to DLPack; the reverse direction (DLPack to candle::Tensor) always copies, since candle has no borrowed CPU tensor type.

use candle_core::Tensor;
use dlpark::{Builder, DlpackFlags, TryFromDlpack, ffi::DLManagedTensorVersioned, versioned};

let tensor = Tensor::new(&[1f32, 2., 3., 4.], &candle_core::Device::Cpu)?;
let dlpack: versioned::Dlpack = Builder::try_from(Box::new(tensor))?.try_build()?;
let dlpack = dlpack.into_foreign();

let tensor2 = unsafe { Tensor::try_from_dlpack(&dlpack)? };
assert_eq!(tensor2.to_vec1::<f32>()?, vec![1., 2., 3., 4.]);

let tensor = Tensor::new(&[1f32, 2., 3., 4.], &candle_core::Device::Cpu)?;
let builder = Builder::try_from(Box::new(tensor))?;
let dlpack = builder
    .insert_flags(DlpackFlags::READ_ONLY)?
    .try_build::<DLManagedTensorVersioned>()?;

cudarc

Zero-copy in both directions between a cudarc CudaSlice<T> and a DLPack tensor. Builder::try_from returns a builder with IS_COPIED and a contiguous 1-D default layout (shape = [len], strides = [1]); replace its metadata for higher-rank tensors. The reverse direction consumes the managed tensor through TryFrom<Local<M>> for BorrowedCudaSlice<M, T>, keeping it alive for as long as the CUDA view exists.

use dlpark::{
    Builder, TryFromDlpack,
    interop::cudarc::BorrowedCudaSlice,
    metadata::CopiedSlice,
    versioned,
};

let dlpack: versioned::Dlpack = Builder::try_from(Box::new(cuda_slice))?
    .metadata(CopiedSlice::new([2, 3], [3, 1]))
    .try_build()?;
let borrowed = unsafe { BorrowedCudaSlice::<_, f32>::try_from_dlpack(dlpack.into_foreign())? };