base/threading/thread_local.h

   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
  51 #include "base/base_export.h"
  52 #include "base/basictypes.h"
  53
  54 #if defined(OS_POSIX)
  55 #include <pthread.h>
  56 #endif
  57
  58 namespace base {
  59
  60 namespace internal {
  61
  62 // Helper functions that abstract the cross-platform APIs.  Do not use directly.
  63 struct BASE_EXPORT 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
  78 template <typename Type>
  79 class 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(
  96         slot_, const_cast<void*>(static_cast<const void*>(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
 107 class 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(val ? this : NULL);
 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_