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// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc. All rights reserved.
// http://code.google.com/p/protobuf/
//
// 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/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

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

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

} // namespace protobuf
} // namespace google

#endif // GOOGLE_PROTOBUF_STUBS_ONCE_H__

Change log

r413 by pli...@google.com on Apr 10, 2012   Diff
Improve GoogleOnceInit() in Protocol
Buffers.

It is based on V8's new CallOnce():
http://codereview.chromium.org/9447052/.

This patch includes the following changes:
  - POD (no static initializer generated)
and faster implementation on Windows.
  - GoogleOnceInit() can now take an
additional parameter which is forwarded to
    the function provided by the user.
...
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Older revisions

r256 by temporal on Dec 18, 2009   Diff
Ensure that 'once' objects are
declared using the macro.  This is
either the third or fourth time I've
screwed this up when down-integrating,
because our internal code does not
...
r146 by ken...@google.com on May 18, 2009   Diff
Fix final leak (win32 only).
r125 by ken...@google.com on Apr 27, 2009   Diff
Make building libprotobuf as a DLL
work again.
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