CODEWITHSUNDEEP
c++rustffi
user7810882
user7810882

Reputation: 233

Rust function that allocates memory and calls a C++ callback crashes

Rust code:

#[repr(C)]
pub struct Tmp {
    pub callback: extern "C" fn(i: i32),
}

#[no_mangle]
pub extern "C" fn set_callback(callback: extern "C" fn(i: i32)) -> *mut Tmp {
    let mut tmp = Box::new(Tmp { callback });
    println!("tmp as ptr: {:p}", tmp); // >> here <<
    &mut *tmp
}

#[no_mangle]
pub extern "C" fn use_callback(tmp_ptr: *mut Tmp) {
    unsafe {
        ((*tmp_ptr).callback)(1);
        ((*tmp_ptr).callback)(3);
    }
}

C++ code:

struct Tmp {
    void (*callback)(int32_t);
};

typedef Tmp*(__stdcall* set_callback_t)(void(*callback_t)(int32_t));
typedef void(__stdcall* use_callback_t)(Tmp*);

void callback(int32_t i) {
    printf("%d\n", i * 2);
}

int main() {
    // ... loading rust part as .dll
    // ... checking if loaded correctly
    Tmp* tmp_ptr = set_callback(callback);
    printf("tmp_ptr %p\n", tmp_ptr);
    use_callback(tmp_ptr);
    // ... freeing the .dll
}

When I compile this the program, it works as expected. The printed values of pointer to Tmp structure in Rust and C++ match. When I comment out the println in Rust, the C++ program crashes, which means that there is something wrong with this (probably Rust part) code.

I am using the Rust code as a .dll. I would like to pass a pointer to C++ function to the set_callback function, and then I would like to use that pointer in the use_callback function when I call use_callback in the C++ code.

For what I understand, at the end I will have to call a Rust function to drop the Tmp structure, but I left that out.

Upvotes: 2

Views: 807

Answers (1)

mcarton
mcarton

Reputation: 29981

Box in Rust is similar to std::unique_ptr in C++. The way you construct tmp, the data pointed to will be freed at the end of the function.

In order to "leak" the pointer into C++ world, you should use Box::into_raw.

Note that as there is no guarantee that Rust and C++ allocate memory the same way; you will have to pass the pointer back to Rust and use Box::from_raw to deallocate it.

Upvotes: 5

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