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/* nbdkit
* Copyright Red Hat
*
* 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 Red Hat 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 RED HAT 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 RED HAT 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.
*/
#ifndef NBDKIT_ONCE_H
#define NBDKIT_ONCE_H
#ifdef HAVE_STDATOMIC_H
#include <stdatomic.h>
#else
/* This is best effort on platforms that don't support atomic.
* 32 bit ints are generally fine in reality.
*/
#define _Atomic /**/
#endif
#include "unique-name.h"
/* Run the statement once (per nbdkit run). */
#define ONCE(stmt) ONCE_1(NBDKIT_UNIQUE_NAME(_once), (stmt))
/* The actual implementation:
*
* The comparison with 0 avoids var wrapping around. Mostly var will
* only be 0 or 1, or in rare cases other small integers.
*
* The atomic increment & comparison with 1 is what only allows a
* single thread to run the statement.
*
* To avoid optimisations: Use 'volatile' so reads and writes are not
* removed, and use 'unsigned' to avoid any with signed overflow.
*/
#define ONCE_1(var, stmt) \
do { \
static volatile _Atomic unsigned var = 0; \
if (var == 0 && ++var == 1) { stmt; } \
} while (0)
#endif /* NBDKIT_ONCE_H */
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