1 // Copyright 2012 the V8 project 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 // emulates google3/base/once.h 6 // 7 // This header is intended to be included only by v8's internal code. Users 8 // should not use this directly. 9 // 10 // This is basically a portable version of pthread_once(). 11 // 12 // This header declares: 13 // * A type called OnceType. 14 // * A macro V8_DECLARE_ONCE() which declares a (global) variable of type 15 // OnceType. 16 // * A function CallOnce(OnceType* once, void (*init_func)()). 17 // This function, when invoked multiple times given the same OnceType object, 18 // will invoke init_func on the first call only, and will make sure none of 19 // the calls return before that first call to init_func has finished. 20 // 21 // Additionally, the following features are supported: 22 // * A macro V8_ONCE_INIT which is expanded into the expression used to 23 // initialize a OnceType. This is only useful when clients embed a OnceType 24 // into a structure of their own and want to initialize it statically. 25 // * The user can provide a parameter which CallOnce() forwards to the 26 // user-provided function when it is called. Usage example: 27 // CallOnce(&my_once, &MyFunctionExpectingIntArgument, 10); 28 // * This implementation guarantees that OnceType is a POD (i.e. no static 29 // initializer generated). 30 // 31 // This implements a way to perform lazy initialization. It's more efficient 32 // than using mutexes as no lock is needed if initialization has already 33 // happened. 34 // 35 // Example usage: 36 // void Init(); 37 // V8_DECLARE_ONCE(once_init); 38 // 39 // // Calls Init() exactly once. 40 // void InitOnce() { 41 // CallOnce(&once_init, &Init); 42 // } 43 // 44 // Note that if CallOnce() is called before main() has begun, it must 45 // only be called by the thread that will eventually call main() -- that is, 46 // the thread that performs dynamic initialization. In general this is a safe 47 // assumption since people don't usually construct threads before main() starts, 48 // but it is technically not guaranteed. Unfortunately, Win32 provides no way 49 // whatsoever to statically-initialize its synchronization primitives, so our 50 // only choice is to assume that dynamic initialization is single-threaded. 51 52 #ifndef V8_BASE_ONCE_H_ 53 #define V8_BASE_ONCE_H_ 54 55 #include <stddef.h> 56 #include <functional> 57 58 #include "src/base/atomicops.h" 59 #include "src/base/base-export.h" 60 61 namespace v8 { 62 namespace base { 63 64 typedef AtomicWord OnceType; 65 66 #define V8_ONCE_INIT 0 67 68 #define V8_DECLARE_ONCE(NAME) ::v8::base::OnceType NAME 69 70 enum { 71 ONCE_STATE_UNINITIALIZED = 0, 72 ONCE_STATE_EXECUTING_FUNCTION = 1, 73 ONCE_STATE_DONE = 2 74 }; 75 76 typedef void (*NoArgFunction)(); 77 typedef void (*PointerArgFunction)(void* arg); 78 79 template <typename T> 80 struct OneArgFunction { 81 typedef void (*type)(T); 82 }; 83 84 V8_BASE_EXPORT void CallOnceImpl(OnceType* once, 85 std::function<void()> init_func); 86 87 inline void CallOnce(OnceType* once, NoArgFunction init_func) { 88 if (Acquire_Load(once) != ONCE_STATE_DONE) { 89 CallOnceImpl(once, init_func); 90 } 91 } 92 93 94 template <typename Arg> 95 inline void CallOnce(OnceType* once, 96 typename OneArgFunction<Arg*>::type init_func, Arg* arg) { 97 if (Acquire_Load(once) != ONCE_STATE_DONE) { 98 CallOnceImpl(once, [=]() { init_func(arg); }); 99 } 100 } 101 102 } // namespace base 103 } // namespace v8 104 105 #endif // V8_BASE_ONCE_H_ 106
