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include/google/protobuf/stubs/once.h

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+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// Author: kenton@google.com (Kenton Varda)
+//
+// emulates google3/base/once.h
+//
+// This header is intended to be included only by internal .cc files and
+// generated .pb.cc files.  Users should not use this directly.
+//
+// This is basically a portable version of pthread_once().
+//
+// This header declares:
+// * A type called ProtobufOnceType.
+// * A macro GOOGLE_PROTOBUF_DECLARE_ONCE() which declares a variable of type
+//   ProtobufOnceType.  This is the only legal way to declare such a variable.
+//   The macro may only be used at the global scope (you cannot create local or
+//   class member variables of this type).
+// * A function GoogleOnceInit(ProtobufOnceType* once, void (*init_func)()).
+//   This function, when invoked multiple times given the same ProtobufOnceType
+//   object, will invoke init_func on the first call only, and will make sure
+//   none of the calls return before that first call to init_func has finished.
+// * The user can provide a parameter which GoogleOnceInit() forwards to the
+//   user-provided function when it is called. Usage example:
+//     int a = 10;
+//     GoogleOnceInit(&my_once, &MyFunctionExpectingIntArgument, &a);
+// * This implementation guarantees that ProtobufOnceType is a POD (i.e. no
+//   static initializer generated).
+//
+// This implements a way to perform lazy initialization.  It's more efficient
+// than using mutexes as no lock is needed if initialization has already
+// happened.
+//
+// Example usage:
+//   void Init();
+//   GOOGLE_PROTOBUF_DECLARE_ONCE(once_init);
+//
+//   // Calls Init() exactly once.
+//   void InitOnce() {
+//     GoogleOnceInit(&once_init, &Init);
+//   }
+//
+// Note that if GoogleOnceInit() is called before main() has begun, it must
+// only be called by the thread that will eventually call main() -- that is,
+// the thread that performs dynamic initialization.  In general this is a safe
+// assumption since people don't usually construct threads before main() starts,
+// but it is technically not guaranteed.  Unfortunately, Win32 provides no way
+// whatsoever to statically-initialize its synchronization primitives, so our
+// only choice is to assume that dynamic initialization is single-threaded.
+
+#ifndef GOOGLE_PROTOBUF_STUBS_ONCE_H__
+#define GOOGLE_PROTOBUF_STUBS_ONCE_H__
+
+#include <google/protobuf/stubs/atomicops.h>
+#include <google/protobuf/stubs/callback.h>
+#include <google/protobuf/stubs/common.h>
+
+namespace google {
+namespace protobuf {
+
+#ifdef GOOGLE_PROTOBUF_NO_THREAD_SAFETY
+
+typedef bool ProtobufOnceType;
+
+#define GOOGLE_PROTOBUF_ONCE_INIT false
+
+inline void GoogleOnceInit(ProtobufOnceType* once, void (*init_func)()) {
+  if (!*once) {
+    *once = true;
+    init_func();
+  }
+}
+
+template <typename Arg>
+inline void GoogleOnceInit(ProtobufOnceType* once, void (*init_func)(Arg),
+    Arg arg) {
+  if (!*once) {
+    *once = true;
+    init_func(arg);
+  }
+}
+
+#else
+
+enum {
+  ONCE_STATE_UNINITIALIZED = 0,
+  ONCE_STATE_EXECUTING_CLOSURE = 1,
+  ONCE_STATE_DONE = 2
+};
+
+typedef internal::AtomicWord ProtobufOnceType;
+
+#define GOOGLE_PROTOBUF_ONCE_INIT ::google::protobuf::ONCE_STATE_UNINITIALIZED
+
+LIBPROTOBUF_EXPORT
+void GoogleOnceInitImpl(ProtobufOnceType* once, Closure* closure);
+
+inline void GoogleOnceInit(ProtobufOnceType* once, void (*init_func)()) {
+  if (internal::Acquire_Load(once) != ONCE_STATE_DONE) {
+    internal::FunctionClosure0 func(init_func, false);
+    GoogleOnceInitImpl(once, &func);
+  }
+}
+
+template <typename Arg>
+inline void GoogleOnceInit(ProtobufOnceType* once, void (*init_func)(Arg*),
+    Arg* arg) {
+  if (internal::Acquire_Load(once) != ONCE_STATE_DONE) {
+    internal::FunctionClosure1<Arg*> func(init_func, false, arg);
+    GoogleOnceInitImpl(once, &func);
+  }
+}
+
+#endif  // GOOGLE_PROTOBUF_NO_THREAD_SAFETY
+
+class GoogleOnceDynamic {
+ public:
+  GoogleOnceDynamic() : state_(GOOGLE_PROTOBUF_ONCE_INIT) { }
+
+  // If this->Init() has not been called before by any thread,
+  // execute (*func_with_arg)(arg) then return.
+  // Otherwise, wait until that prior invocation has finished
+  // executing its function, then return.
+  template<typename T>
+  void Init(void (*func_with_arg)(T*), T* arg) {
+    GoogleOnceInit<T>(&this->state_,
+                      func_with_arg,
+                      arg);
+  }
+ private:
+  ProtobufOnceType state_;
+};
+
+#define GOOGLE_PROTOBUF_DECLARE_ONCE(NAME) \
+  ::google::protobuf::ProtobufOnceType NAME = GOOGLE_PROTOBUF_ONCE_INIT
+
+}  // namespace protobuf
+}  // namespace google
+
+#endif  // GOOGLE_PROTOBUF_STUBS_ONCE_H__