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//! A collection of utilities (functions, traits, data structures, etc ...) to ease conversion between Rust and C-compatible data structures. //! //! Through two **conversion traits**, **`CReprOf`** and **`AsRust`**, this crate provides a framework to convert idiomatic Rust structs to C-compatible structs that can pass through an FFI boundary, and conversely. //! They ensure that the developper uses best practices when performing the conversion in both directions (ownership-wise). //! //! The crate also provides a collection of useful utility functions to perform conversions of types. //! It goes hand in hand with the `ffi-convert-derive` crate as it provides an **automatic derivation** of the `CReprOf` and `AsRust` trait. //! //! # Usage //! When dealing with an FFI frontier, the general philosophy of the crate is : //! - When receiving pointers to structs created by C code, the struct is immediately converted to an owned, idiomatic Rust struct through the use of the `AsRust` trait. //! - To send an idiomatic, owned Rust struct to C code, the struct is converted to C-compatible representation using the CReprOf trait. //! //! ## Example //! //! We want to be able to convert a **`Pizza`** Rust struct that has an idiomatic representation to a **`CPizza`** Rust struct that has a C-compatible representation in memory. //! We start by definining the fields of the `Pizza` struct : //! ``` //! # struct Topping {}; //! # struct Sauce {}; //! pub struct Pizza { //! pub name: String, //! pub toppings: Vec<Topping>, //! pub base: Option<Sauce>, //! pub weight: f32, //! } //!``` //! //! We then create the C-compatible struct by [mapping](#types-representations-mapping) idiomatic Rust types to C-compatible types : //! ``` //! # use ffi_convert::CArray; //! # struct CTopping {}; //! # struct CSauce {}; //! #[repr(C)] //! pub struct CPizza { //! pub name: *const libc::c_char, //! pub toppings: *const CArray<CTopping>, //! pub base: *const CSauce, //! pub weight: libc::c_float, //! } //! ``` //! //! This crate provides two traits that are useful for converting between Pizza to CPizza and conversely. //! //! ```ignore //! CPizza::c_repr_of(pizza) //! <=================| //! //! CPizza Pizza //! //! |=================> //! cpizza.as_rust() //! //! ``` //! Instead of manually writing the body of the conversion traits, we can derive them : //! //! ``` //! # use ffi_convert::{CReprOf, AsRust, CDrop}; //! # use ffi_convert::CArray; //! # use ffi_convert::RawBorrow; //! # struct Sauce {}; //! # #[derive(CReprOf, AsRust, CDrop)] //! # #[target_type(Sauce)] //! # struct CSauce {}; //! # struct Topping {}; //! # #[derive(CReprOf, AsRust, CDrop)] //! # #[target_type(Topping)] //! # struct CTopping {}; //! # //! # struct Pizza { //! # name: String, //! # toppings: Vec<Topping>, //! # base: Sauce, //! # weight: f32 //! # }; //! use libc::{c_char, c_float}; //! //! #[repr(C)] //! #[derive(CReprOf, AsRust, CDrop)] //! #[target_type(Pizza)] //! pub struct CPizza { //! pub name: *const c_char, //! pub toppings: *const CArray<CTopping>, //! pub base: *const CSauce, //! pub weight: c_float, //! } //! ``` //! //! You may have noticed that you have to derive the CDrop trait. //! The CDrop trait needs to be implemented on every C-compatible struct that require manual resource management. //! The release of those resources should be done in the drop method of the CDrop trait. //! //! You can now pass the `CPizza` struct through your FFI boundary ! //! //! ## Types representations mapping //! //! <table> //! <thead> //! <tr> //! <th>C type</th> //! <th>Rust type</th> //! <th>C-compatible Rust type</th> //! </tr> //! </thead> //! <tbody> //! <tr> //! <td><code>const char*</code></td> //! <td><code>String</code></td> //! <td><code>*const libc::c_char</code></td> //! </tr> //! <tr> //! <td><code>const T</code></td> //! <td><code>Option<T></code></td> //! <td><code>*const T</code> (with <code>#[nullable]</code> field annotation)</td> //! </tr> //! <tr> //! <td><code>CArrayT</code></td> //! <td><code>Vec<T></code></td> //! <td><code>CArray<T></code></td> //! </tr> //! </tbody> //! </table> //! //! ```ignore //! typedef struct { //! const T *values; // Pointer to the value of the list //! int32_t size; // Number of T values in the list //! } CArrayT; //! ``` //! ## The CReprOf trait //! The `CReprOf` trait allows to create a C-compatible representation of the reciprocal idiomatic Rust struct by consuming the latter. //! ``` //! # use ffi_convert::{Error, CDrop}; //! pub trait CReprOf<T>: Sized + CDrop { //! fn c_repr_of(input: T) -> Result<Self, Error>; //! } //! ``` //! This shows that the struct implementing it is a `repr(C)` compatible view of the parametrized //! type and can be created from an object of this type. //! ## The AsRust trait //! > When trying to convert a `repr(C)` struct that originated from C, the philosophy is to immediately convert //! > the struct to an **owned** idiomatic representation of the struct via the AsRust trait. //! The `AsRust` trait allows to create an idiomatic Rust struct from a C-compatible struct : //! ``` //! # use ffi_convert::{Error, CDrop}; //! pub trait AsRust<T> { //! fn as_rust(&self) -> Result<T, Error>; //! } //! ``` //! This shows that the struct implementing it is a `repr(C)` compatible view of the parametrized //! type and that an instance of the parametrized type can be created form this struct. //! ## The CDrop trait //! A Trait showing that the `repr(C)` compatible view implementing it can free up its part of memory that are not //! managed by Rust drop mechanism. //! ## Caveats with derivation of CReprOf and AsRust traits //! pub use ffi_convert_derive::*; mod conversions; mod types; pub use conversions::*; pub use types::*; pub use failure::Error;