Cross-Language Performance Interop

In this lesson, you will learn how to design data interfaces that achieve seamless, zero-copy communication between Rust and C/C++ by aligning memory layouts and managing object ownership across language boundaries.

The Challenge of Binary Compatibility

When passing data between Rust and C++, the primary performance killer is the need to "translate" or copy data structures to satisfy the ABI (Application Binary Interface) requirements of each language. To achieve zero-copy performance, you must treat your data interfaces as a shared memory contract, ensuring that both languages interpret the bit patterns in the exact same way.

Struct Alignment and repr(C)

Rust’s default memory layout (repr(Rust)) is free to reorder fields to optimize for size. This is fatal for cross-language interop. To communicate with C/C++, you must use #[repr(C)] on your structs. This forces a deterministic layout that mirrors standard C compiler behavior.

#[repr(C)]
pub struct SensorData {
    pub timestamp: u64,
    pub value: f32,
    pub sensor_id: u32, // Padding will be added automatically to match C alignment
}

Zero-Copy Ownership Tactics

To avoid copying data (like large buffers or arrays), you must define who "owns" the memory. A common architectural pattern is to allocate a buffer in Rust, pass a raw pointer (*const T or *mut T) and a length to the C++ side, and keep the Rust object alive until the C++ operation completes.

Crucially, you must ensure the C++ side does not attempt to free() memory allocated by the Rust allocator, as this leads to heap corruption. Use a dedicated drop_buffer function exported from Rust to handle deallocation:

#[no_mangle]
pub unsafe extern "C" fn release_sensor_buffer(ptr: *mut SensorData, len: usize) {
    let _ = Vec::from_raw_parts(ptr, len, len);
    // The Vec drops here, correctly freeing the memory in the Rust heap
}

By aligning layouts via repr(C) and strictly defining the lifecycle of raw pointers, you eliminate the overhead of data marshalling. The C++ code can access the memory directly as if it were a native struct, and Rust retains safety by strictly managing the buffer's ultimate destruction.

Key Takeaway: Achieve zero-copy interop by forcing stable memory layouts with #[repr(C)] and strictly managing buffer lifetimes to prevent cross-heap corruption.