In multi-threaded computer programming, a function is thread-safe when it can be invoked or accessed concurrently by multiple threads without causing unexpected behavior, race conditions, or data corruption. As in the multi-threaded context where a program executes several threads simultaneously in a shared address space and each of those threads has access to every other thread's memory, thread-safe functions need to ensure that all those threads behave properly and fulfill their design specifications without unintended interaction. There are various strategies for making thread-safe data structures.

Levels of thread safety

Different vendors use slightly different terminology for thread-safety, but the most commonly use thread-safety terminology are: Thread safety guarantees usually also include design steps to prevent or limit the risk of different forms of deadlocks, as well as optimizations to maximize concurrent performance. However, deadlock-free guarantees cannot always be given, since deadlocks can be caused by callbacks and violation of architectural layering independent of the library itself. Software libraries can provide certain thread-safety guarantees. For example, concurrent reads might be guaranteed to be thread-safe, but concurrent writes might not be. Whether a program using such a library is thread-safe depends on whether it uses the library in a manner consistent with those guarantees.

Implementation approaches

Below we discuss two classes of approaches for avoiding race conditions to achieve thread-safety. The first class of approaches focuses on avoiding shared state and includes: The second class of approaches are synchronization-related, and are used in situations where shared state cannot be avoided:

Examples

In the following piece of Java code, the Java keyword synchronized makes the method thread-safe: 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 "mut****ex" (from mutual exclusion). In the following piece of C code, the function is thread-safe, but not reentrant: In the above, can be called by different threads without any problem since a mutex is used to synchronize all access to the shared variable. But if the function is used in a reentrant interrupt handler and a second interrupt arises while the mutex is locked, the second routine will hang forever. As interrupt servicing can disable other interrupts, the whole system could suffer. The same function can be implemented to be both thread-safe and reentrant using the lock-free atomics in C++11: