Is it possible to modify the value of a variable from multiple threads or queues simultaneously?

It is not allowed to change the values of the same variable simultaneously from multiple threads or queues without synchronization, as this leads to race conditions and unpredictable program behavior.

To safely access variables from different threads in iOS, the following are usually used:

• Synchronization via Dispatch Queues: for example, using a serial queue to perform all operations on a variable.
• Locks: NSLock, pthread_mutex, and other locking mechanisms.
• Atomic operations: for example, using OSAtomic functions or std::atomic in C++.

Example using a serial queue:

let serialQueue = DispatchQueue(label: "com.example.serialQueue")
var counter = 0

serialQueue.async {
    counter += 1
}

serialQueue.async {
    counter += 1
}

Thus, operations on the variable will be executed sequentially, preventing simultaneous modification.