How to Use Opaque Pointers for FFI in Rust

Use opaque pointers (*mut c_void or *const c_void) in Rust FFI to hide the concrete type from the C side while maintaining safety via unsafe blocks. Define the pointer type in your Rust struct or function signature, cast it to the concrete type inside an unsafe block, and ensure the C side only passes the raw pointer without dereferencing it.

use std::ffi::c_void;

// Rust side: define a struct and a function accepting an opaque pointer
struct MyData {
    value: i32,
}

#[no_mangle]
pub extern "C" fn process_data(ptr: *mut c_void) {
    unsafe {
        // Cast the opaque pointer to the concrete type
        let data = &mut *(ptr as *mut MyData);
        data.value += 1;
    }
}

On the C side, declare the function to accept void* and pass the address of your struct without casting it to a specific type in C.

Opaque pointers let Rust and C exchange data without agreeing on the exact structure details upfront. Think of it like handing someone a sealed envelope (the pointer) that they can open only with the right key (the Rust code), keeping the contents safe from accidental misuse. You use this when building libraries where C code should not know or care about Rust's internal data layout.