Thread-local storage

Thread-local storage (TLS) is a computer programming method that uses static or global memory local to a thread.

TLS is used in some places where ordinary, single-threaded programs would use global variables but where this would be inappropriate in multithreaded cases. An example of such situations is where functions use a global variable to set an error condition (for example the global variable errno used by many functions of the C library). If errno were a global variable, a call of a system function on one thread may overwrite the value previously set by a call of a system function on a different thread, possibly before following code on that different thread could check for the error condition. The solution is to have errno be a variable that looks like it is global, but in fact exists once per thread -- i.e., it lives in thread-local storage. A second use case would be multiple threads accumulating information into a global variable. To avoid a race condition, every access to this global variable would have to be protected by a mutex. Alternatively, each thread might accumulate into a thread-local variable (that, by definition, can not be read from or written to from other threads, implying that there can be no race conditions). Threads then only have to synchronise a final accumulation from their own thread-local variable into a single, truly global variable.