Deadlocks in C vs Rust: What Does Rust Really Prevent?

Deadlocks are runtime concurrency bugs, not compile-time errors. So how can Rust claim safer multithreading? Here's a breakdown of what Rust prevents—and what it doesn't.

What is a Deadlock?

A deadlock occurs when threads hold resources and wait on each other in a cycle. All 4 Coffman conditions must hold:

1. Mutual exclusion — at least one resource is non-shareable
2. Hold and wait — threads hold one resource and wait for others
3. No preemption — resources can't be forcibly taken
4. Circular wait — a cycle of threads each waiting for the next

Rust does not eliminate deadlocks, but gives you tools that make many of them easier to avoid.

Runtime Deadlock in C vs Rust

In C (Pthreads):

pthread_mutex_lock(&a);
// work
pthread_mutex_lock(&b);  // may deadlock if other thread locked `b` then `a`

In Rust:

let a = Arc::new(Mutex::new(()));
let b = Arc::new(Mutex::new(()));

let t1 = {
    let a = Arc::clone(&a);
    let b = Arc::clone(&b);
    std::thread::spawn(move || {
        let _a = a.lock().unwrap();
        let _b = b.lock().unwrap();  // same problem if lock order differs
    })
};

💥 Both can deadlock if threads acquire locks in different orders.

Rust's Stronger Guarantees

Feature	C (Pthreads)	Rust	Why It Matters
Ownership tracking	❌	✅ (Compiler enforced)	Prevents aliasing lock misuse
Automatic unlocks	❌	✅ (Drop via RAII)	Avoids forgetting to release locks
Safe sharing of locks	❌	✅ (Arc<Mutex<T>>)	Clear thread-safe semantics
Data race prevention	❌	✅ (No races in safe code)	Prevents many deadlock scenarios
Deadlock prevention	❌	❌	Still requires logic from the dev

Lock Lifecycle in Rust

Rust ensures that:

• Locks are released when their guard goes out of scope
• You can't access a mutex without locking it first
• Captured references follow borrowing rules

But: Rust cannot reason about lock acquisition order. If thread A locks a then b, and thread B locks b then a, you can still deadlock.

Compile-Time vs Runtime Safety

Issue	Detected in C?	Detected in Rust?	Compile-Time Safe?
Data races	❌	✅	✅
Use-after-free	❌	✅	✅
Dangling pointers	❌	✅	✅
Circular locking patterns	❌	❌	❌
Deadlocks	❌	❌	❌

Dynamic Tools for Deadlock Detection

Rust doesn't check for lock order at compile time, but you can use tools like:

• loom – test all interleavings of concurrent code
• deadlock – detect runtime deadlocks in debug mode
• Static analyzers (WIP in ecosystem)

Takeaways

✅ Rust gives memory and thread safety, and ownership helps avoid accidental misuse
❌ Deadlocks are still possible — Rust doesn’t enforce lock order
🚀 Write predictable locking code and test interleavings using tools like loom

Try This: What happens if two threads try to lock() Mutex<T>s in different orders?
Answer: If the acquisition order cycles, your program may hang due to a deadlock. Rust won't stop you—but it makes everything else much safer.