Heap vs Stack — How to Rust

In Rust, as in many other programming languages, the decision to store data on the stack or the heap is based on the data’s size, its compile-time known size, and its lifetime. Understanding the distinction between the stack and the heap, and why different types of memory are stored in each, is fundamental to grasping Rust’s ownership system, which aims to ensure memory safety and efficiency without the need for a garbage collector.

The Stack

The stack is a region of memory that stores values in a last-in, first-out (LIFO) manner. It’s extremely fast and efficient because adding or removing data only involves changing the stack pointer. However, the stack has limitations:

• Size: The size of items must be known at compile-time. This allows Rust to allocate space for the data without complex calculations or overhead.
• Lifetime: Data on the stack can only live for as long as the function call in which it was created. Once the function exits, the data is automatically removed.

Stack storage is ideal for small-sized data and for situations where the scope and lifetime of data are clear and constrained. Primitive types like integers and floats, fixed-size arrays, and tuples (containing types with a known size at compile-time) are examples of data typically stored on the stack.

The Heap

The heap is a more flexible, dynamically allocated pool of memory. Unlike the stack, data on the heap can:

• Have a Dynamic Size: The size of data can be determined at runtime. This is essential for types like String and Vec<T>, where the amount of data they hold isn't known until the program runs.
• Live Longer: Heap-allocated data can outlive the function call that created it. This is crucial for passing data around and for building complex data structures.

However, this flexibility comes at a cost:

• Performance: Allocating and deallocating memory on the heap is slower because it involves an allocation algorithm to find a big enough spot to hold the data. This can lead to fragmentation over time.
• Management Complexity: The programmer must ensure that the memory is properly managed (allocated and freed). In Rust, this management is mostly handled by the ownership system, which checks at compile time that every piece of data on the heap has a clear owner and that there’s no dangling pointer or memory leak.

How big is the Stack memory

In Rust, the stack memory size for a thread isn’t defined by the Rust language itself, but by the operating system and sometimes by settings in the runtime environment or the executable’s configuration. Typically, the default stack size for a thread in most operating systems is around 1 to 2 MB, but this can vary:

1. On Windows: The default stack size is usually around 1 MB.
2. On Linux and macOS: The typical default stack size for new threads is about 2 MB.
3. Customizable Size: It’s also possible to adjust the stack size for specific threads if needed, either through system settings or programmatically (such as using attributes in Rust or thread configuration in the runtime).

In Rust, you might see the stack size being important particularly when dealing with recursive functions or large stack-allocated data. If you encounter a stack overflow, you might need to increase the stack size or refactor your approach to use heap allocation instead.

Why Rust Uses Both

Rust’s use of both stack and heap memory is a balance between performance and flexibility. The stack is used for data with a known, fixed size to leverage its speed and efficiency. The heap is used for data that needs to be dynamically sized or live beyond a single function call, despite the overhead of dynamic memory allocation.

Rust’s ownership model, with rules for borrowing, lifetimes, and ownership transfers, is designed to manage heap data safely and efficiently. By requiring clear ownership and preventing data races at compile time, Rust ensures that heap memory is used safely, avoiding common errors like use-after-free, double free, and memory leaks that are prevalent in languages with manual memory management.

In summary, Rust stores some memory on the stack for speed and simplicity and other memory on the heap for flexibility and longevity. Its strict but helpful compiler ensures that developers can manage both types of memory safely and efficiently.