---
title: Races on collections and other APIs
framework: xcode
role: article
role_heading: Article
path: xcode/races-on-collections-and-other-apis
---

# Races on collections and other APIs

Detects when one thread accesses a mutable object while another thread is writing to it.

## Overview

Overview In Xcode 9 and later, the Thread Sanitizer detects unsafe thread accesses of Foundation and Core Foundation framework APIs. This feature applies to the following collection types: NSMutableArray NSMutableDictionary CFMutableArray CFMutableDictionary Collection race with a mutable array In the following example, the code enumerates a mutable array in one thread while writing to the array from another without synchronizing access: Swift let array: NSMutableArray = [] var sum: Int = 0 // Executed on Thread #1 for value in array {     sum += value as! Int } // Executed on Thread #2 array.add(42) Objective-C NSMutableArray *array = [NSMutableArray new]; NSInteger sum = 0; // Executed on Thread #1 for (id value in array) {       sum += [value integerValue]; }  // Executed on Thread #2 [array addObject:@42]; Solution Use Dispatch APIs to coordinate access to array across multiple threads. Collection race with a mutable dictionary In the following example, the code enumerates a mutable dictionary in one thread while writing to the dictionary from another without synchronizing access: let dictionary: NSMutableDictionary = [:] var sum: Int = 0 // Executed on Thread #1 for key in dictionary.keyEnumerator() {     sum += dictionary[key] as! Int } // Executed on Thread #2 dictionary["forty-two"] = 42 Objective-C NSMutableDictionary *dictionary = [NSMutableDictionary new]; NSInteger sum = 0; // Executed on Thread #1 for (id key in dictionary) {     sum += [dictionary[key] integerValue]; } // Executed on Thread #2 dictionary[@"forty-two"] = @42; Solution Use Dispatch APIs to coordinate access to dictionary across multiple threads.

## See Also

### Thread Sanitizer

- [Data races](xcode/data-races.md)
- [Swift access races](xcode/swift-access-races.md)
- [Uninitialized mutexes](xcode/uninitialized-mutexes.md)
- [Thread leaks](xcode/thread-leaks.md)