Memory layout of Rust's data types

Summary from a very good YouTube video- Visualizing memory layout of Rust’s data types Author - Sreekanth

This is mostly for my reference

Summary

Understanding the memory layout of data types in Rust is important for better program design and dealing with compiler errors. The video explores the memory layout of common data types and how they are stored in stack and heap memory.

Highlights

• When writing programs in Rust, understanding the memory layout of data types is crucial for better program design and handling compiler errors.

• When executing a binary, the operating system provides a continuous range of memory addresses called the virtual address space for the program to use.

• The stack memory is used to store data of the currently executing function and is fast and efficient, but has limitations on variable size and lifetime.

• The heap memory is used for dynamic memory allocation and allows for data sharing, but comes with performance overhead and requires explicit memory management.

• Rust’s data types have different memory layouts, including stack-only types, heap-allocated types, and dynamically sized types like slices and trait objects.

• Ownership models in Rust ensure memory safety by enforcing rules for moving and borrowing data, and smart pointers like Rc and Arc enable shared ownership of data across threads.

• Closures in Rust are represented using structs and implement specific trait types depending on their behavior, allowing for different levels of mutability and ownership.

• The executable format specifies different segments of memory, like text segment for instructions, data segment for initialized variables, heap segment for dynamic memory allocation etc.

• The stack and heap grow in opposite directions. The stack grows downwards and is used to store function data, while the heap grows upwards and is used for dynamic memory allocation.

• The memory address range is determined by the CPU’s word size, which is 64 bits on most modern CPUs. This means a 64-bit CPU can address up to 2^48 bytes of memory.

• The video goes over the memory layout of common data types in Rust, including integers, tuples, slices, vectors, strings, structs, enums, smart pointers, trait objects, and function pointers.

  • Structs have memory allocated for each of their fields.
  • Enums have memory allocated to store the variant tag and the data of the chosen variant.
  • Smart pointers like Box and Rc manage heap-allocated memory.
  • Trait objects are fat pointers that consist of two pointers: one to the value and one to the table representing the value’s type.
  • Function pointers store the address of the actual machine code for the function.
  • Closures are represented using structs that store the captured variables and implement the appropriate trait.

  Memory Allocation and Segments:

  When a Rust program executes, the operating system allocates a continuous block of memory, known as the virtual address space, for the program’s exclusive use. This space is further divided into segments, each serving a specific purpose:

  • Text segment: This segment stores the executable instructions that make up the program’s logic.
  • Data segment: This segment houses initialized static variables, which are global variables assigned values at compile time.
  • BSS segment: This segment holds uninitialized global variables, whose values are set to zero during program startup.
  • Stack segment: This segment is crucial for function execution. Each thread possesses its own stack, which grows downwards from a high memory address. This growth direction signifies that function calls add data to the stack, and returning from a function removes data.
  • Heap segment: This segment caters to dynamic memory allocation. It allows the program to request memory during runtime and is shared by all threads within the same process. The heap grows upwards, expanding as needed.

  Stack Frames and Function Execution:

  The stack segment plays a vital role in function execution. Every function call results in a dedicated memory block called a stack frame being created on the stack. This frame stores:

  • Local variables: Variables declared within the function’s scope are allocated space on the stack frame.
  • Function parameters: The arguments passed to the function are also stored on the stack frame.
  • Return address: This crucial piece of data holds the memory address of the next instruction to execute after the function returns.

  Stack memory allocation offers efficiency due to its simplicity. Manipulating the stack pointer allows for quick data pushing and popping, making it ideal for local variables and function parameters with known sizes at compile time. However, there are limitations:

  Only variables with fixed sizes can reside on the stack. Arrays with dynamic sizes cannot be directly stored there. Returning references to local variables defined in other functions is not possible because their memory gets overwritten upon function return.

  Memory Layout of Common Data Types:

  The video delves into the memory layout of various data types commonly used in Rust:

  • Integer types: These are stored in their native size, typically 8 bytes for 64-bit systems.
  • Tuples: The layout of a tuple depends on the size and alignment of its individual elements. Elements are placed sequentially in memory.
  • Slices: A slice is essentially a pointer to an array and its length. The slice struct itself occupies 8 bytes on a 64-bit system.
  • Vectors: Vectors represent dynamically sized arrays. Their memory layout comprises three parts: the vector struct itself (24 bytes on 64-bit systems), a pointer to the allocated memory, and the actual data elements.
  • Strings: Strings in Rust are pointers to a 3-byte length prefix followed by the UTF-8 encoded bytes of the string content.
  • Structs: The memory layout of a struct is determined by the size and alignment of its fields. Fields are placed sequentially in memory, potentially with padding for alignment.
  • Enums: The memory layout of an enum is determined by the size of its largest variant. The chosen variant’s data is stored alongside a tag indicating the variant type.
  • Smart pointers: Smart pointers like Box and Rc manage heap-allocated memory. They typically store a pointer to the allocated memory and additional information like reference count.
  • Trait objects: Trait objects represent values of different types that implement a common trait. They are implemented as fat pointers, consisting of two pointers: one to the value itself and another to the vtable (virtual table) representing the value’s type.
  • Function pointers: These pointers simply store the memory address of the actual machine code for the function, occupying a single machine word.
  • Closures: Closures are represented using structs that store the captured variables and implement the appropriate trait (e.g., fn once, fn mut, or fn).

Additional Considerations:

The video also touches upon marker traits like Send and Sync. These traits are used to specify whether a data type is thread-safe, meaning it can be safely shared across threads without causing data races or other concurrency issues. Understanding thread-safety is crucial when working with data that might be accessed by multiple threads simultaneously.

By comprehensively examining memory allocation, stack frames, and the layout of various data types, this video provides a valuable foundation for understanding how Rust programs interact with memory and manage their resources efficiently.