base/threading/thread_local.h

   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_