c,rust,ffi , How to lend a Rust object to C code for an arbitrary lifetime?


How to lend a Rust object to C code for an arbitrary lifetime?

Question:

Tag: c,rust,ffi

I'm writing a library in Rust that has a C interface. C side must be able to create and destroy Rust objects (C side owns them and controls their lifetime).

I've managed to "leak" an object to C, but I'm not sure how to properly free it:

pub extern "C" fn create() -> *mut Foo {
   let obj = Foo; // oops, a bug
   let ptr = std::mem::transmute(&mut obj);
   std::mem::forget(obj);
   return ptr;
}

pub extern "C" fn destroy(handle: *mut Foo) {
   // get Foo back and Drop it??? 
}

I'm not sure how can I turn pointer back to an object that Rust will call Drop on. Simply dereferencing *handle doesn't compile.


Answer:

Actually, you haven't managed to leak an object to C; you've managed to leak a reference to a (shortly) non-existent stack frame. :D

Here's a full example that should work correctly. I've tried to comment it as appropriate to explain what I'm doing and why.

#![feature(collections)]

pub struct Dramatic(String);

// Implement a destructor just so we can see when the object is destroyed.
impl ::std::ops::Drop for Dramatic {
    fn drop(&mut self) {
        println!("And lo, I, {}, meet a most terrible fate!", self.0);
    }
}

pub extern "C" fn create() -> *mut Dramatic {
    // We **must** heap-allocate the object!  Returning a reference to a local
    // will **almost certainly** break your program!
    let mut obj = Box::new(Dramatic("Roger".to_string()));

    // * derefs the Box into a Dramatic, the &mut re-borrows it into a regular
    // reference.  The constraint ensures we coerce the &mut Dramatic into
    // a *mut Dramatic, which "hides" the reference from the borrow checker.
    let ptr: *mut _ = &mut *obj;

    // Forget discards its argument (passed by-move), without trigger its
    // destructor, if it has one.
    unsafe { ::std::mem::forget(obj) };

    ptr
}

pub extern "C" fn destroy(ptr: &mut *mut Dramatic) {
    // For is_null:
    use std::ptr::PtrExt;

    // First, we **must** check to see if the pointer is null.
    if ptr.is_null() {
        // Do nothing.
        return;
    }

    // Now, we know the pointer is non-null, we can continue.
    let obj: Box<Dramatic> = unsafe { ::std::mem::transmute(*ptr) };

    // We don't *have* to do anything else; once obj goes out of scope, it will
    // be dropped.  I'm going to drop it explicitly, however, for clarity.
    ::std::mem::drop(obj);

    // I am, however, going to null out the `ptr` we were passed just so the
    // calling code is less likely to accidentally re-use the pointer.
    *ptr = ::std::ptr::null_mut();
}

fn main() {
    let mut ptr = create();
    println!("ptr = {:?}", ptr);
    destroy(&mut ptr);
    println!("ptr = {:?}", ptr);
}

(Edited to remove redundant transmute in create.)


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