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Latest Version docs MIT Rust Rust

Interoptopus 🐙

The polyglot bindings generator for your library.

Interoptopus allows you to deliver high-quality system libraries to your users, and enables your users to easily consume those libraries from the language of their choice:

  • Design a single .dll / .so in Rust, consume it from any language.
  • Use patterns (e.g., classes, strings) in languages that have them.
  • Always be fully C compatible.
  • Painless workflow, no external tools required.

We strive to make our generated bindings zero cost. They should be as idiomatic as you could have reasonably written them yourself, but never magic or hiding the interface you actually wanted to expose.

Code you write …

use interoptopus::{ffi_function, ffi_type, Inventory, InventoryBuilder, function};

pub struct Vec2 {
    pub x: f32,
    pub y: f32,

pub extern "C" fn my_function(input: Vec2) {
    println!("{}", input.x);

// Define our FFI interface as `ffi_inventory` containing
// a single function `my_function`. Types are inferred.
pub fn ffi_inventory() -> Inventory {

… Interoptopus generates

1 For the reference project.
2 Create your own backend in just a few hours. No pull request needed. Pinkie promise.

Getting Started 🍼

If you want to …

Supported Rust Constructs

See the reference project for an overview:

  • functions (extern "C" functions and delegates)
  • types (composites, enums, opaques, references, …)
  • constants (primitive constants; results of const evaluation)
  • patterns (ASCII pointers, options, slices, classes, …)

Performance 🏁

Generated low-level bindings are zero cost w.r.t. hand-crafted bindings for that language.

That said, even hand-crafted bindings encounter some target-specific overhead at the FFI boundary (e.g., marshalling or pinning in managed languages). For C# that cost is often nanoseconds, for Python CFFI it can be microseconds.

While ultimately there is nothing you can do about a language’s FFI performance, being aware of call costs can help you design better APIs.

Detailed call cost tables can be found here: 🔥

For a quick overview, this table lists the most common call types in ns / call:

many_args_5(0, 0, 0, 0, 0)10786
callback(x => x, 0)431168

Feature Flags

Gated behind feature flags, these enable:

  • derive - Proc macros such as ffi_type, …
  • serde - Serde attributes on internal types.
  • log - Invoke log on FFI errors.


  • v0.14 - Better inventory UX.
  • v0.13 - Python backend uses ctypes now.
  • v0.12 - Better compat using #[ffi_service_method].
  • v0.11 - C# switch ctors to static methods.
  • v0.10 - C# flavors DotNet and Unity (incl. Burst).
  • v0.9 - 150x faster C# slices, Python type hints.
  • v0.8 - Moved testing functions to respective backends.
  • v0.7 - Make patterns proc macros for better FFI docs.
  • v0.6 - Renamed and clarified many patterns.
  • v0.5 - More ergonomic slice usage in Rust and FFI.
  • v0.4 - Enable logging support in auto-generated FFI calls.
  • v0.3 - Better compatibility with generics.
  • v0.2 - Introduced “patterns”; working interop for C#.
  • v0.1 - First version.

Also see our upgrade instructions.



PRs are welcome.

  • Submit small bug fixes directly. Major changes should be issues first.
  • Anything that makes previously working bindings change behavior or stop compiling is a major change;
  • This doesn’t mean we’re opposed to breaking stuff just that we’d like to talk about it before it happens.
  • New features or patterns must be materialized in the reference project and accompanied by an interop test (i.e., a backend test running C# / Python against a DLL invoking that code) in at least one included backend.


Abstractions for authors of backends.

Optional types that translate to binding with better semantics in languages supporting them.

Test generated bindings for various languages.

Helpers for backend authors.

Types used by backends to produce pretty output.


Defines a callback type, akin to a fn f(T) -> R wrapped in an Option.

Register a constant with an InventoryBuilder.

Register an extra type with an InventoryBuilder.

Register a function with an InventoryBuilder.

Debug macro resolving to the current file and line number.

Writes a line of code, possibly with multiple indentations. Used in backends.

Register a pattern with an InventoryBuilder.


Represents all FFI-relevant items, produced via InventoryBuilder, ingested by backends.

Produces a Inventory inside your inventory function, start here.


Can be observed if something goes wrong.

References to items contained within an Inventory.

Tells the InventoryBuilder what to register.


Main entry point for backends to generate language bindings.


Create a single Inventory from a number of individual inventories.

Returns all functions not belonging to a service pattern.

Attribute Macros

Enables a const to appear in generated bindings.

Enable an extern "C" function to appear in generated bindings.

Creates a FFI service from an impl Service {} block.

Inside a #[ffi_service] block, mark the FFI constructor.

Inside a #[ffi_service] block, don’t emit code for a method.

Inside a #[ffi_service] block, configure the generated FFI helper.

On methods and structs, provide a type helper for foreign types.⚠️


Enable a struct or enum to appear in generated bindings.