base/thread_local.h

   1 // Copyright (c) 2006-2008 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 warpper 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_THREAD_LOCAL_H_
  49 #define BASE_THREAD_LOCAL_H_
  50
  51 #include "base/basictypes.h"
  52
  53 #if defined(OS_POSIX)
  54 #include <pthread.h>
  55 #endif
  56
  57 namespace base {
  58
  59 // Helper functions that abstract the cross-platform APIs.  Do not use directly.
  60 struct ThreadLocalPlatform {
  61 #if defined(OS_WIN)
  62   typedef int SlotType;
  63 #elif defined(OS_POSIX)
  64   typedef pthread_key_t SlotType;
  65 #endif
  66
  67   static void AllocateSlot(SlotType& slot);
  68   static void FreeSlot(SlotType& slot);
  69   static void* GetValueFromSlot(SlotType& slot);
  70   static void SetValueInSlot(SlotType& slot, void* value);
  71 };
  72
  73 template <typename Type>
  74 class ThreadLocalPointer {
  75  public:
  76   ThreadLocalPointer() : slot_() {
  77     ThreadLocalPlatform::AllocateSlot(slot_);
  78   }
  79
  80   ~ThreadLocalPointer() {
  81     ThreadLocalPlatform::FreeSlot(slot_);
  82   }
  83
  84   Type* Get() {
  85     return static_cast<Type*>(ThreadLocalPlatform::GetValueFromSlot(slot_));
  86   }
  87
  88   void Set(Type* ptr) {
  89     ThreadLocalPlatform::SetValueInSlot(slot_, ptr);
  90   }
  91
  92  private:
  93   typedef ThreadLocalPlatform::SlotType SlotType;
  94
  95   SlotType slot_;
  96
  97   DISALLOW_COPY_AND_ASSIGN(ThreadLocalPointer<Type>);
  98 };
  99
 100 class ThreadLocalBoolean {
 101  public:
 102   ThreadLocalBoolean() { }
 103   ~ThreadLocalBoolean() { }
 104
 105   bool Get() {
 106     return tlp_.Get() != NULL;
 107   }
 108
 109   void Set(bool val) {
 110     tlp_.Set(reinterpret_cast<void*>(val ? 1 : 0));
 111   }
 112
 113  private:
 114   ThreadLocalPointer<void> tlp_;
 115
 116   DISALLOW_COPY_AND_ASSIGN(ThreadLocalBoolean);
 117 };
 118
 119 }  // namespace base
 120
 121 #endif  // BASE_THREAD_LOCAL_H_