c++linuxmultithreadingnew-operatorthread-local-storage

Reputation: 43079

How to allocate thread local storage?

I have a variable in my function that is static, but I would like it to be static on a per thread basis.

How can I allocate the memory for my C++ class such that each thread has its own copy of the class instance?

AnotherClass::threadSpecificAction()
{
  // How to allocate this with thread local storage?
  static MyClass *instance = new MyClass();

  instance->doSomething();
}

This is on Linux. I'm not using C++0x and this is gcc v3.4.6.

Upvotes: 58

Answers (9)

Arne

Reputation: 8481

C++11 specifies a thread_local storage type, just use it.

AnotherClass::threadSpecificAction()
{
  thread_local MyClass *instance = new MyClass();
  instance->doSomething();
}

One optional optimization is to also allocate on thread local storage.

Upvotes: 5

chema989

Reputation: 4172

Folly (Facebook Open-source Library) has a portable implementation of Thread Local Storage.

According its authors:

Improved thread local storage for non-trivial types (similar speed as pthread_getspecific but only consumes a single pthread_key_t, and 4x faster than boost::thread_specific_ptr).

If your looking for a portable implementation of Local Storage Thread, this library is a good option.

Upvotes: 2

Deqing

Reputation: 14632

It is worth noting that C++11 introduces the thread_local keyword.

Here is an example from Storage duration specifiers:

#include <iostream>
#include <string>
#include <thread>
#include <mutex>

thread_local unsigned int rage = 1; 
std::mutex cout_mutex;

void increase_rage(const std::string& thread_name)
{
    ++rage;
    std::lock_guard<std::mutex> lock(cout_mutex);
    std::cout << "Rage counter for " << thread_name << ": " << rage << '\n';
}

int main()
{
    std::thread a(increase_rage, "a"), b(increase_rage, "b");
    increase_rage("main");

    a.join();
    b.join();

    return 0;
}

Possible output:

Rage counter for a: 2
Rage counter for main: 2
Rage counter for b: 2

Upvotes: 70

Arek Bal

Reputation: 733

Just a side note... MSVC++ supports declspec(thread) from VSC++2005

#if (_MSC_VER >= 1400)
  #ifndef thread_local     
    #define thread_local __declspec(thread)
  #endif
#endif

Main problem is(which is solved in boost::thread_specific_ptr) variables marked with it can't contain ctor or dtor.

Upvotes: 3

tvn

Reputation: 634

boost::thread_specific_ptr is the best way as it portable solution.

On Linux & GCC you may use __thread modifier.

So your instance variable will look like:

static __thread MyClass *instance = new MyClass();

Upvotes: 15

ravenspoint

Reputation: 20457

#include <boost/thread/tss.hpp>
static boost::thread_specific_ptr< MyClass> instance;
if( ! instance.get() ) {
    // first time called by this thread
    // construct test element to be used in all subsequent calls from this thread
    instance.reset( new MyClass);
}
    instance->doSomething();

Upvotes: 71

Jason

Reputation: 32490

If you're using Pthreads you can do the following:

//declare static data members
pthread_key_t AnotherClass::key_value;
pthread_once_t AnotherClass::key_init_once = PTHREAD_ONCE_INIT;

//declare static function
void AnotherClass::init_key()
{
    //while you can pass a NULL as the second argument, you 
    //should pass some valid destrutor function that can properly
    //delete a pointer for your MyClass
    pthread_key_create(&key_value, NULL);
}

void AnotherClass::threadSpecificAction()
{
  //Initialize the key value
  pthread_once(&key_init_once, init_key);

  //this is where the thread-specific pointer is obtained
  //if storage has already been allocated, it won't return NULL

  MyClass *instance = NULL;
  if ((instance = (MyClass*)pthread_getspecific(key_value)) == NULL)
  {
    instance = new MyClass;
    pthread_setspecific(key_value, (void*)instance);
  }

  instance->doSomething();
}

Upvotes: 12