---
title: Data races
framework: xcode
role: article
role_heading: Article
path: xcode/data-races
---

# Data races

Detects unsynchronized access to mutable state across multiple threads.

## Overview

Overview Use this check to detect when multiple threads access the same memory without synchronization, and at least one access is a write. Available in Xcode 8 and later. Data race with producer and consumer functions In the following example, the producer() function sets the global variable message, and the consumer() function waits for a flag to set before printing the message. Because producer() executes on one thread and consumer() executes on another thread, their execution can be concurrent, creating a data race. var message: String? = nil var messageIsAvailable: Bool = false // Executed on Thread #1 func producer() {     message = "hello!"     messageIsAvailable = true } // Executed on Thread #2 func consumer() {     repeat {         usleep(1000)     } while !messageIsAvailable     print(message) } Solution Use Dispatch APIs to coordinate access to message across multiple threads.

## See Also

### Thread Sanitizer

- [Swift access races](xcode/swift-access-races.md)
- [Races on collections and other APIs](xcode/races-on-collections-and-other-apis.md)
- [Uninitialized mutexes](xcode/uninitialized-mutexes.md)
- [Thread leaks](xcode/thread-leaks.md)