thread_local.h revision ddb351dbec246cf1fab5ec20d2d5520909041de1 
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// 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/base_api.h"
53#include "base/basictypes.h"
54
55#if defined(OS_POSIX)
56#include <pthread.h>
57#endif
58
59namespace base {
60
61namespace internal {
62
63// Helper functions that abstract the cross-platform APIs.  Do not use directly.
64struct BASE_API ThreadLocalPlatform {
65#if defined(OS_WIN)
66  typedef unsigned long SlotType;
67#elif defined(OS_POSIX)
68  typedef pthread_key_t SlotType;
69#endif
70
71  static void AllocateSlot(SlotType& slot);
72  static void FreeSlot(SlotType& slot);
73  static void* GetValueFromSlot(SlotType& slot);
74  static void SetValueInSlot(SlotType& slot, void* value);
75};
76
77}  // namespace internal
78
79template <typename Type>
80class ThreadLocalPointer {
81 public:
82  ThreadLocalPointer() : slot_() {
83    internal::ThreadLocalPlatform::AllocateSlot(slot_);
84  }
85
86  ~ThreadLocalPointer() {
87    internal::ThreadLocalPlatform::FreeSlot(slot_);
88  }
89
90  Type* Get() {
91    return static_cast<Type*>(
92        internal::ThreadLocalPlatform::GetValueFromSlot(slot_));
93  }
94
95  void Set(Type* ptr) {
96    internal::ThreadLocalPlatform::SetValueInSlot(slot_, ptr);
97  }
98
99 private:
100  typedef internal::ThreadLocalPlatform::SlotType SlotType;
101
102  SlotType slot_;
103
104  DISALLOW_COPY_AND_ASSIGN(ThreadLocalPointer<Type>);
105};
106
107class ThreadLocalBoolean {
108 public:
109  ThreadLocalBoolean() { }
110  ~ThreadLocalBoolean() { }
111
112  bool Get() {
113    return tlp_.Get() != NULL;
114  }
115
116  void Set(bool val) {
117    tlp_.Set(reinterpret_cast<void*>(val ? 1 : 0));
118  }
119
120 private:
121  ThreadLocalPointer<void> tlp_;
122
123  DISALLOW_COPY_AND_ASSIGN(ThreadLocalBoolean);
124};
125
126}  // namespace base
127
128#endif  // BASE_THREADING_THREAD_LOCAL_H_
129