Thread-local storage

In computer programming, thread-local storage (TLS) is a memory management method that uses static or global memory local to a thread. The concept allows storage of data that appears to be global in a system with separate threads.

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  used by many functions of the C library. On a modern machine, where multiple threads may be modifying the  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  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.

Windows implementation
The application programming interface (API) function  can be used to obtain an unused TLS slot index; the TLS slot index will then be considered 'used'.

The  and   functions are then used to read and write a memory address to a thread-local variable identified by the TLS slot index. only affects the variable for the current thread. The  function can be called to release the TLS slot index.

There is a Win32 Thread Information Block for each thread. One of the entries in this block is the thread-local storage table for that thread. TlsAlloc returns an index to this table, unique per address space, for each call. Each thread has its own copy of the thread-local storage table. Hence, each thread can independently use TlsSetValue(index) and obtain the specified value via TlsGetValue(index), because these set and look up an entry in the thread's own table.

Apart from TlsXxx function family, Windows executables can define a section which is mapped to a different page for each thread of the executing process. Unlike TlsXxx values, these pages can contain arbitrary and valid addresses. These addresses, however, are different for each executing thread and therefore should not be passed to asynchronous functions (which may execute in a different thread) or otherwise passed to code which assume that a virtual address is unique within the whole process. TLS sections are managed using memory paging and its size is quantized to a page size (4kB on x86 machines). Such sections may only be defined inside a main executable of a program - DLLs should not contain such sections, because they are not correctly initialized when loading with LoadLibrary.

Pthreads implementation
In the Pthreads API, memory local to a thread is designated with the term Thread-specific data.

The functions  and   are used respectively to create and delete a key for thread-specific data. The type of the key is explicitly left opaque and is referred to as. This key can be seen by all threads. In each thread, the key can be associated with thread-specific data via. The data can later be retrieved using.

In addition  can optionally accept a destructor function that will automatically be called at thread exit, if the thread-specific data is not NULL. The destructor receives the value associated with the key as parameter so it can perform cleanup actions (close connections, free memory, etc.). Even when a destructor is specified, the program must still call  to free the thread-specific data at process level (the destructor only frees the data local to the thread).

Language-specific implementation
Apart from relying on programmers to call the appropriate API functions, it is also possible to extend the programming language to support thread local storage (TLS).

C and C++
In C11, the keyword  is used for defining thread-local variables. The header, if supported, defines   as a synonym for that keyword. Example usage:

In C11,  also defines a number of functions for retrieving, changing, and destructing a thread-local storage, using names starting with tss_. In C23,  itself becomes a keyword.

C++11 introduces the keyword which can be used in the following cases
 * Namespace level (global) variables
 * File static variables
 * Function static variables
 * Static member variables

Aside from that, various compiler implementations provide specific ways to declare thread-local variables:
 * Solaris Studio C/C++, IBM XL C/C++, GNU C, llvm-gcc, Clang, and Intel C++ Compiler (Linux systems) use the syntax:
 * Visual C++, Intel C/C++ (Windows systems), C++Builder, Clang, and Digital Mars C++ use the syntax:
 * C++Builder also supports the syntax:
 * C++Builder also supports the syntax:
 * C++Builder also supports the syntax:

On Windows versions before Vista and Server 2008,  works in DLLs only when those DLLs are bound to the executable, and will not work for those loaded with LoadLibrary (a protection fault or data corruption may occur).

Common Lisp and other dialects
Common Lisp provides a feature called dynamically scoped variables.

Dynamic variables have a binding which is private to the invocation of a function and all of the children called by that function.

This abstraction naturally maps to thread-specific storage, and Lisp implementations that provide threads do this. Common Lisp has numerous standard dynamic variables, and so threads cannot be sensibly added to an implementation of the language without these variables having thread-local semantics in dynamic binding.

For instance the standard variable  determines the default radix in which integers are printed. If this variable is overridden, then all enclosing code will print integers in an alternate radix:

If functions can execute concurrently on different threads, this binding has to be properly thread-local, otherwise each thread will fight over who controls a global printing radix.

D
In D version 2, all static and global variables are thread-local by default and are declared with syntax similar to "normal" global and static variables in other languages. Global variables must be explicitly requested using the shared keyword:

The shared keyword works both as the storage class, and as a type qualifier – shared variables are subject to some restrictions which statically enforce data integrity. To declare a "classic" global variable without these restrictions, the unsafe __gshared keyword must be used:

Java
In Java, thread-local variables are implemented by the class object. ThreadLocal holds variable of type T, which is accessible via get/set methods. For example, ThreadLocal variable holding Integer value looks like this:

At least for Oracle/OpenJDK, this does not use native thread-local storage in spite of OS threads being used for other aspects of Java threading. Instead, each Thread object stores a (non-thread-safe) map of ThreadLocal objects to their values (as opposed to each ThreadLocal having a map of Thread objects to values and incurring a performance overhead).

.NET languages: C# and others
In .NET Framework languages such as C#, static fields can be marked with the ThreadStatic attribute:

In .NET Framework 4.0 the System.Threading.ThreadLocal class is available for allocating and lazily loading thread-local variables.

Also an API is available for dynamically allocating thread-local variables.

Object Pascal
In Object Pascal (Delphi) or Free Pascal the threadvar reserved keyword can be used instead of 'var' to declare variables using the thread-local storage.

Objective-C
In Cocoa, GNUstep, and OpenStep, each  object has a thread-local dictionary that can be accessed through the thread's   method.

Perl
In Perl threads were added late in the evolution of the language, after a large body of extant code was already present on the Comprehensive Perl Archive Network (CPAN). Thus, threads in Perl by default take their own local storage for all variables, to minimise the impact of threads on extant non-thread-aware code. In Perl, a thread-shared variable can be created using an attribute:

PureBasic
In PureBasic thread variables are declared with the keyword Threaded.

Threaded Var

Python
In Python version 2.4 or later, local class in threading module can be used to create thread-local storage. Multiple instances of local class can be created to store different sets of variables. Thus, it is not a singleton.

Ruby
Ruby can create/access thread-local variables using /  methods:

Rust
Thread-local variables can be created in Rust using the thread_local! macro provided by the Rust standard library: