Thread safety

Thread safety is a computer programming concept applicable to multi-threaded code. Thread-safe code only manipulates shared data structures in a manner that guarantees safe execution by multiple threads. There are various strategies for making thread-safe data structures.

A program may execute code in several threads simultaneously in a shared address space where each of those threads has access to virtually all of the memory of every other thread. Thread safety is a property that allows code to run in multithreaded environments by re-establishing some of the correspondences between the actual flow of control and the text of the program, by means of synchronization.

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.

Different vendors use slightly different terminology for thread-safety:

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 can not always be given, since deadlocks can be caused by callbacks and violation of architectural layering independent of the library itself.

Below we discuss two 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:

In the following piece of Java code, the function is thread-safe:

In the following piece of C code, the function is thread-safe, but not reentrant:

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