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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/macros.h"
     56 #include "base/threading/thread_local_storage.h"
     57 #include "build/build_config.h"
     58 
     59 #if defined(OS_POSIX)
     60 #include <pthread.h>
     61 #endif
     62 
     63 namespace base {
     64 namespace internal {
     65 
     66 // Helper functions that abstract the cross-platform APIs.  Do not use directly.
     67 struct BASE_EXPORT ThreadLocalPlatform {
     68 #if defined(OS_WIN)
     69   typedef unsigned long SlotType;
     70 #elif defined(OS_ANDROID)
     71   typedef ThreadLocalStorage::StaticSlot SlotType;
     72 #elif defined(OS_POSIX)
     73   typedef pthread_key_t SlotType;
     74 #endif
     75 
     76   static void AllocateSlot(SlotType* slot);
     77   static void FreeSlot(SlotType slot);
     78   static void* GetValueFromSlot(SlotType slot);
     79   static void SetValueInSlot(SlotType slot, void* value);
     80 };
     81 
     82 }  // namespace internal
     83 
     84 template <typename Type>
     85 class ThreadLocalPointer {
     86  public:
     87   ThreadLocalPointer() : slot_() {
     88     internal::ThreadLocalPlatform::AllocateSlot(&slot_);
     89   }
     90 
     91   ~ThreadLocalPointer() {
     92     internal::ThreadLocalPlatform::FreeSlot(slot_);
     93   }
     94 
     95   Type* Get() {
     96     return static_cast<Type*>(
     97         internal::ThreadLocalPlatform::GetValueFromSlot(slot_));
     98   }
     99 
    100   void Set(Type* ptr) {
    101     internal::ThreadLocalPlatform::SetValueInSlot(
    102         slot_, const_cast<void*>(static_cast<const void*>(ptr)));
    103   }
    104 
    105  private:
    106   typedef internal::ThreadLocalPlatform::SlotType SlotType;
    107 
    108   SlotType slot_;
    109 
    110   DISALLOW_COPY_AND_ASSIGN(ThreadLocalPointer<Type>);
    111 };
    112 
    113 class ThreadLocalBoolean {
    114  public:
    115   ThreadLocalBoolean() {}
    116   ~ThreadLocalBoolean() {}
    117 
    118   bool Get() {
    119     return tlp_.Get() != NULL;
    120   }
    121 
    122   void Set(bool val) {
    123     tlp_.Set(val ? this : NULL);
    124   }
    125 
    126  private:
    127   ThreadLocalPointer<void> tlp_;
    128 
    129   DISALLOW_COPY_AND_ASSIGN(ThreadLocalBoolean);
    130 };
    131 
    132 }  // namespace base
    133 
    134 #endif  // BASE_THREADING_THREAD_LOCAL_H_
    135