How to Manage Memory Across FFI Boundaries

Manage memory across FFI boundaries by explicitly transferring ownership using Box<T> to allocate on the heap and std::ptr to pass raw pointers, ensuring the receiving side knows who is responsible for freeing the memory.

use std::ptr;

// Rust side: Allocate on heap and leak to C (or pass ownership)
fn create_string() -> *mut i8 {
    let s = String::from("hello");
    let ptr = s.into_bytes().as_ptr() as *mut i8;
    // Leak the String to prevent Rust from freeing it immediately
    std::mem::forget(s);
    ptr
}

// C side (conceptual): Free the memory
// void free_string(i8* ptr) { /* free(ptr) */ }

For safe ownership transfer, define a C function to free the memory and call it from Rust using unsafe blocks when the pointer is no longer needed.

When Rust code talks to other languages like C, they don't automatically share how to clean up memory. You must manually decide which language owns the data and is responsible for deleting it when done. Think of it like handing a physical object to a friend; you must agree who will throw it away later to avoid clutter.