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:
ThreadLocalPointer()87   ThreadLocalPointer() : slot_() {
88     internal::ThreadLocalPlatform::AllocateSlot(&slot_);
89   }
90 
~ThreadLocalPointer()91   ~ThreadLocalPointer() {
92     internal::ThreadLocalPlatform::FreeSlot(slot_);
93   }
94 
Get()95   Type* Get() {
96     return static_cast<Type*>(
97         internal::ThreadLocalPlatform::GetValueFromSlot(slot_));
98   }
99 
Set(Type * ptr)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:
ThreadLocalBoolean()115   ThreadLocalBoolean() {}
~ThreadLocalBoolean()116   ~ThreadLocalBoolean() {}
117 
Get()118   bool Get() {
119     return tlp_.Get() != NULL;
120   }
121 
Set(bool val)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