Home | History | Annotate | Download | only in threading 
      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_
    129