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      1 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
      2 // Use of this source code is governed by a BSD-style license that can be
      3 // found in the LICENSE file.
      4 
      5 // WARNING: Thread local storage is a bit tricky to get right.  Please make
      6 // sure that this is really the proper solution for what you're trying to
      7 // achieve.  Don't prematurely optimize, most likely you can just use a Lock.
      8 //
      9 // These classes implement a wrapper around the platform's TLS storage
     10 // mechanism.  On construction, they will allocate a TLS slot, and free the
     11 // TLS slot on destruction.  No memory management (creation or destruction) is
     12 // handled.  This means for uses of ThreadLocalPointer, you must correctly
     13 // manage the memory yourself, these classes will not destroy the pointer for
     14 // you.  There are no at-thread-exit actions taken by these classes.
     15 //
     16 // ThreadLocalPointer<Type> wraps a Type*.  It performs no creation or
     17 // destruction, so memory management must be handled elsewhere.  The first call
     18 // to Get() on a thread will return NULL.  You can update the pointer with a
     19 // call to Set().
     20 //
     21 // ThreadLocalBoolean wraps a bool.  It will default to false if it has never
     22 // been set otherwise with Set().
     23 //
     24 // Thread Safety:  An instance of ThreadLocalStorage is completely thread safe
     25 // once it has been created.  If you want to dynamically create an instance,
     26 // you must of course properly deal with safety and race conditions.  This
     27 // means a function-level static initializer is generally inappropiate.
     28 //
     29 // In Android, the system TLS is limited, the implementation is backed with
     30 // ThreadLocalStorage.
     31 //
     32 // Example usage:
     33 //   // My class is logically attached to a single thread.  We cache a pointer
     34 //   // on the thread it was created on, so we can implement current().
     35 //   MyClass::MyClass() {
     36 //     DCHECK(Singleton<ThreadLocalPointer<MyClass> >::get()->Get() == NULL);
     37 //     Singleton<ThreadLocalPointer<MyClass> >::get()->Set(this);
     38 //   }
     39 //
     40 //   MyClass::~MyClass() {
     41 //     DCHECK(Singleton<ThreadLocalPointer<MyClass> >::get()->Get() != NULL);
     42 //     Singleton<ThreadLocalPointer<MyClass> >::get()->Set(NULL);
     43 //   }
     44 //
     45 //   // Return the current MyClass associated with the calling thread, can be
     46 //   // NULL if there isn't a MyClass associated.
     47 //   MyClass* MyClass::current() {
     48 //     return Singleton<ThreadLocalPointer<MyClass> >::get()->Get();
     49 //   }
     50 
     51 #ifndef BASE_THREADING_THREAD_LOCAL_H_
     52 #define BASE_THREADING_THREAD_LOCAL_H_
     53 
     54 #include "base/base_export.h"
     55 #include "base/basictypes.h"
     56 #include "base/threading/thread_local_storage.h"
     57 
     58 #if defined(OS_POSIX)
     59 #include <pthread.h>
     60 #endif
     61 
     62 namespace base {
     63 namespace internal {
     64 
     65 // Helper functions that abstract the cross-platform APIs.  Do not use directly.
     66 struct BASE_EXPORT ThreadLocalPlatform {
     67 #if defined(OS_WIN)
     68   typedef unsigned long SlotType;
     69 #elif defined(OS_ANDROID)
     70   typedef ThreadLocalStorage::StaticSlot SlotType;
     71 #elif defined(OS_POSIX)
     72   typedef pthread_key_t SlotType;
     73 #endif
     74 
     75   static void AllocateSlot(SlotType* slot);
     76   static void FreeSlot(SlotType slot);
     77   static void* GetValueFromSlot(SlotType slot);
     78   static void SetValueInSlot(SlotType slot, void* value);
     79 };
     80 
     81 }  // namespace internal
     82 
     83 template <typename Type>
     84 class ThreadLocalPointer {
     85  public:
     86   ThreadLocalPointer() : slot_() {
     87     internal::ThreadLocalPlatform::AllocateSlot(&slot_);
     88   }
     89 
     90   ~ThreadLocalPointer() {
     91     internal::ThreadLocalPlatform::FreeSlot(slot_);
     92   }
     93 
     94   Type* Get() {
     95     return static_cast<Type*>(
     96         internal::ThreadLocalPlatform::GetValueFromSlot(slot_));
     97   }
     98 
     99   void Set(Type* ptr) {
    100     internal::ThreadLocalPlatform::SetValueInSlot(
    101         slot_, const_cast<void*>(static_cast<const void*>(ptr)));
    102   }
    103 
    104  private:
    105   typedef internal::ThreadLocalPlatform::SlotType SlotType;
    106 
    107   SlotType slot_;
    108 
    109   DISALLOW_COPY_AND_ASSIGN(ThreadLocalPointer<Type>);
    110 };
    111 
    112 class ThreadLocalBoolean {
    113  public:
    114   ThreadLocalBoolean() {}
    115   ~ThreadLocalBoolean() {}
    116 
    117   bool Get() {
    118     return tlp_.Get() != NULL;
    119   }
    120 
    121   void Set(bool val) {
    122     tlp_.Set(val ? this : NULL);
    123   }
    124 
    125  private:
    126   ThreadLocalPointer<void> tlp_;
    127 
    128   DISALLOW_COPY_AND_ASSIGN(ThreadLocalBoolean);
    129 };
    130 
    131 }  // namespace base
    132 
    133 #endif  // BASE_THREADING_THREAD_LOCAL_H_
    134