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 // PLEASE READ: Do you really need a singleton? 6 // 7 // Singletons make it hard to determine the lifetime of an object, which can 8 // lead to buggy code and spurious crashes. 9 // 10 // Instead of adding another singleton into the mix, try to identify either: 11 // a) An existing singleton that can manage your object's lifetime 12 // b) Locations where you can deterministically create the object and pass 13 // into other objects 14 // 15 // If you absolutely need a singleton, please keep them as trivial as possible 16 // and ideally a leaf dependency. Singletons get problematic when they attempt 17 // to do too much in their destructor or have circular dependencies. 18 19 #ifndef BASE_MEMORY_SINGLETON_H_ 20 #define BASE_MEMORY_SINGLETON_H_ 21 22 #include "base/at_exit.h" 23 #include "base/atomicops.h" 24 #include "base/base_export.h" 25 #include "base/memory/aligned_memory.h" 26 #include "base/third_party/dynamic_annotations/dynamic_annotations.h" 27 #include "base/threading/thread_restrictions.h" 28 29 namespace base { 30 namespace internal { 31 32 // Our AtomicWord doubles as a spinlock, where a value of 33 // kBeingCreatedMarker means the spinlock is being held for creation. 34 static const subtle::AtomicWord kBeingCreatedMarker = 1; 35 36 // We pull out some of the functionality into a non-templated function, so that 37 // we can implement the more complicated pieces out of line in the .cc file. 38 BASE_EXPORT subtle::AtomicWord WaitForInstance(subtle::AtomicWord* instance); 39 40 } // namespace internal 41 } // namespace base 42 43 // TODO(joth): Move more of this file into namespace base 44 45 // Default traits for Singleton<Type>. Calls operator new and operator delete on 46 // the object. Registers automatic deletion at process exit. 47 // Overload if you need arguments or another memory allocation function. 48 template<typename Type> 49 struct DefaultSingletonTraits { 50 // Allocates the object. 51 static Type* New() { 52 // The parenthesis is very important here; it forces POD type 53 // initialization. 54 return new Type(); 55 } 56 57 // Destroys the object. 58 static void Delete(Type* x) { 59 delete x; 60 } 61 62 // Set to true to automatically register deletion of the object on process 63 // exit. See below for the required call that makes this happen. 64 static const bool kRegisterAtExit = true; 65 66 // Set to false to disallow access on a non-joinable thread. This is 67 // different from kRegisterAtExit because StaticMemorySingletonTraits allows 68 // access on non-joinable threads, and gracefully handles this. 69 static const bool kAllowedToAccessOnNonjoinableThread = false; 70 }; 71 72 73 // Alternate traits for use with the Singleton<Type>. Identical to 74 // DefaultSingletonTraits except that the Singleton will not be cleaned up 75 // at exit. 76 template<typename Type> 77 struct LeakySingletonTraits : public DefaultSingletonTraits<Type> { 78 static const bool kRegisterAtExit = false; 79 static const bool kAllowedToAccessOnNonjoinableThread = true; 80 }; 81 82 83 // Alternate traits for use with the Singleton<Type>. Allocates memory 84 // for the singleton instance from a static buffer. The singleton will 85 // be cleaned up at exit, but can't be revived after destruction unless 86 // the Resurrect() method is called. 87 // 88 // This is useful for a certain category of things, notably logging and 89 // tracing, where the singleton instance is of a type carefully constructed to 90 // be safe to access post-destruction. 91 // In logging and tracing you'll typically get stray calls at odd times, like 92 // during static destruction, thread teardown and the like, and there's a 93 // termination race on the heap-based singleton - e.g. if one thread calls 94 // get(), but then another thread initiates AtExit processing, the first thread 95 // may call into an object residing in unallocated memory. If the instance is 96 // allocated from the data segment, then this is survivable. 97 // 98 // The destructor is to deallocate system resources, in this case to unregister 99 // a callback the system will invoke when logging levels change. Note that 100 // this is also used in e.g. Chrome Frame, where you have to allow for the 101 // possibility of loading briefly into someone else's process space, and 102 // so leaking is not an option, as that would sabotage the state of your host 103 // process once you've unloaded. 104 template <typename Type> 105 struct StaticMemorySingletonTraits { 106 // WARNING: User has to deal with get() in the singleton class 107 // this is traits for returning NULL. 108 static Type* New() { 109 // Only constructs once and returns pointer; otherwise returns NULL. 110 if (base::subtle::NoBarrier_AtomicExchange(&dead_, 1)) 111 return NULL; 112 113 return new(buffer_.void_data()) Type(); 114 } 115 116 static void Delete(Type* p) { 117 if (p != NULL) 118 p->Type::~Type(); 119 } 120 121 static const bool kRegisterAtExit = true; 122 static const bool kAllowedToAccessOnNonjoinableThread = true; 123 124 // Exposed for unittesting. 125 static void Resurrect() { 126 base::subtle::NoBarrier_Store(&dead_, 0); 127 } 128 129 private: 130 static base::AlignedMemory<sizeof(Type), ALIGNOF(Type)> buffer_; 131 // Signal the object was already deleted, so it is not revived. 132 static base::subtle::Atomic32 dead_; 133 }; 134 135 template <typename Type> base::AlignedMemory<sizeof(Type), ALIGNOF(Type)> 136 StaticMemorySingletonTraits<Type>::buffer_; 137 template <typename Type> base::subtle::Atomic32 138 StaticMemorySingletonTraits<Type>::dead_ = 0; 139 140 // The Singleton<Type, Traits, DifferentiatingType> class manages a single 141 // instance of Type which will be created on first use and will be destroyed at 142 // normal process exit). The Trait::Delete function will not be called on 143 // abnormal process exit. 144 // 145 // DifferentiatingType is used as a key to differentiate two different 146 // singletons having the same memory allocation functions but serving a 147 // different purpose. This is mainly used for Locks serving different purposes. 148 // 149 // Example usage: 150 // 151 // In your header: 152 // template <typename T> struct DefaultSingletonTraits; 153 // class FooClass { 154 // public: 155 // static FooClass* GetInstance(); <-- See comment below on this. 156 // void Bar() { ... } 157 // private: 158 // FooClass() { ... } 159 // friend struct DefaultSingletonTraits<FooClass>; 160 // 161 // DISALLOW_COPY_AND_ASSIGN(FooClass); 162 // }; 163 // 164 // In your source file: 165 // #include "base/memory/singleton.h" 166 // FooClass* FooClass::GetInstance() { 167 // return Singleton<FooClass>::get(); 168 // } 169 // 170 // And to call methods on FooClass: 171 // FooClass::GetInstance()->Bar(); 172 // 173 // NOTE: The method accessing Singleton<T>::get() has to be named as GetInstance 174 // and it is important that FooClass::GetInstance() is not inlined in the 175 // header. This makes sure that when source files from multiple targets include 176 // this header they don't end up with different copies of the inlined code 177 // creating multiple copies of the singleton. 178 // 179 // Singleton<> has no non-static members and doesn't need to actually be 180 // instantiated. 181 // 182 // This class is itself thread-safe. The underlying Type must of course be 183 // thread-safe if you want to use it concurrently. Two parameters may be tuned 184 // depending on the user's requirements. 185 // 186 // Glossary: 187 // RAE = kRegisterAtExit 188 // 189 // On every platform, if Traits::RAE is true, the singleton will be destroyed at 190 // process exit. More precisely it uses base::AtExitManager which requires an 191 // object of this type to be instantiated. AtExitManager mimics the semantics 192 // of atexit() such as LIFO order but under Windows is safer to call. For more 193 // information see at_exit.h. 194 // 195 // If Traits::RAE is false, the singleton will not be freed at process exit, 196 // thus the singleton will be leaked if it is ever accessed. Traits::RAE 197 // shouldn't be false unless absolutely necessary. Remember that the heap where 198 // the object is allocated may be destroyed by the CRT anyway. 199 // 200 // Caveats: 201 // (a) Every call to get(), operator->() and operator*() incurs some overhead 202 // (16ns on my P4/2.8GHz) to check whether the object has already been 203 // initialized. You may wish to cache the result of get(); it will not 204 // change. 205 // 206 // (b) Your factory function must never throw an exception. This class is not 207 // exception-safe. 208 // 209 template <typename Type, 210 typename Traits = DefaultSingletonTraits<Type>, 211 typename DifferentiatingType = Type> 212 class Singleton { 213 private: 214 // Classes using the Singleton<T> pattern should declare a GetInstance() 215 // method and call Singleton::get() from within that. 216 friend Type* Type::GetInstance(); 217 218 // Allow TraceLog tests to test tracing after OnExit. 219 friend class DeleteTraceLogForTesting; 220 221 // This class is safe to be constructed and copy-constructed since it has no 222 // member. 223 224 // Return a pointer to the one true instance of the class. 225 static Type* get() { 226 #ifndef NDEBUG 227 // Avoid making TLS lookup on release builds. 228 if (!Traits::kAllowedToAccessOnNonjoinableThread) 229 base::ThreadRestrictions::AssertSingletonAllowed(); 230 #endif 231 232 base::subtle::AtomicWord value = base::subtle::NoBarrier_Load(&instance_); 233 if (value != 0 && value != base::internal::kBeingCreatedMarker) { 234 // See the corresponding HAPPENS_BEFORE below. 235 ANNOTATE_HAPPENS_AFTER(&instance_); 236 return reinterpret_cast<Type*>(value); 237 } 238 239 // Object isn't created yet, maybe we will get to create it, let's try... 240 if (base::subtle::Acquire_CompareAndSwap( 241 &instance_, 0, base::internal::kBeingCreatedMarker) == 0) { 242 // instance_ was NULL and is now kBeingCreatedMarker. Only one thread 243 // will ever get here. Threads might be spinning on us, and they will 244 // stop right after we do this store. 245 Type* newval = Traits::New(); 246 247 // This annotation helps race detectors recognize correct lock-less 248 // synchronization between different threads calling get(). 249 // See the corresponding HAPPENS_AFTER below and above. 250 ANNOTATE_HAPPENS_BEFORE(&instance_); 251 base::subtle::Release_Store( 252 &instance_, reinterpret_cast<base::subtle::AtomicWord>(newval)); 253 254 if (newval != NULL && Traits::kRegisterAtExit) 255 base::AtExitManager::RegisterCallback(OnExit, NULL); 256 257 return newval; 258 } 259 260 // We hit a race. Wait for the other thread to complete it. 261 value = base::internal::WaitForInstance(&instance_); 262 263 // See the corresponding HAPPENS_BEFORE above. 264 ANNOTATE_HAPPENS_AFTER(&instance_); 265 return reinterpret_cast<Type*>(value); 266 } 267 268 // Adapter function for use with AtExit(). This should be called single 269 // threaded, so don't use atomic operations. 270 // Calling OnExit while singleton is in use by other threads is a mistake. 271 static void OnExit(void* /*unused*/) { 272 // AtExit should only ever be register after the singleton instance was 273 // created. We should only ever get here with a valid instance_ pointer. 274 Traits::Delete( 275 reinterpret_cast<Type*>(base::subtle::NoBarrier_Load(&instance_))); 276 instance_ = 0; 277 } 278 static base::subtle::AtomicWord instance_; 279 }; 280 281 template <typename Type, typename Traits, typename DifferentiatingType> 282 base::subtle::AtomicWord Singleton<Type, Traits, DifferentiatingType>:: 283 instance_ = 0; 284 285 #endif // BASE_MEMORY_SINGLETON_H_ 286