Rust Ffi

Binding Generation

C/C++ → Rust (bindgen)

# Auto-generate bindings
bindgen input.h \
    --output src/bindings.rs \
    --allowlist-type 'my_*' \
    --allowlist-function 'my_*'

Rust → C (cbindgen)

# Generate C header
cbindgen --crate mylib --output include/mylib.h

Solution Patterns

Pattern 1: Calling C Functions

use std::ffi::{CStr, CString};
use libc::c_int;

#[link(name = "curl")]
extern "C" {
    fn curl_version() -> *const libc::c_char;
    fn curl_easy_perform(curl: *mut c_int) -> c_int;
}

// ✅ Safe wrapper
fn get_version() -> String {
    unsafe {
        let ptr = curl_version();
        // SAFETY: curl_version returns valid null-terminated string
        CStr::from_ptr(ptr).to_string_lossy().into_owned()
    }
}

Pattern 2: String Passing

// ✅ Safe way to pass strings
fn process_c_string(s: &CStr) {
    // SAFETY: s is a valid CStr, ptr is valid for call duration
    unsafe {
        some_c_function(s.as_ptr());
    }
}

// Creating CString from Rust
fn get_c_string() -> Result<CString, std::ffi::NulError> {
    CString::new("hello")
}

// ❌ Dangerous: temporary CString
// let ptr = CString::new("hello").unwrap().as_ptr();  // Dangling!

// ✅ Correct: keep CString alive
let c_str = CString::new("hello")?;
let ptr = c_str.as_ptr();
// use ptr...
// c_str dropped here

Pattern 3: Callback Functions

extern "C" fn callback(data: *mut libc::c_void) {
    // SAFETY: data must be a valid pointer to UserData
    // Caller guarantees this invariant
    unsafe {
        let user_data: &mut UserData = &mut *(data as *mut UserData);
        user_data.count += 1;
    }
}

fn register_callback(callback: extern "C" fn(*mut c_void), data: *mut c_void) {
    unsafe {
        some_c_lib_register(callback, data);
    }
}

Pattern 4: C++ Interop with cxx

// Using cxx for safe C++ FFI
use cxx::CxxString;

#[cxx::bridge]
mod ffi {
    unsafe extern "C++" {
        include!("my_library.h");

        type MyClass;

        fn do_something(&self, input: i32) -> i32;
        fn get_data(&self) -> &CxxString;
    }
}

struct RustWrapper {
    inner: cxx::UniquePtr<ffi::MyClass>,
}

impl RustWrapper {
    pub fn new() -> Self {
        Self {
            inner: ffi::create_my_class(),
        }
    }

    pub fn do_something(&self, input: i32) -> i32 {
        self.inner.do_something(input)
    }
}

Data Type Mapping

Error Handling

C Error Codes

fn call_c_api() -> Result<(), Box<dyn std::error::Error>> {
    // SAFETY: c_function is properly initialized
    let result = unsafe { c_function_that_returns_int() };
    if result < 0 {
        return Err(format!("C API error: {}", result).into());
    }
    Ok(())
}

Panic Across FFI

// Panics across FFI boundary = UB
// Must catch or prevent

#[no_mangle]
pub extern "C" fn safe_call() -> i32 {
    let result = std::panic::catch_unwind(|| {
        rust_code_that_might_panic()
    });

    match result {
        Ok(value) => value,
        Err(_) => -1,  // Error code
    }
}

C++ Exceptions

// C++ exceptions → Rust panic (with cxx)
// Must catch at FFI boundary

#[no_mangle]
pub extern "C" fn safe_cpp_call(error_code: *mut i32) -> *const c_char {
    let result = std::panic::catch_unwind(|| {
        unsafe { cpp_function() }
    });

    match result {
        Ok(Ok(value)) => value.as_ptr(),
        Ok(Err(e)) => {
            if !error_code.is_null() {
                unsafe { *error_code = e.code(); }
            }
            std::ptr::null()
        }
        Err(_) => {
            if !error_code.is_null() {
                unsafe { *error_code = -999; }
            }
            std::ptr::null()
        }
    }
}

Memory Management

// ✅ Rust allocates, C borrows
#[no_mangle]
pub extern "C" fn create_buffer(len: usize) -> *mut u8 {
    let mut buf = vec![0u8; len];
    let ptr = buf.as_mut_ptr();
    std::mem::forget(buf);  // Don't drop
    ptr
}

#[no_mangle]
pub extern "C" fn free_buffer(ptr: *mut u8, len: usize) {
    unsafe {
        // SAFETY: ptr was allocated by create_buffer with this len
        let _ = Vec::from_raw_parts(ptr, len, len);
    }  // Vec dropped, memory freed
}

Workflow

Step 1: Choose FFI Strategy

Need to call C code?
  → Simple functions? Manual extern declarations
  → Complex API? Use bindgen
  → C++? Use cxx crate

Exporting to C?
  → Use cbindgen to generate headers
  → Mark functions #[no_mangle]
  → Use extern "C"

Step 2: Define Safety Invariants

For every FFI call:
1. Document pointer validity requirements
2. Document lifetime expectations
3. Document thread safety assumptions
4. Document panic handling

Step 3: Build Safe Wrapper

unsafe FFI calls
  ↓
Safe private functions (validate inputs)
  ↓
Safe public API (no unsafe visible)

Step 4: Test Thoroughly

# Test with Miri
cargo +nightly miri test

# Memory safety check
valgrind ./target/release/program

# Cross-compile test
cargo build --target x86_64-unknown-linux-gnu

Language-Specific Tools

Common Pitfalls

Review Checklist

When reviewing FFI code:

• All extern functions have SAFETY comments
• String conversion uses CStr/CString properly
• Memory ownership is clearly documented
• No panics across FFI boundary (use catch_unwind)
• FFI types use #[repr(C)]
• Raw pointers validated before dereferencing
• Functions exported with #[no_mangle]
• Callbacks have correct ABI (extern "C")
• Tested with Miri for UB detection
• Documentation explains ownership protocol

Verification Commands

# Check safety
cargo +nightly miri test

# Memory leaks
valgrind --leak-check=full ./target/release/program

# Generate bindings
bindgen wrapper.h --output src/ffi.rs

# Generate C header
cbindgen --lang c --output target/mylib.h

# Check exports
nm target/release/libmylib.so | grep my_function

Safety Guidelines

1. Minimize unsafe: Only wrap necessary C calls
2. Defensive programming: Check null pointers, validate ranges
3. Clear documentation: Who owns memory, who frees it
4. Test coverage: FFI bugs are extremely hard to debug
5. Use Miri: Detect undefined behavior early

Related Skills

• rust-unsafe - Unsafe code fundamentals
• rust-ownership - Memory and lifetime management
• rust-coding - Export conventions
• rust-performance - FFI overhead optimization
• rust-web - Using FFI in web services

Localized Reference

• Chinese version: SKILL_ZH.md - 完整中文版本，包含所有内容

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