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Data races

Detects unsynchronized access to mutable state across multiple threads.

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