thread_local.h revision cce46a0c214b37e8da48c522c83037e8ffa4f9fd 
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
63namespace base {
64namespace internal {
65
66// Helper functions that abstract the cross-platform APIs.  Do not use directly.
67struct 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
84template <typename Type>
85class 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
113class 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