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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 // Example usage:
     30 //   // My class is logically attached to a single thread.  We cache a pointer
     31 //   // on the thread it was created on, so we can implement current().
     32 //   MyClass::MyClass() {
     33 //     DCHECK(Singleton<ThreadLocalPointer<MyClass> >::get()->Get() == NULL);
     34 //     Singleton<ThreadLocalPointer<MyClass> >::get()->Set(this);
     35 //   }
     36 //
     37 //   MyClass::~MyClass() {
     38 //     DCHECK(Singleton<ThreadLocalPointer<MyClass> >::get()->Get() != NULL);
     39 //     Singleton<ThreadLocalPointer<MyClass> >::get()->Set(NULL);
     40 //   }
     41 //
     42 //   // Return the current MyClass associated with the calling thread, can be
     43 //   // NULL if there isn't a MyClass associated.
     44 //   MyClass* MyClass::current() {
     45 //     return Singleton<ThreadLocalPointer<MyClass> >::get()->Get();
     46 //   }
     47 
     48 #ifndef BASE_THREADING_THREAD_LOCAL_H_
     49 #define BASE_THREADING_THREAD_LOCAL_H_
     50 
     51 #include "base/base_export.h"
     52 #include "base/basictypes.h"
     53 
     54 #if defined(OS_POSIX)
     55 #include <pthread.h>
     56 #endif
     57 
     58 namespace base {
     59 namespace internal {
     60 
     61 // Helper functions that abstract the cross-platform APIs.  Do not use directly.
     62 struct BASE_EXPORT ThreadLocalPlatform {
     63 #if defined(OS_WIN)
     64   typedef unsigned long SlotType;
     65 #elif defined(OS_POSIX)
     66   typedef pthread_key_t SlotType;
     67 #endif
     68 
     69   static void AllocateSlot(SlotType* slot);
     70   static void FreeSlot(SlotType slot);
     71   static void* GetValueFromSlot(SlotType slot);
     72   static void SetValueInSlot(SlotType slot, void* value);
     73 };
     74 
     75 }  // namespace internal
     76 
     77 template <typename Type>
     78 class ThreadLocalPointer {
     79  public:
     80   ThreadLocalPointer() : slot_() {
     81     internal::ThreadLocalPlatform::AllocateSlot(&slot_);
     82   }
     83 
     84   ~ThreadLocalPointer() {
     85     internal::ThreadLocalPlatform::FreeSlot(slot_);
     86   }
     87 
     88   Type* Get() {
     89     return static_cast<Type*>(
     90         internal::ThreadLocalPlatform::GetValueFromSlot(slot_));
     91   }
     92 
     93   void Set(Type* ptr) {
     94     internal::ThreadLocalPlatform::SetValueInSlot(
     95         slot_, const_cast<void*>(static_cast<const void*>(ptr)));
     96   }
     97 
     98  private:
     99   typedef internal::ThreadLocalPlatform::SlotType SlotType;
    100 
    101   SlotType slot_;
    102 
    103   DISALLOW_COPY_AND_ASSIGN(ThreadLocalPointer<Type>);
    104 };
    105 
    106 class ThreadLocalBoolean {
    107  public:
    108   ThreadLocalBoolean() {}
    109   ~ThreadLocalBoolean() {}
    110 
    111   bool Get() {
    112     return tlp_.Get() != NULL;
    113   }
    114 
    115   void Set(bool val) {
    116     tlp_.Set(val ? this : NULL);
    117   }
    118 
    119  private:
    120   ThreadLocalPointer<void> tlp_;
    121 
    122   DISALLOW_COPY_AND_ASSIGN(ThreadLocalBoolean);
    123 };
    124 
    125 }  // namespace base
    126 
    127 #endif  // BASE_THREADING_THREAD_LOCAL_H_
    128