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Thread-local storage

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

This is sometimes needed because all threads in a process share the same address space. In other words, data in a static or global variable is normally always located at the same memory location, when referred to by threads from the same process. Variables on the stack however are local to threads, because each thread has its own stack, residing in a different memory location.

Sometimes it is desirable that two threads referring to the same static or global variable are actually referring to different memory locations, thereby making the variable thread local, a canonical example being the C error code variable errno.

If it is possible to make at least a memory address sized variable thread local, it is in principle possible to make arbitrarily sized memory blocks thread local, by allocating such a memory block and storing the memory address of that block in a thread local variable.

Windows implementation

The API function TlsAlloc can be used to obtain an unused TLS slot index; the TLS slot index will then be considered ‘used’.

The TlsGetValue and TlsSetValue functions can then be used to read and write a memory address to a thread local variable identified by the TLS slot index. TlsSetValue can only affect the variable for the current thread.

The TlsFree function can be called to release the TLS slot index; the index will then be considered ‘unused’ and a new call to TlsAlloc can return it again.

Pthreads implementation

TLS with Pthreads (Thread-Specific Data in Pthreads nomenclature) is similar to TlsAlloc and related functionality for Windows. pthread_key_create creates a key, with an optional destructor, that can later be associated with thread specific data via pthread_setspecific. The data can be retrieved using pthread_getspecific. If the thread specific value is not NULL, the destructor will be called when the thread exits. Additionally, key must be destroyed with pthread_key_delete.

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 TLS.

Object Pascal

In Delphi or Free Pascal you can use the 'threadvar' reserved keyword instead of 'var' to declare variables using the thread-local storage. 

var
   mydata_process: integer;
threadvar
   mydata_threadlocal: integer;

Java

In Java thread local variables are implemented by the ThreadLocal class. A ThreadLocal object maintains a separate instance of the variable for each thread that calls the object's get or set method. The following example (for J2SE 5.0 or later version of Java) illustrates using a ThreadLocal that holds an Integer object: 

ThreadLocal<Integer> local = new ThreadLocal<Integer>(){
    @Override protected Integer initialValue(){
        return 1;
    }
};

the preceding code declares and instantiates a ThreadLocal object local which returns an initial value of 1 if no other value has been stored for the calling thread. The following code increments the value for the currently executing thread. 

local.set( local.get()+1 );

local.get() returns the current Integer object associated with the current thread, or 1 if no object has yet been associated with the thread. The code calls local.set() to set a new value associated with the thread. (Note that the above example uses both generics and autoboxing—features added to Java in J2SE 5.0.)

Sun Studio C/C++, IBM XL C/C++, GNU C and Intel C/C++ (Linux systems)

The keyword __thread is used like this: 

__thread int number;
  • __thread defines number to be a thread local variable.

Visual C++, Intel C/C++ (Windows systems)

In Visual C++ the keywords declspec(thread) are used like this: 

__declspec(thread) int number;
  • __declspec(thread) defines number to be a thread local variable.
  • On operating systems prior to Vista and Server 2008 __declspec(thread) 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).
  • There are additional rules: "Rules and Limitations for TLS" in MSDN.

Digital Mars C++

In Digital Mars C++ the keywords declspec(thread) are used like this: 

__declspec(thread) int number;
  • __declspec(thread) defines number to be a thread local variable.

D

In the D programming language 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. Regular global variables must be explicitly requested using the __gshared keyword: 

int threadLocal;  // This is a thread local variable.
__gshared int global;  // This is a plain old global variable.

Borland C++ Builder

In Borland C++ Builder the keywords __declspec(thread) are used like this: 

__declspec(thread) int number;

the same in a more elegant way: 

int __thread number;
  • __declspec(thread) defines number to be a thread local variable. __thread is a synonym for __declspec(thread).

GCC

GCC C/C++ implements __thread as above.

The initialiser must be a compile-time constant, even in C++. E.g. 

__thread int number = 1;

but not 

void f(int number)
{
 static __thread int number_copy = number;

or (C++) 

__thread int number = calculate_number();

C# and other .NET languages

Static fields can be marked with the ThreadStatic attribute: 

class FooBar
{
  [ThreadStatic] static int foo;
}

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

Python

In Python version 2.4 or later local class in threading module can be used to create thread-local storage. 

import threading
mydata = threading.local()
mydata.x = 1

Ruby

In Ruby thread local variables can be created/accessed using []=/[] methods. 

Thread.current[:user_id] = 1

Perl

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

use threads;
use threads::shared;

my $localvar;
my $sharedvar :shared;

External links

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