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
62namespace base {
63namespace internal {
64
65// Helper functions that abstract the cross-platform APIs.  Do not use directly.
66struct 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
83template <typename Type>
84class 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
112class 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