Rust FFI and C Memory Ownership Transfer

Question

Integrating Rust with C via the Foreign Function Interface (FFI) requires a strict protocol for managing heap-allocated memory across the language boundary. When Rust transfers ownership of a resource to a C consumer using Box::into_raw, the Rust compiler ceases to track the lifetime of that allocation.

The primary constraint is ensuring that the memory is reclaimed without causing undefined behavior. Since C lacks a native understanding of Rust's ownership model, the responsibility for triggering the destructor falls on the integration layer, typically requiring the pointer to be passed back to a specific Rust function for reconstruction via Box::from_raw.

How can the integration be structured to guarantee that Box::from_raw is called exactly once for every Box::into_raw call?
  What is the recommended pattern for verifying that the pointer passed back from C still matches the original allocation layout expected by the Rust allocator?

Tasadduq Burney · Answer

Guaranteeing exactly once Box::from_raw in Rust FFI
The most reliable structure pairs a dedicated Rust deallocation function with the pointer handed to C. C receives the raw pointer from Box::into_raw, uses it, and later calls the Rust function to reconstruct the Box via Box::from_raw. As long as the C side invokes that function precisely once, the Rust compiler’s lifetime contract is satisfied.
Likely explanation
When Rust calls Box::into_raw the pointer is handed off to C. The Rust compiler drops all ownership tracking. To reclaim the allocation C must route the pointer back through a known entry point. If C ever frees the memory through a different mechanism (e.g. free() or a custom allocator) without informing Rust or if the pointer is dropped twice undefined behavior arises. The exactly once guarantee therefore rests on a single well documented deallocation path that C must follow.
Confirmed facts

Rust's extern C ABI does not convey ownership across the boundary the caller on each side must observe the matching allocation deallocation pair.
  Box::into_raw produces a *mut T that the Rust compiler no longer considers owned omitting Box::from_raw leaks the heap block.
  Box::from_raw is only safe when the pointer originates from a matching Box::into_raw for the same type T.
  Using a mismatched allocator (e.g. C malloc paired with Rust alloc) invokes undefined behavior pair new delete or malloc free across the same language boundary.

Steps needed for this case

Define a pub extern C fn deallocate(ptr: *mut T) in Rust that guards with a null check and calls Box::from_raw ptr.
  Expose the allocated pointer to C as the return value of an extern C function that performed Box::into_raw.
  Document that C must call the deallocate function exactly once after it is finished with the pointer.
  If C may forward the pointer to other modules or code paths consider wrapping the raw pointer in a Rust token type that consumes itself on first deallocation or use std mem forget on the Rust side while providing a corresponding C side free function that the application author guarantees to invoke once.

Practical verification
// Rust FFI pattern
use std::boxed::Box;

#[no_mangle]
pub extern "C" fn rust_allocate() -> *mut i32 {
    let boxed: Box = Box::new(42);
    Box::into_raw(boxed)
}

#[no_mangle]
pub extern "C" fn rust_deallocate(ptr: *mut i32) {
    if !ptr.is_null() {
        let _ = Box::from_raw(ptr);
    }
}

Compile the example and run under valgrind --leak-check=full or cargo miri test to confirm no leaks or double frees appear in the FFI path.
If the C integration may free the pointer through a different code path than the designated rust_deallocate function please specify the exact function signature that receives the pointer the recommendation may shift to a token based guard or a ManuallyDrop wrapped wrapper.

Rust FFI and C Memory Ownership Transfer

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Guaranteeing exactly once Box::from_raw in Rust FFI

Likely explanation

Confirmed facts

Steps needed for this case

Practical verification

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