and are thread-safe since they are local to each thread, even though the code which accesses them might be executed simultaneously by another thread. Immutable objects The state of an object cannot be changed after construction. This implies both that only read-only data is shared and that inherent thread safety is attained. Mutable (non-const) operations can then be implemented in such a way that they create new objects instead of modifying existing ones. This approach is characteristic of functional programming and is also used by the string implementations in Java, C#, and Python. (See Immutable object.) The second class of approaches are synchronization-related, and are used in situations where shared state cannot be avoided: Mutual exclusion Access to shared data is serialized using mechanisms that ensure only one thread reads or writes to the shared data at any time. Incorporation of mutual exclusion needs to be well thought out, since improper usage can lead to side-effects like deadlocks, livelocks, and resource starvation. Atomic operations Shared data is accessed by using atomic operations which cannot be interrupted by other threads. This usually requires using special machine language instructions, which might be available in a runtime library. Since the operations are atomic, the shared data is always kept in a valid state, no matter how other threads access it. Atomic operations form the basis of many thread locking mechanisms, and are used to implement mutual exclusion primitives. Examples In the following piece of Java code, the Java keyword synchronized makes the method thread-safe: class Counter { private int i = 0; public synchronized void inc() { i++; } } In the C programming language, each thread has its own stack. However, a static variable is not kept on the stack; all threads share simultaneous access to it. If multiple threads overlap while running the same function, it is possible that a static variable might be changed by one thread while another is midway through checking it. This difficult-to-diagnose logic error, which may compile and run properly most of the time, is called a race condition. One common way to avoid this is to use another shared variable as a "lock" or "mutex" (from mutual exclusion). In the following piece of C code, the function is thread-safe, but not reentrant: # include <pthread.h> int increment_counter () { static int counter = 0; static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER; // only allow one thread to