base/threading/thread_local.h

// Copyright (c) 2011 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// WARNING: Thread local storage is a bit tricky to get right.  Please make
// sure that this is really the proper solution for what you're trying to
// achieve.  Don't prematurely optimize, most likely you can just use a Lock.
//
// These classes implement a wrapper around the platform's TLS storage
// mechanism.  On construction, they will allocate a TLS slot, and free the
// TLS slot on destruction.  No memory management (creation or destruction) is
// handled.  This means for uses of ThreadLocalPointer, you must correctly
// manage the memory yourself, these classes will not destroy the pointer for
// you.  There are no at-thread-exit actions taken by these classes.
//
// ThreadLocalPointer<Type> wraps a Type*.  It performs no creation or
// destruction, so memory management must be handled elsewhere.  The first call
// to Get() on a thread will return NULL.  You can update the pointer with a
// call to Set().
//
// ThreadLocalBoolean wraps a bool.  It will default to false if it has never
// been set otherwise with Set().
//
// Thread Safety:  An instance of ThreadLocalStorage is completely thread safe
// once it has been created.  If you want to dynamically create an instance,
// you must of course properly deal with safety and race conditions.  This
// means a function-level static initializer is generally inappropiate.
//
// In Android, the system TLS is limited, the implementation is backed with
// ThreadLocalStorage.
//
// Example usage:
//   // My class is logically attached to a single thread.  We cache a pointer
//   // on the thread it was created on, so we can implement current().
//   MyClass::MyClass() {
//     DCHECK(Singleton<ThreadLocalPointer<MyClass> >::get()->Get() == NULL);
//     Singleton<ThreadLocalPointer<MyClass> >::get()->Set(this);
//   }
//
//   MyClass::~MyClass() {
//     DCHECK(Singleton<ThreadLocalPointer<MyClass> >::get()->Get() != NULL);
//     Singleton<ThreadLocalPointer<MyClass> >::get()->Set(NULL);
//   }
//
//   // Return the current MyClass associated with the calling thread, can be
//   // NULL if there isn't a MyClass associated.
//   MyClass* MyClass::current() {
//     return Singleton<ThreadLocalPointer<MyClass> >::get()->Get();
//   }

#ifndef BASE_THREADING_THREAD_LOCAL_H_
#define BASE_THREADING_THREAD_LOCAL_H_

#include "base/base_export.h"
#include "base/basictypes.h"
#include "base/threading/thread_local_storage.h"

#if defined(OS_POSIX)
#include <pthread.h>
#endif

namespace base {
namespace internal {

// Helper functions that abstract the cross-platform APIs.  Do not use directly.
struct BASE_EXPORT ThreadLocalPlatform {
#if defined(OS_WIN)
  typedef unsigned long SlotType;
#elif defined(OS_ANDROID)
  typedef ThreadLocalStorage::StaticSlot SlotType;
#elif defined(OS_POSIX)
  typedef pthread_key_t SlotType;
#endif

  static void AllocateSlot(SlotType* slot);
  static void FreeSlot(SlotType slot);
  static void* GetValueFromSlot(SlotType slot);
  static void SetValueInSlot(SlotType slot, void* value);
};

}  // namespace internal

template <typename Type>
class ThreadLocalPointer {
 public:
  ThreadLocalPointer() : slot_() {
    internal::ThreadLocalPlatform::AllocateSlot(&slot_);
  }

  ~ThreadLocalPointer() {
    internal::ThreadLocalPlatform::FreeSlot(slot_);
  }

  Type* Get() {
    return static_cast<Type*>(
        internal::ThreadLocalPlatform::GetValueFromSlot(slot_));
  }

  void Set(Type* ptr) {
    internal::ThreadLocalPlatform::SetValueInSlot(
        slot_, const_cast<void*>(static_cast<const void*>(ptr)));
  }

 private:
  typedef internal::ThreadLocalPlatform::SlotType SlotType;

  SlotType slot_;

  DISALLOW_COPY_AND_ASSIGN(ThreadLocalPointer<Type>);
};

class ThreadLocalBoolean {
 public:
  ThreadLocalBoolean() {}
  ~ThreadLocalBoolean() {}

  bool Get() {
    return tlp_.Get() != NULL;
  }

  void Set(bool val) {
    tlp_.Set(val ? this : NULL);
  }

 private:
  ThreadLocalPointer<void> tlp_;

  DISALLOW_COPY_AND_ASSIGN(ThreadLocalBoolean);
};

}  // namespace base

#endif  // BASE_THREADING_THREAD_LOCAL_H_