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 29namespace base { 30namespace internal { 31 32// Our AtomicWord doubles as a spinlock, where a value of 33// kBeingCreatedMarker means the spinlock is being held for creation. 34static 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. 38BASE_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. 48template<typename Type> 49struct 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#ifndef NDEBUG 67 // Set to false to disallow access on a non-joinable thread. This is 68 // different from kRegisterAtExit because StaticMemorySingletonTraits allows 69 // access on non-joinable threads, and gracefully handles this. 70 static const bool kAllowedToAccessOnNonjoinableThread = false; 71#endif 72}; 73 74 75// Alternate traits for use with the Singleton<Type>. Identical to 76// DefaultSingletonTraits except that the Singleton will not be cleaned up 77// at exit. 78template<typename Type> 79struct LeakySingletonTraits : public DefaultSingletonTraits<Type> { 80 static const bool kRegisterAtExit = false; 81#ifndef NDEBUG 82 static const bool kAllowedToAccessOnNonjoinableThread = true; 83#endif 84}; 85 86 87// Alternate traits for use with the Singleton<Type>. Allocates memory 88// for the singleton instance from a static buffer. The singleton will 89// be cleaned up at exit, but can't be revived after destruction unless 90// the Resurrect() method is called. 91// 92// This is useful for a certain category of things, notably logging and 93// tracing, where the singleton instance is of a type carefully constructed to 94// be safe to access post-destruction. 95// In logging and tracing you'll typically get stray calls at odd times, like 96// during static destruction, thread teardown and the like, and there's a 97// termination race on the heap-based singleton - e.g. if one thread calls 98// get(), but then another thread initiates AtExit processing, the first thread 99// may call into an object residing in unallocated memory. If the instance is 100// allocated from the data segment, then this is survivable. 101// 102// The destructor is to deallocate system resources, in this case to unregister 103// a callback the system will invoke when logging levels change. Note that 104// this is also used in e.g. Chrome Frame, where you have to allow for the 105// possibility of loading briefly into someone else's process space, and 106// so leaking is not an option, as that would sabotage the state of your host 107// process once you've unloaded. 108template <typename Type> 109struct StaticMemorySingletonTraits { 110 // WARNING: User has to deal with get() in the singleton class 111 // this is traits for returning NULL. 112 static Type* New() { 113 // Only constructs once and returns pointer; otherwise returns NULL. 114 if (base::subtle::NoBarrier_AtomicExchange(&dead_, 1)) 115 return NULL; 116 117 return new(buffer_.void_data()) Type(); 118 } 119 120 static void Delete(Type* p) { 121 if (p != NULL) 122 p->Type::~Type(); 123 } 124 125 static const bool kRegisterAtExit = true; 126 static const bool kAllowedToAccessOnNonjoinableThread = true; 127 128 // Exposed for unittesting. 129 static void Resurrect() { 130 base::subtle::NoBarrier_Store(&dead_, 0); 131 } 132 133 private: 134 static base::AlignedMemory<sizeof(Type), ALIGNOF(Type)> buffer_; 135 // Signal the object was already deleted, so it is not revived. 136 static base::subtle::Atomic32 dead_; 137}; 138 139template <typename Type> base::AlignedMemory<sizeof(Type), ALIGNOF(Type)> 140 StaticMemorySingletonTraits<Type>::buffer_; 141template <typename Type> base::subtle::Atomic32 142 StaticMemorySingletonTraits<Type>::dead_ = 0; 143 144// The Singleton<Type, Traits, DifferentiatingType> class manages a single 145// instance of Type which will be created on first use and will be destroyed at 146// normal process exit). The Trait::Delete function will not be called on 147// abnormal process exit. 148// 149// DifferentiatingType is used as a key to differentiate two different 150// singletons having the same memory allocation functions but serving a 151// different purpose. This is mainly used for Locks serving different purposes. 152// 153// Example usage: 154// 155// In your header: 156// template <typename T> struct DefaultSingletonTraits; 157// class FooClass { 158// public: 159// static FooClass* GetInstance(); <-- See comment below on this. 160// void Bar() { ... } 161// private: 162// FooClass() { ... } 163// friend struct DefaultSingletonTraits<FooClass>; 164// 165// DISALLOW_COPY_AND_ASSIGN(FooClass); 166// }; 167// 168// In your source file: 169// #include "base/memory/singleton.h" 170// FooClass* FooClass::GetInstance() { 171// return Singleton<FooClass>::get(); 172// } 173// 174// And to call methods on FooClass: 175// FooClass::GetInstance()->Bar(); 176// 177// NOTE: The method accessing Singleton<T>::get() has to be named as GetInstance 178// and it is important that FooClass::GetInstance() is not inlined in the 179// header. This makes sure that when source files from multiple targets include 180// this header they don't end up with different copies of the inlined code 181// creating multiple copies of the singleton. 182// 183// Singleton<> has no non-static members and doesn't need to actually be 184// instantiated. 185// 186// This class is itself thread-safe. The underlying Type must of course be 187// thread-safe if you want to use it concurrently. Two parameters may be tuned 188// depending on the user's requirements. 189// 190// Glossary: 191// RAE = kRegisterAtExit 192// 193// On every platform, if Traits::RAE is true, the singleton will be destroyed at 194// process exit. More precisely it uses base::AtExitManager which requires an 195// object of this type to be instantiated. AtExitManager mimics the semantics 196// of atexit() such as LIFO order but under Windows is safer to call. For more 197// information see at_exit.h. 198// 199// If Traits::RAE is false, the singleton will not be freed at process exit, 200// thus the singleton will be leaked if it is ever accessed. Traits::RAE 201// shouldn't be false unless absolutely necessary. Remember that the heap where 202// the object is allocated may be destroyed by the CRT anyway. 203// 204// Caveats: 205// (a) Every call to get(), operator->() and operator*() incurs some overhead 206// (16ns on my P4/2.8GHz) to check whether the object has already been 207// initialized. You may wish to cache the result of get(); it will not 208// change. 209// 210// (b) Your factory function must never throw an exception. This class is not 211// exception-safe. 212// 213template <typename Type, 214 typename Traits = DefaultSingletonTraits<Type>, 215 typename DifferentiatingType = Type> 216class Singleton { 217 private: 218 // Classes using the Singleton<T> pattern should declare a GetInstance() 219 // method and call Singleton::get() from within that. 220 friend Type* Type::GetInstance(); 221 222 // Allow TraceLog tests to test tracing after OnExit. 223 friend class DeleteTraceLogForTesting; 224 225 // This class is safe to be constructed and copy-constructed since it has no 226 // member. 227 228 // Return a pointer to the one true instance of the class. 229 static Type* get() { 230#ifndef NDEBUG 231 // Avoid making TLS lookup on release builds. 232 if (!Traits::kAllowedToAccessOnNonjoinableThread) 233 base::ThreadRestrictions::AssertSingletonAllowed(); 234#endif 235 236 // The load has acquire memory ordering as the thread which reads the 237 // instance_ pointer must acquire visibility over the singleton data. 238 base::subtle::AtomicWord value = base::subtle::Acquire_Load(&instance_); 239 if (value != 0 && value != base::internal::kBeingCreatedMarker) { 240 // See the corresponding HAPPENS_BEFORE below. 241 ANNOTATE_HAPPENS_AFTER(&instance_); 242 return reinterpret_cast<Type*>(value); 243 } 244 245 // Object isn't created yet, maybe we will get to create it, let's try... 246 if (base::subtle::Acquire_CompareAndSwap( 247 &instance_, 0, base::internal::kBeingCreatedMarker) == 0) { 248 // instance_ was NULL and is now kBeingCreatedMarker. Only one thread 249 // will ever get here. Threads might be spinning on us, and they will 250 // stop right after we do this store. 251 Type* newval = Traits::New(); 252 253 // This annotation helps race detectors recognize correct lock-less 254 // synchronization between different threads calling get(). 255 // See the corresponding HAPPENS_AFTER below and above. 256 ANNOTATE_HAPPENS_BEFORE(&instance_); 257 // Releases the visibility over instance_ to the readers. 258 base::subtle::Release_Store( 259 &instance_, reinterpret_cast<base::subtle::AtomicWord>(newval)); 260 261 if (newval != NULL && Traits::kRegisterAtExit) 262 base::AtExitManager::RegisterCallback(OnExit, NULL); 263 264 return newval; 265 } 266 267 // We hit a race. Wait for the other thread to complete it. 268 value = base::internal::WaitForInstance(&instance_); 269 270 // See the corresponding HAPPENS_BEFORE above. 271 ANNOTATE_HAPPENS_AFTER(&instance_); 272 return reinterpret_cast<Type*>(value); 273 } 274 275 // Adapter function for use with AtExit(). This should be called single 276 // threaded, so don't use atomic operations. 277 // Calling OnExit while singleton is in use by other threads is a mistake. 278 static void OnExit(void* /*unused*/) { 279 // AtExit should only ever be register after the singleton instance was 280 // created. We should only ever get here with a valid instance_ pointer. 281 Traits::Delete( 282 reinterpret_cast<Type*>(base::subtle::NoBarrier_Load(&instance_))); 283 instance_ = 0; 284 } 285 static base::subtle::AtomicWord instance_; 286}; 287 288template <typename Type, typename Traits, typename DifferentiatingType> 289base::subtle::AtomicWord Singleton<Type, Traits, DifferentiatingType>:: 290 instance_ = 0; 291 292#endif // BASE_MEMORY_SINGLETON_H_ 293