Crate cglue

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CGlue API Docs Build and test MIT licensed Rustc 1.45

If all code is glued together, our glue is the safest on the market.

FFI-safe trait generation, helper structures, and more!


CGlue bridges Rust traits between C and other languages. It aims to be seamless to integrate - just add a few annotations around your traits, and they should be good to go!

use cglue::*;

// One annotation for the trait.
pub trait InfoPrinter {
    fn print_info(&self);

struct Info {
    value: usize

impl InfoPrinter for Info {
    fn print_info(&self) {
        println!("Info struct: {}", self.value);

fn use_info_printer(printer: &impl InfoPrinter) {
    println!("Printing info:");

fn main() -> () {
    let mut info = Info {
        value: 5

    // Here, the object is fully opaque, and is FFI and ABI safe.
    let obj = trait_obj!(&mut info as InfoPrinter);


Rust does not guarantee your code will work with neither 2 different compiler versions clashing, nor any other minor changes, CGlue glues it all together in a way that works.

This is done by generating wrapper vtables (virtual function tables) for the specified trait, and creating an opaque object with matching table.

cglue_trait annotation generates a InfoPrinterVtbl structure, and all the code needed to construct it for a type implementing the InfoPrinter trait. Then, a CGlueTraitObj is constructed that wraps the input object and implements the InfoPrinter trait.

But that’s not all, you can also group traits together!

use cglue::*;

// Extra trait definitions

pub trait InfoChanger {
    fn change_info(&mut self, new_val: usize);

impl InfoChanger for Info {
    fn change_info(&mut self, new_val: usize) {
        self.value = new_val;

pub trait InfoDeleter {
    fn delete_info(&mut self);

// Define a trait group.
// Here, `InfoPrinter` is mandatory - always required to be implemented,
// whereas `InfoChanger` with `InfoDeleter` are optional traits - a checked
// cast must be performed to access them.
cglue_trait_group!(InfoGroup, InfoPrinter, { InfoChanger, InfoDeleter });

// Implement the group for `Info` structure, defining
// only that `InfoChanger` is optionally implemented.
// This is not required if `unstable` feature is being used!
cglue_impl_group!(Info, InfoGroup, InfoChanger);

let mut info = Info { value: 5 };

let mut obj = group_obj!(info as InfoGroup);

// Object does not implement `InfoDeleter`
assert!(as_ref!(&obj impl InfoDeleter).is_none());

change_info(&mut cast!(obj impl InfoChanger).unwrap(), 20);

fn change_info(change: &mut (impl InfoPrinter + InfoChanger), new_val: usize) {
    println!("Old info:");
    println!("New info:");

And there is much more! Here are some highlights:

  1. Ability to use self-consuming trait functions.

  2. Some standard library traits are exposed (Clone).

  3. Ability to wrap associated trait types into new CGlue trait objects and groups.

  4. The above ability also works with mutable, and const reference associated type returns*.

  5. Generic traits and their groups.

  6. Library reference counting.

  7. Optional runtime ABI/API validation with abi_stable (enable layout_checks feature).

In-depth look

Safety assumptions

This crate relies on the assumption that opaque objects will not be tampered with, that is vtable functions will not be modified. It is being ensured through encapsulation of fields from anywhere by using hidden submodules. However, unverifiable users (C libraries) may still be able to modify the tables. This library assumes they are not malicious and does not perform any runtime verification. API version mismatch checking with abi_stable is an opt-in feature (requires rustc 1.46+).

Other than 2 bits in associated type wrapping, this crate should be safe.

The crate employs a number of unsafe traits that get auto-implemented, or traits with unsafe functions. Their usage inside the code generator should be safe, they are marked in such a way so that manual implementations can not introduce undefined behaviour.

Name generation

#[cglue_trait] macro for MyTrait will generate the following important types:

NamePurposeInstance typeContext
MyTraitBoxRegular owned CGlue object.CBox<c_void>NoContext
MyTraitCtxBox<Ctx>Owned CGlue object with a context.CBox<c_void>Ctx
MyTraitArcBoxOwned CGlue object with a reference counted context.CBox<c_void>CArc<c_void>
MyTraitMutBy-mut-ref CGlue object.&mut c_void.NoContext
MyTraitCtxMut<Ctx>By-mut-ref CGlue object with a context.&mut c_void.Ctx
MyTraitArcMutBy-mut-ref CGlue object with a reference counted context.&mut c_void.CArc<c_void>
MyTraitRefBy-ref (const) CGlue object.&c_void.NoContext
MyTraitCtxRef<Ctx>By-ref (const) CGlue object with a context.&c_void.Ctx
MyTraitArcRefBy-ref (const) CGlue object with a reference counted context.&c_void.CArc<c_void>

Only opaque types provide functionality. Non-opaque types can be used as Into trait bounds and are required to type check trait bounds.

These are the generic types needed for bounds checking:

NamePurposeInstance typeContext
MyTraitBaseBox<T>Base owned CGlue object.CBox<T>NoContext
MyTraitBaseCtxBox<T, Ctx>Base owned CGlue object with some context.CBox<T>Ctx
MyTraitBaseArcBox<T, Ctx>Base owned CGlue object with reference counted context.CBox<T>CArc<Ctx>
MyTraitBaseMut<T>Base by-mut-ref CGlue object.&mut T.NoContext
MyTraitBaseRef<T>Typedef for generic by-ref (const) CGlue object.&T.NoContext
MyTraitBase<Inst, Ctx>Base (non-opaque) CGlue object. It can have any compatible instance and contextInstCtx

Finally, the following underlying types exist, but do not need to be interacted with in Rust:

MyTraitVtbl<C>Table of all functions of the trait. Should be opaque to the user.
MyTraitRetTmp<Ctx>Structure for temporary return values. It should be opaque to the user.

Instead, every opaque CGlue object implements MyTraitOpaqueObj trait, which contains the type of the vtable.

cglue_trait_group! macro for MyGroup will generate the following main types:

NamePurposeInstance typeContext
MyGroupBoxOwned CGlue trait group.CBox<c_void>NoContext
MyGroupCtxBox<Ctx>Owned CGlue trait group with some context.CBox<c_void>Ctx
MyGroupArcBoxTypedef for opaque owned CGlue trait group with reference counted context.CBox<c_void>CArc<c_void>
MyGroupMutTypedef for opaque by-mut-ref CGlue trait group.&mut c_void.NoContext
MyGroupCtxMut<Ctx>Typedef for opaque by-mut-ref CGlue trait group with a custom context.&mut c_void.Ctx
MyGroupArcMutTypedef for opaque by-mut-ref CGlue trait group with a reference counted context.&mut c_void.CArc<c_void>
MyGroupRefTypedef for opaque by-ref (const) CGlue trait group.&c_void.NoContext
MyGroupCtxRef<Ctx>Typedef for opaque by-ref (const) CGlue trait group with a custom context.&c_void.Ctx
MyGroupArcRefTypedef for opaque by-ref (const) CGlue trait group with a reference counted context.&c_void.CArc<c_void>

Base types are as follows:

NamePurposeInstance typeContext
MyGroupBaseBox<T>Base owned CGlue trait group. Its container is a CBox<T>
MyGroupBaseCtxBox<T, Ctx>Base owned CGlue trait group with some context.CBox<T>Ctx
MyGroupBaseArcBox<T, Ctx>Base owned CGlue trait group with reference counted context.CBox<T>CArc<Ctx>
MyGroupBaseMut<T>Base by-mut-ref CGlue trait group.&mut T.NoContext
MyGroupBaseCtxMut<T, Ctx>Base by-mut-ref CGlue trait group with a context.&mut T.Ctx
MyGroupBaseArcMut<T, Ctx>Base by-mut-ref CGlue trait group with a reference counted context.&mut T.CArc<Ctx>
MyGroupBaseRef<T>Base by-ref (const) CGlue trait group.&T.NoContext
MyGroupBaseCtxRef<T, Ctx>Base by-ref (const) CGlue trait group with a context.&T.Ctx
MyGroupBaseArcRef<T, Ctx>Base by-ref (const) CGlue trait group with a reference counted context.&T.CArc<Ctx>
MyGroup<Inst, Ctx>Base definiton of the group. It needs to be manually made opaque.InstCtx

Container type (opaque to Rust users) that is placed within the group:

MyGroupContainer<Inst, Ctx>Stores temporary return storage. Vtables are built for this type.

And finally, the filler trait that is required for an object to be grouppable:

MyGroupVtableFillerTrait that allows an object to specify which optional traits are available, through the use of enable_trait functions.

The macro generation will also generate structures for all combinations of optional traits being used. For more convenient by-macro usage, the names of optional traits inside are sorted in alphabetical order. If not using macros, check MyGroup documentation for underlying conversion function definitions.

Generics in groups

Groups are fairly flexible - they are not limited to basic types. They can also contain generic parameters, associated types, and self returns (this also applies to single-trait objects).

Use of generics in trait groups is rather straightforward, with a couple of tiny nuances.

Define a group with the standard template syntax:

cglue_trait_group!(GenGroup<T>, Getter<T>, { TA });

It is also possible to specify trait bounds:

cglue_trait_group!(GenGroup<T: Eq>, Getter<T>, { TA });


cglue_trait_group!(GenGroup<T> where T: Eq {}, Getter<T>, { TA });

Implement the group on a generic type:

cglue_impl_group!(GA<T: Eq>, GenGroup<T>, { TA });

Note that in the above case, GA<T> will be grouppable, if, and only if it implements both, Getter<T> and TA for T: Eq. If GA implements different sets of optional traits with different type parameters, then provide multiple implementations, with specified types. On each implementation, still add a generic type T, but specify its type with an equality somewhere on the line:

cglue_impl_group!(GA<T = u64>, GenGroup<T>, {});
cglue_impl_group!(GA<T>, GenGroup<T = usize>, { TA });

Here, GA<u64> implements only Getter<T>, while GA<usize> implements both Getter<usize> and TA.

Finally, you can also mix the 2, assuming the most general implementation has the most optional traits defined:

cglue_impl_group!(GA<T: Eq>, GenGroup<T>, { TA });
cglue_impl_group!(GA<T = u64>, GenGroup<T>, {});
Manually implementing groups

NOTE: This is not supported if unstable feature is enabled. Instead, you have to do nothing!

It is also possible to manually implement the groups by implementing MyGroupVtableFiller. Here is what the above 2 macro invocations expand to:

        CGlueInst: ::core::ops::Deref<Target = GA<T>>,
        CGlueCtx: cglue::trait_group::ContextBounds,
        T: Eq,
    > GenGroupVtableFiller<'cglue_a, CGlueInst, CGlueCtx, T> for GA<T>
    Self: TA,
    &'cglue_a TAVtbl<'cglue_a, GenGroupContainer<CGlueInst, CGlueCtx, T>>:
        'cglue_a + Default,
    T: cglue::trait_group::GenericTypeBounds,
    fn fill_table(
        table: GenGroupVtables<'cglue_a, CGlueInst, CGlueCtx, T>,
    ) -> GenGroupVtables<'cglue_a, CGlueInst, CGlueCtx, T> {
        CGlueInst: ::core::ops::Deref<Target = GA<u64>>,
        CGlueCtx: cglue::trait_group::ContextBounds,
    > GenGroupVtableFiller<'cglue_a, CGlueInst, CGlueCtx, u64> for GA<u64>
    fn fill_table(
        table: GenGroupVtables<'cglue_a, CGlueInst, CGlueCtx, u64>,
    ) -> GenGroupVtables<'cglue_a, CGlueInst, CGlueCtx, u64> {

External traits

Certain traits may not be available for #[cglue_trait] annotation. Thus, there are mechanisms in place to allow constructing CGlue objects of external traits. The core primitive is #[cglue_trait_ext]. Essentially the user needs to provide a sufficient definition for the actual trait, like so:

pub trait Clone {
    fn clone(&self) -> Self;

Notice how this trait does not have the clone_from function. Having a separate &Self parameter is not supported, but the trait can still be implemented, because clone_from is merely an optional optimization and there already is a blanket implementation for it.

Usage of external traits is the same when constructing single-trait objects. It gets more complicated when groups are involved. This is how a MaybeClone group would be implemented:

cglue_trait_group!(MaybeClone, { }, { ext::Clone }, {
    pub trait Clone {
        fn clone(&self) -> Self;

The first change is to use ext::Clone. This marks cglue to create external trait glue code. The second bit is the trait definition. Yes, unfortunately the group needs another definition of the trait. CGlue does not have the context of the crate, and it needs to know the function signatures.

This is far from ideal, thus there is an additional mechanism in place - built-in external traits. It is a store of trait definitions that can be used without providing multiple trait definitions. With Clone being both inside the store, and marked as prelude export, the above code gets simplified to just this:

cglue_trait_group!(MaybeClone, { }, { Clone });

For traits not in the prelude, they can be accessed through their fully qualified ::ext path:

cglue_trait_group!(MaybeAsRef<T>, { }, { ::ext::core::convert::AsRef<T> });

Note that use imports do not work - a fully qualified path is required.

The trait store is the least complete part of this system. If you encounter missing traits and wish to use them, please file a pull request with their definitions, and I will be glad to include them.

Type wrapping

As for details, commonly used Rust structures are automatically wrapped in a way that works effectively.

For instance, slices and str types get converted to C-compatible slices.

fn with_slice(&self, slice: &[usize]) {}

// Generated vtable entry:

with_slice: extern "C" fn(&CGlueC, slice: CSlice<usize>),

Option types that can not have nullable pointer optimization are wrapped into COption:

fn non_npo_option(&self, opt: Option<usize>) {}

// Generated vtable entry:

non_npo_option: extern "C" fn(&CGlueC, opt: Option<usize>),

Result is automatically wrapped into CResult:

fn with_cresult(&self) -> Result<usize, usize> {}

// Generated vtable entry:

with_cresult: extern "C" fn(&CGlueC) -> CResult<usize, usize>,

Result with IntError type can return an integer code with Ok value written to a variable:

fn with_int_result(&self) -> Result<usize> {}

// Generated vtable entry:

with_int_result: extern "C" fn(&CGlueC, ok_out: &mut MaybeUninit<usize>) -> i32,

All wrapping and conversion is handled transparently behind the scenes, with user’s control.

Associated type wrapping

Associated types can be wrapped into custom CGlue objects. Below is a minimal example of this in action:

use cglue::*;
pub trait ObjReturn {
    type ReturnType: InfoPrinter + 'static;

    fn or_1(&self) -> Self::ReturnType;

struct InfoBuilder {}

impl ObjReturn for InfoBuilder {
    type ReturnType = Info;

    fn or_1(&self) -> Self::ReturnType {
        Info {
            value: 80

let builder = InfoBuilder {};

let obj = trait_obj!(builder as ObjReturn);

let info_printer = obj.or_1();


This also works if the trait were to return a &Self::ReturnType, or &mut Self::ReturnType. It is done by storing wrapped return value in an intermediate storage, and then returning references to there.

However, there is a SAFETY WARNING:

Wrapping &Self::ReturnType in a function that takes a non-mutable &self technically breaks Rust’s safety rules by potentially overwriting data that is already being borrowed as const. However, in real world a function that takes &self and returns &T will usually return the same reference, and it should be alright, but YOU HAVE BEEN WARNED. TODO: Disallow this?

The above warning does not apply to &mut self functions, because the returned reference is bound to the same lifetime and can not be re-created while being borrowed.

In addition, there is quite a bit of type safety being broken when when wrapping associated types in anonymous lifetime references. It should be okay, but the situation is as follows:

  1. Due to no GAT, CGlueObjRef/Mut<'_> is being promoted to CGlueObjRef/Mut<'static>. This should be okay, given it is not possible to clone non-CBox objects, and these objects are returned by-reference, not value.

  2. Trait bounds are only checked for one lifetime (lifetime of the vtable), and the C function is being cast into a HRTB one unsafely. This is because it is not possible to specify the HRTB upper bound (for<'b: 'a>). It should be okay, since the vtable can be created for the vtable’s lifetime, the returned reference will not outlive the vtable, and the C function is fully type checked otherwise.

However, if there is a glaring issue I am missing, and there is a solution to this unsafety, please file an issue report.

Generally speaking, you will want to use wrap_with_obj/wrap_with_group in Self::ReturnType functions, wrap_with_obj_mut/wrap_with_group_mut in &mut Self::ReturnType functions, and wrap_with_obj_ref/wrap_with_group_ref in &Self::ReturnType functions. It is important to note that if there is a trait that returns a combination of these types, it is not possible to use wrapping, because the underlying object types differ. If possible, split up the type to multiple associated types.

Plugin system

A full example is available in the repo’s examples subdirectory.

CGlue currently does not provide an out-of-the box plugin system, but there are primitives in place for relatively safe trait usage using dynamically loaded libraries. The core primitive is a cloneable context, such as a libloading::Library` Arc, which will keep the library opened until all of the CGlue objects are dropped.

use cglue::prelude::v1::*;

pub trait PluginRoot {
    // ...

impl PluginRoot for () {}

let root = ();
// This could be a `libloading::Library` arc.
let ref_to_count = CArc::from(());
// Merely passing a tuple is enough.
let obj = trait_obj!((root, ref_to_count) as PluginRoot);
// ...

Reference counting the Arc allows to safeguard the dynamically loaded library from being unloaded prematurely.

If PluginRoot were to branch out and build new objects that can be dropped after the instance of PluginRoot, for instance an InfoPrinter object, the Arc gets moved/cloned into the new object.

pub trait PluginRoot {
    type PrinterType: InfoPrinter;

    fn get_printer(&self) -> Self::PrinterType;

impl PluginRoot for () {
    type PrinterType = Info;

    fn get_printer(&self) -> Self::PrinterType {
        Info { value: 42 }

let root = ();
// This could be a `libloading::Library` arc.
let ref_to_count = CArc::from(());
let obj = trait_obj!((root, ref_to_count) as PluginRoot);
let printer = obj.get_printer();
// It is safe to drop obj now:

Note that this is not foolproof, and there may be situations where returned data could depend on the library. The most error prone of which are unhandled Err(E) conditions, where E is some static str. main function could return an error pointing to the memory of the library, unload it, and then attempt to print it out, resulting in a segfault. If possible, try to use IntError types, and mark the trait with #[int_result], which would prevent this particular issue from happening.

Working with cbindgen

cbindgen can be used to generate C and C++ bindings. There is some important setup needed.

In addition, cglue-bindgen provides additional helper method generation, making working with CGlue from C/C++ much more convenient.


Firstly, create a cbindgen.toml, and make sure both cglue, and any crates using cglue are included and have macro expansion enabled:

parse_deps = true
include = ["cglue", "your-crate"]

crates = ["cglue", "your-crate"]

Macro expansion currently requires nightly Rust. Thus, it is then possible to generate bindings like so:

rustup run nightly cbindgen --config cbindgen.toml --crate your_crate --output output_header.h

You can set C or C++ language mode by appending -l c or -l c++ flag. Alternatively, set it in the toml:

language = "C"

Export any shortened typedefs that are not used by any of the extern C functions:

include = ["FeaturesGroupArcBox", "PluginInnerRef", "PluginInnerMut"]

cglue-bindgen is a cbindgen wrapper that attempts to automatically clean up the headers. It also adds an ability to automatically invoke nightly rust with +nightly flag, and also generates vtable wrappers for simpler usage. The change is simple - just move all cbindgen arguments after --:

cglue-bindgen +nightly -- --config cbindgen.toml --crate your_crate --output output_header.h

This wrapper is probably the most fragile part of CGlue - if something does not work, please open up an issue report. In the future, we will aim to integrate CGlue directly with cbindgen.


  1. Associated type function arguments are not possible, because opaque conversion works one-way.

  2. Functions that accept an additional Self types are not possible for the same reason.

  3. Custom generic arguments for cglue traits are not yet supported, but this is to be improved upon.

  4. There probably are some corner cases when it comes to path imports. If you find any, please file an issue report :)

Unstable feature

cglue_impl_group may force you into making conservative optional trait choices, because it is currently not possible to specialize these cases with stable Rust features. But this is not always desirable. You can solve this, by enabling unstable feature.

This feature makes cglue_impl_group a no-op, and automatically enables the widest set of traits for the given object.

To use it you need to either:

  • nightly Rust compiler.

  • Set RUSTC_BOOTSTRAP=try_default environment variable when building.

Do note, however, that Rust’s stability guarantees get invalidated by either of these 2 options.

Projects using CGlue

If you want your project to be added to the list, please open an issue report :)


It is available in file.


FFI-safe Arc.

FFI-safe wrapped box.

FFI compatible callbacks

Built-in external traits.

Forwards a trait on references

From on Into

FFI compatible iterators

FFI safe option.

Import most commonly used types.

Null-terminated transparent C-strings.

FFI safe result.

Slices as C-structs.

Core definitions for traits, and their groups.

Repr(C) tuples


Checked cast to a list of optional traits.

Checked cast to a list of optional traits.

Checked cast to a list of optional traits.

Implement a CGlue group for a specific type.

Define a CGlue trait group.

Convert into a CGlue trait group.

Checked cast to a list of optional traits.

Convert into a CGlue compatible object.

Attribute Macros

Generate forward trait implementation for Fwd.

Generate forward trait implementation for Fwd.

Make a trait CGlue compatible.

Make an external trait CGlue compatible.

Add custom wrapping for a trait impl and the C interface.

Mark the trait or function to use IntResult.

Exclude a single function from using IntResult.

Specify return type conversion with a closure.

Skip reimplementing this function.

Emit a vtable entry, but do not use it in Rust.

Wrap the associated type with a custom type.

Wrap the associated type with a CGlue trait group.

Wrap the associated type with a CGlue trait group mutable reference.

Wrap the associated type with a CGlue trait group reference.

Wrap the associated type with a CGlue trait object.

Wrap the associated type with a CGlue trait object mutable reference.

Wrap the associated type with a CGlue trait object reference.