Detecting Memory Leaks in Unsafe Code

You will employ memory profiling techniques to verify that manually allocated memory is always reclaimed.

In this lesson, you will learn how to verify that your manual memory management in unsafe blocks doesn't introduce leaks by utilizing memory profiling techniques.

When you use unsafe to allocate memory—such as using std::alloc::alloc—you bypass Rust’s ownership system. If you fail to call dealloc exactly once for every allocation, your program will accumulate memory indefinitely. Unlike safe Rust, the compiler cannot help you here, so you must treat your manual management as a verified contract.

Using Memory Profiling

The most reliable way to catch leaks is by comparing the heap state before and after a specific operation. In the Rust ecosystem, valgrind (on Linux) is the gold standard for this, specifically the massif or memcheck tools.

Consider this manual allocation:

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

unsafe {
    let layout = Layout::new::<[u8; 1024]>();
    let ptr = alloc(layout);
    if !ptr.is_null() {
        // We forgot to dealloc(ptr, layout)!
    }
}

To detect this, run your compiled binary through Valgrind: valgrind --leak-check=full ./your_executable

Valgrind will track every malloc and free call (or their equivalents). If you have a leak, the report will point to the exact unsafe block where the pointer was allocated but never released.

Proactive Verification

For automated testing, you can integrate crates like dhat or jemalloc-ctl. These allow you to hook into the allocator during your unit tests. By wrapping your allocation code in a test function, you can query the number of active allocations before and after the function runs:

#[test]
fn test_no_leaks() {
    let start_allocs = get_current_allocations();
    unsafe { perform_manual_work(); }
    assert_eq!(start_allocs, get_current_allocations());
}

By establishing a baseline and asserting that the state returns to zero after cleanup, you turn manual memory management into a testable, verifiable process. Always ensure your test paths cover error scenarios, as unsafe code often leaks precisely when an early return skips the cleanup logic.

Key Takeaway: Because unsafe code bypasses ownership, you must validate manual memory reclamation using external heap profilers or allocator-tracking tests to ensure every allocation has a corresponding deallocation.