Safe Memory Allocation for C

In this lesson, you will learn how to allocate memory within Rust using the system allocator so that it remains compatible with C’s free() function.

The Memory Ownership Problem

When you pass memory across an FFI boundary, the host environment (usually C) often expects to take ownership of that memory. If you allocate memory using Rust’s default Box::new() or Vec, and then attempt to pass that pointer to C, you face a critical failure: C’s free() function is unaware of Rust’s internal memory management or the Rust allocator’s metadata. If C attempts to free Rust-managed memory, the program will likely crash or suffer from heap corruption.

Using the System Allocator

To ensure compatibility, you must use the standard system allocator directly rather than Rust’s high-level abstractions. By using std::alloc::alloc, you can request raw memory from the host operating system's allocator—the same one C uses.

Here is how you allocate a block of memory intended for the C heap:

use std::alloc::{alloc, Layout};

pub unsafe fn allocate_buffer(size: usize) -> *mut u8 {
    // Define the layout (size and alignment requirements)
    let layout = Layout::array::<u8>(size).unwrap();
    
    // Allocate memory via the system allocator
    let ptr = alloc(layout);
    
    if ptr.is_null() {
        panic!("Allocation failed");
    }
    
    ptr
}

The Contract with C

Once you return this *mut u8 pointer to C, Rust effectively "forgets" the allocation. You must provide clear documentation to the C developer that this memory was allocated via the system allocator and must be freed using the standard C free() function.

Crucially, you must ensure that the Layout used for allocation matches the alignment expected by the C code. If you are allocating a struct, use Layout::new::<MyStruct>() to ensure the memory is aligned to the platform's requirements. By using the system allocator, you create a transparent handoff where C treats the buffer as if it had allocated it itself.

Key Takeaway: To pass heap-allocated memory to C, use std::alloc::alloc to obtain a pointer that the C host can safely free(), bypassing Rust’s internal allocator management.