How to fill Rust function pointer from C++

Question

I want to call C++ from Rust. C++ then allocates and fills a buffer, then Rust uses it and deallocates. However, I'd like to deliver back the function pointer to deallocate the data. This is what I tried:

In Rust:

extern "C" {
    pub fn openvpn_receive(
        instance: *mut OpenVpnInstance,
        size: *mut size_t,
        deallocate: extern "C" fn(*mut u8),
    ) -> *mut u8;
}

fn main() {
    let size: *mut size_t;
    let deallocate = extern "C" fn(*mut u8);
    let data: *mut u8 = openvpn_receive(self.instance, size, deallocate);
    //use data
    deallocate(data);
}

In C++:

uint8_t* openvpn_receive(size_t *size, void (*deallocate_function)(uint8_t *))
{
    deallocate_function = &cpp_deallocate_u8;
    uint8*t data = receive_data(size);
}

But the Rust code is not valid. How can I make such thing happen?

Your code is somewhat difficult to follow, since it contains several undeclared names (e.g. a reference to self in the middle of the main() function). It looks like your C++ function signature isn't right – it accepts a function pointer as an input, whereas you want to return a function pointer. You either need to make the deallocate function the return value, or accept a pointer to a function pointer, so you can write the function pointer to that memory location. — Sven Marnach
– Sven Marnach, Commented Nov 10, 2020 at 8:03
The Rust code also has some obvious errors. The declaration of deallocate should use a colon to separate name and type. The variable should be mutable, since you want to pass a mutable pointer to it to your C++ code. And the type should be unsafe extern "C" fn(*mut u8). — Sven Marnach
– Sven Marnach, Commented Nov 10, 2020 at 8:06

Michał Hanusek · Accepted Answer · 2020-11-26 19:55:12Z

Maybe it will help You.

https://doc.rust-lang.org/std/keyword.extern.html

https://rust-embedded.github.io/book/interoperability/rust-with-c.html

C++ -> C -> Rust

use libc::{size_t, c_void};

#[repr(C)]
pub struct OpenVpnInstance();

//#[link(name = "your_c_library")]
extern "C" {
    pub fn openvpn_receive(
        instance: *mut OpenVpnInstance,
        size: *mut size_t,
        deallocate: *mut c_void,
    ) -> *mut u8;
}

pub fn my_callback(arg: *mut u8)
{
    println!("CB!");
}

fn main() 
{
    let size: size_t = 42;
    let instance = OpenVpnInstance{};
    
    let dummy_arg: u8 = 42;
    
    unsafe {
        let cb = my_callback as *mut c_void;
        let instance_ptr = &instance as *const _ as *mut OpenVpnInstance;
        let arg_ptr = &dummy_arg as *const _ as *mut u8;
        openvpn_receive(instance_ptr, size as *mut size_t, cb);
        my_callback(arg_ptr)
    }
}

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