thread_local.h revision 3f50c38dc070f4bb515c1b64450dae14f316474e 
1// Copyright (c) 2010 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#pragma once
51
52#include "base/basictypes.h"
53
54#if defined(OS_POSIX)
55#include <pthread.h>
56#endif
57
58namespace base {
59
60namespace internal {
61
62// Helper functions that abstract the cross-platform APIs.  Do not use directly.
63struct ThreadLocalPlatform {
64#if defined(OS_WIN)
65  typedef unsigned long SlotType;
66#elif defined(OS_POSIX)
67  typedef pthread_key_t SlotType;
68#endif
69
70  static void AllocateSlot(SlotType& slot);
71  static void FreeSlot(SlotType& slot);
72  static void* GetValueFromSlot(SlotType& slot);
73  static void SetValueInSlot(SlotType& slot, void* value);
74};
75
76}  // namespace internal
77
78template <typename Type>
79class ThreadLocalPointer {
80 public:
81  ThreadLocalPointer() : slot_() {
82    internal::ThreadLocalPlatform::AllocateSlot(slot_);
83  }
84
85  ~ThreadLocalPointer() {
86    internal::ThreadLocalPlatform::FreeSlot(slot_);
87  }
88
89  Type* Get() {
90    return static_cast<Type*>(
91        internal::ThreadLocalPlatform::GetValueFromSlot(slot_));
92  }
93
94  void Set(Type* ptr) {
95    internal::ThreadLocalPlatform::SetValueInSlot(slot_, 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
106class 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(reinterpret_cast<void*>(val ? 1 : 0));
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