Once.h

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/*
 * Objectively: Ultra-lightweight object oriented framework for GNU C.
 * Copyright (C) 2014 Jay Dolan <jay@jaydolan.com>
 *
 * This software is provided 'as-is', without any express or implied
 * warranty. In no event will the authors be held liable for any damages
 * arising from the use of this software.
 *
 * Permission is granted to anyone to use this software for any purpose,
 * including commercial applications, and to alter it and redistribute it
 * freely, subject to the following restrictions:
 *
 * 1. The origin of this software must not be misrepresented; you must not
 * claim that you wrote the original software. If you use this software
 * in a product, an acknowledgment in the product documentation would be
 * appreciated but is not required.
 *
 * 2. Altered source versions must be plainly marked as such, and must not be
 * misrepresented as being the original software.
 *
 * 3. This notice may not be removed or altered from any source distribution.
 */
 
#pragma once
 
#include <Objectively/Types.h>
 
typedef long Once;
 
#define do_once(once, block) \
    do { \
      Once _pending = 0; \
      if (__atomic_compare_exchange_n(once, &_pending, -1, 0, __ATOMIC_RELAXED, __ATOMIC_RELAXED)) { \
        block; __atomic_store_n(once, 1, __ATOMIC_RELEASE); \
      } else { \
        while (__atomic_load_n(once, __ATOMIC_ACQUIRE) != 1) ; \
      } \
    } while (0)