Undermind/proxy/cpp/include/google/protobuf/stubs/once.h

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// 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 three things:
// * 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 GogoleOnceInit(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.
//
// 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/common.h>

#ifndef _WIN32
#include <pthread.h>
#endif

namespace google {
namespace protobuf {

#ifdef _WIN32

struct ProtobufOnceInternal;

struct LIBPROTOBUF_EXPORT ProtobufOnceType {
  ProtobufOnceType();
  ~ProtobufOnceType();
  void Init(void (*init_func)());

  volatile bool initialized_;
  ProtobufOnceInternal* internal_;
};

#define GOOGLE_PROTOBUF_DECLARE_ONCE(NAME)                    \
  ::google::protobuf::ProtobufOnceType NAME

inline void GoogleOnceInit(ProtobufOnceType* once, void (*init_func)()) {
  // Note:  Double-checked locking is safe on x86.
  if (!once->initialized_) {
    once->Init(init_func);
  }
}

#else

typedef pthread_once_t ProtobufOnceType;

#define GOOGLE_PROTOBUF_DECLARE_ONCE(NAME)                    \
  pthread_once_t NAME = PTHREAD_ONCE_INIT

inline void GoogleOnceInit(ProtobufOnceType* once, void (*init_func)()) {
  pthread_once(once, init_func);
}

#endif

}  // namespace protobuf
}  // namespace google

#endif  // GOOGLE_PROTOBUF_STUBS_ONCE_H__