Crate trait_ffi 

trait-ffi

A Rust procedural macro library for creating and implementing extern fn with Trait.

Features

This library provides procedural macros to define and implement cross-crate extern func, supporting:

1. Define external traits (#[def_extern_trait]) - Define a trait that can be implemented across crates
2. Generated implementation macros (impl_trait!) - Auto-generated macros for simplified trait implementation
3. Multiple ABI support - Support for both C and Rust ABIs

Usage

Define external traits

Define an external trait in the interface crate:

use trait_ffi::*;

#[def_extern_trait]
pub trait DemeIf {
    fn say_hello(a: usize) -> i32;
}

pub fn if_say_hello(a: usize) -> i32 {
    println!("Hello from DemeIf with value: {}", a);
    demeif::say_hello(a)
}

Implement external traits

Implement the external trait in the implementation crate:

use interface::{DemeIf, impl_trait};

pub struct MyImpl;

impl_trait! {
    impl DemeIf for MyImpl {
        fn say_hello(a: usize) -> i32 {
            println!("Hello from MyImpl with value: {}", a);
            (a * 2) as i32
        }
    }
}

Specify ABI

You can specify the ABI for external traits (default is “rust”):

#[def_extern_trait(abi = "c")]
pub trait MyTrait {
    fn my_function() -> i32;
}

Supported ABI types:

• "rust" - Rust ABI (default)
• "c" - C ABI

How it works

This library uses Rust’s procedural macro system to generate cross-crate external function calls:

1. Definition stage: The #[def_extern_trait] macro generates corresponding external function declarations and call wrappers for each function in the trait
2. Implementation stage: The #[impl_extern_trait] macro generates #[no_mangle] external functions for the implemented functions
3. Linking stage: Through function name prefix conventions, ensures that the interface crate can correctly call functions in the implementation crate

Generated function names use the crate name and version as a prefix, e.g., __mycrate_0_2_function_name, to avoid symbol conflicts and ensure version compatibility.

Version Control

This library implements automatic version control to ensure compatibility across different versions:

• Semantic versioning: Uses the crate’s semantic version (major.minor.patch) from Cargo.toml
• Version-based symbol naming:
  • For version 0.x.y: Uses 0_x as the version prefix (e.g., 0_2 for version 0.2.0)
  • For version x.y.z where x >= 1: Uses major version x as the prefix (e.g., 1 for version 1.2.3)
• Automatic compatibility checking: Generates checker functions to ensure interface and implementation versions match
• Symbol conflict prevention: Different versions generate different symbol names, preventing linking conflicts

Example symbol naming:

• Crate: my-interface, Version: 0.2.0, Function: say_hello
• Generated symbol: __my_interface_0_2_say_hello

Key Features

• Type safety: Maintains complete Rust type system safety
• Zero runtime overhead: All processing is done at compile time
• Multiple ABI support: Supports both C and Rust ABIs
• Automatic symbol management: Automatically handles function name prefixes to avoid symbol conflicts
• Version compatibility management: Automatically manages function symbol versioning based on semantic versioning
• Easy to use: Provides clean macro interfaces

Limitations

• Only supports function-type trait items
• Requires linking implementation crates at compile time
• Function parameters and return values must be FFI-safe types (when using C ABI)
• Interface and implementation crates must use compatible versions (same version prefix)
• Breaking changes in trait definitions require version updates

License

This project is licensed under the MIT License.

Attribute Macros

def_extern_trait

Defines an extern trait that can be called across FFI boundaries.

impl_extern_trait

Implements an extern trait for a type and generates corresponding C function exports.