Handling Deadlocks with DispatchSemaphore and Counters in Swift

Deepak Carpenter
Published in
3 min readJun 1, 2023

Concurrency is a powerful tool for improving the performance of Swift applications, but it can introduce challenges such as deadlocks. Deadlocks occur when multiple threads are blocked indefinitely, waiting for each other to release resources. In this blog post, we will explore the concept of deadlocks, and how to prevent them using DispatchSemaphore and counters in Swift.

Understanding Deadlocks: A deadlock occurs when two or more threads are stuck in a circular wait for resources, resulting in a program freeze. Deadlocks can arise due to improper synchronization or resource management, where threads hold resources and wait for other resources to be released before proceeding. This can lead to a situation where all threads are blocked, unable to make progress.

Using DispatchSemaphore and Counters: DispatchSemaphore, a synchronization primitive in Swift's Grand Central Dispatch (GCD), can help prevent deadlocks by regulating resource access. By combining DispatchSemaphore with counters, we can control the number of threads allowed to access a resource at a given time, ensuring thread safety and preventing deadlocks.

Example: Resource Access Control with Semaphore and Counter

import Dispatch

let semaphore = DispatchSemaphore(value: 3)
var resourceCounter = 0

func accessResource() {
// Increment the resource counter
resourceCounter += 1
// Perform resource-related operations
// ...
// Decrement the resource counter
resourceCounter -= 1

// Spawn concurrent tasks that access the resource
for _ in 1...10 {
DispatchQueue.global().async {

In this example, we create a DispatchSemaphore with an initial value of 3, indicating that three threads can access the resource concurrently. The accessResource() function uses wait() to acquire the semaphore, ensuring that the resource counter is incremented only when a thread can access the resource. After performing resource-related operations, the resource counter is decremented, and the semaphore is signaled using signal() to allow other threads to access the resource.

By employing the semaphore and counter mechanism, we control the number of threads accessing the resource simultaneously, preventing deadlocks and ensuring thread safety.

Final Words:
Deadlocks can significantly impact the stability and performance of concurrent Swift applications. By understanding the nature of deadlocks and using synchronization mechanisms like DispatchSemaphore and counters, we can effectively prevent deadlocks and promote thread safety.

By employing DispatchSemaphore, we can regulate access to shared resources, allowing a controlled number of threads to access them concurrently. Combining it with counters ensures that resources are acquired and released correctly, preventing circular wait situations and potential deadlocks.

When implementing concurrent logic, it is crucial to analyze your code and identify potential areas where deadlocks may occur. By using DispatchSemaphore and counters, you can effectively manage resource access and mitigate the risk of deadlocks, enabling your Swift applications to run smoothly and efficiently.

Remember to carefully consider the concurrency requirements of your application and utilize the appropriate synchronization techniques to ensure thread safety and prevent deadlocks.

Happy Coding!