CS 4414/5416:
Systems Programming

Memory Layouts and Smart Pointers

David Bindel

2026-09-03

Warm-up exercise

fn get_letter_id(c: char) -> Result<usize,char> {
    let c = c.to_ascii_lowercase();
    if c.is_ascii_lowercase() {
        Ok((u32::from(c) as usize)-97)
    } else {
        Err(c)
    }
}

fn get_word_hist(word: &str) -> Result<[u8; 26],char> {
    todo!("Form a histogram of counting a..z instances in word")
}

Recap

Plan for this week

• Tue
  • Traits and generics
  • Error handling
  • Modules and libraries (probably on your own)
• Today
  • Memory layouts
  • Smart pointer types
  • Ownership, lifetimes, borrowing revisited
• Fri: Process registry lab, part 1

Where we’re going (revised)

• Next Tue
  • Testing
  • Iterators and closures
  • Cargo and crates
• Next Thu
  • Dynamic traits and vtables
  • Advanced Rust, further resources
• Fri: Process registry lab, part 2
• And then on to performance tuning.

Logistics

• Project list is posted!
  • First project repository is linked
  • Check-in submission due next Tuesday
  • Final submission due two Mondays hence
• We have Google cloud credits! See Ed for details
• Add deadline is Sep 8 – ticket if you need it!

Logistics

• Write your name and netid on class sheets
  • Print netid, ideally in pen!
• To simplify the end of class
  • Pass class sheets right and up
  • Pass blank sheets left and up

Memory Layouts

Basic setup

• Program runs in own virtual memory
  • Hardware and OS map virtual to physical memory
  • More about virtual memory in a few lectures
• Virtual address space is partitioned into segments

Stack, heap, and global

Segments (high to low):

1. Stack segment
2. Heap segment
3. Global data segment
4. Text segment (program code)

Stack frames

Keep track of per-call state

• Function arguments
• Return address
• Local variables (fixed size)

Frame is popped when call returns

Can’t touch this

int* bad_idea()
{
    int x = 1;
    return &x;
}

• Caller receives an invalid pointer!
  • Only makes sense until the frame is popped
  • Ignores valid lifetime of referenced data
• Legal C (but undefined behavior)
• Borrow checker prevents this in Rust

Heap data

• Place for data that outlives calls
  • Also for variable-length allocations
  • Valid lifetime is decoupled from calls
• Managed (semi)-explicitly in Rust, C/C++

Heap data

• Allocator map says what’s available
  • Bad allocation patterns fragment the heap
  • Allocation/free is inexpensive, not totally free
  • Resizing may cause copying (which can get expensive)
• In Rust, release automatically at end of life
  • drop call when owner goes out of scope
  • Can choose to leak in some cases

Global data

Used for

• String literals
• Other compile-time constants
• static items
  • Associated with 'static lifetime (more on this soon)

Data size/placement is set at compile time.

Memory representations

• Stack, heap, and global are where data is stored
• What about the representations?
• Scalar types should be familiar from 3410
• What about tuples, structs, and enums?
  • Or more elaborate things?

Structs in C

struct pair_t {
    uint8_t tag;
    uint64_t value;
};

Representation in memory:

• 1 byte for tag
• 7 bytes of padding (for alignment of value)
• 8 byes for value

Usual advice: order fields by decreasing size to minimize alignment padding

Struct and tuple representation

fn main() {
    println!("{} vs {} + {}",
             size_of::<(usize,u8)>(),
             size_of::<usize>(), size_of::<u8>());
}

• Mostly just list the fields
  • Compiler is allowed to reorder
  • This is different from C!
• Even with reordering, may need to pad for alignment
  • Ex: a u64 usually starts at an 8-byte boundary
    (an address that is a multiple of 8)

Enum representation

#[repr(u16)]
enum HttpErrorCodes {
    Continue = 100,
    OK = 200,
    Forbidden = 403,
    BadRequest = 404,
}

• Simple enums (no payload) are coded as a small integer
• Can force values with = value (as in C
• The repr macro makes it a specific integer type
• Use code as u16 to get value (as is like a cast)

Enum representation

enum ArticlePages {
    Pages(u32, u32),
    Online
}

fn main() {
    println!("{}", std::mem::size_of::<ArticlePages>());
}

• Enums with payloads are coded as a tagged union
• Must reserve space for tag plus largest variant

String layout example

fn foo(s3: &str) { /* ... */ }
fn main() {
    let s1 = "Hello";
    let s2 = String::from(s1);
    foo(&s2[1..5]);
}

Pretend this was not optimized to nothing.
What is the memory picture?

String layout picture

String layout notes

• Types of metadata objects
  • Raw pointers (and references) are just addresses
  • Slices are “fat pointers” (address and length)
  • String is a struct with capacity, address, length
• Capacity and length parameters are in bytes
  • Note UTF-8 coded characters vary in length
• Metadata is stored separately from data

Strings and ownership

For a String:

• The metadata object owns the heap data
• Transfer of ownership means copying metadata object
  • Or just renaming it – up to the compiler
• Only one owner avoids multiple frees
• Compilers track lifetime to avoid use-after-free

Strings and cloning

fn hello(s: String) {
    println!("Hello, {s}");
}

fn main() {
    let s = String::from("World");
    hello(s.clone());
    hello(s);
}

The clone method (trait Clone) usually makes a deep copy

• Copy the data and make new metadata
  • New metadata has different pointer, to new data!
• hello consumes s and clone (call drop at return)

Exercise

• String::from(s) copies data from s. Why?
• What is the size of an Option<u16>? An Option<u32>?
• Did the person next to you ever have pets? What type?

Pointer types

Rust pointer types

• Basic pointer types (non-owning)
  • References: &T and &mut T
  • Slices: &[T] and &mut [T]
  • Raw pointers: *const T and *mut T (unsafe)
• Smart pointer types
  • Box<T>: Owning pointer
  • Rc<T>: Reference counted smart pointer
  • RefCell<T>: Run-time checked references
• Many containers (String, Vec) use pointers internally.

Box type

fn main() {
    let mut v = Box::new(5u8);
    *v += 1;
    println!("{}", v);
}

• A Box<T> is an owning pointer to a T on the heap
• Implements the Deref trait, * accesses contents
  • Can therefore call T methods directly on Box<T>
• Also gives us borrow operator (& and &mut)

Box type and linked lists

#[derive(Debug)]
enum List<T> {
    Link {data: T, next: Box<List<T>>},
    Empty
}

fn main() {
    let mut l: List<u8> = List::Empty;
    for i in (0..5).rev() {
        l = List::Link {data: i, next: Box::new(l)};
    }
    println!("{:?}", l);
}

Rc types

use std::rc::Rc;

fn main() {
    let p1: Rc<u8> = Rc::new(String::from("cat"));
    let p2 = Rc::clone(&p1);
    println("{p1:?} and {p2:?}");
}

Rc types

• rc is the strong reference count
  • For concurrent code, need atomic updates of rc
  • Will talk about Arc (and Mutex) later in semester

Rc types

• Both p1 and p2 point to the same heap data
• Data is stored along with a reference counter
• Can lead to leakage with cycles
  • But cycles are hard to create
  • Requires interior mutability

RefCell

• Single owner (like Box<T>)
• Usual rules apply
  • One mutable ref or many immutable (but not both)
  • References must always be valid
• But with RefCell, enforce borrow safety at runtime
• Good for a little mutability (e.g. logging)

RefCell

use std::cell::RefCell;

fn main() {
    let x = RefCell::new(5);
    {
        let mut y = x.borrow_mut();
        *y += 1;
    }
    println!("{x:?}");
}

• Can change contents of x even though x is immutable
• This is interior mutability
• OK, but violating borrow rules causes runtime panic

Exercise

use std::cell::RefCell;

fn main() {
    let x = RefCell::new(String::from("cat"));
    let mut y = x.borrow_mut();
}

What might memory look like for representing this?

Borrowing and Lifetimes

Borrow revisited

Basic rules

• Each entity has one owner
• One mutable ref or many immutable refs (not both)
• References must always be valid

… with some addenda for smart pointers

Passing and returning

/// Get anagram class by index
pub fn get_class(&self, id: usize) -> &[usize] {
    let lo = self.class_offsets[id];
    let hi = self.class_offsets[id + 1];
    &self.word_ids[lo..hi]
}

• So far, we mostly passed references into functions
• Can’t drop reference passed from caller while in callee!
• What about references passed out of functions?

What could go wrong?

Passing out references seems dangerous!

• Could I violate reference exclusivity rules?
• What about reference validity rules?

Want to relate input and output references somehow.

Why does this work?

/// Get anagram class by index
pub fn get_class(&self, id: usize) -> &[usize] {
    let lo = self.class_offsets[id];
    let hi = self.class_offsets[id + 1];
    &self.word_ids[lo..hi]
}

• One reference goes in (&self)
• One slice comes out
• Assume you borrowed out of &self
  • So &self (and return value) remain borrowed on return
  • This is a lifetime constraint
• If assumption is wrong, you need to annotate

Implicit lifetime tracking

#[derive(Debug)]
struct Point2d(f64, f64);

fn borrow1(p: &Point2d) -> &f64 { return &p.0; }
fn get1(p: &Point2d) -> f64     { return p.0;  }
fn change2(p: &mut Point2d)     { p.1 = 1.0;   }

fn main() {
    let mut p = Point2d(0.0, 0.0);
    //let x = borrow1(&p); // Compile error
    let x = get1(&p);      // This is OK
    change2(&mut p);
    println!("{x} {p:?}");
}

Explicit lifetimes

fn ifelse<'a>(cond: bool, x: &'a str, y: &'a str) -> &'a str {
    if cond { x } else { y }
}

fn main() {
    let x = "vanilla";
    let y = "chocolate";
    println!("{}", ifelse(false, x, y));
}

• 'a is a lifetime parameter
• Name doesn’t really matter; where it appears does
• Here, compiler assumes both inputs borrowed on output

Lifetimes in compound types

struct Highlight<'doc>(&'doc str);

fn main() {
    let doc = String::from("I like rabbits!");
    let rabbit = Highlight(&doc[7..13]);
    println!("{}", rabbit.0);
}

Lifetime elision rules

When returning a reference, can skip lifetime parameter if

• Only one reference is passed into a function
• Lifetime of return is the same as &self in a method

Otherwise, need an explicit parameter.

Wrap-up

Upcoming

• Project 1 is launched!
  • Check-in is Tuesday, Sep 8, 11:59 PM
  • Project is due Sep 14 at 11:59 PM
• Partner finding social today, Gates 114, 4:30-5:30
• Consulting hours and Ed Discussions are open!
• Lab 2 is tomorrow

Outro

What would you like to hear about from me
(course material or otherwise)?

• Write your name and netid on class sheets
  • Print netid, ideally in pen!
• To simplify the end of class
  • Pass class sheets right and up
  • Pass blank sheets left and up