What difficulties are encountered when accessing a variable from multiple threads of execution?

Problems arising from accessing the same variable from multiple threads are related to race conditions. Main difficulties:

• Unpredictable behavior: if multiple threads read and write to the variable simultaneously, the final value can be unpredictable.

• Lack of atomicity: read and write operations can be interrupted, leading to partially updated data.

• Visibility issues: changes made in one thread may not be immediately visible to others due to caching and CPU optimizations.

To avoid these problems, synchronization mechanisms are used:

• Mutexes (locks): block access to the variable, allowing only one thread to work with it at a time.

• Volatile variables: guarantee visibility of changes between threads (in Java/Android).

• Atomic operations: special classes and methods that ensure atomic updates without locks.

Example in Kotlin for Android using @Volatile and synchronized:

@Volatile
var sharedVar = 0

fun increment() {
    synchronized(this) {
        sharedVar++
    }
}

Thus, proper synchronization is necessary for correct operation with shared variables in a multithreaded environment.