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30 
31 // Author: kenton@google.com (Kenton Varda)
32 //
33 // emulates google3/base/once.h
34 //
35 // This header is intended to be included only by internal .cc files and
36 // generated .pb.cc files.  Users should not use this directly.
37 //
38 // This is basically a portable version of pthread_once().
39 //
40 // This header declares:
41 // * A type called ProtobufOnceType.
42 // * A macro GOOGLE_PROTOBUF_DECLARE_ONCE() which declares a variable of type
43 //   ProtobufOnceType.  This is the only legal way to declare such a variable.
44 //   The macro may only be used at the global scope (you cannot create local or
45 //   class member variables of this type).
46 // * A function GoogleOnceInit(ProtobufOnceType* once, void (*init_func)()).
47 //   This function, when invoked multiple times given the same ProtobufOnceType
48 //   object, will invoke init_func on the first call only, and will make sure
49 //   none of the calls return before that first call to init_func has finished.
50 // * The user can provide a parameter which GoogleOnceInit() forwards to the
51 //   user-provided function when it is called. Usage example:
52 //     int a = 10;
53 //     GoogleOnceInit(&my_once, &MyFunctionExpectingIntArgument, &a);
54 // * This implementation guarantees that ProtobufOnceType is a POD (i.e. no
55 //   static initializer generated).
56 //
57 // This implements a way to perform lazy initialization.  It's more efficient
58 // than using mutexes as no lock is needed if initialization has already
59 // happened.
60 //
61 // Example usage:
62 //   void Init();
63 //   GOOGLE_PROTOBUF_DECLARE_ONCE(once_init);
64 //
65 //   // Calls Init() exactly once.
66 //   void InitOnce() {
67 //     GoogleOnceInit(&once_init, &Init);
68 //   }
69 //
70 // Note that if GoogleOnceInit() is called before main() has begun, it must
71 // only be called by the thread that will eventually call main() -- that is,
72 // the thread that performs dynamic initialization.  In general this is a safe
73 // assumption since people don't usually construct threads before main() starts,
74 // but it is technically not guaranteed.  Unfortunately, Win32 provides no way
75 // whatsoever to statically-initialize its synchronization primitives, so our
76 // only choice is to assume that dynamic initialization is single-threaded.
77 
78 #ifndef GOOGLE_PROTOBUF_STUBS_ONCE_H__
79 #define GOOGLE_PROTOBUF_STUBS_ONCE_H__
80 
81 #include <google/protobuf/stubs/atomicops.h>
82 #include <google/protobuf/stubs/common.h>
83 
84 namespace google {
85 namespace protobuf {
86 
87 #ifdef GOOGLE_PROTOBUF_NO_THREAD_SAFETY
88 
89 typedef bool ProtobufOnceType;
90 
91 #define GOOGLE_PROTOBUF_ONCE_INIT false
92 
GoogleOnceInit(ProtobufOnceType * once,void (* init_func)())93 inline void GoogleOnceInit(ProtobufOnceType* once, void (*init_func)()) {
94   if (!*once) {
95     *once = true;
96     init_func();
97   }
98 }
99 
100 template <typename Arg>
GoogleOnceInit(ProtobufOnceType * once,void (* init_func)(Arg),Arg arg)101 inline void GoogleOnceInit(ProtobufOnceType* once, void (*init_func)(Arg),
102     Arg arg) {
103   if (!*once) {
104     *once = true;
105     init_func(arg);
106   }
107 }
108 
109 #else
110 
111 enum {
112   ONCE_STATE_UNINITIALIZED = 0,
113   ONCE_STATE_EXECUTING_CLOSURE = 1,
114   ONCE_STATE_DONE = 2
115 };
116 
117 typedef internal::AtomicWord ProtobufOnceType;
118 
119 #define GOOGLE_PROTOBUF_ONCE_INIT ::google::protobuf::ONCE_STATE_UNINITIALIZED
120 
121 LIBPROTOBUF_EXPORT
122 void GoogleOnceInitImpl(ProtobufOnceType* once, Closure* closure);
123 
124 inline void GoogleOnceInit(ProtobufOnceType* once, void (*init_func)()) {
125   if (internal::Acquire_Load(once) != ONCE_STATE_DONE) {
126     internal::FunctionClosure0 func(init_func, false);
127     GoogleOnceInitImpl(once, &func);
128   }
129 }
130 
131 template <typename Arg>
132 inline void GoogleOnceInit(ProtobufOnceType* once, void (*init_func)(Arg*),
133     Arg* arg) {
134   if (internal::Acquire_Load(once) != ONCE_STATE_DONE) {
135     internal::FunctionClosure1<Arg*> func(init_func, false, arg);
136     GoogleOnceInitImpl(once, &func);
137   }
138 }
139 
140 #endif  // GOOGLE_PROTOBUF_NO_THREAD_SAFETY
141 
142 class GoogleOnceDynamic {
143  public:
GoogleOnceDynamic()144   GoogleOnceDynamic() : state_(GOOGLE_PROTOBUF_ONCE_INIT) { }
145 
146   // If this->Init() has not been called before by any thread,
147   // execute (*func_with_arg)(arg) then return.
148   // Otherwise, wait until that prior invocation has finished
149   // executing its function, then return.
150   template<typename T>
Init(void (* func_with_arg)(T *),T * arg)151   void Init(void (*func_with_arg)(T*), T* arg) {
152     GoogleOnceInit<T>(&this->state_,
153                       func_with_arg,
154                       arg);
155   }
156  private:
157   ProtobufOnceType state_;
158 };
159 
160 #define GOOGLE_PROTOBUF_DECLARE_ONCE(NAME) \
161   ::google::protobuf::ProtobufOnceType NAME = GOOGLE_PROTOBUF_ONCE_INIT
162 
163 }  // namespace protobuf
164 }  // namespace google
165 
166 #endif  // GOOGLE_PROTOBUF_STUBS_ONCE_H__
167