Race Conditions

A race condition occurs when the program's behavior depends on the relative timing of events, such as the order in which threads execute.

The Problem

Thread A:           Thread B:
read x (= 0)        read x (= 0)
x = x + 1           x = x + 1
write x (= 1)       write x (= 1)

Expected: x = 2
Actual:   x = 1  ← Lost update!

Both threads read the same initial value, compute independently, and overwrite each other.

Types of Race Conditions

Type	Description
Read-Modify-Write	Check-then-act on shared state
Check-Then-Act	Condition changes between check and action
Compound Operations	Multiple operations that should be atomic

Detection

Go: go run -race or go test -race
C/C++: ThreadSanitizer (TSan)
Rust: Compile-time prevention via ownership system

Prevention Strategies

Strategy	How
Mutex/Lock	Ensure exclusive access
Atomic operations	Hardware-level indivisibility
Immutability	No shared mutable state
Message passing	Actors/channels — no shared memory
Thread-local storage	Each thread owns its data

In Practice

Bad:

var counter int

func increment() {
    counter++  // Race condition!
}