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Rust – FFI and Interoperability

We will explore FFI (Foreign Function Interface) and interoperability capabilities in RUST and how to interface with C and other languages.

One of Rust’s strengths is its ability to interoperate with other programming languages, especially with C and C++. The Foreign Function Interface (FFI) is a key enabler of this interoperability which allows you to call functions written in other languages from Rust code and vice versa.

Table of Contents:

  1. FFI Basics and Terminology
  2. Calling C Functions from Rust
  3. Calling Rust Functions from C
  4. Working with Complex Data Types
  5. Error Handling in FFI
  6. Tips for FFI Safety

FFI Basics and Terminology:

The Foreign Function Interface (FFI) is a mechanism that allows different programming languages to communicate with each other by calling functions and sharing data structures. In Rust, FFI is primarily used for interfacing with C, as C libraries are widely available and C-compatible interfaces are supported by many languages.

Some key FFI-related terms to know:

  • Bindings: Rust code that defines function signatures and data structures for a foreign library.
  • cbindgen: A tool that generates C bindings for Rust code.
  • bindgen: A tool that generates Rust bindings for C and C++ code.

Calling C Functions from Rust:

To call C functions from Rust, follow given below steps:

  1. Create a C library or locate an existing one to use.
  2. Write or generate Rust bindings for the C functions and data structures.
  3. Use the extern keyword to declare the foreign functions in Rust.
  4. Use the unsafe keyword to call the foreign functions.


File: example.c

#include <stdint.h>

int32_t add_ints(int32_t a, int32_t b) {

    return a + b;



extern "C" {

    fn add_ints(a: i32, b: i32) -> i32;


fn main() {

    let result = unsafe { add_ints(2, 3) };

    println!("Result: {}", result);


Calling Rust Functions from C:

To call Rust functions from C:

  1. Export Rust functions using the pub keyword and #[no_mangle] attribute.
  2. Ensure Rust functions have the extern keyword and C-compatible signatures.
  3. Generate a C header file with cbindgen or write one manually.
  4. Include the header file in your C code and call the Rust functions.



pub extern "C" fn add_ints(a: i32, b: i32) -> i32 {

    a + b


File: main.c

#include <stdio.h>

#include "rust_functions.h"

int main() {

    int32_t result = add_ints(2, 3);

    printf("Result: %d\n", result);

    return 0;


Working with Complex Data Types:

When dealing with FFI, it’s essential to understand how complex data types are represented and passed between Rust and the target language. Here are some common complex data types and their representations:

  • Strings: In Rust, strings are usually represented as &str or String. In C, strings are usually represented as const char* or char*. When passing strings across FFI boundaries, you’ll need to convert between these representations.
  • Structs: Rust and C language structs can have different memory layouts. To ensure compatibility, use the #[repr(C)] attribute in Rust to enforce C-compatible memory layout.
  • Enums: Rust’s enums can be more complex than C’s, as they can store data. To achieve compatibility, use the #[repr(C)] attribute to create a C-compatible enum.

To convert complex data types between Rust and C, you’ll need to handle data conversions and manage memory.

Example: Passing a String from Rust to C

use std::ffi::CString;

use std::os::raw::c_char;

extern "C" {

    fn print_c_string(c_string: *const c_char);


fn main() {

    let rust_string = "Hello, FFI!";

    let c_string = CString::new(rust_string).unwrap();

    unsafe {




In this example, we use CString from the std::ffi module to convert a Rust &str to a C-compatible string (*const c_char) before passing it to the C function print_c_string.

Example: Passing a Struct from Rust to C

use std::os::raw::c_int;

pub struct Point {

    x: c_int,

    y: c_int,


extern "C" {

    fn calculate_distance(point_a: *const Point, point_b: *const Point) -> f64;


fn main() {

    let point_a = Point { x: 0, y: 0 };

    let point_b = Point { x: 3, y: 4 };

    let distance = unsafe { calculate_distance(&point_a, &point_b) };

    println!("Distance: {}", distance);


In this example, we define a Point struct with the #[repr(C)] attribute, ensuring a C-compatible memory layout. We then pass pointers to the struct instances to the C function calculate_distance.

Tips for FFI Safety:

Understand the Target Language’s Semantics:

Grasping the target language’s semantics is vital for ensuring FFI safety. Familiarize yourself with the language’s memory management, error handling, data types, and calling conventions. This knowledge will enable you to make informed decisions when interfacing with foreign functions and correctly translate data types and errors between Rust and the target language.

Use the unsafe Keyword Wisely:

Rust requires the use of the unsafe keyword when calling foreign functions or performing operations that could compromise memory safety. When using unsafe, ensure you understand the potential risks and take necessary precautions to maintain safety. Keep unsafe blocks as small as possible and encapsulate them in safe abstractions, allowing for easier auditing and better separation of concerns.

Validate and Sanitize Input Data:

When passing data between Rust and foreign functions, validate and sanitize input data to prevent undefined behavior, crashes, or security vulnerabilities. Ensure that the data being passed is within the expected range, size, or format before passing it across the FFI boundary.

Manage Memory Allocation and Deallocation:

When working with FFI, it’s essential to ensure memory is correctly allocated and deallocated to prevent memory leaks, double-frees, or undefined behavior. When sharing memory between Rust and foreign functions, make sure you know which side is responsible for memory management and follow the appropriate conventions.

Use C-compatible Data Types and Representations:

To maintain compatibility across FFI boundaries, use data types and representations that are compatible with C, the most common language for FFI interaction. Rust provides several C-compatible data types in the std::os::raw module. Additionally, use the #[repr(C)] attribute to enforce C-compatible memory layouts for structs and enums.

Handle Errors Gracefully:

Error handling is a critical aspect of programming, and it becomes even more important when working with FFI. Understand the target language’s error handling mechanism and adopt strategies such as using return values or out-parameters to handle errors. Ensure you handle errors gracefully in Rust, preventing crashes or undefined behavior.

Be Mindful of Platform Differences:

When working with FFI, keep in mind the differences between platforms, such as endianness, word size, and alignment. Be cautious when making assumptions about the target platform’s characteristics, and use Rust’s built-in mechanisms for platform-specific code when necessary.

Test Your FFI Code Thoroughly:

Thorough testing is crucial for ensuring FFI safety. Write comprehensive test cases that cover various edge cases, input data, and potential errors. Use tools like Valgrind or sanitizers to detect memory leaks, undefined behavior, or other issues that might not be apparent during regular testing.

Relevant Rust topics:


Posted in Rust

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