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Thread-local storage

Method for computer memory management

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Many systems impose restrictions on the size of the thread-local memory block, in fact often rather tight limits. On the other hand, if a system can provide at least a memory address (pointer) sized variable thread-local, then this allows the use of arbitrarily sized memory blocks in a thread-local manner, by allocating such a memory block dynamically and storing the memory address of that block in the thread-local variable. On RISC machines, the calling convention often reserves a thread pointer register for this use.

Usage

While the use of global variables is generally discouraged in modern programming, some older operating systems such as UNIX were originally designed for uniprocessor hardware and often use global variables to store important values. An example is the errno used by many functions of the C library. On a modern machine, where multiple threads may be modifying the errno 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 as if it is global, but is physically stored in a per-thread memory pool, the 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. Instead, each thread might accumulate into a thread-local variable, thereby eliminating any possibility of a race condition, thereby removing the need for locking. The threads then only have to synchronise a final accumulation from their own thread-local variable into a single global variable.

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